Separation of crack extension modes in orthotropic delamination models

NASA Technical Reports Server (NTRS)

Beuth, Jack L.

1995-01-01

In the analysis of an interface crack between dissimilar elastic materials, the mode of crack extension is typically not unique, due to oscillatory behavior of near-tip stresses and displacements. This behavior currently limits the applicability of interfacial fracture mechanics as a means to predict composite delamination. The Virtual Crack Closure Technique (VCCT) is a method used to extract mode 1 and mode 2 energy release rates from numerical fracture solutions. The mode of crack extension extracted from an oscillatory solution using the VCCT is not unique due to the dependence of mode on the virtual crack extension length, Delta. In this work, a method is presented for using the VCCT to extract Delta-independent crack extension modes for the case of an interface crack between two in-plane orthotropic materials. The method does not involve altering the analysis to eliminate its oscillatory behavior. Instead, it is argued that physically reasonable, Delta-independent modes of crack extension can be extracted from oscillatory solutions. Knowledge of near-tip fields is used to determine the explicit Delta dependence of energy release rate parameters. Energy release rates are then defined that are separated from the oscillatory dependence on Delta. A modified VCCT using these energy release rate definitions is applied to results from finite element analyses, showing that Delta-independent modes of crack extension result. The modified technique has potential as a consistent method for extracting crack extension modes from numerical solutions. The Delta-independent modes extracted using this technique can also serve as guides for testing the convergence of finite element models. Direct applications of this work include the analysis of planar composite delamination problems, where plies or debonded laminates are modeled as in-plane orthotropic materials.

Crack closure on rehydration of glass-ionomer materials.

PubMed

Sidhu, Sharanbir K; Pilecki, Peter; Sherriff, Martyn; Watson, Timothy F

2004-10-01

Moisture-sensitivity of immature glass-ionomer cements suggests that hydration-induced volumetric expansion might close and potentially heal established cracks. Crack closure in glass-ionomer cements (GICs) was observed following rehydration. Circular cavities were prepared in 15 teeth: 10 were restored with resin-modified GICs (5 with Fuji II LC and 5 with Photac-Fil) and 5 were restored with a conventional GIC (Fuji IX); all were dehydrated for 1 min with air and imaged immediately by confocal microscopy. Crack formation in each was located, after which water was placed on the surface and observed for 15 min via a CCD camera. Dehydration caused cracks with measurable gaps, while rehydration resulted in varying degrees of closure: closure was limited in the conventional GIC, and complete or near complete along part/s of the crack in the resin-modified GICs. In all, closure movement became imperceptible after the first 10 min. Statistical analysis indicated no significant difference between the closure behavior of all materials. However, the resin-modified GICs appeared to show a greater potential for closure of established cracks than the conventional GIC upon rehydration.

Analysis of local delaminations caused by angle ply matrix cracks

NASA Technical Reports Server (NTRS)

Salpekar, Satish A.; Obrien, T. Kevin; Shivakumar, K. N.

1993-01-01

Two different families of graphite/epoxy laminates with similar layups but different stacking sequences, (0,theta,-theta) sub s and (-theta/theta/0) sub s were analyzed using three-dimensional finite element analysis for theta = 15 and 30 degrees. Delaminations were modeled in the -theta/theta interface, bounded by a matrix crack and the stress free edge. The total strain energy release rate, G, along the delamination front was computed using three different techniques: the virtual crack closure technique (VCCT), the equivalent domain Integral (EDI) technique, and a global energy balance technique. The opening fracture mode component of the strain energy release rate, Gl, along the delamination front was also computed for various delamination lengths using VCCT. The effect of residual thermal and moisture stresses on G was evaluated.

Overload retardation due to plasticity-induced crack closure

NASA Technical Reports Server (NTRS)

Fleck, N. A.; Shercliff, H. R.

1989-01-01

Experiments are reported which show that plasticity-induced crack closure can account for crack growth retardation following an overload. The finite element method is used to provide evidence which supports the experimental observations of crack closure. Finally, a simple model is presented which predicts with limited success the retardation transient following an overload.

Fatigue crack growth behaviour of semi-elliptical surface cracks for an API 5L X65 gas pipeline under tension

NASA Astrophysics Data System (ADS)

Shaari, M. S.; Akramin, M. R. M.; Ariffin, A. K.; Abdullah, S.; Kikuchi, M.

2018-02-01

The paper is presenting the fatigue crack growth (FCG) behavior of semi-elliptical surface cracks for API X65 gas pipeline using S-version FEM. A method known as global-local overlay technique was used in this study to predict the fatigue behavior that involve of two separate meshes each specifically for global (geometry) and local (crack). The pre-post program was used to model the global geometry (coarser mesh) known as FAST including the material and boundary conditions. Hence, the local crack (finer mesh) will be defined the exact location and the mesh control accordingly. The local mesh was overlaid along with the global before the numerical computation taken place to solve the engineering problem. The stress intensity factors were computed using the virtual crack closure-integral method (VCCM). The most important results is the behavior of the fatigue crack growth, which contains the crack depth (a), crack length (c) and stress intensity factors (SIF). The correlation between the fatigue crack growth and the SIF shows a good growth for the crack depth (a) and dissimilar for the crack length (c) where stunned behavior was resulted. The S-version FEM will benefiting the user due to the overlay technique where it will shorten the computation process.

Analytical and Experimental Study of Near-Threshold Interactions Between Crack Closure Mechanisms

NASA Technical Reports Server (NTRS)

Newman, John A.; Riddell, William T.; Piascik, Robert S.

2003-01-01

The results of an analytical closure model that considers contributions and interactions between plasticity-, roughness-, and oxide-induced crack closure mechanisms are presented and compared with experimental data. The analytical model is shown to provide a good description of the combined influences of crack roughness, oxide debris, and plasticity in the near-threshold regime. Furthermore, analytical results indicate that closure mechanisms interact in a non-linear manner such that the total amount of closure is not the sum of closure contributions for each mechanism.

Fatigue crack closure: a review of the physical phenomena

PubMed Central

Pippan, R.

2017-01-01

Abstract Plasticity‐induced, roughness‐induced and oxide‐induced crack closures are reviewed. Special attention is devoted to the physical origin, the consequences for the experimental determination and the prediction of the effective crack driving force for fatigue crack propagation. Plasticity‐induced crack closure under plane stress and plane strain conditions require, in principle, a different explanation; however, both types are predictable. This is even the case in the transition region from the plane strain to the plane stress state and all types of loading conditions including constant and variable amplitude loading, the short crack case or the transition from small‐scale to large‐scale yielding. In contrast, the prediction of roughness‐induced and oxide‐induced closures is not as straightforward. PMID:28616624

Effect of Stitching on Debonding in Composite Structural Elements

NASA Technical Reports Server (NTRS)

Raju, I. S.; Glaessgen, E. H.

2001-01-01

Stitched multiaxial warp knit materials have been suggested as viable alternatives to laminated prepreg materials for large aircraft structures such as wing skins. Analyses have been developed to quantify the effectiveness of stitching for reducing strain energy release rates in skin-stiffener debond, lap joint and sandwich debond configurations. Strain energy release rates were computed using the virtual crack closure technique. In all configurations, the stitches were shown to significantly reduce the strain energy release rate.

Influence of Finite Element Software on Energy Release Rates Computed Using the Virtual Crack Closure Technique

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Goetze, Dirk; Ransom, Jonathon (Technical Monitor)

2006-01-01

Strain energy release rates were computed along straight delamination fronts of Double Cantilever Beam, End-Notched Flexure and Single Leg Bending specimens using the Virtual Crack Closure Technique (VCCT). Th e results were based on finite element analyses using ABAQUS# and ANSYS# and were calculated from the finite element results using the same post-processing routine to assure a consistent procedure. Mixed-mode strain energy release rates obtained from post-processing finite elem ent results were in good agreement for all element types used and all specimens modeled. Compared to previous studies, the models made of s olid twenty-node hexahedral elements and solid eight-node incompatible mode elements yielded excellent results. For both codes, models made of standard brick elements and elements with reduced integration did not correctly capture the distribution of the energy release rate acr oss the width of the specimens for the models chosen. The results suggested that element types with similar formulation yield matching results independent of the finite element software used. For comparison, m ixed-mode strain energy release rates were also calculated within ABAQUS#/Standard using the VCCT for ABAQUS# add on. For all specimens mod eled, mixed-mode strain energy release rates obtained from ABAQUS# finite element results using post-processing were almost identical to re sults calculated using the VCCT for ABAQUS# add on.

Three-dimensional measurements of fatigue crack closure

NASA Technical Reports Server (NTRS)

Ray, S. K.; Grandt, A. F., Jr.

1984-01-01

Fatigue crack growth and retardation experiments conducted in polycarbonate test specimen are described. The transparent test material allows optical interferometry measurements of the fatigue crack opening (and closing) profiles. Crack surface displacements are obtained through the specimen thickness and three dimensional aspects of fatigue crack closure are discussed.

Evaluation of Elber's Crack Closure Model as an Explanation of Train Load Sequence Effects on Crack Growth Rates

DOT National Transportation Integrated Search

1990-06-01

Elber's crack closure model is studied in relation to the results of laboratory spectrum crack growth tests on compact tension specimens (CTS) fabricated from rail effected by mean of an analysis of a center cracked panel (CCP) subjected to an equiva...

A Crack Closure Model and Its Application to Vibrothermography Nondestructive Evaluation

NASA Astrophysics Data System (ADS)

Schiefelbein, Bryan Edward

Vibrothermography nondestructive evaluation (NDE) is in the early stages of research and development, and there exists uncertainty in the fundamental mechanisms and processes by which heat generation occurs. Holland et al. have developed a set of tools which simulate and predict the outcome of a vibrothermography inspection by breaking the inspection into three distinct processes: vibrational excitation, heat generation, and thermal imaging. The stage of vibrothermography which is not well understood is the process by which vibrations are converted to heat at the crack surface. It has been shown that crack closure and closure state impact the resulting heat generation. Despite this, research into the link between partial crack closure and vibrothermography is limited. This work seeks to rectify this gap in knowledge by modeling the behavior of a partially closed crack in response to static external loading and a dynamic vibration. The residual strains left by the plastic wake during fatigue crack growth manifest themselves as contact stresses acting at the crack surface interface. In response to an applied load below the crack opening stress, the crack closure state will evolve, but the crack will remain partially closed. The crack closure model developed in this work is based in linear elastic fracture mechanics (LEFM) and describes the behavior of a partially closed crack in response to a tensile external load and non-uniform closure stress distribution. The model builds on work by Fleck to describe the effective length, crack opening displacement, and crack tip stress field for a partially closed crack. These quantities are solved for by first establishing an equilibrium condition which governs the effective or apparent length of the partially closed crack. The equilibrium condition states that, under any external or crack surface loading, the effective crack tip will be located where the effective stress intensity factor is zero. In LEFM, this is equivalent to saying that the effective crack tip is located where the stress singularity vanishes. If the closure stresses are unknown, the model provides an algorithm with which to solve for the distribution, given measurements of the effective crack length as a function of external load. Within literature, a number of heating mechanisms have been proposed as being dominant in vibrothermography. These include strain hysteresis, adhesion hysteresis, plastic flow, thermoelasticity, and sliding friction. Based on experimental observation and theory, this work eliminates strain hysteresis, thermoelasticity, and plastic flow as plausible heating mechanisms. This leaves friction and adhesion hysteresis as the only plausible mechanisms. Frictional heating is based on the classical Coulomb friction model, while adhesion hysteresis heating comes from irreversibility in surface adhesion. Adhesion hysteresis only satisfies the experimental observation that heating vanishes for high compressive loading if surface roughness and the instability of surface adhesion is considered. By understanding the fundamental behavior of a partially closed crack in response to non-uniform loading, and the link between crack surface motion and heat generation, we are one step closer to a fully predictive vibrothermography heat generation model. Future work is needed to extend the crack closure model to a two-dimensional semi-elliptical surface crack and better understand the distinction between frictional and adhesion heating.

Closure of fatigue cracks at high strains

NASA Technical Reports Server (NTRS)

Iyyer, N. S.; Dowling, N. E.

1985-01-01

Experiments were conducted on smooth specimens to study the closure behavior of short cracks at high cyclic strains under completely reversed cycling. Testing procedures and methodology, and closure measurement techniques, are described in detail. The strain levels chosen for the study cover from predominantly elastic to grossly plastic strains. Crack closure measurements are made at different crack lengths. The study reveals that, at high strains, cracks close only as the lowest stress level in the cycle is approached. The crack opening is observed to occur in the compressive part of the loading cycle. The applied stress needed to open a short crack under high strain is found to be less than for cracks under small scale yielding. For increased plastic deformations, the value of sigma sub op/sigma sub max is observed to decrease and approaches the value of R. Comparison of the experimental results with existing analysis is made and indicates the limitations of the small scale yielding approach where gross plastic deformation behavior occurs.

Modeling Progressive Damage Using Local Displacement Discontinuities Within the FEAMAC Multiscale Modeling Framework

NASA Technical Reports Server (NTRS)

Ranatunga, Vipul; Bednarcyk, Brett A.; Arnold, Steven M.

2010-01-01

A method for performing progressive damage modeling in composite materials and structures based on continuum level interfacial displacement discontinuities is presented. The proposed method enables the exponential evolution of the interfacial compliance, resulting in unloading of the tractions at the interface after delamination or failure occurs. In this paper, the proposed continuum displacement discontinuity model has been used to simulate failure within both isotropic and orthotropic materials efficiently and to explore the possibility of predicting the crack path, therein. Simulation results obtained from Mode-I and Mode-II fracture compare the proposed approach with the cohesive element approach and Virtual Crack Closure Techniques (VCCT) available within the ABAQUS (ABAQUS, Inc.) finite element software. Furthermore, an eccentrically loaded 3-point bend test has been simulated with the displacement discontinuity model, and the resulting crack path prediction has been compared with a prediction based on the extended finite element model (XFEM) approach.

40 CFR 63.964 - Inspection and monitoring requirements.

Code of Federal Regulations, 2011 CFR

2011-07-01

.... Defects include, but are not limited to, visible cracks, holes, or gaps in the closure devices; broken..., visible cracks, holes, or gaps in the closure devices; broken, cracked, or otherwise damaged seals or..., visible cracks, holes, gaps, or other open spaces in the sewer line joints, seals, or other emission...

40 CFR 63.964 - Inspection and monitoring requirements.

Code of Federal Regulations, 2010 CFR

2010-07-01

.... Defects include, but are not limited to, visible cracks, holes, or gaps in the closure devices; broken..., visible cracks, holes, or gaps in the closure devices; broken, cracked, or otherwise damaged seals or..., visible cracks, holes, gaps, or other open spaces in the sewer line joints, seals, or other emission...

Environmental fatigue in aluminum-lithium alloys

NASA Technical Reports Server (NTRS)

Piascik, Robert S.

1992-01-01

Aluminum-lithium alloys exhibit similar environmental fatigue crack growth characteristics compared to conventional 2000 series alloys and are more resistant to environmental fatigue compared to 7000 series alloys. The superior fatigue crack growth behavior of Al-Li alloys 2090, 2091, 8090, and 8091 is due to crack closure caused by tortuous crack path morphology and crack surface corrosion products. At high R and reduced closure, chemical environment effects are pronounced resulting in accelerated near threshold da/dN. The beneficial effects of crack closure are minimized for small cracks resulting in rapid growth rates. Limited data suggest that the 'chemically small crack' effect, observed in other alloy system, is not pronounced in Al-Li alloys. Modeling of environmental fatigue in Al-Li-Cu alloys related accelerated fatigue crack growth in moist air and salt water to hydrogen embrittlement.

Influence of load interactions on crack growth as related to state of stress and crack closure

NASA Technical Reports Server (NTRS)

Telesman, J.

1985-01-01

Fatigue crack propagation (FCP) after an application of a low-high loading sequence was investigated as a function of specimen thickness and crack closure. No load interaction effects were detected for specimens in a predominant plane strain state. However, for the plane stress specimens, initially high FCP rates after transition to a higher stress intensity range were observed. The difference in observed behavior was explained by examining the effect of the resulting closure stress intensity values on the effective stress intensity range.

Elastic-plastic analysis of a propagating crack under cyclic loading

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Armen, H., Jr.

1974-01-01

Development and application of a two-dimensional finite-element analysis to predict crack-closure and crack-opening stresses during specified histories of cyclic loading. An existing finite-element computer program which accounts for elastic-plastic material behavior under cyclic loading was modified to account for changing boundary conditions - crack growth and intermittent contact of crack surfaces. This program was subsequently used to study the crack-closure behavior under constant-amplitude and simple block-program loading.

On the measurement of the crack tip stress field as a means of determining Delta K(sub eff) under conditions of fatigue crack closure

NASA Technical Reports Server (NTRS)

Wallhead, Ian R.; Edwards, Lyndon; Poole, Peter

1994-01-01

The optical method of caustics has been successfully extended to enable stress intensity factors as low as 1MPa square root of m to be determined accurately for central fatigue cracks in 2024-T3 aluminium alloy test panels. The feasibility of using this technique to study crack closure, and to determine the effective stress intensity factor range, Delta K(sub eff), has been investigated. Comparisons have been made between the measured values of stress intensity factor, K(sub caus), and corresponding theoretical values, K(sub theo), for a range of fatigue cracks grown under different loading conditions. The values of K(sub caus) and K(sub theo) were in good agreement at maximum stress, where the cracks are fully open, while K(sub caus) exceeded K(sub theo) at minimum stress, due to crack closure. However, the levels of crack closure and values of Delta K(sub eff) obtained could not account for the variations of crack growth rate with loading conditions. It is concluded that the values of Delta K(sub eff), based on caustic measurements in a 1/square root of r stress field well outside the plastic zone, do not fully reflect local conditions which control crack tip behavior.

Guidelines for VCCT-Based Interlaminar Fatigue and Progressive Failure Finite Element Analysis

NASA Technical Reports Server (NTRS)

Deobald, Lyle R.; Mabson, Gerald E.; Engelstad, Steve; Prabhakar, M.; Gurvich, Mark; Seneviratne, Waruna; Perera, Shenal; O'Brien, T. Kevin; Murri, Gretchen; Ratcliffe, James;

40 CFR 63.946 - Inspection and monitoring requirements.

Code of Federal Regulations, 2010 CFR

2010-07-01

... its foundation mountings; broken, cracked, or otherwise damaged seals or gaskets on closure devices; and broken or missing hatches, access covers, caps, or other closure devices. (2) The owner or...; broken, cracked, or otherwise damaged seals or gaskets on closure devices; and broken or missing hatches...

A crack-closure model for predicting fatigue-crack growth under aircraft spectrum loading

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1981-01-01

The development and application of an analytical model of cycle crack growth is presented that includes the effects of crack closure. The model was used to correlate crack growth rates under constant amplitude loading and to predict crack growth under aircraft spectrum loading on 2219-T851 aluminum alloy sheet material. The predicted crack growth lives agreed well with experimental data. The ratio of predicted to experimental lives ranged from 0.66 to 1.48. These predictions were made using data from an ASTM E24.06.01 Round Robin.

Characterization of Residual Stress Effects on Fatigue Crack Growth of a Friction Stir Welded Aluminum Alloy

NASA Technical Reports Server (NTRS)

Newman, John A.; Smith, Stephen W.; Seshadri, Banavara R.; James, Mark A.; Brazill, Richard L.; Schultz, Robert W.; Donald, J. Keith; Blair, Amy

2015-01-01

An on-line compliance-based method to account for residual stress effects in stress-intensity factor and fatigue crack growth property determinations has been evaluated. Residual stress intensity factor results determined from specimens containing friction stir weld induced residual stresses are presented, and the on-line method results were found to be in excellent agreement with residual stress-intensity factor data obtained using the cut compliance method. Variable stress-intensity factor tests were designed to demonstrate that a simple superposition model, summing the applied stress-intensity factor with the residual stress-intensity factor, can be used to determine the total crack-tip stress-intensity factor. Finite element, VCCT (virtual crack closure technique), and J-integral analysis methods have been used to characterize weld-induced residual stress using thermal expansion/contraction in the form of an equivalent delta T (change in local temperature during welding) to simulate the welding process. This equivalent delta T was established and applied to analyze different specimen configurations to predict residual stress distributions and associated residual stress-intensity factor values. The predictions were found to agree well with experimental results obtained using the crack- and cut-compliance methods.

40 CFR 63.1047 - Inspection and monitoring requirements.

Code of Federal Regulations, 2010 CFR

2010-07-01

... separator wall; broken, cracked, or otherwise damaged seals or gaskets on closure devices; and broken or... the surface of the liquid in the separator; broken, cracked, or otherwise damaged seals or gaskets on closure devices; and broken or missing hatches, access covers, caps, or other closure devices. (ii) The...

A nonlinear fracture mechanics approach to the growth of small cracks

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1983-01-01

An analytical model of crack closure is used to study the crack growth and closure behavior of small cracks in plates and at notches. The calculated crack opening stresses for small and large cracks, together with elastic and elastic plastic fracture mechanics analyses, are used to correlate crack growth rate data. At equivalent elastic stress intensity factor levels, calculations predict that small cracks in plates and at notches should grow faster than large cracks because the applied stress needed to open a small crack is less than that needed to open a large crack. These predictions agree with observed trends in test data. The calculations from the model also imply that many of the stress intensity factor thresholds that are developed in tests with large cracks and with load reduction schemes do not apply to the growth of small cracks. The current calculations are based upon continuum mechanics principles and, thus, some crack size and grain structure exist where the underlying fracture mechanics assumptions become invalid because of material inhomogeneity (grains, inclusions, etc.). Admittedly, much more effort is needed to develop the mechanics of a noncontinuum. Nevertheless, these results indicate the importance of crack closure in predicting the growth of small cracks from large crack data.

Crack-closure and crack-growth measurements in surface-flawed titanium alloy Ti6Al-4V

NASA Technical Reports Server (NTRS)

Elber, W.

1975-01-01

The crack-closure and crack-growth characteristics of the titanium alloy Ti-6Al-4V were determined experimentally on surface-flawed plate specimens. Under cyclic loading from zero to tension, cracks deeper than 1 mm opened at approximately 50 percent of the maximum load. Cracks shallower than 1 mm opened at higher loads. The correlation between crack-growth rate and the total stress-intensity range showed a lower threshold behavior. This behavior was attributed to the high crack-opening loads at short cracks because the lower threshold was much less evident in correlations between the crack-growth rates and the effective stress-intensity range.

Microcrack closure in rocks under stress - Direct observation

NASA Technical Reports Server (NTRS)

Batzle, M. L.; Simmons, G.; Siegfried, R. W.

1980-01-01

Direct observations of the closure of microcracks in rocks under increasing stress are reported. Uniaxial stresses up to 300 bars were applied to untreated and previously heated samples of Westerly granite and Frederick diabase by a small hydraulic press which fit entirely within a scanning electron microscope. Crack closure characteristics are found to depend on crack orientation, with cracks perpendicular to the applied stress closing and those parallel tending to open, as well as crack aspect ratio, crack intersection properties, stress concentrations and surface roughness. Uniaxial and hydrostatic stress measurements are found to be strongly dependent on fracture content as observed by SEM, and the observed hysteresis in strain measurements in the first stress cycles is also related to microscopic processes

Fatigue crack closure behavior at high stress ratios

NASA Technical Reports Server (NTRS)

Turner, C. Christopher; Carman, C. Davis; Hillberry, Ben M.

1988-01-01

Fatigue crack delay behavior at high stress ratio caused by single peak overloads was investigated in two thicknesses of 7475-T731 aluminum alloy. Closure measurements indicated no closure occurred before or throughout the overload plastic zones following the overload. This was further substantiated by comparing the specimen compliance following the overload with the compliance of a low R ratio test when the crack was fully open. Scanning electron microscope studies revealed that crack tunneling and possibly reinitiation of the crack occurred, most likely a result of crack-tip blunting. The number of delay cycles was greater for the thinner mixed mode stress state specimen than for the thicker plane strain stress state specimen, which is similar to low R ratio test results and may be due to a larger plastic zone for the mixed mode cased.

Investigation of problems of closing of geophysical cracks in thermoelastic media in the case of flow of fluids with impurities

NASA Astrophysics Data System (ADS)

Martirosyan, A. N.; Davtyan, A. V.; Dinunts, A. S.; Martirosyan, H. A.

2018-04-01

The purpose of this article is to investigate a problem of closing cracks by building up a layer of sediments on surfaces of a crack in an infinite thermoelastic medium in the presence of a flow of fluids with impurities. The statement of the problem of closing geophysical cracks in the presence of a fluid flow is presented with regard to the thermoelastic stress and the influence of the impurity deposition in the liquid on the crack surfaces due to thermal diffusion at the fracture closure. The Wiener–Hopf method yields an analytical solution in the special case without friction. Numerical calculations are performed in this case and the dependence of the crack closure time on the coordinate is plotted. A similar spatial problem is also solved. These results generalize the results of previous studies of geophysical cracks and debris in rocks, where the closure of a crack due to temperature effects is studied without taking the elastic stresses into account.

Analyses of Fatigue Crack Growth and Closure Near Threshold Conditions for Large-Crack Behavior

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1999-01-01

A plasticity-induced crack-closure model was used to study fatigue crack growth and closure in thin 2024-T3 aluminum alloy under constant-R and constant-K(sub max) threshold testing procedures. Two methods of calculating crack-opening stresses were compared. One method was based on a contact-K analyses and the other on crack-opening-displacement (COD) analyses. These methods gave nearly identical results under constant-amplitude loading, but under threshold simulations the contact-K analyses gave lower opening stresses than the contact COD method. Crack-growth predictions tend to support the use of contact-K analyses. Crack-growth simulations showed that remote closure can cause a rapid rise in opening stresses in the near threshold regime for low-constraint and high applied stress levels. Under low applied stress levels and high constraint, a rise in opening stresses was not observed near threshold conditions. But crack-tip-opening displacement (CTOD) were of the order of measured oxide thicknesses in the 2024 alloy under constant-R simulations. In contrast, under constant-K(sub max) testing the CTOD near threshold conditions were an order-of-magnitude larger than measured oxide thicknesses. Residual-plastic deformations under both constant-R and constant-K(sub max) threshold simulations were several times larger than the expected oxide thicknesses. Thus, residual-plastic deformations, in addition to oxide and roughness, play an integral part in threshold development.

A re-evaluation of finite-element models and stress-intensity factors for surface cracks emanating from stress concentrations

NASA Technical Reports Server (NTRS)

Tan, P. W.; Raju, I. S.; Shivakumar, K. N.; Newman, J. C., Jr.

1988-01-01

A re-evaluation of the 3-D finite-element models and methods used to analyze surface crack at stress concentrations is presented. Previous finite-element models used by Raju and Newman for surface and corner cracks at holes were shown to have ill-shaped elements at the intersection of the hole and crack boundaries. These ill-shaped elements tended to make the model too stiff and, hence, gave lower stress-intensity factors near the hole-crack intersection than models without these elements. Improved models, without these ill-shaped elements, were developed for a surface crack at a circular hole and at a semi-circular edge notch. Stress-intensity factors were calculated by both the nodal-force and virtual-crack-closure methods. Both methods and different models gave essentially the same results. Comparisons made between the previously developed stress-intensity factor equations and the results from the improved models agreed well except for configurations with large notch-radii-to-plate-thickness ratios. Stress-intensity factors for a semi-elliptical surface crack located at the center of a semi-circular edge notch in a plate subjected to remote tensile loadings were calculated using the improved models. The ratio of crack depth to crack length ranged form 0.4 to 2; the ratio of crack depth to plate thickness ranged from 0.2 to 0.8; and the ratio of notch radius to the plate thickness ranged from 1 to 3. The models had about 15,000 degrees-of-freedom. Stress-intensity factors were calculated by using the nodal-force method.

A shape memory polymer concrete crack closure system activated by electrical current

NASA Astrophysics Data System (ADS)

Teall, Oliver; Pilegis, Martins; Davies, Robert; Sweeney, John; Jefferson, Tony; Lark, Robert; Gardner, Diane

2018-07-01

The presence of cracks has a negative impact on the durability of concrete by providing paths for corrosive materials to the embedded steel reinforcement. Cracks in concrete can be closed using shape memory polymers (SMP) which produce a compressive stress across the crack faces. This stress has been previously found to enhance the load recovery associated with autogenous self-healing. This paper details the experiments undertaken to incorporate SMP tendons containing polyethylene terephthalate (PET) filaments into reinforced and unreinforced 500 × 100 × 100 mm structural concrete beam samples. These tendons are activated via an electrical supply using a nickel-chrome resistance wire heating system. The set-up, methodology and results of restrained shrinkage stress and crack closure experiments are explained. Crack closure of up to 85% in unreinforced beams and 26%–39% in reinforced beams is measured using crack-mouth opening displacement, microscope and digital image correlation equipment. Conclusions are made as to the effectiveness of the system and its potential for application within industry.

Small fatigue cracks; Proceedings of the Second International Conference/Workshop, Santa Barbara, CA, Jan. 5-10, 1986

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ritchie, R.O.; Lankford, J.

Topics discussed in this volume include crack initiation and stage I growth, microstructure effects, crack closure, environment effects, the role of notches, analytical modeling, fracture mechanics characterization, experimental techniques, and engineering applications. Papers are presented on fatigue crack initiation along slip bands, the effect of microplastic surface deformation on the growth of small cracks, short fatigue crack behavior in relation to three-dimensional aspects and the crack closure effect, the influence of crack depth on crack electrochemistry and fatigue crack growth, and nondamaging notches in fatigue. Consideration is also given to models of small fatigue cracks, short crack theory, assessment ofmore » the growth of small flaws from residual strength data, the relevance of short crack behavior to the integrity of major rotating aero engine components, and the relevance of short fatigue crack growth data to the durability and damage tolerance analyses of aircraft.« less

Growth of Small Cracks in Aeroengine Disc Materials.

DTIC Science & Technology

1988-06-01

parameters is given in Figure 4-12. Al though the load required to open the crack (Pop) is the basic measurement in any crack closure study, this measurement...See Section 6.3). The latter observation supports the previous hypothesis of Hicks et al [4.11] that crack closure in these alloys is controlled by...cyclic stress relaxation on the local mean stress can be estimated from the measurements of Lindholm et al [5.21, obtained on the superalloy B1900+Hf

Crack Development in Cross-Ply Laminates Under Uniaxial Tension

NASA Technical Reports Server (NTRS)

Gyekenyesi, Andrew L.

1996-01-01

This study addresses matrix-dominated failures in carbon fiber/polymer matrix composite laminates in a cross-ply lay-up. The events of interest are interlaminar fracture in the form of transverse cracks in the 90' plies and longitudinal splitting in the 0 deg plies and interlaminar fracture in the form of 0/90 delamination. These events were observed using various nondestructive evaluation (NDE) techniques during static tensile tests. Acoustic emission (AE) x radiography, and edge view microscopy were the principal ones utilized in a real-time environment. A comparison of the NDE results with an analytical model based on the classical linear fracture mechanics concept of strain energy release rate as a criterion for crack growth was performed. The virtual crack closure theory was incorporated with a finite element model to generate strain energy release rate curves for the analytical case. Celion carbon fiber/polyimide matrix (G30-500/PMR-15) was the material tested with cross-ply lay-ups of (0(2)/90(6))s and (0(4)/90(4))s. The test specimens contained thermally induced cracks caused by the high-temperature processing. The analytical model was updated to compensate for the initial damage and to study further accumulation by taking into account the crack interactions. By correlating the experimental and analytical data, the critical energy release rates were found for the observable events of interest.

surf3d: A 3-D finite-element program for the analysis of surface and corner cracks in solids subjected to mode-1 loadings

NASA Technical Reports Server (NTRS)

Raju, I. S.; Newman, J. C., Jr.

1993-01-01

A computer program, surf3d, that uses the 3D finite-element method to calculate the stress-intensity factors for surface, corner, and embedded cracks in finite-thickness plates with and without circular holes, was developed. The cracks are assumed to be either elliptic or part eliptic in shape. The computer program uses eight-noded hexahedral elements to model the solid. The program uses a skyline storage and solver. The stress-intensity factors are evaluated using the force method, the crack-opening displacement method, and the 3-D virtual crack closure methods. In the manual the input to and the output of the surf3d program are described. This manual also demonstrates the use of the program and describes the calculation of the stress-intensity factors. Several examples with sample data files are included with the manual. To facilitate modeling of the user's crack configuration and loading, a companion program (a preprocessor program) that generates the data for the surf3d called gensurf was also developed. The gensurf program is a three dimensional mesh generator program that requires minimal input and that builds a complete data file for surf3d. The program surf3d is operational on Unix machines such as CRAY Y-MP, CRAY-2, and Convex C-220.

Effect of Shear Deformation and Continuity on Delamination Modelling with Plate Elements

NASA Technical Reports Server (NTRS)

Glaessgen, E. H.; Riddell, W. T.; Raju, I. S.

1998-01-01

The effects of several critical assumptions and parameters on the computation of strain energy release rates for delamination and debond configurations modeled with plate elements have been quantified. The method of calculation is based on the virtual crack closure technique (VCCT), and models that model the upper and lower surface of the delamination or debond with two-dimensional (2D) plate elements rather than three-dimensional (3D) solid elements. The major advantages of the plate element modeling technique are a smaller model size and simpler geometric modeling. Specific issues that are discussed include: constraint of translational degrees of freedom, rotational degrees of freedom or both in the neighborhood of the crack tip; element order and assumed shear deformation; and continuity of material properties and section stiffness in the vicinity of the debond front, Where appropriate, the plate element analyses are compared with corresponding two-dimensional plane strain analyses.

Fatigue Life Analysis of Tapered Hybrid Composite Flexbeams

NASA Technical Reports Server (NTRS)

Murri, Gretchen B.; Schaff, Jeffery R.; Dobyns, Alan L.

2002-01-01

Nonlinear-tapered flexbeam laminates from a full-size composite helicopter rotor hub flexbeam were tested under combined constant axial tension and cyclic bending loads. The two different graphite/glass hybrid configurations tested under cyclic loading failed by delamination in the tapered region. A 2-D finite element model was developed which closely approximated the flexbeam geometry, boundary conditions, and loading. The analysis results from two geometrically nonlinear finite element codes, ANSYS and ABAQUS, are presented and compared. Strain energy release rates (G) obtained from the above codes using the virtual crack closure technique (VCCT) at a resin crack location in the flexbeams are presented for both hybrid material types. These results compare well with each other and suggest that the initial delamination growth from the resin crack toward the thick region of the flexbeam is strongly mode II. The peak calculated G values were used with material characterization data to calculate fatigue life curves and compared with test data. A curve relating maximum surface strain to number of loading cycles at delamination onset compared reasonably well with the test results.

Low cracking concretes for the closure pours and overlays of the Dunlap Creek Bridge : final report.

DOT National Transportation Integrated Search

2017-10-01

Joints, wide cracks, and poor-quality concretes facilitate the intrusion of chlorides, causing corrosion in bridge decks and substructures. In this study, joints were replaced with closure pours (link slabs) consisting of low permeability fiber-reinf...

Fatigue Life Prediction Based on Crack Closure and Equivalent Initial Flaw Size

PubMed Central

Wang, Qiang; Zhang, Wei; Jiang, Shan

2015-01-01

Failure analysis and fatigue life prediction are necessary and critical for engineering structural materials. In this paper, a general methodology is proposed to predict fatigue life of smooth and circular-hole specimens, in which the crack closure model and equivalent initial flaw size (EIFS) concept are employed. Different effects of crack closure on small crack growth region and long crack growth region are considered in the proposed method. The EIFS is determined by the fatigue limit and fatigue threshold stress intensity factor △Kth. Fatigue limit is directly obtained from experimental data, and △Kth is calculated by using a back-extrapolation method. Experimental data for smooth and circular-hole specimens in three different alloys (Al2024-T3, Al7075-T6 and Ti-6Al-4V) under multiple stress ratios are used to validate the method. In the validation section, Semi-circular surface crack and quarter-circular corner crack are assumed to be the initial crack shapes for the smooth and circular-hole specimens, respectively. A good agreement is observed between model predictions and experimental data. The detailed analysis and discussion are performed on the proposed model. Some conclusions and future work are given. PMID:28793625

Crack Growth Behavior in the Threshold Region for High Cyclic Loading

NASA Technical Reports Server (NTRS)

Forman, R.; Figert, J.; Beek, J.; Ventura, J.; Martinez, J.; Samonski, F.

2011-01-01

The present studies show that fanning in the threshold regime is likely caused by other factors than a plastic wake developed during load shedding. The cause of fanning at low R-values is a result of localized roughness, mainly formation of a faceted crack surface morphology , plus crack bifurcations which alters the crack closure at low R-values. The crack growth behavior in the threshold regime involves both crack closure theory and the dislocation theory of metals. Research will continue in studying numerous other metal alloys and performing more extensive analysis, such as the variation in dislocation properties (e.g., stacking fault energy) and its effects in different materials.

Growth rate models for short surface cracks in AI 2219-T851

NASA Astrophysics Data System (ADS)

Morris, W. L.; James, M. R.; Buck, O.

1981-01-01

Rates of fatigue propagation of short Mode I surface cracks in Al 2219-T851 are measured as a function of crack length and of the location of the surface crack tips relative to the grain boundaries. The measured rates are then compared to values predicted from crack growth models. The crack growth rate is modeled with an underlying assumption that slip responsible for early propagation does not extend in significant amounts beyond the next grain boundary in the direction of crack propagation. Two models that contain this assumption are combined: 1) cessation of propagation into a new grain until a mature plastic zone is developed; 2) retardation of propagation by crack closure stress, with closure stress calculated from the location of a crack tip relative to the grain boundary. The transition from short to long crack growth behavior is also discussed.

Evaluation of high-performance fiber-reinforced concrete for bridge deck connections, closure pours, and joints : final report.

DOT National Transportation Integrated Search

2017-01-01

Connections, closure pours, and joints in bridges are often sources of distress because of cracks and openings. Wide separation facilitates the penetration of harmful solutions that can lead to costly repairs. Cracks are caused by volumetric changes ...

On the variation in crack-opening stresses at different locations in a three-dimensional body

NASA Technical Reports Server (NTRS)

Chermahini, R. G.; Blom, Anders F.

1990-01-01

Crack propagation and closure behavior of thin, and thick middle crack tension specimens under constant amplitude loading were investigated using a three dimensional elastic plastic finite element analysis of fatigue crack propagation and closure. In the thin specimens the crack front closed first on the exterior (free) surface and closed last in the interior during the unloading portion of cyclic loading; a load reduced displacement technique was used to determine crack opening stresses at specified locations in the plate from the displacements calculated after the seven cycle. All the locations were on the plate external surface and were located near the crack tip, behind the crack tip, at the centerline of the crack. With this technique, the opening stresses at the specified points were found to be 0.52, 0.42, and 0.39 times the maximum applied stress.

Modeling of crack bridging in a unidirectional metal matrix composite

NASA Technical Reports Server (NTRS)

Ghosn, Louis J.; Kantzos, Pete; Telesman, Jack

1991-01-01

The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches; the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, Delta K(sup eff), was calculated using both models to correlate the measure crack growth rate in the composite. The calculated Delta K(sup eff) from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.

Modeling of crack bridging in a unidirectional metal matrix composite

NASA Technical Reports Server (NTRS)

Ghosn, Louis J.; Kantzos, Pete; Telesman, Jack

1992-01-01

The effective fatigue crack driving force and crack opening profiles were determined analytically for fatigue tested unidirectional composite specimens exhibiting fiber bridging. The crack closure pressure due to bridging was modeled using two approaches: the fiber pressure model and the shear lag model. For both closure models, the Bueckner weight function method and the finite element method were used to calculate crack opening displacements and the crack driving force. The predicted near crack tip opening profile agreed well with the experimentally measured profiles for single edge notch SCS-6/Ti-15-3 metal matrix composite specimens. The numerically determined effective crack driving force, Delta K(eff), was calculated using both models to correlate the measure crack growth rate in the composite. The calculated Delta K(eff) from both models accounted for the crack bridging by showing a good agreement between the measured fatigue crack growth rates of the bridged composite and that of unreinforced, unbridged titanium matrix alloy specimens.

Implementation of equivalent domain integral method in the two-dimensional analysis of mixed mode problems

NASA Technical Reports Server (NTRS)

Raju, I. S.; Shivakumar, K. N.

1989-01-01

An equivalent domain integral (EDI) method for calculating J-intergrals for two-dimensional cracked elastic bodies is presented. The details of the method and its implementation are presented for isoparametric elements. The total and product integrals consist of the sum of an area of domain integral and line integrals on the crack faces. The line integrals vanish only when the crack faces are traction free and the loading is either pure mode 1 or pure mode 2 or a combination of both with only the square-root singular term in the stress field. The EDI method gave accurate values of the J-integrals for two mode I and two mixed mode problems. Numerical studies showed that domains consisting of one layer of elements are sufficient to obtain accurate J-integral values. Two procedures for separating the individual modes from the domain integrals are presented. The procedure that uses the symmetric and antisymmetric components of the stress and displacement fields to calculate the individual modes gave accurate values of the integrals for all problems analyzed. The EDI method when applied to a problem of an interface crack in two different materials showed that the mode 1 and mode 2 components are domain dependent while the total integral is not. This behavior is caused by the presence of the oscillatory part of the singularity in bimaterial crack problems. The EDI method, thus, shows behavior similar to the virtual crack closure method for bimaterial problems.

The Significance of Small Cracks in Fatigue Design Concepts as Related to Rotorcraft Metallic Dynamic Components

NASA Technical Reports Server (NTRS)

Everett, R. A., Jr.; Elber, W.

2000-01-01

In this paper the significance of the "small" crack effect as defined in fracture mechanics will be discussed as it relates to life managing rotorcraft dynamic components using the conventional safe-life, the flaw tolerant safe-life, and the damage tolerance design philosophies. These topics will be introduced starting with an explanation of the small-crack theory, then showing how small-crack theory has been used to predict the total fatigue life of fatigue laboratory test coupons with and without flaws, and concluding with how small cracks can affect the crack-growth damage tolerance design philosophy. As stated in this paper the "small" crack effect is defined in fracture mechanics where it has been observed that cracks on the order of 300 microns or less in length will propagate at higher growth rates than long cracks and also will grow at AK values below the long crack AK threshold. The small-crack effect is illustrated herein as resulting from a lack of crack closure and is explained based on continuum mechanics principles using crack-closure concepts in fracture mechanics.

Finite element analysis of the end notched flexure specimen for measuring Mode II fracture toughness

NASA Technical Reports Server (NTRS)

Gillespie, J. W., Jr.; Carlsson, L. A.; Pipes, R. B.

1986-01-01

The paper presents a finite element analysis of the end-notched flexure (ENF) test specimen for Mode II interlaminar fracture testing of composite materials. Virtual crack closure and compliance techniques employed to calculate strain energy release rates from linear elastic two-dimensional analysis indicate that the ENF specimen is a pure Mode II fracture test within the constraints of small deflection theory. Furthermore, the ENF fracture specimen is shown to be relatively insensitive to process-induced cracks, offset from the laminate midplane. Frictional effects are investigated by including the contact problem in the finite element model. A parametric study investigating the influence of delamination length, span, thickness, and material properties assessed the accuracy of beam theory expressions for compliance and strain energy release rate, GII. Finite element results indicate that data reduction schemes based upon beam theory underestimate GII by approximately 20-40 percent for typical unidirectional graphite fiber composite test specimen geometries. Consequently, an improved data reduction scheme is proposed.

An Enriched Shell Element for Delamination Simulation in Composite Laminates

NASA Technical Reports Server (NTRS)

McElroy, Mark

2015-01-01

A formulation is presented for an enriched shell finite element capable of delamination simulation in composite laminates. The element uses an adaptive splitting approach for damage characterization that allows for straightforward low-fidelity model creation and a numerically efficient solution. The Floating Node Method is used in conjunction with the Virtual Crack Closure Technique to predict delamination growth and represent it discretely at an arbitrary ply interface. The enriched element is verified for Mode I delamination simulation using numerical benchmark data. After determining important mesh configuration guidelines for the vicinity of the delamination front in the model, a good correlation was found between the enriched shell element model results and the benchmark data set.

The effect of thickness on fatigue crack propagation in 7475-T731 aluminum alloy sheet

NASA Technical Reports Server (NTRS)

Daiuto, R. A.; Hillberry, B. M.

1984-01-01

Tests were conducted on three thicknesses of 7475-T731 aluminum alloy sheet to investigate the effect of thickness on fatigue crack propagation under constant amplitude loading conditions and on retardation following a single peak overload. Constant amplitude loading tests were performed at stress ratios of 0.05 and 0.75 to obtain data for conditions with crack closure and without crack closure, respectively. At both stress ratios a thickness effect was clearly evident, with thicker specimens exhibiting higher growth rates in the transition from plane strain to plane stress region. The effect of thickness for a stress ratio of 0.05 corresponded well with the fracturing mode transitions observed on the specimens. A model based on the strain energy release rate which accounted for the fracture mode transition was found to correlate the thickness effects well. The specimens tested at the stress ratio of 0.75 did not make the transition from tensile mode to shear mode, indicating that another mechanism besides crack closure or fracture mode transition was active.

Fatigue crack propagation behavior of stainless steel welds

NASA Astrophysics Data System (ADS)

Kusko, Chad S.

The fatigue crack propagation behavior of austenitic and duplex stainless steel base and weld metals has been investigated using various fatigue crack growth test procedures, ferrite measurement techniques, light optical microscopy, stereomicroscopy, scanning electron microscopy, and optical profilometry. The compliance offset method has been incorporated to measure crack closure during testing in order to determine a stress ratio at which such closure is overcome. Based on this method, an empirically determined stress ratio of 0.60 has been shown to be very successful in overcoming crack closure for all da/dN for gas metal arc and laser welds. This empirically-determined stress ratio of 0.60 has been applied to testing of stainless steel base metal and weld metal to understand the influence of microstructure. Regarding the base metal investigation, for 316L and AL6XN base metals, grain size and grain plus twin size have been shown to influence resulting crack growth behavior. The cyclic plastic zone size model has been applied to accurately model crack growth behavior for austenitic stainless steels when the average grain plus twin size is considered. Additionally, the effect of the tortuous crack paths observed for the larger grain size base metals can be explained by a literature model for crack deflection. Constant Delta K testing has been used to characterize the crack growth behavior across various regions of the gas metal arc and laser welds at the empirically determined stress ratio of 0.60. Despite an extensive range of stainless steel weld metal FN and delta-ferrite morphologies, neither delta-ferrite morphology significantly influence the room temperature crack growth behavior. However, variations in weld metal da/dN can be explained by local surface roughness resulting from large columnar grains and tortuous crack paths in the weld metal.

Small Crack Growth and Fatigue Life Predictions for High-Strength Aluminium Alloys. Part 1; Experimental and Fracture Mechanics Analysis

NASA Technical Reports Server (NTRS)

Wu, X. R.; Newman, J. C.; Zhao, W.; Swain, M. H.; Ding, C. F.; Phillips, E. P.

1998-01-01

The small crack effect was investigated in two high-strength aluminium alloys: 7075-T6 bare and LC9cs clad alloy. Both experimental and analytical investigations were conducted to study crack initiation and growth of small cracks. In the experimental program, fatigue tests, small crack and large crack tests A,ere conducted under constant amplitude and Mini-TWIST spectrum loading conditions. A pronounced small crack effect was observed in both materials, especially for the negative stress ratios. For all loading conditions, most of the fatigue life of the SENT specimens was shown to be crack propagation from initial material defects or from the cladding layer. In the analysis program, three-dimensional finite element and A weight function methods were used to determine stress intensity factors and to develop SIF equations for surface and corner cracks at the notch in the SENT specimens. A plastisity-induced crack-closure model was used to correlate small and large crack data, and to make fatigue life predictions, Predicted crack-growth rates and fatigue lives agreed well with experiments. A total fatigue life prediction method for the aluminum alloys was developed and demonstrated using the crack-closure model.

Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading

NASA Technical Reports Server (NTRS)

Forman, Royce G.; Zanganehgheshlaghi, Mohannad

2014-01-01

The research results described in this paper presents a new understanding of the behavior of fatigue crack growth in the threshold region. It is believed by some crack growth experts that the ASTM load shedding test method does not produce true or valid threshold properties. The concern involves the observed fanning of threshold region da/dN data plots for some materials in which the low R-ratio data fans out or away from the high R-ratio data. This data fanning or elevation of threshold values is obviously caused by an increase in crack closure in the low R-ratio tested specimens. This increase in crack closure is assumed by some investigators to be caused by a plastic wake on the crack surfaces that was created during the load shedding test phase. This study shows that the increase in crack closure is the result of an extensive occurrence of crack bifurcation behavior in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the particular fanning behavior in aluminum alloys is a function of intrinsic dislocation property of the materials and that the fanned data represents valid material properties. However, for corrosion sensitive steel alloys used in this study the fanning was caused by a build-up of iron oxide at the crack tip from fretting corrosion.

The application of Newman crack-closure model to predicting fatigue crack growth

NASA Astrophysics Data System (ADS)

Si, Erjian

1994-09-01

Newman crack-closure model and the relevant crack growth program were applied to the analysis of crack growth under constant amplitude and aircraft spectrum loading on a number of aluminum alloy materials. The analysis was performed for available test data of 2219-T851, 2024-T3, 2024-T351, 7075-T651, 2324-T39, and 7150-T651 aluminum materials. The results showed that the constraint factor is a significant factor in the method. The determination of the constraint factor is discussed. For constant amplitude loading, satisfactory crack growth lives could be predicted. For the above aluminum specimens, the ratio of predicted to experimental lives, Np/Nt, ranged from 0.74 to 1.36. The mean value of Np/Nt was 0.97. For a specified complex spectrum loading, predicted crack growth lives are not in very good agreement with the test data. Further effort is needed to correctly simulate the transition between plane strain and plane stress conditions, existing near the crack tip.

Application of the cohesive zone model for the evaluation of stiffness losses in a rotor with a transverse breathing crack

NASA Astrophysics Data System (ADS)

Toni Liong, Rugerri; Proppe, Carsten

2013-04-01

The breathing mechanism of a transversely cracked rotor and its influence on a rotor system that appears due to shaft weight and inertia forces is studied. A method is proposed for the evaluation of the stiffness losses in the cross-section that contains the crack. This method is based on a cohesive zone model (CZM) instead of linear elastic fracture mechanics (LEFM). The CZM is developed for mode-I plane strain conditions and accounts explicitly for triaxiality of the stress state by using constitutive relations. The breathing crack is modelled by a parabolic shape. As long as the relative crack depth is small, a crack closure straight line model may be used, while the crack closure parabolic line should be used in the case of a deep crack. The CZM is also implemented in a one-dimensional continuum rotor model by means of finite element (FE) discretisation in order to predict and to analyse the dynamic behavior of a cracked rotor. The proposed method provides a useful tool for the analysis of rotor systems containing cracks.

A Semi-Analytical Method for Determining the Energy Release Rate of Cracks in Adhesively-Bonded Single-Lap Composite Joints

NASA Technical Reports Server (NTRS)

Yang, Charles; Sun, Wenjun; Tomblin, John S.; Smeltzer, Stanley S., III

2007-01-01

A semi-analytical method for determining the strain energy release rate due to a prescribed interface crack in an adhesively-bonded, single-lap composite joint subjected to axial tension is presented. The field equations in terms of displacements within the joint are formulated by using first-order shear deformable, laminated plate theory together with kinematic relations and force equilibrium conditions. The stress distributions for the adherends and adhesive are determined after the appropriate boundary and loading conditions are applied and the equations for the field displacements are solved. Based on the adhesive stress distributions, the forces at the crack tip are obtained and the strain energy release rate of the crack is determined by using the virtual crack closure technique (VCCT). Additionally, the test specimen geometry from both the ASTM D3165 and D1002 test standards are utilized during the derivation of the field equations in order to correlate analytical models with future test results. The system of second-order differential field equations is solved to provide the adherend and adhesive stress response using the symbolic computation tool, Maple 9. Finite element analyses using J-integral as well as VCCT were performed to verify the developed analytical model. The finite element analyses were conducted using the commercial finite element analysis software ABAQUS. The results determined using the analytical method correlated well with the results from the finite element analyses.

Elasto-plastic bending of cracked plates, including the effects of crack closure. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Jones, D. P.

1972-01-01

A capability for solving elasto-plastic plate bending problems is developed using assumptions consistent with Kirchhoff plate theory. Both bending and extensional modes of deformation are admitted with the two modes becoming coupled as yielding proceeds. Equilibrium solutions are obtained numerically by determination of the stationary point of a functional which is analogous to the potential strain energy. The stationary value of the functional for each load increment is efficiently obtained through use of the conjugate gradient. This technique is applied to the problem of a large centrally through cracked plate subject to remote circular bending. Comparison is drawn between two cases of the bending problem. The first neglects the possibility of crack face interference with bending, and the second includes a kinematic prohibition against the crack face from passing through the symmetry plane. Results are reported which isolate the effects of elastoplastic flow and crack closure.

Delamination growth in composite materials

NASA Technical Reports Server (NTRS)

Gillespie, J. W., Jr.; Carlsson, L. A.; Pipes, R. B.; Rothschilds, R.; Trethewey, B.; Smiley, A.

1986-01-01

The Double Cantilever Beam (DCB) and the End Notched Flexure (ENF) specimens are employed to characterize MODE I and MODE II interlaminar fracture resistance of graphite/epoxy (CYCOM 982) and graphite/PEEK (APC2) composites. Sizing of test specimen geometries to achieve crack growth in the linear elastic regime is presented. Data reduction schemes based upon beam theory are derived for the ENF specimen and include the effects of shear deformation and friction between crack surfaces on compliance, C, and strain energy release rate, G sub II. Finite element (FE) analyses of the ENF geometry including the contact problem with friction are presented to assess the accuracy of beam theory expressions for C and G sub II. Virtual crack closure techniques verify that the ENF specimen is a pure Mode II test. Beam theory expressions are shown to be conservative by 20 to 40 percent for typical unidirectional test specimen geometries. A FE parametric study investigating the influence of delamination length and depth, span, thickness and material properties on G sub II is presented. Mode I and II interlaminar fracture test results are presented. Important experimental parameters are isolated, such as precracking techniques, rate effects, and nonlinear load-deflection response. It is found that subcritical crack growth and inelastic materials behavior, responsible for the observed nonlinearities, are highly rate-dependent phenomena with high rates generally leading to linear elastic response.

Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading

NASA Technical Reports Server (NTRS)

Forman, R. G.; Zanganeh, M.

2014-01-01

This paper describes the results of a research program conducted to improve the understanding of fatigue crack growth rate behavior in the threshold growth rate region and to answer a question on the validity of threshold region test data. The validity question relates to the view held by some experimentalists that using the ASTM load shedding test method does not produce valid threshold test results and material properties. The question involves the fanning behavior observed in threshold region of da/dN plots for some materials in which the low R-ratio data fans out from the high R-ratio data. This fanning behavior or elevation of threshold values in the low R-ratio tests is generally assumed to be caused by an increase in crack closure in the low R-ratio tests. Also, the increase in crack closure is assumed by some experimentalists to result from using the ASTM load shedding test procedure. The belief is that this procedure induces load history effects which cause remote closure from plasticity and/or roughness changes in the surface morphology. However, experimental studies performed by the authors have shown that the increase in crack closure is a result of extensive crack tip bifurcations that can occur in some materials, particularly in aluminum alloys, when the crack tip cyclic yield zone size becomes less than the grain size of the alloy. This behavior is related to the high stacking fault energy (SFE) property of aluminum alloys which results in easier slip characteristics. Therefore, the fanning behavior which occurs in aluminum alloys is a function of intrinsic dislocation property of the alloy, and therefore, the fanned data does represent the true threshold properties of the material. However, for the corrosion sensitive steel alloys tested in laboratory air, the occurrence of fanning results from fretting corrosion at the crack tips, and these results should not be considered to be representative of valid threshold properties because the fanning is eliminated when testing is performed in dry air.

Debond Analyses for Stitched Composite Structures

NASA Technical Reports Server (NTRS)

Glaessgen, E. H.; Raju, I. S.; Poe, C. C., Jr.

1998-01-01

The effect of stitching on mode I and mode II strain energy release rates for debond configurations is studied using an analysis based on plate finite elements and the virtual crack closure technique. The stitches were modeled as discrete nonlinear fastener elements with a compliance determined by experiment. The axial and shear behavior of the stitches was considered with both the compliances and failure loads assumed to be independent. The mode I strain energy release rate, G(sub I), was shown to decrease once the debond had grown beyond the first row of stitches and was reduced to zero for long debonds, however, the mode II strain energy release rate, G(sub II), continued to be of significant magnitude over the range of debond lengths considered.

Definition of Mutually Optimum NDI and Proof Test Criteria for 2219 Aluminum Pressure Vessels. Volume 2: Optimization and Fracture Studies

NASA Technical Reports Server (NTRS)

Schwartzberg, F. R.; Toth, C., Jr.; King, R. G.; Todd, P. H., Jr.

1979-01-01

Certain behavioral aspects associated with fracture and crack extension that cannot be studied using other techniques were evaluated with the ultrasonic method. Characterization of collimated beam techniques showed that significant beam width reduction could be accomplished. Techniques for collimation are given. The crack-opening displacement-gage correction-factor study showed that displacement resulting from crack opening and that from plasticity could be readily differentiated. Crack closure studies using both ultrasonic and crack-opening displacement measurements showed an opening and closing behavior associated with load-unload curves. The results of this work were in general agreement with the closure concepts of Elber. Ultrasonic measurements used to study the nature of flaw extension characteristics associated with failure of the ligament between the flaw front and back surface showed that penetration could occur by an abrupt fracturing after subcritical growth or by continuous growth.

The Effects of Loading History and Closure of the Fatigue Characteristics of Heat Treated Ti-6Al-2Sn-4Zr-6Mo

DTIC Science & Technology

1988-12-01

closure is dependent on the measurement technique used. Macha and others (24:213 ; 25:195) noted that the accuracy in determining the closure load was...Crack Closure, ASTM STP 982. 247-259. Philadelphia: American Society for Testing and Materials, 1988. 24. Macha , D. E. and others. "On the Variation of

Comparison of mixed-mode stress-intensity factors obtained through displacement correlation, J-integral formulation, and modified crack-closure integral

NASA Astrophysics Data System (ADS)

Bittencourt, Tulio N.; Barry, Ahmabou; Ingraffea, Anthony R.

This paper presents a comparison among stress-intensity factors for mixed-mode two-dimensional problems obtained through three different approaches: displacement correlation, J-integral, and modified crack-closure integral. All mentioned procedures involve only one analysis step and are incorporated in the post-processor page of a finite element computer code for fracture mechanics analysis (FRANC). Results are presented for a closed-form solution problem under mixed-mode conditions. The accuracy of these described methods then is discussed and analyzed in the framework of their numerical results. The influence of the differences among the three methods on the predicted crack trajectory of general problems is also discussed.

Fatigue-Crack-Growth Structural Analysis

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1986-01-01

Elastic and plastic deformations calculated under variety of loading conditions. Prediction of fatigue-crack-growth lives made with FatigueCrack-Growth Structural Analysis (FASTRAN) computer program. As cyclic loads are applied to initial crack configuration, FASTRAN predicts crack length and other parameters until complete break occurs. Loads are tensile or compressive and of variable or constant amplitude. FASTRAN incorporates linear-elastic fracture mechanics with modifications of load-interaction effects caused by crack closure. FASTRAN considered research tool, because of lengthy calculation times. FASTRAN written in FORTRAN IV for batch execution.

Sub-10-micrometer toughening and crack tip toughness of dental enamel.

PubMed

Ang, Siang Fung; Schulz, Anja; Pacher Fernandes, Rodrigo; Schneider, Gerold A

2011-04-01

In previous studies, enamel showed indications to occlude small cracks in-vivo and exhibited R-curve behaviors for bigger cracks ex-vivo. This study quantifies the crack tip's toughness (K(I0),K(III0)), the crack's closure stress and the cohesive zone size at the crack tip of enamel and investigates the toughening mechanisms near the crack tip down to the length scale of a single enamel crystallite. The crack-opening-displacement (COD) profile of cracks induced by Vickers indents on mature bovine enamel was studied using atomic force microscopy (AFM). The mode I crack tip toughness K(I0) of cracks along enamel rod boundaries and across enamel rods exhibit a similar range of values: K(I0,Ir)=0.5-1.6MPa m(0.5) (based on Irwin's 'near-field' solution) and K(I0,cz)=0.8-1.5MPa m(0.5) (based on the cohesive zone solution of the Dugdale-Muskhelishvili (DM) crack model). The mode III crack tip toughness K(III0,Ir) was computed as 0.02-0.15MPa m(0.5). The crack-closure stress at the crack tip was computed as 163-770 MPa with a cohesive zone length and width 1.6-10.1μm and 24-44 nm utilizing the cohesive zone solution. Toughening elements were observed under AFM and SEM: crack bridging due to protein ligament and hydroxyapatite fibres (micro- and nanometer scale) as well as microcracks were identified. Crown Copyright © 2010. Published by Elsevier Ltd. All rights reserved.

Numerical and analytical modeling of the end-loaded split (ELS) test specimens made of multi-directional coupled composite laminates

NASA Astrophysics Data System (ADS)

Samborski, Sylwester; Valvo, Paolo S.

2018-01-01

The paper deals with the numerical and analytical modelling of the end-loaded split test for multi-directional laminates affected by the typical elastic couplings. Numerical analysis of three-dimensional finite element models was performed with the Abaqus software exploiting the virtual crack closure technique (VCCT). The results show possible asymmetries in the widthwise deflections of the specimen, as well as in the strain energy release rate (SERR) distributions along the delamination front. Analytical modelling based on a beam-theory approach was also conducted in simpler cases, where only bending-extension coupling is present, but no out-of-plane effects. The analytical results matched the numerical ones, thus demonstrating that the analytical models are feasible for test design and experimental data reduction.

Near-threshold fatigue behavior of copper alloys in air and aqueous environments: A high cyclic frequency study

NASA Astrophysics Data System (ADS)

Ahmed, Tawfik M.

The near-threshold fatigue crack propagation behavior of alpha-phase copper alloys in desiccated air and several aqueous environments has been investigated. Three commercial alloys of nominal composition Cu-30Ni (Cu-Ni), Cu-30Zn (Cu-Zn) and 90Cu-7Al-3Fe (Cu-Al) were tested. Fatigue tests were conducted using standard prefatigued single edged notched (SEN) specimens loaded in tension at a high frequency of ˜100 Hz. Different R-ratios were employed, mostly at R-ratios of 0.5. Low loading levels were used that corresponded to the threshold and near-threshold regions where Delta Kth ≤ DeltaK ≤ 11 MPa√m. Fatigue tests in the aqueous solutions showed that the effect of different corrosive environments during high frequency testing (˜100 Hz) was not as pronounced as was expected when compared relative to air. Further testing revealed that environmental effects were present and fatigue crack growth rates were influenced by the fluid-induced closure effects which are generally reported in the fatigue literature to be operative only in viscous liquids, not in aqueous solutions. It was concluded that high frequency testing in aqueous environments consistently decreased crack growth rates in a manner similar to crack retardation effects in viscous fluids. Several theoretical models reported in the literature have underestimated, if not failed, to adequately predict the fluid induced closure in aqueous solutions. Results from the desiccated air tests confirmed that, under closure-free conditions (high R-ratios), both threshold values and fatigue crack growth rate of stage II can be related to Young's modulus, in agreement with results from the literature. The role of different mechanical and environmental variables on fatigue behavior becomes most visible in the low R -ratio regime, and contribute to various closure processes.

Crack Closure and Fatigue Crack Growth in 2219-T851 Aluminum Alloy

DTIC Science & Technology

1976-08-01

assumes the length of the crack perimeter to remain es - ’I sentially constant. At the maximum load, the crack is ap- proximately parabolic (or ellipical...for center cracked j specimens) in shape. With unloading, the parabola (or el- lipse) is collapsed. The resulting change in shape produces an apparent...reloading process, the electrical potential remained es - j sentially constant initially and was less than that at the corresponding load during unloading

A theoretical approach to quantify the effect of random cracks on rock deformation in uniaxial compression

NASA Astrophysics Data System (ADS)

Zhou, Shuwei; Xia, Caichu; Zhou, Yu

2018-06-01

Cracks have a significant effect on the uniaxial compression of rocks. Thus, a theoretically analytical approach was proposed to assess the effects of randomly distributed cracks on the effective Young’s modulus during the uniaxial compression of rocks. Each stage of the rock failure during uniaxial compression was analyzed and classified. The analytical approach for the effective Young’s modulus of a rock with only a single crack was derived while considering the three crack states under stress, namely, opening, closure-sliding, and closure-nonsliding. The rock was then assumed to have many cracks with randomly distributed direction, and the effect of crack shape and number during each stage of the uniaxial compression on the effective Young’s modulus was considered. Thus, the approach for the effective Young’s modulus was used to obtain the whole stress-strain process of uniaxial compression. Afterward, the proposed approach was employed to analyze the effects of related parameters on the whole stress-stain curve. The proposed approach was eventually compared with some existing rock tests to validate its applicability and feasibility. The proposed approach has clear physical meaning and shows favorable agreement with the rock test results.

Selection of finite-element mesh parameters in modeling the growth of hydraulic fracturing cracks

NASA Astrophysics Data System (ADS)

Kurguzov, V. D.

2016-12-01

The effect of the mesh geometry on the accuracy of solutions obtained by the finite-element method for problems of linear fracture mechanics is investigated. The guidelines have been formulated for constructing an optimum mesh for several routine problems involving elements with linear and quadratic approximation of displacements. The accuracy of finite-element solutions is estimated based on the degree of the difference between the calculated stress-intensity factor (SIF) and its value obtained analytically. In problems of hydrofracturing of oil-bearing formation, the pump-in pressure of injected water produces a distributed load on crack flanks as opposed to standard fracture mechanics problems that have analytical solutions, where a load is applied to the external boundaries of the computational region and the cracks themselves are kept free from stresses. Some model pressure profiles, as well as pressure profiles taken from real hydrodynamic computations, have been considered. Computer models of cracks with allowance for the pre-stressed state, fracture toughness, and elastic properties of materials are developed in the MSC.Marc 2012 finite-element analysis software. The Irwin force criterion is used as a criterion of brittle fracture and the SIFs are computed using the Cherepanov-Rice invariant J-integral. The process of crack propagation in a linearly elastic isotropic body is described in terms of the elastic energy release rate G and modeled using the VCCT (Virtual Crack Closure Technique) approach. It has been found that the solution accuracy is sensitive to the mesh configuration. Several parameters that are decisive in constructing effective finite-element meshes, namely, the minimum element size, the distance between mesh nodes in the vicinity of a crack tip, and the ratio of the height of an element to its length, have been established. It has been shown that a mesh that consists of only small elements does not improve the accuracy of the solution.

Effect of additional heat treatment of 2024-T3 on the growth of fatigue crack in air and in vacuum

NASA Technical Reports Server (NTRS)

Louwaard, E. P.

1986-01-01

In order to determine the influence of ductility on the fatigue crack growth rate of aluminum alloys, fatigue tests were carried out on central notched specimens of 2024-T3 and 2024-T8 sheet material. The 2024-T8 material was obtained by an additional heat treatment applied on 2024-T3 (18 hours at 192 C), which increased the static yield strength from 43.6 to 48.9 kgf/sq mm. A change in the ultimate strength was not observed. Fatigue tests were carried out on both materials in humid air and in high vacuum. According to a new crack propagation model, crack extension is supported to be caused by a slip-related process and debonding triggered by the environment. This model predicts an effect of the ductility on the crack growth rate which should be smaller in vacuum than in humid air; however, this was not confirmed. In humid air the crack-growth rate in 2024-T8 was about 2 times faster than in 2024-T3, while in vacuum the ratio was about 2.5. Crack closure measurements gave no indications that crack closure played a significant role in both materials. Some speculative explanations are briefly discussed.

Mitigation of Crack Damage in Metallic Materials

NASA Technical Reports Server (NTRS)

Leser, Patrick E.; Newman, John A.; Smith, Stephen W.; Leser, William P.; Wincheski, Russell A.; Wallace, Terryl A.; Glaessgen, Edward H.; Piascik, Robert S.

2014-01-01

A system designed to mitigate or heal crack damage in metallic materials has been developed where the protected material or component is coated with a low-melting temperature film. After a crack is formed, the material is heated, melting the film which then infiltrates the crack opening through capillary action. Upon solidification, the healing material inhibits further crack damage in two ways. While the crack healing material is intact, it acts like an adhesive that bonds or bridges the crack faces together. After fatigue loading damages, the healing material in the crack mouth inhibits further crack growth by creating artificially-high crack closure levels. Mechanical test data show that this method sucessfully arrests or retards crack growth in laboratory specimens.

Strain energy release rate analysis of the end-notched flexure specimen using the finite-element method

NASA Technical Reports Server (NTRS)

Salpekar, S. A.; Raju, I. S.; O'Brien, T. K.

1988-01-01

Two-dimensional finite-element analysis of the end-notched flexure specimen was performed using 8-node isoparametric, parabolic elements to evaluate compliance and mode II strain energy release rates, G sub II. The G sub II values were computed using two different techniques: the virtual crack-closure technique (VCCT) and the rate of change of compliance with crack length (compliance derivative method). The analysis was performed for various crack-length-to-semi-span (a/L) ratios ranging from 0.2 to 0.9. Three material systems representing a wide range of material properties were analyzed. The compliance and strain energy release rates of the specimen calculated with the present finite-element analysis agree very well with beam theory equations including transverse shear. The G sub II values calculated using the compliance derivative method compared extremely well with those calculated using the VCCT. The G sub II values obtained by the compliance derivative method using the top or bottom beam deflections agreed closely with each other. The strain energy release rates from a plane-stress analysis were higher than the plane-strain values by only a small percentage, indicating that either assumption may be used in the analysis. The G sub II values for one material system calculated from the finte-element analysis agreed with one solution in the literature and disagreed with the other solution in the literature.

Cold Spray for Repair of Magnesium Components

DTIC Science & Technology

2011-11-01

powder material. Other advantages of the Cold Spray process include:  It provides extremely dense coatings with virtually no inclusions or cracks ... crack on insertion of Rosan fitting and does not reclaim the mechanical properties of the Mg alloy. It is expected that the use of Cold Spray coating...Spray process include:  Extremely dense coatings with virtually no inclusions or cracks .  Retains properties and microstructure of initial powder

On the interaction of ultrasound with cracks: Applications to fatigue crack growth

NASA Technical Reports Server (NTRS)

Buck, O.; Thompson, R. B.; Rehbein, D. K.

1986-01-01

Partial contact of two rough fatigue crack surfaces leads to transmission and diffraction of an acoustic signal at those contacts. Recent experimental and theoretical efforts to understand and quantify such contact in greater detail are discussed. The objective is to develop an understanding of the closure phenomenon and its application to the interpretation of fatigue data, in particular the R-ratio, spike overload/underload and threshold effects on crack propagation.

Fatigue crack sizing in rail steel using crack closure-induced acoustic emission waves

NASA Astrophysics Data System (ADS)

Li, Dan; Kuang, Kevin Sze Chiang; Ghee Koh, Chan

2017-06-01

The acoustic emission (AE) technique is a promising approach for detecting and locating fatigue cracks in metallic structures such as rail tracks. However, it is still a challenge to quantify the crack size accurately using this technique. AE waves can be generated by either crack propagation (CP) or crack closure (CC) processes and classification of these two types of AE waves is necessary to obtain more reliable crack sizing results. As the pre-processing step, an index based on wavelet power (WP) of AE signal is initially established in this paper in order to distinguish between the CC-induced AE waves and their CP-induced counterparts. Here, information embedded within the AE signal was used to perform the AE wave classification, which is preferred to the use of real-time load information, typically adopted in other studies. With the proposed approach, it renders the AE technique more amenable to practical implementation. Following the AE wave classification, a novel method to quantify the fatigue crack length was developed by taking advantage of the CC-induced AE waves, the count rate of which was observed to be positively correlated with the crack length. The crack length was subsequently determined using an empirical model derived from the AE data acquired during the fatigue tests of the rail steel specimens. The performance of the proposed method was validated by experimental data and compared with that of the traditional crack sizing method, which is based on CP-induced AE waves. As a significant advantage over other AE crack sizing methods, the proposed novel method is able to estimate the crack length without prior knowledge of the initial crack length, integration of AE data or real-time load amplitude. It is thus applicable to the health monitoring of both new and existing structures.

Finite element modelling for mode-I fracture behaviour of CFRP

NASA Astrophysics Data System (ADS)

Chetan, H. C.; Kattimani, Subhaschandra; Murigendrappa, S. M.

2018-04-01

Debonding is a major failure mechanism in Carbon Fiber Reinforced Polymer (CFRP) due to presence of many adhesion joins, in between many layers. In the current study a finite element simulation is carried out using Virtual Crack Closure Technique (VCCT) and Cohesive Zone Modelling (CZM) using Abaqus as analysis tool. A comparative study is performed in to order analyze convergence of results from CZM and VCCT. It was noted that CZM results matched well with published literature. The results from VCCT were also in good comparison with experimental data of published literature, but were seen to be overestimated. Parametric study is performed to evaluate the variation of input parameters like initial stiffness, element size, peak stress and energy release rate `G'. From the numerical evaluation, it was noted that CZM simulation relies largely on element size and peak stress.

Results of the second Round Robin on opening-load measurement conducted by ASTM Task Group E24.04.04 on crack closure measurement and analysis

NASA Technical Reports Server (NTRS)

Phillips, E. P.

1993-01-01

A second experimental Round Robin on the measurement of the crack opening load in fatigue crack growth tests has been completed by the ASTM Task Group E24.04.04 on Crack Closure Measurement and Analysis. Fourteen laboratories participated in the testing of aluminum alloy compact tension specimens. Opening-load measurements were made at three crack lengths during constant Delta K, constant stress ratio tests by most of the participants. Four participants made opening-load measurements during threshold tests. All opening-load measurements were based on the analysis of specimens compliance behavior, where the displacement/strain was measured either at the crack mouth or the mid-height back face location. The Round Robin data were analyzed for opening load using two non-subjective analysis methods: the compliance offset and the correlation coefficient methods. The scatter in the opening load results was significantly reduced when some of the results were excluded from the analysis population based on an accept/reject criterion for raw data quality. The compliance offset and correlation coefficient opening load analysis methods produced similar results for data populations that had been screened to eliminate poor quality data.

Modeling Delamination in Postbuckled Composite Structures Under Static and Fatigue Loads

NASA Technical Reports Server (NTRS)

Bisagni, Chiara; Brambilla, Pietro; Bavila, Carlos G.

2013-01-01

The ability of the Abaqus progressive Virtual Crack Closure Technique (VCCT) to model delamination in composite structures was investigated for static, postbuckling, and fatigue loads. Preliminary evaluations were performed using simple Double Cantilever Beam (DCB) and Mixed-Mode Bending (MMB) specimens. The nodal release sequences that describe the propagation of the delamination front were investigated. The effect of using a sudden or a gradual nodal release was evaluated by considering meshes aligned with the crack front as well as misaligned meshes. Fatigue simulations were then performed using the Direct Cyclic Fatigue (DCF) algorithm. It was found that in specimens such as the DCB, which are characterized by a nearly linear response and a pure fracture mode, the algorithm correctly predicts the Paris Law rate of propagation. However, the Abaqus DCF algorithm does not consider different fatigue propagation laws in different fracture modes. Finally, skin/stiffener debonding was studied in an aircraft fuselage subcomponent in which debonding occurs deep into post-buckling deformation. VCCT was shown to be a robust tool for estimating the onset propagation. However, difficulties were found with the ability of the current implementation of the Abaqus progressive VCCT to predict delamination propagation within structures subjected to postbuckling deformations or fatigue loads.

Direct observation of the residual plastic deformation caused by a single tensile overload

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bichler, C.; Pippan, R.

1999-07-01

The fatigue crack growth behavior following single tensile overloads at high stress intensity ranges in a cold-rolled austenitic steel has been studied experimentally. After tensile overloads, fatigue cracks initially accelerate, followed by significant retardation, before the growth rates return to their baseline level. The initial acceleration was attributed to an immediate reduction in near-tip closure. Scanning electron micrography and stereophotogrammetric reconstruction of the fracture surface were applied to study the residual plastic deformation caused by a single tensile overload in the mid-thickness of the specimen. The measured residual opening displacement of the crack as a function of the overload ismore » presented and compared with simple estimations. Also, free specimen surface observations of the residual plastic deformation and crack growth rate were performed. In the midsection of the specimens the striation spacing-length, i.e., the microscopic growth rates, were measured before and after the applied overload. It will be shown that the measured plasticity-induced wedges from the single overload and the observed propagation behavior support the significance of the concept of crack closure.« less

Improved method for determining the stress relaxation at the crack tip

NASA Astrophysics Data System (ADS)

Grinevich, A. V.; Erasov, V. S.; Avtaev, V. V.

2017-10-01

A technique is suggested to determine the stress relaxation at the crack tip during tests of a specimen of a new type at a constant crack opening fixed by a stay bolt. The shape and geometry of the specimen make it possible to set the load and to determine the crack closure force after long-term exposure using the force transducer of a tensile-testing machine. The stress relaxation at the crack tip is determined in a V95pchT2 alloy specimen at elevated temperatures.

Nonlinear crack analysis with finite elements

NASA Technical Reports Server (NTRS)

Armen, H., Jr.; Saleme, E.; Pifko, A.; Levine, H. S.

1973-01-01

The application of finite element techniques to the analytic representation of the nonlinear behavior of arbitrary two-dimensional bodies containing cracks is discussed. Specific methods are proposed using which it should be possible to obtain information concerning: the description of the maximum, minimum, and residual near-tip stress and strain fields; the effects of crack closure on the near-tip behavior of stress and strain fields during cyclic loading into the plastic range; the stress-strain and displacement field behavior associated with a nonstationary crack; and the effects of large rotation near the crack tip.

Fatigue crack growth in unidirectional metal matrix composite

NASA Technical Reports Server (NTRS)

Ghosn, Louis J.; Telesman, Jack; Kantzos, Peter

1990-01-01

The weight function method was used to determine the effective stress intensity factor and the crack opening profile for a fatigue tested composite which exhibited fiber bridging. The bridging mechanism was modeled using two approaches; the crack closure approach and the shear lag approach. The numerically determined stress intensity factor values from both methods were compared and correlated with the experimentally obtained crack growth rates for SiC/Ti-15-3 (0)(sub 8) oriented composites. The near crack tip opening profile was also determined for both methods and compared with the experimentally obtained measurements.

The noncontinuum crack tip deformation behavior of surface microcracks

NASA Astrophysics Data System (ADS)

Morris, W. L.

1980-07-01

The crack tip opening displacement (CTOD) of small surface fatigue cracks (lengths of the grain size) in Al 2219-T851 depends upon the location of a crack relative to the grain boundaries. Both CTOD and crack tip closure stress are greatest when the crack tip is a large distance from the next grain boundary in the direction of crack propagation. Contrary to behavioral trends predicted by continuum fracture mechanics, crack length has no detectable effect on the contribution of plastic deformation to CTOD. It is apparent from these observations that the region of significant plastic deformation is confined by the grain boundaries, resulting in a plastic zone size that is insensitive to crack length and to external load.

Effect of Water Vapor Pressure on Fatigue Crack Growth in Al-Zn-Cu-Mg Alloy Over Wide-Range Stress Intensity Factor Loading

DTIC Science & Technology

2014-05-07

impacts: (a) crack closure, (b) transport of water vapor molecules within the fatigue crack (47], and (c) tensile stress-plastic strain range...sealed stainless steel UHV chamber. Pure water vapor was introduced from a sealed glass flask containing triply distilled water, via a precision leak...lamellar for H1 flow in a fatigue crack in steel ; specifically, flow is dominated by the low dynamic viscosity of a gas (particularly at low pressures) and

40 CFR 63.906 - Inspection and monitoring requirements.

Code of Federal Regulations, 2010 CFR

2010-07-01

... between the roof and the tank wall; broken, cracked, or otherwise damaged seals or gaskets on closure devices; and broken or missing hatches, access covers, caps, or other closure devices. (2) The owner or... air pollution control equipment designated as such a written explanation of the reasons why the...

Results of the Round Robin on opening-load measurement conducted by ASTM Task Group E24.04.04 on Crack Closure Measurement and Analysis

NASA Technical Reports Server (NTRS)

Phillips, Edward P.

1989-01-01

An experimental Round Robin on the measurement of the opening load in fatigue crack growth tests was conducted on Crack Closure Measurement and Analysis. The Round Robin evaluated the current level of consistency of opening load measurements among laboratories and to identify causes for observed inconsistency. Eleven laboratories participated in the testing of compact and middle-crack specimens. Opening-load measurements were made for crack growth at two stress-intensity factor levels, three crack lengths, and following an overload. All opening-load measurements were based on the analysis of specimen compliance data. When all of the results reported (from all participants, all measurement methods, and all data analysis methods) for a given test condition were pooled, the range of opening loads was very large--typically spanning the lower half of the fatigue loading cycle. Part of the large scatter in the reported opening-load results was ascribed to consistent differences in results produced by the various methods used to measure specimen compliance and to evaluate the opening load from the compliance data. Another significant portion of the scatter was ascribed to lab-to-lab differences in producing the compliance data when using nominally the same method of measurement.

Analysis of the status of pre-release cracks in prestressed concrete structures using long-gauge sensors

NASA Astrophysics Data System (ADS)

Abdel-Jaber, H.; Glisic, B.

2015-02-01

Prestressed structures experience limited tensile stresses in concrete, which limits or completely eliminates the occurrence of cracks. However, in some cases, large tensile stresses can develop during the early age of the concrete due to thermal gradients and shrinkage effects. Such stresses can cause early-age cracks, termed ‘pre-release cracks’, which occur prior to the transfer of the prestressing force. When the prestressing force is applied to the cross-section, it is assumed that partial or full closure of the cracks occurs by virtue of the force transfer through the cracked cross-section. Verification of the closure of the cracks after the application of the prestressing force is important as it can either confirm continued structural integrity or indicate and approximate reduced structural capacity. Structural health monitoring (SHM) can be used for this purpose. This paper researches an SHM method that can be applied to prestressed beam structures to assess the condition of pre-release cracks. The sensor network used in this method consists of parallel long-gauge fiber optic strain sensors embedded in the concrete cross-sections at various locations. The same network is used for damage detection, i.e. detection and characterization of the pre-release cracks, and for monitoring the prestress force transfer. The method is validated on a real structure, a curved continuous girder. Results from the analysis confirm the safety and integrity of the structure. The method and its application are presented in this paper.

Constant amplitude and post-overload fatigue crack growth behavior in PM aluminum alloy AA 8009

NASA Technical Reports Server (NTRS)

Reynolds, A. P.

1992-01-01

A recently developed, rapidly solidified, powder metallurgy, dispersion strengthened aluminum alloy, AA 8009, was fatigue tested at room temperature in lab air. Constant amplitude/constant delta kappa and single spike overload conditions were examined. High fatigue crack growth rates and low crack closure levels compared to typical ingot metallurgy aluminum alloys were observed. It was proposed that minimal crack roughness, crack path delection, and limited slip reversibility, resulting from ultra-fine microstructure, were responsible for the relatively poor da/dN-delta kappa performance of AA 8009 as compared to that of typical IM aluminum alloys.

Constant amplitude and post-overload fatigue crack growth behavior in PM aluminum alloy AA 8009

NASA Technical Reports Server (NTRS)

Reynolds, A. P.

1991-01-01

A recently developed, rapidly solidified, powder metallurgy, dispersion strengthened aluminum alloy, AA 8009, was fatigue tested at room temperature in lab air. Constant amplitude/constant delta kappa and single spike overload conditions were examined. High fatigue crack growth rates and low crack closure levels compared to typical ingot metallurgy aluminum alloys were observed. It was proposed that minimal crack roughness, crack path deflection, and limited slip reversibility, resulting from ultra-fine microstructure, were responsible for the relatively poor da/dN-delta kappa performance of AA 8009 as compared to that of typical IM aluminum alloys.

Fracture Mechanics Analysis of Stitched Stiffener-Skin Debonding

NASA Technical Reports Server (NTRS)

Glaessgen, E. H.; Raju, I. S.; Poe, C. C., Jr.

1998-01-01

An analysis based on plate finite elements and the virtual crack closure technique has been implemented to study the effect of stitching on mode I and mode II strain energy release rates for debond configurations. The stitches were modeled as discrete nonlinear fastener elements with a compliance determined by experiment. The axial and shear behavior of the stitches was considered, however, the two compliances and failure loads were assumed to be independent. Both a double cantilever beam (mode I) and a mixed mode skin-stiffener debond configuration were studied. In the double cantilever beam configurations, G(sub I) began to decrease once the debond had grown beyond the first row of stitches and was reduced to zero for long debonds. In the mixed-mode skin-stiffener configurations, G(sub I) showed a similar behavior as in the double cantilever beam configurations, however, G(sub u), continued to increase with increasing debond length.

Analysis of an Aircraft Honeycomb Sandwich Panel with Circular Face Sheet/Core Disbond Subjected to Ground-Air Pressurization

NASA Technical Reports Server (NTRS)

Rinker, Martin; Krueger, Ronald; Ratcliffe, James

2013-01-01

The ground-air pressurization of lightweight honeycomb sandwich structures caused by alternating pressure differences between the enclosed air within the honeycomb core and the ambient environment is a well-known and controllable loading condition of aerospace structures. However, initial face sheet/core disbonds intensify the face sheet peeling effect of the internal pressure load significantly and can decrease the reliability of the sandwich structure drastically. Within this paper, a numerical parameter study was carried out to investigate the criticality of initial disbonds in honeycomb sandwich structures under ground-air pressurization. A fracture mechanics approach was used to evaluate the loading at the disbond front. In this case, the strain energy release rate was computed via the Virtual Crack Closure Technique. Special attention was paid to the pressure-deformation coupling which can decrease the pressure load within the disbonded sandwich section significantly when the structure is highly deformed.

Mesh Convergence Requirements for Composite Damage Models

NASA Technical Reports Server (NTRS)

Davila, Carlos G.

2016-01-01

The ability of the finite element method to accurately represent the response of objects with intricate geometry and loading renders the finite element method as an extremely versatile analysis technique for structural analysis. Finite element analysis is routinely used in industry to calculate deflections, stress concentrations, natural frequencies, buckling loads, and much more. The method works by discretizing complex problems into smaller, simpler approximations that are valid over small uniform domains. For common analyses, the maximum size of the elements that can be used is often be determined by experience. However, to verify the quality of a solution, analyses with several levels of mesh refinement should be performed to ensure that the solution has converged. In recent years, the finite element method has been used to calculate the resistance of structures, and in particular that of composite structures. A number of techniques such as cohesive zone modeling, the virtual crack closure technique, and continuum damage modeling have emerged that can be used to predict cracking, delaminations, fiber failure, and other composite damage modes that lead to structural collapse. However, damage models present mesh refinement requirements that are not well understood. In this presentation, we examine different mesh refinement issues related to the representation of damage in composite materials. Damage process zone sizes and their corresponding mesh requirements will be discussed. The difficulties of modeling discontinuities and the associated need for regularization techniques will be illustrated, and some unexpected element size constraints will be presented. Finally, some of the difficulties in constructing models of composite structures capable of predicting transverse matrix cracking will be discussed. It will be shown that to predict the initiation and propagation of transverse matrix cracks, their density, and their saturation may require models that are significantly more refined than those that have been contemplated in the past.

Three Dimensional Constraint Effects on the Estimated (Delta)CTOD during the Numerical Simulation of Different Fatigue Threshold Testing Techniques

NASA Technical Reports Server (NTRS)

Seshadri, Banavara R.; Smith, Stephen W.

2007-01-01

Variation in constraint through the thickness of a specimen effects the cyclic crack-tip-opening displacement (DELTA CTOD). DELTA CTOD is a valuable measure of crack growth behavior, indicating closure development, constraint variations and load history effects. Fatigue loading with a continual load reduction was used to simulate the load history associated with fatigue crack growth threshold measurements. The constraint effect on the estimated DELTA CTOD is studied by carrying out three-dimensional elastic-plastic finite element simulations. The analysis involves numerical simulation of different standard fatigue threshold test schemes to determine how each test scheme affects DELTA CTOD. The American Society for Testing and Materials (ASTM) prescribes standard load reduction procedures for threshold testing using either the constant stress ratio (R) or constant maximum stress intensity (K(sub max)) methods. Different specimen types defined in the standard, namely the compact tension, C(T), and middle cracked tension, M(T), specimens were used in this simulation. The threshold simulations were conducted with different initial K(sub max) values to study its effect on estimated DELTA CTOD. During each simulation, the DELTA CTOD was estimated at every load increment during the load reduction procedure. Previous numerical simulation results indicate that the constant R load reduction method generates a plastic wake resulting in remote crack closure during unloading. Upon reloading, this remote contact location was observed to remain in contact well after the crack tip was fully open. The final region to open is located at the point at which the load reduction was initiated and at the free surface of the specimen. However, simulations carried out using the constant Kmax load reduction procedure did not indicate remote crack closure. Previous analysis results using various starting K(sub max) values and different load reduction rates have indicated DELTA CTOD is independent of specimen size. A study of the effect of specimen thickness and geometry on the measured DELTA CTOD for various load reduction procedures and its implication in the estimation of fatigue crack growth threshold values is discussed.

Development of high shrinkage polyethylene terephthalate (PET) shape memory polymer tendons for concrete crack closure

NASA Astrophysics Data System (ADS)

Teall, Oliver; Pilegis, Martins; Sweeney, John; Gough, Tim; Thompson, Glen; Jefferson, Anthony; Lark, Robert; Gardner, Diane

2017-04-01

The shrinkage force exerted by restrained shape memory polymers (SMPs) can potentially be used to close cracks in structural concrete. This paper describes the physical processing and experimental work undertaken to develop high shrinkage die-drawn polyethylene terephthalate (PET) SMP tendons for use within a crack closure system. The extrusion and die-drawing procedure used to manufacture a series of PET tendon samples is described. The results from a set of restrained shrinkage tests, undertaken at differing activation temperatures, are also presented along with the mechanical properties of the most promising samples. The stress developed within the tendons is found to be related to the activation temperature, the cross-sectional area and to the draw rate used during manufacture. Comparisons with commercially-available PET strip samples used in previous research are made, demonstrating an increase in restrained shrinkage stress by a factor of two for manufactured PET filament samples.

Flexible, liquid core light guide with focusing and light shaping attachments

DOEpatents

Wojcik, Randolph Frank; Majewski, Stanislaw; Zorn, Carl John; Kross, Brian

1999-01-01

A liquid light guide system for ultraviolet light is disclosed that has a light shaping arrangement for the emitted light, a stable liquid core and sheath and reliable and effective end closures. The end closures include a metal crimping arrangement that utilizes two layers of deformable materials to prevent cracking of endplugs.

Stress Ratio Effects on Crack Opening Loads and Crack Growth Rates in Aluminum Alloy 2024

NASA Technical Reports Server (NTRS)

Riddell, William T.; Piascik, Robert S.

1998-01-01

The effects of stress ratio (R) and crack opening behavior on fatigue crack growth rates (da/dN) for aluminum alloy (AA) 2024-T3 were investigated using constant-delta K testing, closure measurements, and fractography. Fatigue crack growth rates were obtained for a range of delta K and stress ratios. Results show that constant delta K fatigue crack growth for R ranging from near 0 to 1 is divided into three regions. In Region 1, at low R, da/dN increases with increasing R. In Region 2, at intermediate R, fatigue crack growth rates are relatively independent of R. In Region 3, at high R, further increases in da/dN are observed with increasing R.

Generating Fatigue Crack Growth Thresholds with Constant Amplitude Loads

NASA Technical Reports Server (NTRS)

Forth, Scott C.; Newman, James C., J.; Forman, Royce G.

2002-01-01

The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. Some experimental procedures tend to induce load history effects that result in remote crack closure from plasticity. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor, K, will increase, as will the crack growth rate, da/dN. A fatigue crack growth threshold test procedure is developed and experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R.

Gear crack propagation investigations

NASA Technical Reports Server (NTRS)

Lewicki, David G.; Ballarini, Roberto

1996-01-01

Analytical and experimental studies were performed to investigate the effect of gear rim thickness on crack propagation life. The FRANC (FRacture ANalysis Code) computer program was used to simulate crack propagation. The FRANC program used principles of linear elastic fracture mechanics, finite element modeling, and a unique re-meshing scheme to determine crack tip stress distributions, estimate stress intensity factors, and model crack propagation. Various fatigue crack growth models were used to estimate crack propagation life based on the calculated stress intensity factors. Experimental tests were performed in a gear fatigue rig to validate predicted crack propagation results. Test gears were installed with special crack propagation gages in the tooth fillet region to measure bending fatigue crack growth. Good correlation between predicted and measured crack growth was achieved when the fatigue crack closure concept was introduced into the analysis. As the gear rim thickness decreased, the compressive cyclic stress in the gear tooth fillet region increased. This retarded crack growth and increased the number of crack propagation cycles to failure.

Corrosion fatigue crack propagation in metals

NASA Technical Reports Server (NTRS)

Gangloff, Richard P.

1990-01-01

This review assesses fracture mechanics data and mechanistic models for corrosion fatigue crack propagation in structural alloys exposed to ambient temperature gases and electrolytes. Extensive stress intensity-crack growth rate data exist for ferrous, aluminum and nickel based alloys in a variety of environments. Interactive variables (viz., stress intensity range, mean stress, alloy composition and microstructure, loading frequency, temperature, gas pressure and electrode potential) strongly affect crack growth kinetics and complicate fatigue control. Mechanistic models to predict crack growth rates were formulated by coupling crack tip mechanics with occluded crack chemistry, and from both the hydrogen embrittlement and anodic dissolution/film rupture perspectives. Research is required to better define: (1) environmental effects near threshold and on crack closure; (2) damage tolerant life prediction codes and the validity of similitude; (3) the behavior of microcrack; (4) probes and improved models of crack tip damage; and (5) the cracking performance of advanced alloys and composites.

Flexible, liquid core light guide with focusing and light shaping attachments

DOEpatents

Wojcik, R.F.; Majewski, S.; Zorn, C.J.; Kross, B.

1999-04-20

A liquid light guide system for ultraviolet light is disclosed that has a light shaping arrangement for the emitted light, a stable liquid core and sheath and reliable and effective end closures. The end closures include a metal crimping arrangement that utilizes two layers of deformable materials to prevent cracking of endplugs. 19 figs.

On Generating Fatigue Crack Growth Thresholds

NASA Technical Reports Server (NTRS)

Forth, Scott C.; Newman, James, Jr.; Forman, Royce G.

2003-01-01

The fatigue crack growth threshold, defining crack growth as either very slow or nonexistent, has been traditionally determined with standardized load reduction methodologies. These experimental procedures can induce load history effects that result in crack closure. This history can affect the crack driving force, i.e. during the unloading process the crack will close first at some point along the wake or blunt at the crack tip, reducing the effective load at the crack tip. One way to reduce the effects of load history is to propagate a crack under constant amplitude loading. As a crack propagates under constant amplitude loading, the stress intensity factor range, Delta K, will increase, as will the crack growth rate. da/dN. A fatigue crack growth threshold test procedure is experimentally validated that does not produce load history effects and can be conducted at a specified stress ratio, R. The authors have chosen to study a ductile aluminum alloy where the plastic deformations generated during testing may be of the magnitude to impact the crack opening.

The fatigue response of the aluminium-lithium alloy, 8090

NASA Technical Reports Server (NTRS)

Birt, M. J.; Beevers, C. J.

1989-01-01

The fatigue response of an Al-Li-Cu-Mg-Zr (8090) alloy has been studied at room temperature. The initiation and growth of small and long cracks has been examined at R = 0.1 and at a frequency of 100 Hz. Initiation was observed to occur dominantly at sub-grain boundaries. The growth of the small cracks was crystallographic in character and exhibited little evidence of retardation or arrest at the grain boundaries. The long crack data showed the alloy to have a high resistance to fatigue crack growth with underaging providing the optimum heat treatment for fatigue crack growth resistance. In general, this can be attributed to high levels of crack closure which resulted from the presence of extensive microstructurally related asperities.

Fractographic Observations on the Mechanism of Fatigue Crack Growth in Aluminium Alloys

NASA Astrophysics Data System (ADS)

Alderliesten, R. C.; Schijve, J.; Krkoska, M.

Special load histories are adopted to obtain information about the behavior of the moving crack tip during the increasing and decreasing part of a load cycle. It is associated with the crack opening and closure of the crack tip. Secondly, modern SEM techniques are applied for observations on the morphology of the fractures surfaces of a fatigue crack. Information about the cross section profiles of striations are obtained. Corresponding locations of the upper and the lower fracture surface are also explored in view of the crack extension mechanism. Most experiments are carried out on sheet specimens of aluminum alloys 2024-T3, but 7050-T7451 specimens are also tested in view of a different ductility of the two alloys.

Fatigue life and crack growth prediction methodology

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Phillips, E. P.; Everett, R. A., Jr.

1993-01-01

The capabilities of a plasticity-induced crack-closure model and life-prediction code to predict fatigue crack growth and fatigue lives of metallic materials are reviewed. Crack-tip constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta(K(sub eff))) under constant-amplitude loading. Some modifications to the delta(K(sub eff))-rate relations were needed in the near threshold regime to fit small-crack growth rate behavior and endurance limits. The model was then used to calculate small- and large-crack growth rates, and in some cases total fatigue lives, for several aluminum and titanium alloys under constant-amplitude, variable-amplitude, and spectrum loading. Fatigue lives were calculated using the crack growth relations and microstructural features like those that initiated cracks. Results from the tests and analyses agreed well.

Application of a substructuring technique to the problem of crack extension and closure

NASA Technical Reports Server (NTRS)

Armen, H., Jr.

1974-01-01

A substructuring technique, originally developed for the efficient reanalysis of structures, is incorporated into the methodology associated with the plastic analysis of structures. An existing finite-element computer program that accounts for elastic-plastic material behavior under cyclic loading was modified to account for changing kinematic constraint conditions - crack growth and intermittent contact of crack surfaces in two dimensional regions. Application of the analysis is presented for a problem of a centercrack panel to demonstrate the efficiency and accuracy of the technique.

An adaptively refined XFEM with virtual node polygonal elements for dynamic crack problems

NASA Astrophysics Data System (ADS)

Teng, Z. H.; Sun, F.; Wu, S. C.; Zhang, Z. B.; Chen, T.; Liao, D. M.

2018-02-01

By introducing the shape functions of virtual node polygonal (VP) elements into the standard extended finite element method (XFEM), a conforming elemental mesh can be created for the cracking process. Moreover, an adaptively refined meshing with the quadtree structure only at a growing crack tip is proposed without inserting hanging nodes into the transition region. A novel dynamic crack growth method termed as VP-XFEM is thus formulated in the framework of fracture mechanics. To verify the newly proposed VP-XFEM, both quasi-static and dynamic cracked problems are investigated in terms of computational accuracy, convergence, and efficiency. The research results show that the present VP-XFEM can achieve good agreement in stress intensity factor and crack growth path with the exact solutions or experiments. Furthermore, better accuracy, convergence, and efficiency of different models can be acquired, in contrast to standard XFEM and mesh-free methods. Therefore, VP-XFEM provides a suitable alternative to XFEM for engineering applications.

Mixed-mode crack tip loading and crack deflection in 1D quasicrystals

NASA Astrophysics Data System (ADS)

Wang, Zhibin; Scheel, Johannes; Ricoeur, Andreas

2016-12-01

Quasicrystals (QC) are a new class of materials besides crystals and amorphous solids and have aroused much attention of researchers since they were discovered. This paper presents a generalized fracture theory including the J-integral and crack closure integrals, relations between J1, J2 and the stress intensity factors as well as the implementation of the near-tip stress and displacement solutions of 1D QC. Different crack deflection criteria, i.e. the J-integral and maximum circumferential stress criteria, are investigated for mixed-mode loading conditions accounting for phonon-phason coupling. One focus is on the influence of phason stress intensity factors on crack deflection angles.

Behavior and influence of desiccation cracking in loess landfill covers

NASA Astrophysics Data System (ADS)

Wu, Tao; Lan, Ji-wu; Qiu, Qing-wen; He, Hai-jie; Li, He

2017-11-01

In the northwest region of China, loess was the main closure cover material of local landfills. Tests in a full-scale testing facility were conducted to investigate the behavior and influence of desiccation cracking in loess landfill covers. The desiccation cracks in the loess landfill cover intersected with T-shape, and the intersection angles were close to 90 degrees. The desiccation cracks formed as a result of drying, and would heal with the increase of moisture content of the loess. In addition, desiccation cracking in loess covers would promote the formation of preferential flow channels. As a consequence, the gas permeability of the loess cover was improved, and methane emissions increased obviously.

The mechanics of delamination in fiber-reinforced composite materials. Part 1: Stress singularities and solution structure

NASA Technical Reports Server (NTRS)

Wang, S. S.; Choi, I.

1983-01-01

The fundamental mechanics of delamination in fiber composite laminates is studied. Mathematical formulation of the problem is based on laminate anisotropic elasticity theory and interlaminar fracture mechanics concepts. Stress singularities and complete solution structures associated with general composite delaminations are determined. For a fully open delamination with traction-free surfaces, oscillatory stress singularities always appear, leading to physically inadmissible field solutions. A refined model is introduced by considering a partially closed delamination with crack surfaces in finite-length contact. Stress singularities associated with a partially closed delamination having frictional crack-surface contact are determined, and are found to be diferent from the inverse square-root one of the frictionless-contact case. In the case of a delamination with very small area of crack closure, a simplified model having a square-root stress singularity is employed by taking the limit of the partially closed delamination. The possible presence of logarithmic-type stress singularity is examined; no logarithmic singularity of any kind is found in the composite delamination problem. Numerical examples of dominant stress singularities are shown for delaminations having crack-tip closure with different frictional coefficients between general (1) and (2) graphite-epoxy composites.

Analysis of crack propagation in roller bearings using the boundary integral equation method - A mixed-mode loading problem

NASA Technical Reports Server (NTRS)

Ghosn, L. J.

1988-01-01

Crack propagation in a rotating inner raceway of a high-speed roller bearing is analyzed using the boundary integral method. The model consists of an edge plate under plane strain condition upon which varying Hertzian stress fields are superimposed. A multidomain boundary integral equation using quadratic elements was written to determine the stress intensity factors KI and KII at the crack tip for various roller positions. The multidomain formulation allows the two faces of the crack to be modeled in two different subregions, making it possible to analyze crack closure when the roller is positioned on or close to the crack line. KI and KII stress intensity factors along any direction were computed. These calculations permit determination of crack growth direction along which the average KI times the alternating KI is maximum.

Proof test and fatigue crack growth modeling on 2024-T3 aluminum alloy

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Poe, C. C., Jr.; Dawicke, D. S.

1990-01-01

Pressure proof testing of aircraft fuselage structures has been suggested as a means of screening critical crack sizes and of extending their useful life. The objective of this paper is to study the proof-test concept and to model the crack-growth process on a ductile material. Simulated proof and operational fatigue life tests have been conducted on cracked panels made of 2024-T3 aluminum alloy sheet material. A fatigue crack-closure model was modified to simulate the proof test and operational fatigue cycling. Using crack-growth rate and resistance-curve data, the model was able to predict crack growth during and after the proof load. These tests and analyses indicate that the proof test increases fatigue life; but the beneficial life, after a 1.33 or 1.5 proof, was less than a few hundred cycles.

Advances in Fatigue and Fracture Mechanics Analyses for Aircraft Structures

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1999-01-01

This paper reviews some of the advances that have been made in stress analyses of cracked aircraft components, in the understanding of the fatigue and fatigue-crack growth process, and in the prediction of residual strength of complex aircraft structures with widespread fatigue damage. Finite-element analyses of cracked structures are now used to determine accurate stress-intensity factors for cracks at structural details. Observations of small-crack behavior at open and rivet-loaded holes and the development of small-crack theory has lead to the prediction of stress-life behavior for components with stress concentrations under aircraft spectrum loading. Fatigue-crack growth under simulated aircraft spectra can now be predicted with the crack-closure concept. Residual strength of cracked panels with severe out-of-plane deformations (buckling) in the presence of stiffeners and multiple-site damage can be predicted with advanced elastic-plastic finite-element analyses and the critical crack-tip-opening angle (CTOA) fracture criterion. These advances are helping to assure continued safety of aircraft structures.

Advances in Fatigue and Fracture Mechanics Analyses for Metallic Aircraft Structures

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

2000-01-01

This paper reviews some of the advances that have been made in stress analyses of cracked aircraft components, in the understanding of the fatigue and fatigue-crack growth process, and in the prediction of residual strength of complex aircraft structures with widespread fatigue damage. Finite-element analyses of cracked metallic structures are now used to determine accurate stress-intensity factors for cracks at structural details. Observations of small-crack behavior at open and rivet-loaded holes and the development of small-crack theory has lead to the prediction of stress-life behavior for components with stress concentrations under aircraft spectrum loading. Fatigue-crack growth under simulated aircraft spectra can now be predicted with the crack-closure concept. Residual strength of cracked panels with severe out-of-plane deformations (buckling) in the presence of stiffeners and multiple-site damage can be predicted with advanced elastic-plastic finite-element analyses and the critical crack-tip-opening angle (CTOA) fracture criterion. These advances are helping to assure continued safety of aircraft structures.

Hybrid-finite-element analysis of some nonlinear and 3-dimensional problems of engineering fracture mechanics

NASA Technical Reports Server (NTRS)

Atluri, S. N.; Nakagaki, M.; Kathiresan, K.

1980-01-01

In this paper, efficient numerical methods for the analysis of crack-closure effects on fatigue-crack-growth-rates, in plane stress situations, and for the solution of stress-intensity factors for arbitrary shaped surface flaws in pressure vessels, are presented. For the former problem, an elastic-plastic finite element procedure valid for the case of finite deformation gradients is developed and crack growth is simulated by the translation of near-crack-tip elements with embedded plastic singularities. For the latter problem, an embedded-elastic-singularity hybrid finite element method, which leads to a direct evaluation of K-factors, is employed.

3013 DE INNER CONTAINER CLOSURE WELD CORROSION EVALUATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mickalonis, J.

Destructive evaluation (DE) of 3013 containers is one part of the U. S. Department of Energy Integrated Surveillance Program. During standard DE of 3013 containers, visual examinations for pitting and stress corrosion cracking (SCC) are performed on the accessible surfaces of the outer, inner, and convenience containers, which make up the 3013 container. As a result of 3013 DE additional analysis, the area near the inner container closure weld has been identified as being a region of increased corrosion susceptibility, which may provide a pathway for corrosive gases to the outer container. This area has a higher residual stress, anmore » altered microstructure, and less corrosion resistant weld oxides as a result of the welding process as well as a lower temperature than other areas of the container, which may increase the absorption of moisture on the surface. The deposition of moisture in this stressed region could lead to pitting and stress corrosion cracking. During FY2013, the inner container closure weld area was more closely evaluated on several archived samples from DE containers. These containers included FY09 DE2, FY12 DE4, FY12 DE6 and FY12 DE7 and the Hanford High Moisture Container. The additional examinations included visual observations with a stereomicroscope, scanning electron microscopy along with energy dispersive spectroscopy for chemical analysis, and serial metallography of the sidewall and lid that are part of the inner container closure weld region. Pitting was observed in all the samples taken from the closure weld regions of the examined inner containers. This pitting was generally less 20 μm with most less than 5m. These pits were similar in depth to those observed in the vapor exposed surfaces of teardrops in the shelf life corrosion testing. Cracking was not observed on either the vapor-exposed surfaces of the teardrop coupons or the inner container closure weld region. Further testing is necessary to determine if the conditions in the welded inner container could support SCC during the 50 year life time for the 3013 container.« less

Fatique crack growth behavior of a single crystal alloy as observed through an in situ fatigue loading stage

NASA Technical Reports Server (NTRS)

Telesman, Jack; Kantzos, Peter

1988-01-01

An in situ fatigue loading stage inside a scanning electron microscope (SEM) was used to determine the fatigue crack growth behavior of a PWA 1480 single-crystal nickel-based superalloy. The loading stage permits real-time viewing of the fatigue damage processes at high magnification. The PWA 1480 single-crystal, single-edge notch specimens were tested with the load axis parallel to the (100) orientation. Two distinct fatigue failure mechanisms were identified. The crack growth rate differed substantially when the failure occurred on a single slip system in comparison to multislip system failure. Two processes by which crack branching is produced were identified and are discussed. Also discussed are the observed crack closure mechanisms.

Elevated temperature crack growth

NASA Technical Reports Server (NTRS)

Kim, K. S.; Vanstone, R. H.; Malik, S. N.; Laflen, J. H.

1988-01-01

A study was performed to examine the applicability of path-independent (P-I) integrals to crack growth problems in hot section components of gas turbine aircraft engines. Alloy 718 was used and the experimental parameters included combined temperature and strain cycling, thermal gradients, elastic-plastic strain levels, and mean strains. A literature review was conducted of proposed P-I integrals, and those capable of analyzing hot section component problems were selected and programmed into the postprocessor of a finite element code. Detailed elastic-plastic finite element analyses were conducted to simulate crack growth and crack closure of the test specimen, and to evaluate the P-I integrals. It was shown that the selected P-I integrals are very effective for predicting crack growth for isothermal conditions.

Panel Stiffener Debonding Analysis using a Shell/3D Modeling Technique

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Ratcliffe, James G.; Minguet, Pierre J.

2008-01-01

A shear loaded, stringer reinforced composite panel is analyzed to evaluate the fidelity of computational fracture mechanics analyses of complex structures. Shear loading causes the panel to buckle. The resulting out -of-plane deformations initiate skin/stringer separation at the location of an embedded defect. The panel and surrounding load fixture were modeled with shell elements. A small section of the stringer foot, web and noodle as well as the panel skin near the delamination front were modeled with a local 3D solid model. Across the width of the stringer fo to, the mixed-mode strain energy release rates were calculated using the virtual crack closure technique. A failure index was calculated by correlating the results with a mixed-mode failure criterion of the graphite/epoxy material. The objective was to study the effect of the fidelity of the local 3D finite element model on the computed mixed-mode strain energy release rates and the failure index.

Panel-Stiffener Debonding and Analysis Using a Shell/3D Modeling Technique

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Ratcliffe, James G.; Minguet, Pierre J.

2007-01-01

A shear loaded, stringer reinforced composite panel is analyzed to evaluate the fidelity of computational fracture mechanics analyses of complex structures. Shear loading causes the panel to buckle. The resulting out-of-plane deformations initiate skin/stringer separation at the location of an embedded defect. The panel and surrounding load fixture were modeled with shell elements. A small section of the stringer foot, web and noodle as well as the panel skin near the delamination front were modeled with a local 3D solid model. Across the width of the stringer foot, the mixed-mode strain energy release rates were calculated using the virtual crack closure technique. A failure index was calculated by correlating the results with a mixed-mode failure criterion of the graphite/epoxy material. The objective was to study the effect of the fidelity of the local 3D finite element model on the computed mixed-mode strain energy release rates and the failure index.

Detection of Cracking Levels in Brittle Rocks by Parametric Analysis of the Acoustic Emission Signals

NASA Astrophysics Data System (ADS)

Moradian, Zabihallah; Einstein, Herbert H.; Ballivy, Gerard

2016-03-01

Determination of the cracking levels during the crack propagation is one of the key challenges in the field of fracture mechanics of rocks. Acoustic emission (AE) is a technique that has been used to detect cracks as they occur across the specimen. Parametric analysis of AE signals and correlating these parameters (e.g., hits and energy) to stress-strain plots of rocks let us detect cracking levels properly. The number of AE hits is related to the number of cracks, and the AE energy is related to magnitude of the cracking event. For a full understanding of the fracture process in brittle rocks, prismatic specimens of granite containing pre-existing flaws have been tested in uniaxial compression tests, and their cracking process was monitored with both AE and high-speed video imaging. In this paper, the characteristics of the AE parameters and the evolution of cracking sequences are analyzed for every cracking level. Based on micro- and macro-crack damage, a classification of cracking levels is introduced. This classification contains eight stages (1) crack closure, (2) linear elastic deformation, (3) micro-crack initiation (white patch initiation), (4) micro-crack growth (stable crack growth), (5) micro-crack coalescence (macro-crack initiation), (6) macro-crack growth (unstable crack growth), (7) macro-crack coalescence and (8) failure.

Effect of propellant deformation on ignition and combustion processes in solid propellant cracks

NASA Technical Reports Server (NTRS)

Kumar, M.; Kuo, K. K.

1980-01-01

A comprehensive theoretical model was formulated to study the development of convective burning in a solid propellant crack which continually deforms due to burning and pressure loading. In the theoretical model, the effect of interrelated structural deformation and combustion processes was taken into account by considering (1) transient, one dimensional mass, momentum, and energy conservation equations in the gas phase; (2) a transient, one dimensional heat conduction equation in the solid phase; and (3) quasi-static deformation of the two dimensional, linear viscoelastic propellant crack caused by pressure loading. Partial closures may generate substantial local pressure peaks along the crack, implying a strong coupling between chamber pressurization, crack combustion, and propellant deformation, especially when the cracks are narrow and the chamber pressurization rates high. The maximum pressure in the crack cavity is generally higher than that in the chamber. The initial flame-spreading process is not affected by propellant deformation.

The growth of small corrosion fatigue cracks in alloy 2024

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Willard, Scott A.

1993-01-01

The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminum alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by crack initiation at constituent particle pits, intergranular microcracking for a less than 100 micrometers, and transgranular small crack growth for a micrometer. In aqueous 1 percent NaCl and at a constant anodic potential of -700 mV(sub SCE), small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of delta-K (less than 1 MPa square root of m) below long crack delta-K (sub th). When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Results suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks (a less than or equal to 100 micrometers). This is evidenced by similar small and long crack growth behavior at various levels of R. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for Al by 2024 exposed to salt water.

Submittal of Final Post-Closure Inspection Letter Report for Corrective Action Unit 112: Area 23 Hazardous Waste Trenches, Nevada Test Site, Nevada, Revision 0, December 2007; DOE/NV--1103

DOE Office of Scientific and Technical Information (OSTI.GOV)

NSTec Environmental Restoration

This letter serves as the post-closure monitoring letter report for the above Corrective Action Unit (CAU) for the period October 2006-September 2007. Quarterly inspections were conducted on December 20,2006; March 29,2007; June 13,2007; and September 27, 2007, to observe the condition of the gate, use restriction warning signs, monuments, fencing, trenches, soil covers, and monitoring well covers. The first inspection was conducted on December 20,2006. Signs, fencing, riprap, monuments, and monitoring well covers were in excellent condition. No cracking, erosion, or subsidence was observed on the covers. No issues or concerns were identified, and no corrective actions were recommended. Themore » second inspection was conducted on March 29,2007. Signs, fencing, riprap, monuments, and monitoring well covers were in excellent condition. No cracking, erosion, or subsidence was observed on the covers. Vegetation that was observed to be growing in several locations on the cover was the only identified concern, for which removal was recommended. Vegetation was removed on May 24,2007. The third inspection was conducted on June 13,2007. Signs, fencing, riprap, monuments, and monitoring well covers were in excellent condition. No cracking, erosion, or subsidence was observed on the covers. No issues or concerns were identified, and no corrective actions were recommended. The fourth inspection was conducted on September 27,2007. Signs, fencing, riprap, monuments, and monitoring well covers were in excellent condition. No cracking, erosion, or subsidence was observed on the covers. No issues or concerns were identified, and no corrective actions were recommended. The post-closure inspection checklists for CAU 112 are attached. Photographs and field notes taken during site inspections are maintained in the project files.« less

Thermal-mechanical fatigue crack growth in Inconel X-750

NASA Technical Reports Server (NTRS)

Marchand, N.; Pelloux, R. M.

1984-01-01

Thermal-mechanical fatigue crack growth (TMFCG) was studied in a gamma-gamma' nickel base superalloy Inconel X-750 under controlled load amplitude in the temperature range from 300 to 650 C. In-phase (T sub max at sigma sub max), out-of-phase (T sub min at sigma sub max), and isothermal tests at 650 C were performed on single-edge notch bars under fully reversed cyclic conditions. A dc electrical potential method was used to measure crack length. The electrical potential response obtained for each cycle of a given wave form and R value yields information on crack closure and crack extension per cycle. The macroscopic crack growth rates are reported as a function of delta k and the relative magnitude of the TMFCG are discussed in the light of the potential drop information and of the fractographic observations.

Demonstrating the self-healing behaviour of some selected ceramics under combustion chamber conditions

NASA Astrophysics Data System (ADS)

Farle, A.; Boatemaa, L.; Shen, L.; Gövert, S.; Kok, J. B. W.; Bosch, M.; Yoshioka, S.; van der Zwaag, S.; Sloof, W. G.

2016-08-01

Closure of surface cracks by self-healing of conventional and MAX phase ceramics under realistic turbulent combustion chamber conditions is presented. Three ceramics namely; Al2O3, Ti2AlC and Cr2AlC are investigated. Healing was achieved in Al2O3 by even dispersion of TiC particles throughout the matrix as the MAX phases, Ti2AlC and Cr2AlC exhibit intrinsic self-healing. Fully dense samples (>95%) were sintered by spark plasma sintering and damage was introduced by indentation, quenching and low perpendicular velocity impact methods. The samples were exposed to the oxidizing atmosphere in the post flame zone of a turbulent flame in a combustion chamber to heal at temperatures of approx. 1000 °C at low pO2 levels for 4 h. Full crack-gap closure was observed for cracks up to 20 mm in length and more than 10 μm in width. The reaction products (healing agents) were analysed by scanning electron microscope, x-ray microanalysis and XRD. A semi-quantification of the healing showed that cracks in Al2O3/TiC composite (width 1 μm and length 100 μm) were fully filled with TiO2. In Ti2AlC large cracks were fully filled with a mixture of TiO2 and Al2O3. And in the Cr2AlC, cracks of up to 1.0 μm in width and more than 100 μm in length were also completely filled with Al2O3.

The mechanics of delamination in fiber-reinforced composite materials. I - Stress singularities and solution structure

NASA Technical Reports Server (NTRS)

Wang, S. S.; Choi, I.

1983-01-01

The fundamental mechanics of delamination in fiber composite laminates is studied. Mathematical formulation of the problem is based on laminate anisotropic elasticity theory and interlaminar fracture mechanics concepts. Stress singularities and complete solution structures associated with general composite delaminations are determined. For a fully open delamination with traction-free surfaces, oscillatory stress singularities always appear, leading to physically inadmissible field solutions. A refined model is introduced by considering a partially closed delamination with crack surfaces in finite-length contact. Stress singularities associated with a partially closed delamination having frictional crack-surface contact are determined, and are found to be different from the inverse square-root one of the frictionless-contact case. In the case of a delamination with very small area of crack closure, a simplified model having a square-root stress singularity is employed by taking the limit of the partially closed delamination. The possible presence of logarithmic-type stress singularity is examined; no logarithmic singularity of any kind is found in the composite delamination problem. Numerical examples of dominant stress singularities are shown for delaminations having crack-tip closure with different frictional coefficients between general (1) and (2) graphite-epoxy composites. Previously announced in STAR as N84-13221

Fatigue threshold studies in Fe, Fe-Si, and HSLA steel: Part II. thermally activated behavior of the effective stress intensity at threshold

NASA Astrophysics Data System (ADS)

Yu, W.; Esaklul, K.; Gerberich, W. W.

1984-05-01

It is shown that closure mechanisms alone cannot fully explain increasing fatigue thresholds with decreasing test temperature for a sequence of Fe-Si binary alloys and an HSLA steel. Implications are that fatigue crack propagation near threshold is a thermally activated process. The effective threshold stress intensity, which was obtained by subtracting the closure portion from the fatigue threshold, was examined. This effective stress intensity was found to correlate very well to the thermal component of the flow stress. A detailed fractographic study of the fatigue surface was performed. Water vapor in the room air was found to promote the formation of oxide and intergranular crack growth. At lower temperature, a brittle-type cyclic cleavage fatigue surface was observed but the ductile process persisted even at 123 K. Arrest marks were found on all three modes of fatigue crack growth. The regular spacings between these lines and dislocation modeling suggested that fatigue crack growth was controlled by the subcell structure near threshold. A model based on the slip-off of dislocations was examined. From this, it is shown that the effective fatigue threshold may be related to the square root of (one plus the strain rate sensitivity).

Intrinsic fatigue crack propagation in aluminum-lithium alloys - The effect of gaseous environments

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1989-01-01

Gaseous environmental effects on intrinsic fatigue crack growth are significant for the Al-Li-Cu alloy 2090, peak aged. For both moderate Delta K-low R and low Delta K-high R regimes, crack growth rates decrease according to the environment order: purified water vapor, moist air, helium and oxygen. Gaseous environmental effects are pronounced near threshold and are not closure dominated. Here, embrittlement by low levels of H2O (ppm) supports hydrogen embrittlement and suggests that molecular transport controlled cracking, established for high Delta K-low R, is modified near threshold. Localized crack tip reaction sites or high R crack opening shape may enable the strong, environmental effect at low levels of Delta K. Similar crack growth in He and O2 eliminates the contribution of surface films to fatigue damage in alloy 2090. While 2090 and 7075 exhibit similar environmental trends, the Al-Li-Cu alloy is more resistant to intrinsic corrosion fatigue crack growth.

Fatigue Analyses Under Constant- and Variable-Amplitude Loading Using Small-Crack Theory

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Phillips, E. P.; Everett, R. A., Jr.

1999-01-01

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily "crack growth" from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using "small-crack theory" under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta-Keff) under constant-amplitude loading. Modifications to the delta-Keff-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small-and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

Fatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modules

PubMed Central

Paggi, Marco; Berardone, Irene; Infuso, Andrea; Corrado, Mauro

2014-01-01

Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells. PMID:24675974

Nonlinear dynamics and health monitoring of 6-DOF breathing cracked Jeffcott rotor

NASA Astrophysics Data System (ADS)

Zhao, Jie; DeSmidt, Hans; Yao, Wei

2015-04-01

Jeffcott rotor is employed to study the nonlinear vibration characteristics of breathing cracked rotor system and explore the possibility of further damage identification. This paper is an extension work of prior study based on 4 degree-of-freedom Jeffcott rotor system. With consideration of disk tilting and gyroscopic effect, 6-dof EOM is derived and the crack model is established using SERR (strain energy release rate) in facture mechanics. Same as the prior work, the damaged stiffness matrix is updated by computing the instant crack closure line through Zero Stress Intensity Factor method. The breathing crack area is taken as a variable to analyze the breathing behavior in terms of eccentricity phase and shaft speed. Furthermore, the coupled vibration among lateral, torsional and longitudinal d.o.f is studied under torsional/axial excitation. The final part demonstrates the possibility of using vibration signal of damaged system for the crack diagnosis and health monitoring.

Self-healing of cracks formed in Silicon-Aluminum anodes electrochemically cycled at high lithiation rates

NASA Astrophysics Data System (ADS)

Bhattacharya, Sandeep; Alpas, Ahmet T.

2016-10-01

Lithiation-induced volume changes in Si result in fracture and fragmentation of Si anodes in Li-ion batteries. This paper reports the self-healing behaviour of cracks observed in micron-sized Si particles dispersed in a ductile Al matrix of a Si-Al electrode electrochemically cycled vs. Li/Li+ using a high lithiation rate of 15.6 C. Cross-sectional high-resolution transmission electron microscopy and Raman spectroscopy revealed that an amorphous layer with a depth up to ∼100 nm was formed at the surface of Si particles. In-situ optical microscopy performed during electrochemical experiments revealed development of cracks in Si particles as the voltage decreased to 0.02 V during lithiation. Self-healing of cracks in Si particles occurred in two steps: i) arresting of the crack growth at the Si/Al interface as the surrounding Al matrix had a higher fracture toughness and thus acted as a barrier to crack propagation, and ii) closure of cracks due to compressive stresses applied to the crack faces by the amorphous zones formed on each side of the crack paths.

Fatigue-Life Prediction Methodology Using Small-Crack Theory

NASA Technical Reports Server (NTRS)

Newmann, James C., Jr.; Phillips, Edward P.; Swain, M. H.

1997-01-01

This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using 'small-crack theory' for various materials and loading conditions. Crack-tip constraint factors, to account for three-dimensional state-of-stress effects, were selected to correlate large-crack growth rate data as a function of the effective-stress-intensity factor range (delta K(eff)) under constant-amplitude loading. Some modifications to the delta k(eff)-rate relations were needed in the near-threshold regime to fit measured small-crack growth rate behavior and fatigue endurance limits. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens made of two aluminum alloys and a steel under constant-amplitude and spectrum loading. Fatigue lives were calculated using the crack-growth relations and microstructural features like those that initiated cracks for the aluminum alloys and steel for edge-notched specimens. An equivalent-initial-flaw-size concept was used to calculate fatigue lives in other cases. Results from the tests and analyses agreed well.

Design of a biomimetic self-healing superalloy composite

NASA Astrophysics Data System (ADS)

Files, Bradley Steven

1997-10-01

Use of systems engineering concepts to design technologically advanced materials has allowed ambitious goals of self-healing alloys to be realized. Shape memory alloy reinforcements are embedded in an alloy matrix to demonstrate concepts of stable crack growth and matrix crack closure. Computer methods are used to design thermodynamically compatible iron-based alloys using bio-inspired concepts of crack bridging and self-healing. Feasibility of crack closure and stable crack growth is shown in a prototype system with a Sn-Bi matrix and TiNi fibers. Design of Fe-Ni-Co-Ti-Al alloys using thermodynamic models to determine stabilities and phase equilibria allows for a methodical system designing compatible multicomponent alloys for composite systems. Final alloy computations for this project led to the alloy Fe-27.6Ni-18.2Co-4.1Ti-1.6Al as a compatible shape memory a with a 650sp°C 90 minute heat treatment leading to martensite and austenite start temperatures (Msbs and Asbs) near room temperature. Thin slices of this alloy were able to fully recover at least 5% strain upon unloading heating. Composites made from the designed shape memory alloy and a compatible Fe-based B2 matrix were used to test self-healing concepts in the superalloy system. Diffusion couple experiments verified thermodynamic compatibility between matrix and reinforcement alloys at the solution treatment temperature of 1100sp°C. Concepts of stable crack growth and crack bridging were demonstrated in the composite, leading to enhanced toughness of the brittle matrix. However, healing behavior in this system was limited by intergranular fracture of the reinforcement alloy. It is believed that use of rapidly solidified powders could eliminate intergranular fracture, leading to greatly enhanced properties of toughening and healing. Crack clamping and stable crack growth were achieved in a feasibility study using a Sn-Bi matrix reinforced with TiNi fibers. Tensile specimens with less than 1% fibers showed an ability upon heating to recover over 80% of the plastic deformation induced during a tensile test. Further straining proved that stable crack growth can be realized in this system due to crack bridging of the shape memory fibers. Macroscopic cracks were clamped shut after heating of the material above the TiNi reversion temperature.

Virtual hybrid test control of sinuous crack

NASA Astrophysics Data System (ADS)

Jailin, Clément; Carpiuc, Andreea; Kazymyrenko, Kyrylo; Poncelet, Martin; Leclerc, Hugo; Hild, François; Roux, Stéphane

2017-05-01

The present study aims at proposing a new generation of experimental protocol for analysing crack propagation in quasi brittle materials. The boundary conditions are controlled in real-time to conform to a predefined crack path. Servo-control is achieved through a full-field measurement technique to determine the pre-set fracture path and a simple predictor model based on linear elastic fracture mechanics to prescribe the boundary conditions on the fly so that the actual crack path follows at best the predefined trajectory. The final goal is to identify, for instance, non-local damage models involving internal lengths. The validation of this novel procedure is performed via a virtual test-case based on an enriched damage model with an internal length scale, a prior chosen sinusoidal crack path and a concrete sample. Notwithstanding the fact that the predictor model selected for monitoring the test is a highly simplified picture of the targeted constitutive law, the proposed protocol exhibits a much improved sensitivity to the sought parameters such as internal lengths as assessed from the comparison with other available experimental tests.

Discrete crack growth analysis methodology for through cracks in pressurized fuselage structures

NASA Technical Reports Server (NTRS)

Potyondy, David O.; Wawrzynek, Paul A.; Ingraffea, Anthony R.

1994-01-01

A methodology for simulating the growth of long through cracks in the skin of pressurized aircraft fuselage structures is described. Crack trajectories are allowed to be arbitrary and are computed as part of the simulation. The interaction between the mechanical loads acting on the superstructure and the local structural response near the crack tips is accounted for by employing a hierarchical modeling strategy. The structural response for each cracked configuration is obtained using a geometrically nonlinear shell finite element analysis procedure. Four stress intensity factors, two for membrane behavior and two for bending using Kirchhoff plate theory, are computed using an extension of the modified crack closure integral method. Crack trajectories are determined by applying the maximum tangential stress criterion. Crack growth results in localized mesh deletion, and the deletion regions are remeshed automatically using a newly developed all-quadrilateral meshing algorithm. The effectiveness of the methodology and its applicability to performing practical analyses of realistic structures is demonstrated by simulating curvilinear crack growth in a fuselage panel that is representative of a typical narrow-body aircraft. The predicted crack trajectory and fatigue life compare well with measurements of these same quantities from a full-scale pressurized panel test.

Virtual reality 3D echocardiography in the assessment of tricuspid valve function after surgical closure of ventricular septal defect.

PubMed

Bol Raap, Goris; Koning, Anton H J; Scohy, Thierry V; ten Harkel, A Derk-Jan; Meijboom, Folkert J; Kappetein, A Pieter; van der Spek, Peter J; Bogers, Ad J J C

2007-02-16

This study was done to investigate the potential additional role of virtual reality, using three-dimensional (3D) echocardiographic holograms, in the postoperative assessment of tricuspid valve function after surgical closure of ventricular septal defect (VSD). 12 data sets from intraoperative epicardial echocardiographic studies in 5 operations (patient age at operation 3 weeks to 4 years and bodyweight at operation 3.8 to 17.2 kg) after surgical closure of VSD were included in the study. The data sets were analysed as two-dimensional (2D) images on the screen of the ultrasound system as well as holograms in an I-space virtual reality (VR) system. The 2D images were assessed for tricuspid valve function. In the I-Space, a 6 degrees-of-freedom controller was used to create the necessary projectory positions and cutting planes in the hologram. The holograms were used for additional assessment of tricuspid valve leaflet mobility. All data sets could be used for 2D as well as holographic analysis. In all data sets the area of interest could be identified. The 2D analysis showed no tricuspid valve stenosis or regurgitation. Leaflet mobility was considered normal. In the virtual reality of the I-Space, all data sets allowed to assess the tricuspid leaflet level in a single holographic representation. In 3 holograms the septal leaflet showed restricted mobility that was not appreciated in the 2D echocardiogram. In 4 data sets the posterior leaflet and the tricuspid papillary apparatus were not completely included. This report shows that dynamic holographic imaging of intraoperative postoperative echocardiographic data regarding tricuspid valve function after VSD closure is feasible. Holographic analysis allows for additional tricuspid valve leaflet mobility analysis. The large size of the probe, in relation to small size of the patient, may preclude a complete data set. At the moment the requirement of an I-Space VR system limits the applicability in virtual reality 3D echocardiography in clinical practice.

78 FR 48851 - Wireline Competition Bureau Announces Closing of the Bureau's Cost Model Virtual Workshop

Federal Register 2010, 2011, 2012, 2013, 2014

2013-08-12

... questions through Public Notice. DATES: Virtual workshop closure effective August 12, 2013. ADDRESSES: You.... The filing hours are 8:00 a.m. to 7:00 p.m. All hand deliveries must be held together with rubber...

Transient radon signals driven by fluid pressure pulse, micro-crack closure, and failure during granite deformation experiments

NASA Astrophysics Data System (ADS)

Girault, Frédéric; Schubnel, Alexandre; Pili, Éric

2017-09-01

In seismically active fault zones, various crustal fluids including gases are released at the surface. Radon-222, a radioactive gas naturally produced in rocks, is used in volcanic and tectonic contexts to illuminate crustal deformation or earthquake mechanisms. At some locations, intriguing radon signals have been recorded before, during, or after tectonic events, but such observations remain controversial, mainly because physical characterization of potential radon anomalies from the upper crust is lacking. Here we conducted several month-long deformation experiments under controlled dry upper crustal conditions with a triaxial cell to continuously monitor radon emission from crustal rocks affected by three main effects: a fluid pressure pulse, micro-crack closure, and differential stress increase to macroscopic failure. We found that these effects are systematically associated with a variety of radon signals that can be explained using a first-order advective model of radon transport. First, connection to a source of deep fluid pressure (a fluid pressure pulse) is associated with a large transient radon emission increase (factor of 3-7) compared with the background level. We reason that peak amplitude is governed by the accumulation time and the radon source term, and that peak duration is controlled by radioactive decay, permeability, and advective losses of radon. Second, increasing isostatic compression is first accompanied by an increase in radon emission followed by a decrease beyond a critical pressure representing the depth below which crack closure hampers radon emission (150-250 MPa, ca. 5.5-9.5 km depth in our experiments). Third, the increase of differential stress, and associated shear and volumetric deformation, systematically triggers significant radon peaks (ca. 25-350% above background level) before macroscopic failure, by connecting isolated cracks, which dramatically enhances permeability. The detection of transient radon signals before rupture indicates that connection of initially isolated cracks in crustal rocks may occur before rupture and potentially lead to radon transients measurable at the surface in tectonically active regions. This study offers thus an experimental and physical basis for understanding predicted or reported radon anomalies.

Transient radon signals driven by fluid pressure pulse, micro-crack closure, and failure during granite deformation experiments

NASA Astrophysics Data System (ADS)

Schubnel, A.; Girault, F.; Pili, E.

2017-12-01

In seismically active fault zones, various crustal fluids including gases are released at the surface. Radon-222, a radioactive gas naturally produced in rocks, is used in volcanic and tectonic contexts to illuminate crustal deformation or earthquake mechanisms. At some locations, intriguing radon signals have been recorded before, during, or after tectonic events, but such observations remain controversial, mainly because physical characterization of potential radon anomalies from the upper crust is lacking. Here we conducted several month-long deformation experiments under controlled dry upper crustal conditions with a triaxial cell to continuously monitor radon emission from crustal rocks affected by three main effects: a fluid pressure pulse, micro-crack closure, and differential stress increase to macroscopic failure. We found that these effects are systematically associated with a variety of radon signals that can be explained using a first-order advective model of radon transport. First, connection to a source of deep fluid pressure (a fluid pressure pulse) is associated with a large transient radon emission increase (factor of 3-7) compared with the background level. We reason that peak amplitude is governed by the accumulation time and the radon source term, and that peak duration is controlled by radioactive decay, permeability, and advective losses of radon. Second, increasing isostatic compression is first accompanied by an increase in radon emission followed by a decrease beyond a critical pressure representing the depth below which crack closure hampers radon emission (150-250 MPa, ca. 5.5-9.5 km depth in our experiments). Third, the increase of differential stress, and associated shear and volumetric deformation, systematically triggers significant radon peaks (ca. 25-350% above background level) before macroscopic failure, by connecting isolated cracks, which dramatically enhances permeability. The detection of transient radon signals before rupture indicates that connection of initially isolated cracks in crustal rocks may occur before rupture and potentially lead to radon transients measurable at the surface in tectonically active regions. This study offers thus an experimental and physical basis for understanding predicted or reported radon anomalies.

Crack deflection: Implications for the growth of long and short fatigue cracks

NASA Astrophysics Data System (ADS)

Suresh, S.

1983-11-01

The influences of crack deflection on the growth rates of nominally Mode I fatigue cracks are examined. Previous theoretical analyses of stress intensity solutions for kinked elastic cracks are reviewed. Simple elastic deflection models are developed to estimate the growth rates of nonlinear fatigue cracks subjected to various degrees of deflection, by incorporating changes in the effective driving force and in the apparent propagation rates. Experimental data are presented for intermediate-quenched and step-quenched conditions of Fe/2Si/0.1C ferrite-martensite dual phase steel, where variations in crack morphology alone influence considerably the fatigue crack propagation rates and threshold stress intensity range values. Such results are found to be in good quantitative agreement with the deflection model predictions of propagation rates for nonlinear cracks. Experimental information on crack deflection, induced by variable amplitude loading, is also provided for 2020-T651 aluminum alloy. It is demonstrated with the aid of elastic analyses and experiments that crack deflection models offer a physically-appealing rationale for the apparently slower growth rates of long fatigue cracks subjected to constant and variable amplitude loading and for the apparent deceleration and/or arrest of short cracks. The changes in the propagation rates of deflected fatigue cracks are discussed in terms of the local mode of crack advance, microstructure, effective driving force, growth mechanisms, mean stress, slip characteristics, and crack closure.

Concurrent Structural Fatigue Damage Prognosis Under Uncertainty

DTIC Science & Technology

2014-04-30

stage is manufactured by Ernest F. Fullam Inc., which is now merged to MTI Instruments Inc.. The maximum gage length between mechanical grips is...closure measurement techniques, Vol. 31, Issue 4, 1988, pp. 703–712 23. M.N. James, M.N. Pacey, L.W. Wei,E.A. Patterson , Characterisation of...34. International Journal of Fatigue, 1999, pp. S35–S46. 39. Newman JC., Jr ."A crack opening stress equation for fatigue crack growth" International

Micromechanical Analysis of Crack Closure Mechanism for Intelligent Material Containing TiNi Fibers

NASA Astrophysics Data System (ADS)

Araki, Shigetoshi; Ono, Hiroyuki; Saito, Kenji

In our previous study, the micromechanical modeling of an intelligent material containing TiNi fibers was performed and the stress intensity factor KI at the tip of the crack in the material was expressed in terms of the magnitude of the shape memory shrinkage of the fibers and the thermal expansion strain in the material. In this study, the value of KI at the tip of the crack in the TiNi/epoxy material is calculated numerically by using analytical expressions obtained in our first report. As a result, we find that the KI value decreases with increasing shrink strain of the fibers, and this tendency agrees with that of the experimental result obtained by Shimamoto etal.(Trans. Jpn. Soc. Mech. Eng., Vol. 65, No. 634 (1999), pp. 1282-1286). Moreover, there exists an optimal value of the shrink strain of the fibers to make the KI value zero. The change in KI with temperature during the heating process from the reference temperature to the inverse austenitic finishing temperature of TiNi fiber is also consistent with the experimental result. These results can be explained by the changes in the shrink strain, the thermal expansion strain, and the elastic moduli of TiNi fiber with temperature. These results may be useful in designing intelligent materials containing TiNi fibers from the viewpoint of crack closure.

Simulation of fatigue crack growth under large scale yielding conditions

NASA Astrophysics Data System (ADS)

Schweizer, Christoph; Seifert, Thomas; Riedel, Hermann

2010-07-01

A simple mechanism based model for fatigue crack growth assumes a linear correlation between the cyclic crack-tip opening displacement (ΔCTOD) and the crack growth increment (da/dN). The objective of this work is to compare analytical estimates of ΔCTOD with results of numerical calculations under large scale yielding conditions and to verify the physical basis of the model by comparing the predicted and the measured evolution of the crack length in a 10%-chromium-steel. The material is described by a rate independent cyclic plasticity model with power-law hardening and Masing behavior. During the tension-going part of the cycle, nodes at the crack-tip are released such that the crack growth increment corresponds approximately to the crack-tip opening. The finite element analysis performed in ABAQUS is continued for so many cycles until a stabilized value of ΔCTOD is reached. The analytical model contains an interpolation formula for the J-integral, which is generalized to account for cyclic loading and crack closure. Both simulated and estimated ΔCTOD are reasonably consistent. The predicted crack length evolution is found to be in good agreement with the behavior of microcracks observed in a 10%-chromium steel.

Analyses of Fatigue and Fatigue-Crack Growth under Constant- and Variable-Amplitude Loading

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1999-01-01

Studies on the growth of small cracks have led to the observation that fatigue life of many engineering materials is primarily crack growth from micro-structural features, such as inclusion particles, voids, slip-bands or from manufacturing defects. This paper reviews the capabilities of a plasticity-induced crack-closure model to predict fatigue lives of metallic materials using small-crack theory under various loading conditions. Constraint factors, to account for three-dimensional effects, were selected to correlate large-crack growth rate data as a function of the effective stress-intensity factor range (delta K(sub eff)) under constant-amplitude loading. Modifications to the delta K(sub eff)-rate relations in the near-threshold regime were needed to fit measured small-crack growth rate behavior. The model was then used to calculate small- and large-crack growth rates, and to predict total fatigue lives, for notched and un-notched specimens under constant-amplitude and spectrum loading. Fatigue lives were predicted using crack-growth relations and micro-structural features like those that initiated cracks in the fatigue specimens for most of the materials analyzed. Results from the tests and analyses agreed well.

Fatigue Resistance of Liquid-assisted Self-repairing Aluminum Alloys Reinforced with Shape Memory Alloys

NASA Technical Reports Server (NTRS)

Wright, M. Clara; Manuel, Michele; Wallace, Terryl

2013-01-01

A self-repairing aluminum-based composite system has been developed using a liquid-assisted healing theory in conjunction with the shape memory effect of wire reinforcements. The metal-metal composite was thermodynamically designed to have a matrix with a relatively even dispersion of a low-melting eutectic phase, allowing for repair of cracks at a predetermined temperature. Additionally, shape memory alloy (SMA) wire reinforcements were used within the composite to provide crack closure. Investigators focused the research on fatigue cracks propagating through the matrix in order to show a proof-of-concept Shape Memory Alloy Self-Healing (SMASH) technology for aeronautical applications.

North Korea's 2017 Test and its Nontectonic Aftershock

NASA Astrophysics Data System (ADS)

Liu, J.; Li, L.; Zahradník, J.; Sokos, E.; Liu, C.; Tian, X.

2018-04-01

Seismology illuminates physical processes occurring during underground explosions, not all yet fully understood. The thus-far strongest North Korean test of 3 September 2017 was followed by a moderate seismic event (mL 4.1) after 8.5 min. Here we provide evidence that this aftershock was a nontectonic event which radiated seismic waves as a buried horizontal closing crack. This vigorous crack closure, occurring shortly after the blast, is studied in the North Korea test site for the first time. The event can be qualitatively explained as rapid destruction of an explosion-generated cracked rock chimney due to cavity collapse, although other compaction processes cannot be ruled out.

A Review of Fatigue Crack Growth in Metallics at Low Stress Intensities.

DTIC Science & Technology

1983-01-01

received R 0,35 * 257 C 2i 0*C b -sT 625C 0 0 10 0 ~1O90 co 0 C 2 2,5 3 4 5 67 8 910C12 15 20 A K MN rfiV2 Figure 31 The influence of test temperature...34 L.2. 11111 L25I MICROCOPY RESOLUTION TEST CHART NATIONAL. BUREAU OF STANDARDS 1963-A A 44-7 F 49620-82-C-0088 A REVIEW OF FATIGUE CRACK GROWTH IN...relation to the following variables, mean stress, yield stress, test temperature, test frequency, fatigue crack closure, microstructure, environment and

In Search of a Time Efficient Approach to Crack and Delamination Growth Predictions in Composites

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Carvalho, Nelson

2016-01-01

Analysis benchmarking was used to assess the accuracy and time efficiency of algorithms suitable for automated delamination growth analysis. First, the Floating Node Method (FNM) was introduced and its combination with a simple exponential growth law (Paris Law) and Virtual Crack Closure technique (VCCT) was discussed. Implementation of the method into a user element (UEL) in Abaqus/Standard(Registered TradeMark) was also presented. For the assessment of growth prediction capabilities, an existing benchmark case based on the Double Cantilever Beam (DCB) specimen was briefly summarized. Additionally, the development of new benchmark cases based on the Mixed-Mode Bending (MMB) specimen to assess the growth prediction capabilities under mixed-mode I/II conditions was discussed in detail. A comparison was presented, in which the benchmark cases were used to assess the existing low-cycle fatigue analysis tool in Abaqus/Standard(Registered TradeMark) in comparison to the FNM-VCCT fatigue growth analysis implementation. The low-cycle fatigue analysis tool in Abaqus/Standard(Registered TradeMark) was able to yield results that were in good agreement with the DCB benchmark example. Results for the MMB benchmark cases, however, only captured the trend correctly. The user element (FNM-VCCT) always yielded results that were in excellent agreement with all benchmark cases, at a fraction of the analysis time. The ability to assess the implementation of two methods in one finite element code illustrated the value of establishing benchmark solutions.

Adhesive-Bonded Composite Joint Analysis with Delaminated Surface Ply Using Strain-Energy Release Rate

NASA Technical Reports Server (NTRS)

Chadegani, Alireza; Yang, Chihdar; Smeltzer, Stanley S. III

2012-01-01

This paper presents an analytical model to determine the strain energy release rate due to an interlaminar crack of the surface ply in adhesively bonded composite joints subjected to axial tension. Single-lap shear-joint standard test specimen geometry with thick bondline is followed for model development. The field equations are formulated by using the first-order shear-deformation theory in laminated plates together with kinematics relations and force equilibrium conditions. The stress distributions for the adherends and adhesive are determined after the appropriate boundary and loading conditions are applied and the equations for the field displacements are solved. The system of second-order differential equations is solved to using the symbolic computation tool Maple 9.52 to provide displacements fields. The equivalent forces at the tip of the prescribed interlaminar crack are obtained based on interlaminar stress distributions. The strain energy release rate of the crack is then determined by using the crack closure method. Finite element analyses using the J integral as well as the crack closure method are performed to verify the developed analytical model. It has been shown that the results using the analytical method correlate well with the results from the finite element analyses. An attempt is made to predict the failure loads of the joints based on limited test data from the literature. The effectiveness of the inclusion of bondline thickness is justified when compared with the results obtained from the previous model in which a thin bondline and uniform adhesive stresses through the bondline thickness are assumed.

Mechanisms of fatigue crack retardation following single tensile overloads in powder metallurgy aluminum alloys

NASA Technical Reports Server (NTRS)

Bray, G. H.; Reynolds, A. P.; Starke, E. A., Jr.

1992-01-01

In ingot metallurgy (IM) alloys, the number of delay cycles following a single tensile overload typically increases from a minimum at an intermediate baseline stress intensity range, Delta-K(B), with decreasing Delta-K(B) approaching threshold and increasing Delta-K(B) approaching unstable fracture to produce a characteristic 'U' shaped curve. Two models have been proposed to explain this behavior. One model is based on the interaction between roughness and plasticity-induced closure, while the other model only utilizes plasticity-induced closure. This article examines these models, using experimental results from constant amplitude and single overload fatigue tests performed on two powder metallurgy (PM) aluminum alloys, AL-905XL and AA 8009. The results indicate that the 'U'-shaped curve is primarily due to plasticity-induced closure, and that the plasticity-induced retardation effect is through-thickness in nature, occurring in both the surface and interior regions. However, the retardation effect is greater at the surface, because the increase in plastic strain at the crack tip and overload plastic zone size are larger in the plane-stress surface regions than in the plane-strain interior regions. These results are not entirely consistent with either of the proposed models.

Biochemical process of ureolysis-based microbial CaCO3 precipitation and its application in self-healing concrete.

PubMed

Xu, Jing; Wang, Xianzhi; Wang, Binbin

2018-04-01

Urea hydrolysis has already been considered as the most effective pathway for microbially induced CaCO 3 precipitation (MICP). The present work first studied the combination of several key factors including initial pH, temperature, and dosage of urea, which contribute to the biochemical process of MICP. Under an amiable condition of pH and temperature, the dosage of urea has a significant impact on the rate of urea degradation and CaCO 3 precipitation. A bacteria-based self-healing system was developed by loading healing agents on ceramsite carriers. The self-healing efficiency was evaluated by visual inspection on crack closure, compressive strength regain, and capillary water absorption. A preferable healing effectiveness was obtained when the bacteria and organic nutrients were co-immobilized in carriers. Image analysis showed that cracks up to 273 μm could be healed with a crack closure ratio of 86% in 28 days. The compressive strength regain increased 24% and the water absorption coefficient decreased 27% compared to the reference. The findings indicated a promising application of ureolysis-based MICP in restoring the mechanical properties and enhancing the durability of concrete.

Thermal fatigue behaviour for a 316 L type steel

NASA Astrophysics Data System (ADS)

Fissolo, A.; Marini, B.; Nais, G.; Wident, P.

1996-10-01

This paper deals with initiation and growth of cracks produced by thermal fatigue loadings on 316 L steel, which is a reference material for the first wall of the next fusion reactor ITER. Two types of facilities have been built. As for true components, thermal cycles have been repeatedly applied on the surface of the specimen. The first is mainly concerned with initiation, which is detected with a light microscope. The second allows one to determine the propagation of a single crack. Crack initiation is analyzed using the French RCC-MR code procedure, and the strain-controlled isothermal fatigue curves. To predict crack growth, a model previously proposed by Haigh and Skelton is applied. This is based on determination of effective stress intensity factors, which takes into account both plastic strain and crack closure phenomena. It is shown that estimations obtained with such methodologies are in good agreement with experimental data.

An evaluation of the pressure proof test concept for 2024-T3 aluminium alloy sheet

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Poe, C. C., Jr.; Newman, J. C.; Harris, C. E.

1991-01-01

The concept of pressure proof testing of fuselage structures with fatigue cracks to insure structural integrity was evaluated from a fracture mechanics viewpoint. A generic analytical and experimental investigation was conducted on uniaxially loaded flat panels with crack configurations and stress levels typical of longitudinal lap splice joints in commercial transport aircraft fuselages. The results revealed that the remaining fatigue life after a proof cycle was longer than that without the proof cycle because of crack growth retardation due to increased crack closure. However, based on a crack length that is slightly less than the critical value at the maximum proof stress, the minimum assured life or proof test interval must be no more than 550 pressure cycles for a 1.33 proof factor and 1530 pressure cycles for a 1.5 proof factor to prevent in-flight failures.

An evaluation of the pressure proof test concept for thin sheet 2024-T3

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Poe, C. C., Jr.; Newman, J. C., Jr.; Harris, C. E.

1990-01-01

The concept of pressure proof testing of fuselage structures with fatigue cracks to insure structural integrity was evaluated from a fracture mechanics viewpoint. A generic analytical and experimental investigation was conducted on uniaxially loaded flat panels with crack configurations and stress levels typical of longitudinal lap-splice joints in commercial transport aircraft fuselage. The results revealed that the remaining fatigue life after a proof test was longer than that without the proof test because of crack growth retardation due to increased crack closure. However, based on a crack length that is slightly less than the critical value at the maximum proof test stress, the minimum assured life or proof test interval must be no more than 550 pressure cycles for a 1.33 proof factor and 1530 pressure cycles for a 1.5 proof factor to prevent in-flight failures.

An evaluation of the pressure proof test concept for thin sheet 2024-T3

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Poe, C. C., Jr.; Newman, James C., Jr.; Harris, Charles E.

1990-01-01

The concept of pressure proof testing of fuselage structures with fatigue cracks to insure structural integrity was evaluated from a fracture mechanics viewpoint. A generic analytical and experimental investigation was conducted on uniaxially loaded flat panels with crack configurations and stress levels typical of longitudinal lap splice joints in commercial transport aircraft fuselages. The results revealed that the remaining fatigue life after a proof test was longer than that without the proof test because of crack growth retardation due to increased crack closure. However, based on a crack length that is slightly less than the critical value at the maximum proof test stress, the minimum assured life or proof test interval must be no more than 550 pressure cycles for a 1.33 proof factor and 1530 pressure cycles for a 1.5 proof factor to prevent in-flight failures.

Predicting Fecal Indicator Bacteria Concentrations in the South Fork Broad River Watershed Using Virtual Beach

EPA Science Inventory

Virtual Beach (VB) is a decision support tool that constructs site-specific statistical models to predict fecal indicator bacteria (FIB) at recreational beaches. Although primarily designed for making decisions regarding beach closures or issuance of swimming advisories based on...

Experimental Investigation and Finite Element Analysis on Fatigue Behavior of Aluminum Alloy 7050 Single-Lap Joints

NASA Astrophysics Data System (ADS)

Zhou, Bing; Cui, Hao; Liu, Haibo; Li, Yang; Liu, Gaofeng; Li, Shujun; Zhang, Shangzhou

2018-03-01

The fatigue behavior of single-lap four-riveted aluminum alloy 7050 joints was investigated by using high-frequency fatigue test and scanning electron microscope (SEM). Stress distributions obtained by finite element (FE) analysis help explain the fatigue performance. The fatigue test results showed that the fatigue lives of the joints depend on cold expansion and applied cyclic loads. FE analysis and fractography indicated that the improved fatigue lives can be attributed to the reduction in maximum stress and evolution of fatigue damage at the critical location. The beneficial effects of strengthening techniques result in tearing ridges or lamellar structure on fracture surface, decrease in fatigue striations spacing, delay of fatigue crack initiation, crack deflection in fatigue crack propagation and plasticity-induced crack closure.

Fracture mechanics by three-dimensional crack-tip synchrotron X-ray microscopy

PubMed Central

Withers, P. J.

2015-01-01

To better understand the relationship between the nucleation and growth of defects and the local stresses and phase changes that cause them, we need both imaging and stress mapping. Here, we explore how this can be achieved by bringing together synchrotron X-ray diffraction and tomographic imaging. Conventionally, these are undertaken on separate synchrotron beamlines; however, instruments capable of both imaging and diffraction are beginning to emerge, such as ID15 at the European Synchrotron Radiation Facility and JEEP at the Diamond Light Source. This review explores the concept of three-dimensional crack-tip X-ray microscopy, bringing them together to probe the crack-tip behaviour under realistic environmental and loading conditions and to extract quantitative fracture mechanics information about the local crack-tip environment. X-ray diffraction provides information about the crack-tip stress field, phase transformations, plastic zone and crack-face tractions and forces. Time-lapse CT, besides providing information about the three-dimensional nature of the crack and its local growth rate, can also provide information as to the activation of extrinsic toughening mechanisms such as crack deflection, crack-tip zone shielding, crack bridging and crack closure. It is shown how crack-tip microscopy allows a quantitative measure of the crack-tip driving force via the stress intensity factor or the crack-tip opening displacement. Finally, further opportunities for synchrotron X-ray microscopy are explored. PMID:25624521

Electromagnetic modulation of the ultrasonic signal for nondestructive detection of small defects and ferromagnetic inclusions in thin wall structures

NASA Astrophysics Data System (ADS)

Finkel, Peter

2008-03-01

We report on new nondestructive evaluation technique based on electromagnetic modulation of ultrasonic signal for detection of the small crack, flaws and inclusions in thin-walled parts. The electromagnetically induced high density current pulse produces stresses which alter the ultrasonic waves scanning the part with the defect and modulate ultrasonic signal. The excited electromagnetic field can produces crack-opening due to Lorentz forces that increase the ultrasonic reflection. The Joule heating associated with the high density current, and consequent thermal stresses may cause both crack-closure, as well as crack-opening, depending on various factors. Experimental data is presented here for the case of a small crack near holes in thin-walled structures. The measurements were taken at 2-10 MHz with a Lamb wave wedge transducer. It is shown that electromagnetic transient modulation of the ultrasonic echo pulse tone-burst suggest that this method could be used to enhance detection of small cracks and ferromagnetic inclusions in thin walled metallic structures.

Development and Application of Benchmark Examples for Mixed-Mode I/II Quasi-Static Delamination Propagation Predictions

NASA Technical Reports Server (NTRS)

Krueger, Ronald

2012-01-01

The development of benchmark examples for quasi-static delamination propagation prediction is presented. The example is based on a finite element model of the Mixed-Mode Bending (MMB) specimen for 50% mode II. The benchmarking is demonstrated for Abaqus/Standard, however, the example is independent of the analysis software used and allows the assessment of the automated delamination propagation prediction capability in commercial finite element codes based on the virtual crack closure technique (VCCT). First, a quasi-static benchmark example was created for the specimen. Second, starting from an initially straight front, the delamination was allowed to propagate under quasi-static loading. Third, the load-displacement as well as delamination length versus applied load/displacement relationships from a propagation analysis and the benchmark results were compared, and good agreement could be achieved by selecting the appropriate input parameters. The benchmarking procedure proved valuable by highlighting the issues associated with choosing the input parameters of the particular implementation. Overall, the results are encouraging, but further assessment for mixed-mode delamination fatigue onset and growth is required.

Delamination Modeling of Composites for Improved Crash Analysis

NASA Technical Reports Server (NTRS)

Fleming, David C.

1999-01-01

Finite element crash modeling of composite structures is limited by the inability of current commercial crash codes to accurately model delamination growth. Efforts are made to implement and assess delamination modeling techniques using a current finite element crash code, MSC/DYTRAN. Three methods are evaluated, including a straightforward method based on monitoring forces in elements or constraints representing an interface; a cohesive fracture model proposed in the literature; and the virtual crack closure technique commonly used in fracture mechanics. Results are compared with dynamic double cantilever beam test data from the literature. Examples show that it is possible to accurately model delamination propagation in this case. However, the computational demands required for accurate solution are great and reliable property data may not be available to support general crash modeling efforts. Additional examples are modeled including an impact-loaded beam, damage initiation in laminated crushing specimens, and a scaled aircraft subfloor structures in which composite sandwich structures are used as energy-absorbing elements. These examples illustrate some of the difficulties in modeling delamination as part of a finite element crash analysis.

A Numerical Study on the Edgewise Compression Strength of Sandwich Structures with Facesheet-Core Disbonds

NASA Technical Reports Server (NTRS)

Bergan, Andrew C.

2017-01-01

Damage tolerant design approaches require determination of critical damage modes and flaw sizes in order to establish nondestructive evaluation detection requirements. A finite element model is developed to assess the effect of circular facesheet-core disbonds on the strength of sandwich specimens subjected to edgewise compressive loads for the purpose of predicting the critical flaw size for a variety of design parameters. Postbuckling analyses are conducted in which an initial imperfection is seeded using results from a linear buckling analysis. Both the virtual crack closure technique (VCCT) and cohesive elements are considered for modeling disbond growth. Predictions from analyses using the VCCT and analyses using cohesive elements are in good correlation. A series of parametric analyses are conducted to investigate the effect of core thickness and material, facesheet layup, facesheet-core interface properties, and curvature on the criticality of facesheet-core disbonds of various sizes. The results from these analyses provide a basis for determining the critical flaw size for facesheet-core disbonds subjected to edgewise compression loads and, therefore, nondestructive evaluation flaw detection requirements for this configuration.

Predicting E. Coli and Enterococci Concentrations in the South Fork Broad River Watershed Using Virtual Beach

EPA Science Inventory

Virtual Beach (VB) is a decision support tool that constructs site-specific statistical models to predict fecal indicator bacteria (FIB) at locations of exposure. Although primarily designed for making decisions regarding beach closures or issuance of swimming advisories based on...

Advances in Structural Integrity Analysis Methods for Aging Metallic Airframe Structures with Local Damage

NASA Technical Reports Server (NTRS)

Starnes, James H., Jr.; Newman, James C., Jr.; Harris, Charles E.; Piascik, Robert S.; Young, Richard D.; Rose, Cheryl A.

2003-01-01

Analysis methodologies for predicting fatigue-crack growth from rivet holes in panels subjected to cyclic loads and for predicting the residual strength of aluminum fuselage structures with cracks and subjected to combined internal pressure and mechanical loads are described. The fatigue-crack growth analysis methodology is based on small-crack theory and a plasticity induced crack-closure model, and the effect of a corrosive environment on crack-growth rate is included. The residual strength analysis methodology is based on the critical crack-tip-opening-angle fracture criterion that characterizes the fracture behavior of a material of interest, and a geometric and material nonlinear finite element shell analysis code that performs the structural analysis of the fuselage structure of interest. The methodologies have been verified experimentally for structures ranging from laboratory coupons to full-scale structural components. Analytical and experimental results based on these methodologies are described and compared for laboratory coupons and flat panels, small-scale pressurized shells, and full-scale curved stiffened panels. The residual strength analysis methodology is sufficiently general to include the effects of multiple-site damage on structural behavior.

Effects of load and thermal histories on mechanical behavior of materials; Proceedings of the Symposium, Denver, CO, Feb. 25, 26, 1987

DOE Office of Scientific and Technical Information (OSTI.GOV)

Liaw, P.K.; Nicholas, T.

This volume includes topics on fatigue crack propagation; isothermal and thermal-mechanical fatigue; and microstructure, fracture, and damage. Papers are presented on transients in fatigue crack growth, elevated-temperature fatigue crack propagation, the role of crack closure in crack retardation in P/M and I/M aluminum alloys, the acoustic interrogation of fatigue overload effects, and the effects of frequency and environment on crack growth in Inconel 718. Special attention is given to isothermal fatigue failure mechanisms in low-tin lead-based solder, the stress and strain controlled low-cycle fatigue of Pb-Sn solder for electronic packaging applications, load sequence effects on the deformation of isolated microplasticmore » grains, and thermal fatigue of stainless steel. Other papers are on the influence of thermal aging on the creep crack growth behavior of a Cr-Mo steel, the effect of cyclic loading on the fracture toughness of a modified 4340 steel, and the effects of hot rolling condition and boron microalloying on phase transformation and microstructure in niobium-bearing interstitial free steel.« less

Elastic-Plastic Finite Element Analysis of Fatigue Crack Growth in Mode 1 and Mode 2 Conditions

NASA Technical Reports Server (NTRS)

Nakagaki, M.; Atluri, S. N.

1978-01-01

Presented is an alternate cost-efficient and accurate elastic-plastic finite element procedure to analyze fatigue crack closure and its effects under general spectrum loading. Both Modes 1 and 2 type cycling loadings are considered. Also presented are the results of an investigation, using the newly developed procedure, of various factors that cause crack growth acceleration or retardation and delay effects under high-to-low, low-to-high, single overload, and constant amplitude type cyclic loading in a Mode 1 situation. Further, the results of an investigation of a centercracked panel under external pure shear (Mode 2) cyclic loading, of constant amplitude, are reported.

Self-Repairing Fatigue Damage in Metallic Structures for Aerospace Vehicles Using Shape Memory Alloy Self-healing (SMASH) Technology

NASA Technical Reports Server (NTRS)

Wright, M. Clara; Manuel, Michele; Wallace, Terryl; Newman, Andy; Brinson, Kate

2015-01-01

This DAA is for the Phase II webinar presentation of the ARMD-funded SMASH technology. A self-repairing aluminum-based composite system has been developed using liquid-assisted healing theory in conjunction with the shape memory effect of wire reinforcements. The metal matrix composite was thermodynamically designed to have a matrix with a relatively even dispersion of low-melting phase, allowing for repair of cracks at a pre-determined temperature. Shape memory alloy wire reinforcements were used within the composite to provide crack closure. Investigators focused the research on fatigue cracks propagating through the matrix in order to optimize and computer model the SMASH technology for aeronautical applications.

An elastic failure model of indentation damage. [of brittle structural ceramics

NASA Technical Reports Server (NTRS)

Liaw, B. M.; Kobayashi, A. S.; Emery, A. F.

1984-01-01

A mechanistically consistent model for indentation damage based on elastic failure at tensile or shear overloads, is proposed. The model accommodates arbitrary crack orientation, stress relaxation, reduction and recovery of stiffness due to crack opening and closure, and interfacial friction due to backward sliding of closed cracks. This elastic failure model was implemented by an axisymmetric finite element program which was used to simulate progressive damage in a silicon nitride plate indented by a tungsten carbide sphere. The predicted damage patterns and the permanent impression matched those observed experimentally. The validation of this elastic failure model shows that the plastic deformation postulated by others is not necessary to replicate the indentation damage of brittle structural ceramics.

Effect of Compressive Mode I on the Mixed Mode I/II Fatigue Crack Growth Rate of 42CrMo4

NASA Astrophysics Data System (ADS)

Heirani, Hasan; Farhangdoost, Khalil

2018-01-01

Subsurface cracks in mechanical contact loading components are subjected to mixed mode I/II, so it is necessary to evaluate the fatigue behavior of materials under mixed mode loading. For this purpose, fatigue crack propagation tests are performed with compact tension shear specimens for several stress intensity factor (SIF) ratios of mode I and mode II. The effect of compressive mode I loading on mixed mode I/II crack growth rate and fracture surface is investigated. Tests are carried out for the pure mode I, pure mode II, and two different mixed mode loading angles. On the basis of the experimental results, mixed mode crack growth rate parameters are proposed according to Tanaka and Richard with Paris' law. Results show neither Richard's nor Tanaka's equivalent SIFs are very useful because these SIFs depend strongly on the loading angle, but Richard's equivalent SIF formula is more suitable than Tanaka's formula. The compressive mode I causes the crack closure, and the friction force between the crack surfaces resists against the crack growth. In compressive loading with 45° angle, d a/d N increases as K eq decreases.

Assessments of Fracture Toughness of Monolithic Ceramics-SEPB Versus SEVNB Methods

NASA Technical Reports Server (NTRS)

Choi, Sung R.; Gyekenyesi, John P.

2006-01-01

Fracture toughness of a total of 13 advanced monolithic ceramics including silicon nitrides, silicon carbide, aluminas, and glass ceramic was determined at ambient temperature by using both single edge precracked beam (SEPB) and single edge v-notched beam (SEVNB) methods. Relatively good agreement in fracture toughness between the two methods was observed for advanced ceramics with flat R-curves; whereas, poor agreement in fracture toughness was seen for materials with rising R-curves. The discrepancy in fracture toughness between the two methods was due to stable crack growth with crack closure forces acting in the wake region of cracks even in SEVNB test specimens. The effect of discrepancy in fracture toughness was analyzed in terms of microstructural feature (grain size and shape), toughening exponent in R-curve, and stable crack growth determined using back-face strain gaging.

Small-crack effects in high-strength aluminum alloys

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Wu, X. R.; Venneri, S. L.; Li, C. G.

1994-01-01

The National Aeronautics and Space Administration and the Chinese Aeronautical Establishment participated in a Fatigue and Fracture Mechanics Cooperative Program. The program objectives were to identify and characterize crack initiation and growth of small cracks (10 microns to 2 mm long) in commonly used US and PRC aluminum alloys, to improve fracture mechanics analyses of surface- and corner-crack configurations, and to develop improved life-prediction methods. Fatigue and small-crack tests were performed on single-edgenotch tension (SENT) specimens and large-crack tests were conducted on center-crack tension specimens for constant-amplitude (stress ratios of -1, 0, and 0.5) and Mini-TWIST spectrum loading. The plastic replica method was used to monitor the initiation and growth of small fatigue cracks at the semicircular notch. Crack growth results from each laboratory on 7075-T6 bare and LC9cs clad aluminum alloys agreed well and showed that fatigue life was mostly crack propagation from a material defect (inclusion particles or void) or from the cladding layer. Finite-element and weight-function methods were used to determine stress intensity factors for surface and corner cracks in the SENT specimens. Equations were then developed and used in a crack growth and crack-closure model to correlate small- and large-crack data and to make life predictions for various load histories. The cooperative program produced useful experimental data and efficient analysis methods for improving life predictions. The results should ultimately improve aircraft structural reliability and safety.

The merging of fatigue and fracture mechanics concepts: a historical perspective

NASA Astrophysics Data System (ADS)

Newman, J. C.

1998-07-01

In this review, some of the technical developments that have occurred during the past 40 years are presented which have led to the merger of fatigue and fracture mechanics concepts. This review is made from the viewpoint of “crack propagation”. As methods to observe the “fatigue” process have improved, the formation of fatigue micro-cracks have been observed earlier in life and the measured crack sizes have become smaller. These observations suggest that fatigue damage can now be characterized by “crack size”. In parallel, the crack-growth analysis methods, using stress-intensity factors, have also improved. But the effects of material inhomogeneities, crack-fracture mechanisms, and nonlinear behavior must now be included in these analyses. The discovery of crack-closure mechanisms, such as plasticity, roughness, and oxide/corrosion/fretting product debris, and the use of the effective stress-intensity factor range, has provided an engineering tool to predict small- and large-crack-growth rate behavior under service loading conditions. These mechanisms have also provided a rationale for developing new, damage-tolerant materials. This review suggests that small-crack growth behavior should be viewed as typical behavior, whereas large-crack threshold behavior should be viewed as the anomaly. Small-crack theory has unified “fatigue” and “fracture mechanics” concepts; and has bridged the gap between safe-life and durability/damage-tolerance design concepts.

The Merging of Fatigue and Fracture Mechanics Concepts: A Historical Perspective

NASA Technical Reports Server (NTRS)

Newman, James C., Jr.

1997-01-01

The seventh Jerry L. Swedlow Memorial Lecture presents a review of some of the technical developments, that have occurred during the past 40 years, which have led to the merger of fatigue and fracture mechanics concepts. This review is made from the viewpoint of 'crack propagation.' As methods to observe the 'fatigue' process have improved, the formation of fatigue micro-cracks have been observed earlier in life and the measured crack sizes have become smaller. These observations suggest that fatigue damage can now be characterized by 'crack size.' In parallel, the crack-growth analysis methods, using stress-intensity factors, have also improved. But the effects of material inhomogeneities, crack-fracture mechanisms, and nonlinear behavior must now be included in these analyses. The discovery of crack-closure mechanisms, such as plasticity, roughness, and oxide/corrosion/fretting product debris, and the use of the effective stress-intensity factor range, has provided an engineering tool to predict small- and large-crack-growth rate behavior under service loading, conditions. These mechanisms have also provided a rationale for developing, new, damage-tolerant materials. This review suggests that small-crack growth behavior should be viewed as typical behavior, whereas large-crack threshold behavior should be viewed as the anomaly. Small-crack theory has unified 'fatigue' and 'fracture mechanics' concepts; and has bridged the cap between safe-life and durability/damage-tolerance design concepts.

A Technique for Mapping Characteristic Lengths to Preserve Energy Dissipated via Strain Softening in a Multiscale Analysis

NASA Technical Reports Server (NTRS)

Pineda, Evan J.; Bednarcyk, Brett A.; Arnold, Steven M.

2014-01-01

It is often advantageous to account for the microstructure of the material directly using multiscale modeling. For computational tractability, an idealized repeating unit cell (RUC) is used to capture all of the pertinent features of the microstructure. Typically, the RUC is dimensionless and depends only on the relative volume fractions of the different phases in the material. This works well for non-linear and inelastic behavior exhibiting a positive-definite constitutive response. Although, once the material exhibits strain softening, or localization, a mesh objective failure theories, such as smeared fracture theories, nodal and element enrichment theories (XFEM), cohesive elements or virtual crack closure technique (VCCT), can be utilized at the microscale, but the dimensions of the RUC must then be defined. One major challenge in multiscale progressive damage modeling is relating the characteristic lengths across the scales in order to preserve the energy that is dissipated via localization at the microscale. If there is no effort to relate the size of the macroscale element to the microscale RUC, then the energy that is dissipated will remain mesh dependent at the macroscale, even if it is regularized at the microscale. Here, a technique for mapping characteristic lengths across the scales is proposed. The RUC will be modeled using the generalized method of cells (GMC) micromechanics theory, and local failure in the matrix constituent subcells will be modeled using the crack band theory. The subcell characteristic lengths used in the crack band calculations will be mapped to the macroscale finite element in order to regularize the local energy in a manner consistent with the global length scale. Examples will be provided with and without the regularization, and they will be compared to a baseline case where the size and shape of the element and RUC are coincident (ensuring energy is preserved across the scales).

Use of fiber reinforced concrete for concrete pavement slab replacement : [summary].

DOT National Transportation Integrated Search

2014-03-01

Replacing cracked concrete in roadways requires : lanes to be closed and traff c disrupted. One way : to reduce road closure time is to reduce concrete : curing time. To accelerate curing time, pavement : engineers mix a very low water-cement ratio w...

Use of Several Thermal Analysis Techniques to Study the Cracking of an Nitrile Butadiene Rubber (NBR) Insulator on the Booster Separation Motor (BSM) of the Space Shuttle

NASA Technical Reports Server (NTRS)

Wingard, Charles D.; Whitaker, Ann F. (Technical Monitor)

2000-01-01

Two different vendor rubber formulations have been used to produce the silica-filled NBR insulators for the BSM used on both of the Solid Rocket Boosters (SRBs) of the Space Shuttle. A number of lots of the BSM insulator in 1998-99 exhibited surface cracks and/or crazing. Each insulator is bonded to the BSM aluminum aft closure with an epoxy adhesive. Induced insulator stresses from adhesive cure are likely greatest where the insulator/adhesive contour is the greatest, thus showing increased insulator surface cracking in this area. Thermal analysis testing by Dynamic Mechanical Analyzer (DMA) and Thermomechanical Analysis (TMA) was performed on one each of the two vendor BSM insulators previously bonded that exhibited the surface cracking. The TMA data from the film/fiber technique yielded the most meaningful results, with thin insulator surface samples containing cracks having roughly the same modulus (stiffness) as thin insulator bulk samples just underneath.

Development of a Practical Methodology for Elastic-Plastic and Fully Plastic Fatigue Crack Growth

NASA Technical Reports Server (NTRS)

McClung, R. C.; Chell, G. G.; Lee, Y. -D.; Russell, D. A.; Orient, G. E.

1999-01-01

A practical engineering methodology has been developed to analyze and predict fatigue crack growth rates under elastic-plastic and fully plastic conditions. The methodology employs the closure-corrected effective range of the J-integral, delta J(sub eff) as the governing parameter. The methodology contains original and literature J and delta J solutions for specific geometries, along with general methods for estimating J for other geometries and other loading conditions, including combined mechanical loading and combined primary and secondary loading. The methodology also contains specific practical algorithms that translate a J solution into a prediction of fatigue crack growth rate or life, including methods for determining crack opening levels, crack instability conditions, and material properties. A critical core subset of the J solutions and the practical algorithms has been implemented into independent elastic-plastic NASGRO modules. All components of the entire methodology, including the NASGRO modules, have been verified through analysis and experiment, and limits of applicability have been identified.

Development of a Practical Methodology for Elastic-Plastic and Fully Plastic Fatigue Crack Growth

NASA Technical Reports Server (NTRS)

McClung, R. C.; Chell, G. G.; Lee, Y.-D.; Russell, D. A.; Orient, G. E.

1999-01-01

A practical engineering methodology has been developed to analyze and predict fatigue crack growth rates under elastic-plastic and fully plastic conditions. The methodology employs the closure-corrected effective range of the J-integral, (Delta)J(sub eff), as the governing parameter. The methodology contains original and literature J and (Delta)J solutions for specific geometries, along with general methods for estimating J for other geometries and other loading conditions, including combined mechanical loading and combined primary and secondary loading. The methodology also contains specific practical algorithms that translate a J solution into a prediction of fatigue crack growth rate or life, including methods for determining crack opening levels, crack instability conditions, and material properties. A critical core subset of the J solutions and the practical algorithms has been implemented into independent elastic-plastic NASGRO modules. All components of the entire methodology, including the NASGRO modules, have been verified through analysis and experiment, and limits of applicability have been identified.

Nonlinear fracture of concrete and ceramics

NASA Technical Reports Server (NTRS)

Kobayashi, Albert S.; Du, Jia-Ji; Hawkins, Niel M.; Bradt, Richard C.

1989-01-01

The nonlinear fracture process zones in an impacted unnotched concrete bend specimen, a prenotched ceramic bend specimen, and an unnotched ceramic/ceramic composite bend specimen were estimated through hybrid experimental numerical analysis. Aggregate bridging in concrete, particulate bridging in ceramics, and fiber bridging in ceramic/ceramic composite are modeled by Barenblatt-type cohesive zones which are incorporated into the finite-element models of the bend specimens. Both generation and propagation analyses are used to estimate the distribution of crack closure stresses in the nonlinear fracture process zones. The finite-element models are then used to simulate fracture tests consisting of rapid crack propagation in an impacted concrete bend specimen, and stable crack growth and strain softening in a ceramic and ceramic/ceramic composite bend specimens.

Computational fluid dynamic comparison between patch-based and primary closure techniques after carotid endarterectomy.

PubMed

Domanin, Maurizio; Bissacco, Daniele; Le Van, Davide; Vergara, Christian

2018-03-01

The aim of the study was to provide, by means of computational fluid dynamics, a comparative analysis after carotid endarterectomy (CEA) between patch graft (PG) and primary closure (PC) techniques performed in real carotid geometries to identify disturbed flow conditions potentially involved in the development of restenosis. Eight carotid geometries in seven asymptomatic patients who underwent CEA were analyzed. In six cases (A-F), CEA was performed using PG closure; in two cases (G and H), PC was performed. Three-dimensional carotid geometries, derived from postoperative magnetic resonance angiography, were reconstructed, and a computational fluid dynamics analysis was performed. A virtual scenario with PC closure was designed in patients in whom PG was originally inserted and vice versa. This allowed us to compare for each patient hemodynamic effects in the PG and PC scenarios in terms of oscillatory shear index (OSI) and relative residence time (RRT), considered indicators of disturbed flow. For the six original PG cases, the mean averaged-in-space OSI was 0.07 ± 0.01 for PG and 0.03 ± 0.02 for virtual-PC (difference, 0.04 ± 0.01; P = .0016). The mean of the percentage of area (%A) with OSI >0.2 resulted in 10.08% ± 3.38% for PG and 3.80% ± 3.22% for virtual-PC (difference, 6.28 ± 1.91; P = .008). For the same cases, the mean of the averaged-in-space RRT resulted in 5.48 ± 3.40 1/Pa for PG and 2.62 ± 1.12 1/Pa for virtual-PC (difference, 2.87 ± 1.46; P = .097). The mean of %A RRT >4.0 1/Pa resulted in 26.53% ± 12.98% for PG and 9.95% ± 6.53% for virtual-PC (difference, 16.58 ± 5.93; P = .025). For the two original PC cases, the averaged-in-space OSIs were 0.02 and 0.04 for PC and 0.03 and 0.02 for virtual-PG; the %A OSIs >0.2 were 0.9% and 7.6% for PC and 3.0% and 2.2% for virtual-PG; the averaged-in-space RRTs were 1.8 and 2.0 1/Pa for PC and 2.9 and 1.9 1/Pa for virtual-PG; the %A RRTs >4.0 1/Pa were 6.8% and 9.8% for PC and 9.4% and 6.2% for virtual-PG. These results revealed generally higher disturbed flows in the PG configurations with respect to the PC ones. OSI and RRT values were generally higher in PG cases with respect to PC, especially for high carotids or when the arteriotomy is mainly at the bulb region. Thus, an elective use of patch should be considered to prevent disturbed flows. Copyright © 2017 Society for Vascular Surgery. Published by Elsevier Inc. All rights reserved.

Passive detection and localization of fatigue cracking in aluminum plates using Green's function reconstruction from ambient noise.

PubMed

Yang, Yang; Xiao, Li; Qu, Wenzhong; Lu, Ye

2017-11-01

Recent theoretical and experimental studies have demonstrated that a local Green's function can be retrieved from the cross-correlation of ambient noise field. This technique can be used to detect fatigue cracking in metallic structures, owing to the fact that the presence of crack can lead to a change in Green's function. This paper presents a method of structural fatigue cracking characterization method by measuring Green's function reconstruction from noise excitation and verifies the feasibility of crack detection in poor noise source distribution. Fatigue cracks usually generate nonlinear effects, in which different wave amplitudes and frequency compositions can cause different nonlinear responses. This study also undertakes analysis of the capacity of the proposed approach to identify fatigue cracking under different noise amplitudes and frequency ranges. Experimental investigations of an aluminum plate are conducted to assess the cross-correlations of received noise between sensor pairs and finally to detect the introduced fatigue crack. A damage index is proposed according to the variation between cross-correlations obtained from the pristine crack closed state and the crack opening-closure state when sufficient noise amplitude is used to generate nonlinearity. A probability distribution map of damage is calculated based on damage indices. The fatigue crack introduced in the aluminum plate is successfully identified and oriented, verifying that a fatigue crack can be detected by reconstructing Green's functions from an imperfect diffuse field in which ambient noise sources exist locally. Copyright © 2017 Elsevier B.V. All rights reserved.

FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (UNIX VERSION)

NASA Technical Reports Server (NTRS)

Newman, J. C.

1994-01-01

Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied loads may be either tensile or compressive. Several standardized aircraft flight-load histories, such as TWIST, Mini-TWIST, FALSTAFF, Inverted FALSTAFF, Felix and Gaussian, are included as options. FASTRAN II also includes two other methods that will help the user input spectrum load histories. The two methods are: (1) a list of stress points, and (2) a flight-by-flight history of stress points. Examples are provided in the user manual. Developed as a research program, FASTRAN II has successfully predicted crack growth in many metallic materials under various aircraft spectrum loading. A computer program DKEFF which is a part of the FASTRAN II package was also developed to analyze crack growth rate data from laboratory specimens to obtain the effective stress-intensity factor against crack growth rate relations used in FASTRAN II. FASTRAN II is written in standard FORTRAN 77. It has been successfully compiled and implemented on Sun4 series computers running SunOS and on IBM PC compatibles running MS-DOS using the Lahey F77L FORTRAN compiler. Sample input and output data are included with the FASTRAN II package. The UNIX version requires 660K of RAM for execution. The standard distribution medium for the UNIX version (LAR-14865) is a .25 inch streaming magnetic tape cartridge in UNIX tar format. It is also available on a 3.5 inch diskette in UNIX tar format. The standard distribution medium for the MS-DOS version (LAR-14944) is a 5.25 inch 360K MS-DOS format diskette. The contents of the diskette are compressed using the PKWARE archiving tools. The utility to unarchive the files, PKUNZIP.EXE, is included. The program was developed in 1984 and revised in 1992. Sun4 and SunOS are trademarks of Sun Microsystems, Inc. IBM PC is a trademark of International Business Machines Corp. MS-DOS is a trademark of Microsoft, Inc. F77L is a trademark of the Lahey Computer Systems, Inc. UNIX is a registered trademark of AT&T Bell Laboratories. PKWARE and PKUNZIP are trademarks of PKWare, Inc.

FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (IBM PC VERSION)

NASA Technical Reports Server (NTRS)

Newman, J. C.

1994-01-01

Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material to be left along the crack surfaces as the crack grows. Plane stress and plane strain conditions, as well as conditions between these two, can be simulated in FASTRAN II by using a constraint factor on tensile yielding at the crack front to approximately account for three-dimensional stress states. FASTRAN II contains seventeen predefined crack configurations (standard laboratory fatigue crack growth rate specimens and many common crack configurations found in structures); and the user can define one additional crack configuration. The baseline crack growth rate properties (effective stress-intensity factor against crack growth rate) may be given in either equation or tabular form. For three-dimensional crack configurations, such as surface cracks or corner cracks at holes or notches, the fatigue crack growth rate properties may be different in the crack depth and crack length directions. Final failure of the cracked structure can be modelled with fracture toughness properties using either linear-elastic fracture mechanics (brittle materials), a two-parameter fracture criterion (brittle to ductile materials), or plastic collapse (extremely ductile materials). The crack configurations in FASTRAN II can be subjected to either constant-amplitude, variable-amplitude or spectrum loading. The applied loads may be either tensile or compressive. Several standardized aircraft flight-load histories, such as TWIST, Mini-TWIST, FALSTAFF, Inverted FALSTAFF, Felix and Gaussian, are included as options. FASTRAN II also includes two other methods that will help the user input spectrum load histories. The two methods are: (1) a list of stress points, and (2) a flight-by-flight history of stress points. Examples are provided in the user manual. Developed as a research program, FASTRAN II has successfully predicted crack growth in many metallic materials under various aircraft spectrum loading. A computer program DKEFF which is a part of the FASTRAN II package was also developed to analyze crack growth rate data from laboratory specimens to obtain the effective stress-intensity factor against crack growth rate relations used in FASTRAN II. FASTRAN II is written in standard FORTRAN 77. It has been successfully compiled and implemented on Sun4 series computers running SunOS and on IBM PC compatibles running MS-DOS using the Lahey F77L FORTRAN compiler. Sample input and output data are included with the FASTRAN II package. The UNIX version requires 660K of RAM for execution. The standard distribution medium for the UNIX version (LAR-14865) is a .25 inch streaming magnetic tape cartridge in UNIX tar format. It is also available on a 3.5 inch diskette in UNIX tar format. The standard distribution medium for the MS-DOS version (LAR-14944) is a 5.25 inch 360K MS-DOS format diskette. The contents of the diskette are compressed using the PKWARE archiving tools. The utility to unarchive the files, PKUNZIP.EXE, is included. The program was developed in 1984 and revised in 1992. Sun4 and SunOS are trademarks of Sun Microsystems, Inc. IBM PC is a trademark of International Business Machines Corp. MS-DOS is a trademark of Microsoft, Inc. F77L is a trademark of the Lahey Computer Systems, Inc. UNIX is a registered trademark of AT&T Bell Laboratories. PKWARE and PKUNZIP are trademarks of PKWare, Inc.

Three Dimensional Numerical Simulation and Characterization of Crack Growth in the Weld Region of a Friction Stir Welded Structure

NASA Technical Reports Server (NTRS)

Seshadri, Banavara R.; Smith, Stephen W.; Newman, John A.

2013-01-01

Friction stir welding (FSW) fabrication technology is being adopted in aerospace applications. The use of this technology can reduce production cost, lead-times, reduce structural weight and need for fasteners and lap joints, which are typically the primary locations of crack initiation and multi-site fatigue damage in aerospace structures. FSW is a solid state welding process that is well-suited for joining aluminum alloy components; however, the process introduces residual stresses (both tensile and compressive) in joined components. The propagation of fatigue cracks in a residual stress field and the resulting redistribution of the residual stress field and its effect on crack closure have to be estimated. To insure the safe insertion of complex integral structures, an accurate understanding of the fatigue crack growth behavior and the complex crack path process must be understood. A life prediction methodology for fatigue crack growth through the weld under the influence of residual stresses in aluminum alloy structures fabricated using FSW will be detailed. The effects and significance of the magnitude of residual stress at a crack tip on the estimated crack tip driving force are highlighted. The location of the crack tip relative to the FSW and the effect of microstructure on fatigue crack growth are considered. A damage tolerant life prediction methodology accounting for microstructural variation in the weld zone and residual stress field will lead to the design of lighter and more reliable aerospace structures

49 CFR 173.35 - Hazardous materials in IBCs.

Code of Federal Regulations, 2014 CFR

2014-10-01

..., cracks, cuts, or other damage which would render it unable to pass the prescribed design type test to... of this subchapter. Additional marking allowed for each design type may be present. Required markings... percent of its water capacity. (e) Where two or more closure systems are fitted in series, the system...

Pattern formation during healing of fluid-filled cracks: an analog experiment

DOE Office of Scientific and Technical Information (OSTI.GOV)

F. Renard; D. K. Dysthe; J. G. Feder

2009-11-01

The formation and subsequent healing of cracks and crack networks may control such diverse phenomena as the strengthening of fault zones between earthquakes, fluid migrations in the Earth's crust, or the transport of radioactive materials in nuclear waste disposal. An intriguing pattern-forming process can develop during healing of fluid-filled cracks, where pockets of fluid remain permanently trapped in the solid as the crack tip is displaced driven by surface energy. Here, we present the results of analog experiments in which a liquid was injected into a colloidal inorganic gel to obtain penny-shaped cracks that were subsequently allowed to close andmore » heal under the driving effect of interfacial tension. Depending on the properties of the gel and the injected liquid, two modes of healing were obtained. In the first mode, the crack healed completely through a continuous process. The second mode of healing was discontinuous and was characterized by a 'zipper-like' closure of a front that moved along the crack perimeter, trapping fluid that may eventually form inclusions trapped in the solid. This instability occurred only when the velocity of the crack tip decreased to zero. Our experiments provide a cheap and simple analog to reveal how aligned arrays of fluid inclusions may be captured along preexisting fracture planes and how small amounts of fluids can be permanently trapped in solids, modifying irreversibly their material properties.« less

Analytical and experimental studies on detection of longitudinal, L and inverted T cracks in isotropic and bi-material beams based on changes in natural frequencies

NASA Astrophysics Data System (ADS)

Ravi, J. T.; Nidhan, S.; Muthu, N.; Maiti, S. K.

2018-02-01

An analytical method for determination of dimensions of longitudinal crack in monolithic beams, based on frequency measurements, has been extended to model L and inverted T cracks. Such cracks including longitudinal crack arise in beams made of layered isotropic or composite materials. A new formulation for modelling cracks in bi-material beams is presented. Longitudinal crack segment sizes, for L and inverted T cracks, varying from 2.7% to 13.6% of length of Euler-Bernoulli beams are considered. Both forward and inverse problems have been examined. In the forward problems, the analytical results are compared with finite element (FE) solutions. In the inverse problems, the accuracy of prediction of crack dimensions is verified using FE results as input for virtual testing. The analytical results show good agreement with the actual crack dimensions. Further, experimental studies have been done to verify the accuracy of the analytical method for prediction of dimensions of three types of crack in isotropic and bi-material beams. The results show that the proposed formulation is reliable and can be employed for crack detection in slender beam like structures in practice.

Application of Benchmark Examples to Assess the Single and Mixed-Mode Static Delamination Propagation Capabilities in ANSYS

NASA Technical Reports Server (NTRS)

Krueger, Ronald

2012-01-01

The application of benchmark examples for the assessment of quasi-static delamination propagation capabilities is demonstrated for ANSYS. The examples are independent of the analysis software used and allow the assessment of the automated delamination propagation in commercial finite element codes based on the virtual crack closure technique (VCCT). The examples selected are based on two-dimensional finite element models of Double Cantilever Beam (DCB), End-Notched Flexure (ENF), Mixed-Mode Bending (MMB) and Single Leg Bending (SLB) specimens. First, the quasi-static benchmark examples were recreated for each specimen using the current implementation of VCCT in ANSYS . Second, the delamination was allowed to propagate under quasi-static loading from its initial location using the automated procedure implemented in the finite element software. Third, the load-displacement relationship from a propagation analysis and the benchmark results were compared, and good agreement could be achieved by selecting the appropriate input parameters. The benchmarking procedure proved valuable by highlighting the issues associated with choosing the input parameters of the particular implementation. Overall the results are encouraging, but further assessment for three-dimensional solid models is required.

On the R-curve behavior of human tooth enamel.

PubMed

Bajaj, Devendra; Arola, Dwayne D

2009-08-01

In this study the crack growth resistance behavior and fracture toughness of human tooth enamel were quantified using incremental crack growth measures and conventional fracture mechanics. Results showed that enamel undergoes an increase in crack growth resistance (i.e. rising R-curve) with crack extension from the outer to the inner enamel, and that the rise in toughness is a function of distance from the dentin enamel junction (DEJ). The outer enamel exhibited the lowest apparent toughness (0.67+/-0.12 MPam(0.5)), and the inner enamel exhibited a rise in the growth toughness from 1.13 MPam(0.5)/mm to 3.93 MPam(0.5)/mm. The maximum crack growth resistance at fracture (i.e. fracture toughness (K(c))) ranged from 1.79 to 2.37 MPam(0.5). Crack growth in the inner enamel was accompanied by a host of mechanisms operating from the micro- to the nano-scale. Decussation in the inner enamel promoted crack deflection and twist, resulting in a reduction of the local stress intensity at the crack tip. In addition, extrinsic mechanisms such as bridging by unbroken ligaments of the tissue and the organic matrix promoted crack closure. Microcracking due to loosening of prisms was also identified as an active source of energy dissipation. In summary, the unique microstructure of enamel in the decussated region promotes crack growth toughness that is approximately three times that of dentin and over ten times that of bone.

Residual Strength Prediction of Fuselage Structures with Multiple Site Damage

NASA Technical Reports Server (NTRS)

Chen, Chuin-Shan; Wawrzynek, Paul A.; Ingraffea, Anthony R.

1999-01-01

This paper summarizes recent results on simulating full-scale pressure tests of wide body, lap-jointed fuselage panels with multiple site damage (MSD). The crack tip opening angle (CTOA) fracture criterion and the FRANC3D/STAGS software program were used to analyze stable crack growth under conditions of general yielding. The link-up of multiple cracks and residual strength of damaged structures were predicted. Elastic-plastic finite element analysis based on the von Mises yield criterion and incremental flow theory with small strain assumption was used. A global-local modeling procedure was employed in the numerical analyses. Stress distributions from the numerical simulations are compared with strain gage measurements. Analysis results show that accurate representation of the load transfer through the rivets is crucial for the model to predict the stress distribution accurately. Predicted crack growth and residual strength are compared with test data. Observed and predicted results both indicate that the occurrence of small MSD cracks substantially reduces the residual strength. Modeling fatigue closure is essential to capture the fracture behavior during the early stable crack growth. Breakage of a tear strap can have a major influence on residual strength prediction.

Electromagnetic stimulation of the ultrasonic signal for nondestructive detection of the ferromagnetic inclusions and flaws

NASA Astrophysics Data System (ADS)

Finkel, Peter

2007-03-01

It was recently shown that thermal or optical stimulation can be used to increase sensitivity of the conventional nondestructive ultrasonic detection of the small crack, flaws and inclusions in a ferromagnetic thin-walled parts. We proposed another method based on electromagnetic modulation of the ultrasonic scattered signal from the inclusions or defects. The electromagnetically induced high density current pulse produces stresses which alter the ultrasonic waves scanning the part with the defect and modulate ultrasonic signal. The excited electromagnetic field can produces crack-opening due to Lorentz forces that increase the ultrasonic reflection. The Joule heating associated with the high density current, and consequent thermal stresses may cause both crack-closure, as well as crack-opening, depending on various factors. Experimental data is presented here for the case of a small cracks near small holes in thin-walled structures. The measurements were taken at 2-10 MHz with a Lamb wave wedge transducer. It is shown that electromagnetic transient modulation of the ultrasonic echo pulse tone-burst suggest that this method could be used to enhance detection of small cracks and ferromagnetic inclusions in thin walled metallic structures.

The mechanics of delamination in fiber-reinforced composite materials. Part 2: Delamination behavior and fracture mechanics parameters

NASA Technical Reports Server (NTRS)

Wang, S. S.; Choi, I.

1983-01-01

Based on theories of laminate anisotropic elasticity and interlaminar fracture, the complete solution structure associated with a composite delamination is determined. Fracture mechanics parameters characterizing the interlaminar crack behavior are defined from asymptotic stress solutions for delaminations with different crack-tip deformation configurations. A numerical method employing singular finite elements is developed to study delaminations in fiber composites with any arbitrary combinations of lamination, material, geometric, and crack variables. The special finite elements include the exact delamination stress singularity in its formulation. The method is shown to be computationally accurate and efficient, and operationally simple. To illustrate the basic nature of composite delamination, solutions are shown for edge-delaminated (0/-0/-0/0) and (+ or - 0/+ or - 0/90/90 deg) graphite-epoxy systems under uniform axial extenstion. Three-dimensional crack-tip stress intensity factors, associated energy release rates, and delamination crack-closure are determined for each individual case. The basic mechanics and mechanisms of composite delamination are studied, and fundamental characteristics unique to recently proposed tests for interlaminar fracture toughness of fiber composite laminates are examined.

Predicting overload-affected fatigue crack growth in steels

DOE Office of Scientific and Technical Information (OSTI.GOV)

Skorupa, M.; Skorupa, A.; Ladecki, B.

1996-12-01

The ability of semi-empirical crack closure models to predict the effect of overloads on fatigue crack growth in low-alloy steels has been investigated. With this purpose, the CORPUS model developed for aircraft metals and spectra has been checked first through comparisons between the simulated and observed results for a low-alloy steel. The CORPUS predictions of crack growth under several types of simple load histories containing overloads appeared generally unconservative which prompted the authors to formulate a new model, more suitable for steels. With the latter approach, the assumed evolution of the crack opening stress during the delayed retardation stage hasmore » been based on experimental results reported for various steels. For all the load sequences considered, the predictions from the proposed model appeared to be by far more accurate than those from CORPUS. Based on the analysis results, the capability of semi-empirical prediction concepts to cover experimentally observed trends that have been reported for sequences with overloads is discussed. Finally, possibilities of improving the model performance are considered.« less

Neutron Radiography Based Visualization and Profiling of Water Uptake in (Un)cracked and Autonomously Healed Cementitious Materials

PubMed Central

Van den Heede, Philip; Van Belleghem, Bjorn; Alderete, Natalia; Van Tittelboom, Kim; De Belie, Nele

2016-01-01

Given their low tensile strength, cement-based materials are very susceptible to cracking. These cracks serve as preferential pathways for corrosion inducing substances. For large concrete infrastructure works, currently available time-consuming manual repair techniques are not always an option. Often, one simply cannot reach the damaged areas and when making those areas accessible anyway (e.g., by redirecting traffic), the economic impacts involved would be enormous. Under those circumstances, it might be useful to have concrete with an embedded autonomous healing mechanism. In this paper, the effectiveness of incorporating encapsulated high and low viscosity polyurethane-based healing agents to ensure (multiple) crack healing has been investigated by means of capillary absorption tests on mortar while monitoring the time-dependent water ingress with neutron radiography. Overall visual interpretation and water front/sample cross-section area ratios as well as water profiles representing the area around the crack and their integrals do not show a preference for the high or low viscosity healing agent. Another observation is that in presence of two cracks, only one is properly healed, especially when using the latter healing agent. Exposure to water immediately after release of the healing agent stimulates the foaming reaction of the polyurethane and ensures a better crack closure. PMID:28773436

An autonomous surface discontinuity detection and quantification method by digital image correlation and phase congruency

NASA Astrophysics Data System (ADS)

Cinar, A. F.; Barhli, S. M.; Hollis, D.; Flansbjer, M.; Tomlinson, R. A.; Marrow, T. J.; Mostafavi, M.

2017-09-01

Digital image correlation has been routinely used to measure full-field displacements in many areas of solid mechanics, including fracture mechanics. Accurate segmentation of the crack path is needed to study its interaction with the microstructure and stress fields, and studies of crack behaviour, such as the effect of closure or residual stress in fatigue, require data on its opening displacement. Such information can be obtained from any digital image correlation analysis of cracked components, but it collection by manual methods is quite onerous, particularly for massive amounts of data. We introduce the novel application of Phase Congruency to detect and quantify cracks and their opening. Unlike other crack detection techniques, Phase Congruency does not rely on adjustable threshold values that require user interaction, and so allows large datasets to be treated autonomously. The accuracy of the Phase Congruency based algorithm in detecting cracks is evaluated and compared with conventional methods such as Heaviside function fitting. As Phase Congruency is a displacement-based method, it does not suffer from the noise intensification to which gradient-based methods (e.g. strain thresholding) are susceptible. Its application is demonstrated to experimental data for cracks in quasi-brittle (Granitic rock) and ductile (Aluminium alloy) materials.

40 CFR 165.65 - Registrants who distribute or sell pesticide products in refillable containers.

Code of Federal Regulations, 2010 CFR

2010-07-01

... formula that is required under FIFRA section 3. (c) What information must I develop? For each pesticide... its confidential statement of formula that is required under FIFRA section 3. (ii) If the refilling..., metal fatigue, damaged threads or closures, or other significant defects. (iii) The container has cracks...

40 CFR 63.1043 - Standards-Separator floating roof.

Code of Federal Regulations, 2010 CFR

2010-07-01

... secondary seal shall be mounted above the primary seal and cover the annular space between the floating roof... visible cracks, holes, gaps, or other open spaces in the closure device or between the perimeter of the... membrane fabric cover that covers at least 90 percent of the area of the opening or a flexible fabric...

40 CFR 63.1043 - Standards-Separator floating roof.

Code of Federal Regulations, 2011 CFR

2011-07-01

... secondary seal shall be mounted above the primary seal and cover the annular space between the floating roof... visible cracks, holes, gaps, or other open spaces in the closure device or between the perimeter of the... membrane fabric cover that covers at least 90 percent of the area of the opening or a flexible fabric...

75 FR 39168 - Special Regulations; Areas of the National Park System

Federal Register 2010, 2011, 2012, 2013, 2014

2010-07-08

... his death in 1972. During the closure, we completed four projects: installation of a new HVAC system... to either bulge or crack. The existing HVAC system installed in 1985 failed to maintain a stable...; Areas of the National Park System AGENCY: National Park Service. ACTION: Final Rule. SUMMARY: The...

78 FR 49906 - Airworthiness Directives; The Boeing Company Airplanes

Federal Register 2010, 2011, 2012, 2013, 2014

2013-08-16

.... ACTION: Final rule. SUMMARY: We are adopting a new airworthiness directive (AD) for all The Boeing Company Model 727 airplanes. This AD was prompted by a report of cracking in the left-side chord of the fin closure rib on the vertical stabilizer. This AD requires repetitive inspections of the left and...

78 FR 25662 - Airworthiness Directives; The Boeing Company Airplanes

Federal Register 2010, 2011, 2012, 2013, 2014

2013-05-02

...). SUMMARY: We propose to adopt a new airworthiness directive (AD) for all The Boeing Company Model 727 series airplanes. This proposed AD was prompted by a report of cracking in the left-side chord of the fin closure rib on the vertical stabilizer. This proposed AD would require repetitive inspections of the left...

A dynamic model of a cantilever beam with a closed, embedded horizontal crack including local flexibilities at crack tips

NASA Astrophysics Data System (ADS)

Liu, J.; Zhu, W. D.; Charalambides, P. G.; Shao, Y. M.; Xu, Y. F.; Fang, X. M.

2016-11-01

As one of major failure modes of mechanical structures subjected to periodic loads, embedded cracks due to fatigue can cause catastrophic failure of machineries. Understanding the dynamic characteristics of a structure with an embedded crack is helpful for early crack detection and diagnosis. In this work, a new three-segment beam model with local flexibilities at crack tips is developed to investigate the vibration of a cantilever beam with a closed, fully embedded horizontal crack, which is assumed to be not located at its clamped or free end or distributed near its top or bottom side. The three-segment beam model is assumed to be a linear elastic system, and it does not account for the nonlinear crack closure effect; the top and bottom segments always stay in contact at their interface during the beam vibration. It can model the effects of local deformations in the vicinity of the crack tips, which cannot be captured by previous methods in the literature. The middle segment of the beam containing the crack is modeled by a mechanically consistent, reduced bending moment. Each beam segment is assumed to be an Euler-Bernoulli beam, and the compliances at the crack tips are analytically determined using a J-integral approach and verified using commercial finite element software. Using compatibility conditions at the crack tips and the transfer matrix method, the nature frequencies and mode shapes of the cracked cantilever beam are obtained. The three-segment beam model is used to investigate the effects of local flexibilities at crack tips on the first three natural frequencies and mode shapes of the cracked cantilever beam. A stationary wavelet transform (SWT) method is used to process the mode shapes of the cracked cantilever beam; jumps in single-level SWT decomposition detail coefficients can be used to identify the length and location of an embedded horizontal crack.

Method for Assessment of Changes in the Width of Cracks in Cement Composites with Use of Computer Image Processing and Analysis

NASA Astrophysics Data System (ADS)

Tomczak, Kamil; Jakubowski, Jacek; Fiołek, Przemysław

2017-06-01

Crack width measurement is an important element of research on the progress of self-healing cement composites. Due to the nature of this research, the method of measuring the width of cracks and their changes over time must meet specific requirements. The article presents a novel method of measuring crack width based on images from a scanner with an optical resolution of 6400 dpi, subject to initial image processing in the ImageJ development environment and further processing and analysis of results. After registering a series of images of the cracks at different times using SIFT conversion (Scale-Invariant Feature Transform), a dense network of line segments is created in all images, intersecting the cracks perpendicular to the local axes. Along these line segments, brightness profiles are extracted, which are the basis for determination of crack width. The distribution and rotation of the line of intersection in a regular layout, automation of transformations, management of images and profiles of brightness, and data analysis to determine the width of cracks and their changes over time are made automatically by own code in the ImageJ and VBA environment. The article describes the method, tests on its properties, sources of measurement uncertainty. It also presents an example of application of the method in research on autogenous self-healing of concrete, specifically the ability to reduce a sample crack width and its full closure within 28 days of the self-healing process.

Application of fiber bridging models to fatigue crack growth in unidirectional titanium matrix composites

NASA Technical Reports Server (NTRS)

Bakuckas, J. G., Jr.; Johnson, W. S.

1992-01-01

Several fiber bridging models were reviewed and applied to study the matrix fatigue crack growth behavior in center notched (0)(sub 8) SCS-6/Ti-15-3 and (0)(sub 4) SCS-6/Ti-6Al-4V laminates. Observations revealed that fatigue damage consisted primarily of matrix cracks and fiber matrix interfacial failure in the (0)(sub 8) SCS-6/Ti-15-3 laminates. Fiber-matrix interface failure included fracture of the brittle reaction zone and cracking between the two carbon rich fiber coatings. Intact fibers in the wake of the matrix cracks reduce the stress intensity factor range. Thus, an applied stress intensity factor range is inappropriate to characterize matrix crack growth behavior. Fiber bridging models were used to determine the matrix stress intensity factor range in titanium metal matrix composites. In these models, the fibers in the wake of the crack are idealized as a closure pressure. An unknown constant frictional shear stress is assumed to act along the debond or slip length of the bridging fibers. The frictional shear stress was used as a curve fitting parameter to available data (crack growth data, crack opening displacement data, and debond length data). Large variations in the frictional shear stress required to fit the experimental data indicate that the fiber bridging models in their present form lack predictive capabilities. However, these models provide an efficient and relatively simple engineering method for conducting parametric studies of the matrix growth behavior based on constituent properties.

Crack Growth Behavior in the Threshold Region for High Cycle Fatigue Loading

NASA Technical Reports Server (NTRS)

Forman, Royce G.; Figert, J.; Beek, J.; Ventura, J.; Martinez, J.; Samonski, F.

2011-01-01

This presentation describes results obtained from a research project conducted at the NASA Johnson Space Center (JSC) that was jointly supported by the FAA Technical Center and JSC. The JSC effort was part of a multi-task FAA program involving several U.S. laboratories and initiated for the purpose of developing enhanced analysis tools to assess damage tolerance of rotorcraft and aircraft propeller systems. The research results to be covered in this presentation include a new understanding of the behavior of fatigue crack growth in the threshold region. This behavior is important for structural life analysis of aircraft propeller systems and certain rotorcraft structural components (e.g., the mast). These components are often designed to not allow fatigue crack propagation to exceed an experimentally determined fatigue crack growth threshold value. During the FAA review meetings for the program, disagreements occurred between the researchers regarding the observed fanning (spread between the da/dN curves of constant R) in the threshold region at low stress ratios, R. Some participants believed that the fanning was a result of the ASTM load shedding test method for threshold testing, and thus did not represent the true characteristics of the material. If the fanning portion of the threshold value is deleted or not included in a life analysis, a significant penalty in the calculated life and design of the component would occur. The crack growth threshold behavior was previously studied and reported by several research investigators in the time period: 1970-1980. Those investigators used electron microscopes to view the crack morphology of the fatigue fracture surfaces. Their results showed that just before reaching threshold, the crack morphology often changed from a striated to a faceted or cleavage-like morphology. This change was reported to have been caused by particular dislocation properties of the material. Based on the results of these early investigations, a program was initiated at JSC to repeat these examinations on a number of aircraft structural alloys that were currently being tested for obtaining fatigue crack growth properties. These new scanning electron microscope (SEM) examinations of the fatigue fracture faces confirmed the change in crack morphology in the threshold crack tip region. In addition, SEM examinations were further performed in the threshold crack-tip region before breaking the specimens open (not done in the earlier published studies). In these examinations, extensive crack forking and even 90-degree crack bifurcations were found to have occurred in the final threshold crack-tip region. The forking and bifurcations caused numerous closure points to occur that prevented full crack closure in the threshold region, and thus were the cause of the fanning at low-R values. Therefore, we have shown that the fanning behavior was caused by intrinsic dislocation properties of the different alloy materials and were not the result of a plastic wake that remains from the load-shedding test phase. Also, to accommodate the use of da/dN data which includes fanning at low R-values, an updated fanning factor term has been developed and will be implemented into the NASGRO fatigue crack growth software. The term can be set to zero if it is desired that the fanning behavior is not be modeled for particular cases, such as when fanning is not a result of the intrinsic properties of a material.

Virtual containment system for composite flywheels

NASA Astrophysics Data System (ADS)

Shiue, Fuh-Wen

2001-07-01

There is much interest in advanced composite flywheel systems for use on satellites mainly because of the potential for considerable weight savings associated with combined energy and momentum management. The additional weight of a containment system needed to protect the satellite in the event of a flywheel failure, however, could negate the potential savings. Therefore, the development of a condition monitoring and virtual containment system is essential to ensure the wide acceptance of flywheel batteries for spacecraft applications. A virtual containment system is a near real-time condition monitoring system, plus additional logic to adjust the operating conditions (maximum rotational speed) accordingly when a flaw or fault is detected. Flaws of primary interest in this study are those unique to composite flywheels, such as delamination and debonding of interfaces. Such flaws change the balance state of a flywheel through small, but detectable, motion of the mass center and principal axes of inertia. A proposed monitoring technique determines the existence and the extent of such flaws by a method similar to the influence-coefficient rotor balancing method. Because of the speed-dependence of the imbalance caused by elastic flaws, a normalized imbalance change, which is a direct measure of the flaw size, was defined. To account for the possibility that flaw growth could actually improve the balance state of a rotor, a new concept of accumulated imbalance change was also introduced. Laboratory tests showed the proposed method was able to detect small simulated flaws that result in as little as 2--3 microns of mass center movement. Fracture mechanics concepts were used to evaluate the severity and growth rate of the detected flaw. An interesting discovery that coincided with some experimental observations reported in the literature was the energy release rate reduction with a large crack. This finding indicates a possible stress relief and crack arrest when a circumferential crack grows over certain size. This phenomenon is largely due to crack curvature unique to filament-wound composite flywheels. Several virtual containment strategies were investigated numerically to demonstrate the feasibility of virtual containment systems. Once a flaw is detected during flywheel operation, the maximum operating speed can be reduced to prevent catastrophic failure, achieve a specific design life, and maximize energy storage capacity over the remaining life. A numerical example showed 4--5 times of improvement in cumulative energy storage through lifetime with a virtual containment. A closed-loop speed controller using condition monitoring sensor feedback was investigated numerically to account for possible imperfection of the fracture mechanics model. Finally, an integrated virtual containment system without any complex fracture mechanics analysis was also developed and successfully demonstrated experimentally.

Teaching and learning in a winter wonderland.

PubMed

Tucker, Cheryl A; Bradshaw, Martha J; Ketcham, Nan

2013-01-01

During a record-breaking Texas ice storm, one school of nursing kept accelerated BSN students on schedule despite 5 days of school closure. The students were diverted from hazardous travels to the safety of warm homes with virtual classes and clinicals. The authors discuss their creation of a virtual experience that leveraged smartphones, laptops, eBooks, and Internet resources with existing university technology, allowing students to stay on track.

Self-protected nitrate reducing culture for intrinsic repair of concrete cracks.

PubMed

Erşan, Yusuf Ç; Gruyaert, Elke; Louis, Ghislain; Lors, Christine; De Belie, Nele; Boon, Nico

2015-01-01

Attentive monitoring and regular repair of concrete cracks are necessary to avoid further durability problems. As an alternative to current maintenance methods, intrinsic repair systems which enable self-healing of cracks have been investigated. Exploiting microbial induced CaCO3 precipitation (MICP) using (protected) axenic cultures is one of the proposed methods. Yet, only a few of the suggested healing agents were economically feasible for in situ application. This study presents a [Formula: see text] reducing self-protected enrichment culture as a self-healing additive for concrete. Concrete admixtures Ca(NO3)2 and Ca(HCOO)2 were used as nutrients. The enrichment culture, grown as granules (0.5-2 mm) consisting of 70% biomass and 30% inorganic salts were added into mortar without any additional protection. Upon 28 days curing, mortar specimens were subjected to direct tensile load and multiple cracks (0.1-0.6 mm) were achieved. Cracked specimens were immersed in water for 28 days and effective crack closure up to 0.5 mm crack width was achieved through calcite precipitation. Microbial activity during crack healing was monitored through weekly NOx analysis which revealed that 92 ± 2% of the available [Formula: see text] was consumed. Another set of specimens were cracked after 6 months curing, thus the effect of curing time on healing efficiency was investigated, and mineral formation at the inner crack surfaces was observed, resulting in 70% less capillary water absorption compared to healed control specimens. In conclusion, enriched mixed denitrifying cultures structured in self-protecting granules are very promising strategies to enhance microbial self-healing.

Self-protected nitrate reducing culture for intrinsic repair of concrete cracks

PubMed Central

Erşan, Yusuf Ç.; Gruyaert, Elke; Louis, Ghislain; Lors, Christine; De Belie, Nele; Boon, Nico

2015-01-01

Attentive monitoring and regular repair of concrete cracks are necessary to avoid further durability problems. As an alternative to current maintenance methods, intrinsic repair systems which enable self-healing of cracks have been investigated. Exploiting microbial induced CaCO3 precipitation (MICP) using (protected) axenic cultures is one of the proposed methods. Yet, only a few of the suggested healing agents were economically feasible for in situ application. This study presents a NO3− reducing self-protected enrichment culture as a self-healing additive for concrete. Concrete admixtures Ca(NO3)2 and Ca(HCOO)2 were used as nutrients. The enrichment culture, grown as granules (0.5–2 mm) consisting of 70% biomass and 30% inorganic salts were added into mortar without any additional protection. Upon 28 days curing, mortar specimens were subjected to direct tensile load and multiple cracks (0.1–0.6 mm) were achieved. Cracked specimens were immersed in water for 28 days and effective crack closure up to 0.5 mm crack width was achieved through calcite precipitation. Microbial activity during crack healing was monitored through weekly NOx analysis which revealed that 92 ± 2% of the available NO3− was consumed. Another set of specimens were cracked after 6 months curing, thus the effect of curing time on healing efficiency was investigated, and mineral formation at the inner crack surfaces was observed, resulting in 70% less capillary water absorption compared to healed control specimens. In conclusion, enriched mixed denitrifying cultures structured in self-protecting granules are very promising strategies to enhance microbial self-healing. PMID:26583015

A comparison of fatigue life prediction methodologies for rotorcraft

NASA Technical Reports Server (NTRS)

Everett, R. A., Jr.

1990-01-01

Because of the current U.S. Army requirement that all new rotorcraft be designed to a 'six nines' reliability on fatigue life, this study was undertaken to assess the accuracy of the current safe life philosophy using the nominal stress Palmgrem-Miner linear cumulative damage rule to predict the fatigue life of rotorcraft dynamic components. It has been shown that this methodology can predict fatigue lives that differ from test lives by more than two orders of magnitude. A further objective of this work was to compare the accuracy of this methodology to another safe life method called the local strain approach as well as to a method which predicts fatigue life based solely on crack growth data. Spectrum fatigue tests were run on notched (k(sub t) = 3.2) specimens made of 4340 steel using the Felix/28 tests fairly well, being slightly on the unconservative side of the test data. The crack growth method, which is based on 'small crack' crack growth data and a crack-closure model, also predicted the fatigue lives very well with the predicted lives being slightly longer that the mean test lives but within the experimental scatter band. The crack growth model was also able to predict the change in test lives produced by the rainflow reconstructed spectra.

Degradation in the fatigue crack growth resistance of human dentin by lactic acid

PubMed Central

Orrego, Santiago; Xu, Huakun; Arola, Dwayne

2017-01-01

The oral cavity frequently undergoes localized changes in chemistry and level of acidity, which threatens the integrity of the restorative material and supporting hard tissue. The focus of this study was to evaluate the changes in fatigue crack growth resistance of dentin and toughening mechanisms caused by lactic acid exposure. Compact tension specimens of human dentin were prepared from unrestored molars and subjected to Mode I opening mode cyclic loads. Fatigue crack growth was achieved in samples from mid- and outer-coronal dentin immersed in either a lactic acid solution or neutral conditions. An additional evaluation of the influence of sealing the lumens by dental adhesive was also conducted. A hybrid analysis combining experimental results and finite element modeling quantified the contribution of the toughening mechanisms for both environments. The fatigue crack growth responses showed that exposure to lactic acid caused a significant reduction (p≤0.05) of the stress intensity threshold for cyclic crack extension, and a significant increase (p≤0.05) in the incremental fatigue crack growth rate for both regions of coronal dentin. Sealing the lumens had negligible influence on the fatigue resistance. The hybrid analysis showed that the acidic solution was most detrimental to the extrinsic toughening mechanisms, and the magnitude of crack closure stresses operating in the crack wake. Exposing dentin to acidic environments contributes to the development of caries, but it also increases the chance of tooth fractures via fatigue-related failure and at lower mastication forces. PMID:28183665

The mechanics of delamination in fiber-reinforced composite materials. II - The delamination behavior and fracture mechanics parameters

NASA Technical Reports Server (NTRS)

Wang, S. S.; Choi, I.

1983-01-01

Based on theories of laminate anisotropic elasticity and interlaminar fracture, the complete solution structure associated with a composite delamination is determined. Fracture mechanics parameters characterizing the interlaminar crack behavior are defined from asymptotic stress solutions for delaminations with different crack-tip deformation configurations. A numerical method employing singular finite elements is developed to study delaminations in fiber composites with any arbitrary combinations of lamination, material, geometric, and crack variables. The special finite elements include the exact delamination stress singularity in its formulation. The method is shown to be computationally accurate and efficient, and operationally simple. To illustrate the basic nature of composite delamination, solutions are shown for edge-delaminated (0/-0/-0/0) and (+ or - 0/+ or - 0/90/90 deg) graphite-epoxy systems under uniform axial extension. Three-dimensional crack-tip stress intensity factors, associated energy release rates, and delamination crack-closure are determined for each individual case. The basic mechanics and mechanisms of composite delamination are studied, and fundamental characteristics unique to recently proposed tests for interlaminar fracture toughness of fiber composite laminates are examined. Previously announced in STAR as N84-13222

Oxidation Kinetics and Strength Degradation of Carbon Fibers in a Cracked Ceramic Matrix Composite

NASA Technical Reports Server (NTRS)

Halbig, Michael C.

2003-01-01

Experimental results and oxidation modeling will be presented to discuss carbon fiber susceptibility to oxidation, the oxidation kinetics regimes and composite strength degradation and failure due to oxidation. Thermogravimetric Analysis (TGA) was used to study the oxidation rates of carbon fiber and of a pyro-carbon interphase. The analysis was used to separately obtain activation energies for the carbon constituents within a C/SiC composite. TGA was also conducted on C/SiC composite material to study carbon oxidation and crack closure as a function of temperature. In order to more closely match applications conditions C/SiC tensile coupons were also tested under stressed oxidation conditions. The stressed oxidation tests show that C/SiC is much more susceptible to oxidation when the material is under an applied load where the cracks are open and allow for oxygen ingress. The results help correlate carbon oxidation with composite strength reduction and failure.

Stress-induced, time-dependent fracture closure at hydrothermal conditions

USGS Publications Warehouse

Beeler, N.M.; Hickman, S.H.

2004-01-01

Time-dependent closure of fractures in quartz was measured in situ at 22-530??C temperature and 0.1-150 MPa water pressure. Unlike previous crack healing and rock permeability studies, in this study, fracture aperture is monitored directly and continuously using a windowed pressure vessel, a long-working-distance microscope, and reflected-light interferometry. Thus the fracture volume and geometry can be measured as a function of time, temperature, and water pressure. Relatively uniform closure occurs rapidly at temperatures and pressures where quartz becomes significantly soluble in water. During closure the aperture is reduced by as much as 80% in a few hours. We infer that this closure results from the dissolution of small particles or asperities that prop the fracture open. The driving force for closure via dissolution of the prop is the sum of three chemical potential terms: (1) the dissolution potential, proportional to the logarithm of the degree of undersaturation of the solution; (2) the coarsening potential, proportional to the radius of curvature of the prop; and (3) the pressure solution potential, proportional to the effective normal stress at the contact between propping particles and the fracture wall. Our observations suggest that closure is controlled by a pressure solution-like process. The aperture of dilatant fractures and microcracks in the Earth that are similar to those in our experiments, such as ones generated from thermal stressing or brittle failure during earthquake rupture and slip, will decrease rapidly with time, especially if the macroscopic stress is nonhydrostatic.

Stress-induced, time-dependent fracture closure at hydrothermal conditions

NASA Astrophysics Data System (ADS)

Beeler, N. M.; Hickman, S. H.

2004-02-01

Time-dependent closure of fractures in quartz was measured in situ at 22-530°C temperature and 0.1-150 MPa water pressure. Unlike previous crack healing and rock permeability studies, in this study, fracture aperture is monitored directly and continuously using a windowed pressure vessel, a long-working-distance microscope, and reflected-light interferometry. Thus the fracture volume and geometry can be measured as a function of time, temperature, and water pressure. Relatively uniform closure occurs rapidly at temperatures and pressures where quartz becomes significantly soluble in water. During closure the aperture is reduced by as much as 80% in a few hours. We infer that this closure results from the dissolution of small particles or asperities that prop the fracture open. The driving force for closure via dissolution of the prop is the sum of three chemical potential terms: (1) the dissolution potential, proportional to the logarithm of the degree of undersaturation of the solution; (2) the coarsening potential, proportional to the radius of curvature of the prop; and (3) the pressure solution potential, proportional to the effective normal stress at the contact between propping particles and the fracture wall. Our observations suggest that closure is controlled by a pressure solution-like process. The aperture of dilatant fractures and microcracks in the Earth that are similar to those in our experiments, such as ones generated from thermal stressing or brittle failure during earthquake rupture and slip, will decrease rapidly with time, especially if the macroscopic stress is nonhydrostatic.

Modeling and monitoring of tooth fillet crack growth in dynamic simulation of spur gear set

NASA Astrophysics Data System (ADS)

Guilbault, Raynald; Lalonde, Sébastien; Thomas, Marc

2015-05-01

This study integrates a linear elastic fracture mechanics analysis of the tooth fillet crack propagation into a nonlinear dynamic model of spur gear sets. An original formulation establishes the rigidity of sound and damaged teeth. The formula incorporates the contribution of the flexible gear body and real crack trajectories in the fillet zone. The work also develops a KI prediction formula. A validation of the equation estimates shows that the predicted KI are in close agreement with published numerical and experimental values. The representation also relies on the Paris-Erdogan equation completed with crack closure effects. The analysis considers that during dN fatigue cycles, a harmonic mean of ΔK assures optimal evaluations. The paper evaluates the influence of the mesh frequency distance from the resonances of the system. The obtained results indicate that while the dependence may demonstrate obvious nonlinearities, the crack progression rate increases with a mesh frequency augmentation. The study develops a tooth fillet crack propagation detection procedure based on residual signals (RS) prepared in the frequency domain. The proposed approach accepts any gear conditions as reference signature. The standard deviation and mean values of the RS are evaluated as gear condition descriptors. A trend tracking of their responses obtained from a moving linear regression completes the analysis. Globally, the results show that, regardless of the reference signal, both descriptors are sensitive to the tooth fillet crack and sharply react to tooth breakage. On average, the mean value detected the crack propagation after a size increase of 3.69 percent as compared to the reference condition, whereas the standard deviation required crack progressions of 12.24 percent. Moreover, the mean descriptor shows evolutions closer to the crack size progression.

Three-Dimensional Effects of Crack Closure in Laminated Composite Plates Subjected to Bending Loads

DTIC Science & Technology

1994-06-01

Approved by: •UW. Kwon, Thesis Advisor wathe D.K~elleher, Chairman Department of Mechanical Engineering ii ABSTRACT Fracture is one of the dominant...5 A. OVERVIEW .......................................... 5 B. CONSTITUTIVE EQUATION .............................. 9 1. Isotropic...the elemental nodes. B. CONSTITUTIVE EQUATION The material property matrix [D] is a symmetric matrix which includes elasticity moduli and Poisson’s

Comninou contact zones for a crack parallel to an interface

DOE Office of Scientific and Technical Information (OSTI.GOV)

Joseph, P.F.; Gadi, K.S.; Erdogen, F.

One of the interesting features in studying the state of stress in elastic solids near singular points, is the so called complex singularity that gives rise to an apparent local oscillatory behavior in the stress and displacement fields. The region in which this occurs is very small, much smaller than any plastic zone would be, and therefore the oscillations can be ignored in practical applications. Nevertheless, it is a matter of interesting theoretical investigation. The Comninou model of a small contact zone near the crack tip appears to correct for this anomaly within the framework of the linear theory. Thismore » model seems to make sense out of a {open_quotes}solution{close_quotes} that violates the boundary conditions. Erdogan and Joseph, showed (to themselves anyway) that the Comninou model actually has a physical basis. They considered a crack parallel to an interface where the order of the singularity is always real. With great care in solving the singular integral equations, it was shown that as the crack approaches the interface, a pinching effect is observed at the crack tip. This pinching effect proves that in the limit as the crack approaches the interface, the correct way to handle the problem is to consider crack surface contact. In this way, the issue of {open_quotes}oscillations{close_quotes} is never encountered for the interface crack problem. In the present study, the value of h/a that corresponds to crack closure (zero value of the stress intensity factor) will be determined for a given material pair for tensile loading. An asymptotic numerical method for the solution of singular integral equations making use of is used to obtain this result. Results for the crack opening displacement near the tip of the crack and the behavior of the stress intensity factor for cracks very close to the interface are presented. Among other interesting issues to be discussed, this solution shows that the semi-infinite crack parallel to an interface is closed.« less

Comparison of full 3-D, thin-film 3-D, and thin-film plate analyses of a postbuckled embedded delamination

NASA Technical Reports Server (NTRS)

Whitcomb, John D.

1989-01-01

Strain-energy release rates are often used to predict when delamination growth will occur in laminates under compression. Because of the inherently high computational cost of performing such analyses, less rigorous analyses such as thin-film plate analysis were used. The assumptions imposed by plate theory restrict the analysis to the calculation of total strain energy, G(sub t). The objective is to determine the accuracy of thin-film plate analysis by comparing the distribution of G(sub t) calculated using fully three dimensional (3D), thin-film 3D, and thin-film plate analyses. Thin-film 3D analysis is the same as thin-film plate analysis, except 3D analysis is used to model the sublaminate. The 3D stress analyses were performed using the finite element program NONLIN3D. The plate analysis results were obtained from published data, which used STAGS. Strain-energy release rates were calculated using variations of the virtual crack closure technique. The results demonstrate that thin-film plate analysis can predict the distribution of G(sub t) quite well, at least for the configurations considered. Also, these results verify the accuracy of the strain-energy release rate procedure for plate analysis.

Development of Benchmark Examples for Delamination Onset and Fatigue Growth Prediction

NASA Technical Reports Server (NTRS)

Krueger, Ronald

2011-01-01

An approach for assessing the delamination propagation and growth capabilities in commercial finite element codes was developed and demonstrated for the Virtual Crack Closure Technique (VCCT) implementations in ABAQUS. The Double Cantilever Beam (DCB) specimen was chosen as an example. First, benchmark results to assess delamination propagation capabilities under static loading were created using models simulating specimens with different delamination lengths. For each delamination length modeled, the load and displacement at the load point were monitored. The mixed-mode strain energy release rate components were calculated along the delamination front across the width of the specimen. A failure index was calculated by correlating the results with the mixed-mode failure criterion of the graphite/epoxy material. The calculated critical loads and critical displacements for delamination onset for each delamination length modeled were used as a benchmark. The load/displacement relationship computed during automatic propagation should closely match the benchmark case. Second, starting from an initially straight front, the delamination was allowed to propagate based on the algorithms implemented in the commercial finite element software. The load-displacement relationship obtained from the propagation analysis results and the benchmark results were compared. Good agreements could be achieved by selecting the appropriate input parameters, which were determined in an iterative procedure.

High-fidelity Modeling of Local Effects of Damage for Derated Offshore Wind Turbines

NASA Astrophysics Data System (ADS)

Richards, Phillip W.; Griffith, D. Todd; Hodges, Dewey H.

2014-06-01

Offshore wind power production is an attractive clean energy option, but the difficulty of access can lead to expensive and rare opportunities for maintenance. As part of the Structural Health and Prognostics Management (SHPM) project at Sandia National Laboratories, smart loads management (controls) are investigated for their potential to increase the fatigue life of offshore wind turbine rotor blades. Derating refers to altering the rotor angular speed and blade pitch to limit power production and loads on the rotor blades. High- fidelity analysis techniques like 3D finite element modeling (FEM) should be used alongside beam models of wind turbine blades to characterize these control strategies in terms of their effect to mitigate fatigue damage and extend life of turbine blades. This study will consider a commonly encountered damage type for wind turbine blades, the trailing edge disbond, and show how FEM can be used to quantify the effect of operations and control strategies designed to extend the fatigue life of damaged blades. The Virtual Crack Closure Technique (VCCT) will be used to post-process the displacement and stress results to provide estimates of damage severity/criticality and provide a means to estimate the fatigue life under a given operations and control strategy.

Development and Application of Benchmark Examples for Mixed-Mode I/II Quasi-Static Delamination Propagation Predictions

NASA Technical Reports Server (NTRS)

Krueger, Ronald

2012-01-01

The development of benchmark examples for quasi-static delamination propagation prediction is presented and demonstrated for a commercial code. The examples are based on finite element models of the Mixed-Mode Bending (MMB) specimen. The examples are independent of the analysis software used and allow the assessment of the automated delamination propagation prediction capability in commercial finite element codes based on the virtual crack closure technique (VCCT). First, quasi-static benchmark examples were created for the specimen. Second, starting from an initially straight front, the delamination was allowed to propagate under quasi-static loading. Third, the load-displacement relationship from a propagation analysis and the benchmark results were compared, and good agreement could be achieved by selecting the appropriate input parameters. Good agreement between the results obtained from the automated propagation analysis and the benchmark results could be achieved by selecting input parameters that had previously been determined during analyses of mode I Double Cantilever Beam and mode II End Notched Flexure specimens. The benchmarking procedure proved valuable by highlighting the issues associated with choosing the input parameters of the particular implementation. Overall the results are encouraging, but further assessment for mixed-mode delamination fatigue onset and growth is required.

Enhanced fatigue endurance of metallic glasses through a staircase-like fracture mechanism.

PubMed

Gludovatz, Bernd; Demetriou, Marios D; Floyd, Michael; Hohenwarter, Anton; Johnson, William L; Ritchie, Robert O

2013-11-12

Bulk-metallic glasses (BMGs) are now candidate materials for structural applications due to their exceptional strength and toughness. However, their fatigue resistance can be poor and inconsistent, severely limiting their potential as reliable structural materials. As fatigue limits are invariably governed by the local arrest of microscopically small cracks at microstructural features, the lack of microstructure in monolithic glasses, often coupled with other factors, such as the ease of crack formation in shear bands or a high susceptibility to corrosion, can lead to low fatigue limits (some ~1/20 of their tensile strengths) and highly variable fatigue lives. BMG-matrix composites can provide a solution here as their duplex microstructures can arrest shear bands at a second phase to prevent cracks from exceeding critical size; under these conditions, fatigue limits become comparable with those of crystalline alloys. Here, we report on a Pd-based glass that similarly has high fatigue resistance but without a second phase. This monolithic glass displays high intrinsic toughness from extensive shear-band proliferation with cavitation and cracking effectively obstructed. We find that this property can further promote fatigue resistance through extrinsic crack-tip shielding, a mechanism well known in crystalline metals but not previously reported in BMGs, whereby cyclically loaded cracks propagate in a highly "zig-zag" manner, creating a rough "staircase-like" profile. The resulting crack-surface contact (roughness-induced crack closure) elevates fatigue properties to those comparable to crystalline alloys, and the accompanying plasticity helps to reduce flaw sensitivity in the glass, thereby promoting structural reliability.

Enhanced fatigue endurance of metallic glasses through a staircase-like fracture mechanism

PubMed Central

Gludovatz, Bernd; Demetriou, Marios D.; Floyd, Michael; Hohenwarter, Anton; Johnson, William L.; Ritchie, Robert O.

2013-01-01

Bulk-metallic glasses (BMGs) are now candidate materials for structural applications due to their exceptional strength and toughness. However, their fatigue resistance can be poor and inconsistent, severely limiting their potential as reliable structural materials. As fatigue limits are invariably governed by the local arrest of microscopically small cracks at microstructural features, the lack of microstructure in monolithic glasses, often coupled with other factors, such as the ease of crack formation in shear bands or a high susceptibility to corrosion, can lead to low fatigue limits (some ∼1/20 of their tensile strengths) and highly variable fatigue lives. BMG-matrix composites can provide a solution here as their duplex microstructures can arrest shear bands at a second phase to prevent cracks from exceeding critical size; under these conditions, fatigue limits become comparable with those of crystalline alloys. Here, we report on a Pd-based glass that similarly has high fatigue resistance but without a second phase. This monolithic glass displays high intrinsic toughness from extensive shear-band proliferation with cavitation and cracking effectively obstructed. We find that this property can further promote fatigue resistance through extrinsic crack-tip shielding, a mechanism well known in crystalline metals but not previously reported in BMGs, whereby cyclically loaded cracks propagate in a highly “zig-zag” manner, creating a rough “staircase-like” profile. The resulting crack-surface contact (roughness-induced crack closure) elevates fatigue properties to those comparable to crystalline alloys, and the accompanying plasticity helps to reduce flaw sensitivity in the glass, thereby promoting structural reliability. PMID:24167284

Role of transient water pressure in quarrying: A subglacial experiment using acoustic emissions

USGS Publications Warehouse

Cohen, D.; Hooyer, T.S.; Iverson, N.R.; Thomason, J.F.; Jackson, M.

2006-01-01

Probably the most important mechanism of glacial erosion is quarrying: the growth and coalescence of cracks in subglacial bedrock and dislodgement of resultant rock fragments. Although evidence indicates that erosion rates depend on sliding speed, rates of crack growth in bedrock may be enhanced by changing stresses on the bed caused by fluctuating basal water pressure in zones of ice-bed separation. To study quarrying in real time, a granite step, 12 cm high with a crack in its stoss surface, was installed at the bed of Engabreen, Norway. Acoustic emission sensors monitored crack growth events in the step as ice slid over it. Vertical stresses, water pressure, and cavity height in the lee of the step were also measured. Water was pumped to the lee of the step several times over 8 days. Pumping initially caused opening of a leeward cavity, which then closed after pumping was stopped and water pressure decreased. During cavity closure, acoustic emissions emanating mostly from the vicinity of the base of the crack in the step increased dramatically. With repeated pump tests this crack grew with time until the step's lee surface was quarried. Our experiments indicate that fluctuating water pressure caused stress thresholds required for crack growth to be exceeded. Natural basal water pressure fluctuations should also concentrate stresses on rock steps, increasing rates of crack growth. Stress changes on the bed due to water pressure fluctuations will increase in magnitude and duration with cavity size, which may help explain the effect of sliding speed on erosion rates. Copyright 2006 by the American Geophysical Union.

Simulation of Crack Propagation in Engine Rotating Components under Variable Amplitude Loading

NASA Technical Reports Server (NTRS)

Bonacuse, P. J.; Ghosn, L. J.; Telesman, J.; Calomino, A. M.; Kantzos, P.

1998-01-01

The crack propagation life of tested specimens has been repeatedly shown to strongly depend on the loading history. Overloads and extended stress holds at temperature can either retard or accelerate the crack growth rate. Therefore, to accurately predict the crack propagation life of an actual component, it is essential to approximate the true loading history. In military rotorcraft engine applications, the loading profile (stress amplitudes, temperature, and number of excursions) can vary significantly depending on the type of mission flown. To accurately assess the durability of a fleet of engines, the crack propagation life distribution of a specific component should account for the variability in the missions performed (proportion of missions flown and sequence). In this report, analytical and experimental studies are described that calibrate/validate the crack propagation prediction capability ]or a disk alloy under variable amplitude loading. A crack closure based model was adopted to analytically predict the load interaction effects. Furthermore, a methodology has been developed to realistically simulate the actual mission mix loading on a fleet of engines over their lifetime. A sequence of missions is randomly selected and the number of repeats of each mission in the sequence is determined assuming a Poisson distributed random variable with a given mean occurrence rate. Multiple realizations of random mission histories are generated in this manner and are used to produce stress, temperature, and time points for fracture mechanics calculations. The result is a cumulative distribution of crack propagation lives for a given, life limiting, component location. This information can be used to determine a safe retirement life or inspection interval for the given location.

BACTERIA, BEACHES AND SWIMMABLE WATERS: INTRODUCING VIRTUAL BEACH

EPA Science Inventory

Safe beaches meet water quality standards and are valued for their aesthetics and the recreational opportunities that they afford. In the United States recreational water quality assessments and beach closure decisions are presently based on samples of enterococci or Escherichia ...

The Rejuvenating Effect in Hot Asphalt Recycling by Mortar Transfer Ratio and Image Analysis.

PubMed

Wang, Fusong; Wang, Zipeng; Li, Chao; Xiao, Yue; Wu, Shaopeng; Pan, Pan

2017-05-24

Using a rejuvenator to improve the performance of asphalt pavement is an effective and economic way of hot asphalt recycling. This research analyzes the rejuvenating effect on aged asphalt by means of a Mortar Transfer Ratio (MTR) test, which concerns the ratio of asphalt mortar that moves from recycled aggregates (RAP aggregates) to fresh added aggregates when aged asphalt is treated with a regenerating agent and comes into contact with fresh aggregates. The proposed MTR test analyzes the regeneration in terms of the softening degree on aged asphalt when the rejuvenator is applied. The covered area ratio is studied with an image analyzing tool to understand the possibility of mortar transferring from RAP aggregates to fresh aggregates. Additionally, a micro-crack closure test is conducted and observed through a microscope. The repairing ability and diffusion characteristics of micro-cracks can therefore be analyzed. The test results demonstrate that the proposed mortar transfer ratio is a feasible way to evaluate rejuvenator diffusion during hot recycling. The mortar transfer ratio and uncovered area ratio on fresh aggregates are compatible, and can be used to quantify the contribution of the rejuvenator. Within a certain temperature range, the diffusing effect of the rejuvenator is better when the diffusing temperature is higher. The diffusion time of the rejuvenator is optimum when diffusion occurs for 4-8 h. When the rejuvenator is properly applied, the rough and cracking surface can be repaired, resulting in better covered aggregates. The micro-closure analysis visually indicates that rejuvenators can be used to repair the RAP aggregates during hot recycling.

The Rejuvenating Effect in Hot Asphalt Recycling by Mortar Transfer Ratio and Image Analysis

PubMed Central

Wang, Fusong; Wang, Zipeng; Li, Chao; Xiao, Yue; Wu, Shaopeng; Pan, Pan

2017-01-01

Using a rejuvenator to improve the performance of asphalt pavement is an effective and economic way of hot asphalt recycling. This research analyzes the rejuvenating effect on aged asphalt by means of a Mortar Transfer Ratio (MTR) test, which concerns the ratio of asphalt mortar that moves from recycled aggregates (RAP aggregates) to fresh added aggregates when aged asphalt is treated with a regenerating agent and comes into contact with fresh aggregates. The proposed MTR test analyzes the regeneration in terms of the softening degree on aged asphalt when the rejuvenator is applied. The covered area ratio is studied with an image analyzing tool to understand the possibility of mortar transferring from RAP aggregates to fresh aggregates. Additionally, a micro-crack closure test is conducted and observed through a microscope. The repairing ability and diffusion characteristics of micro-cracks can therefore be analyzed. The test results demonstrate that the proposed mortar transfer ratio is a feasible way to evaluate rejuvenator diffusion during hot recycling. The mortar transfer ratio and uncovered area ratio on fresh aggregates are compatible, and can be used to quantify the contribution of the rejuvenator. Within a certain temperature range, the diffusing effect of the rejuvenator is better when the diffusing temperature is higher. The diffusion time of the rejuvenator is optimum when diffusion occurs for 4–8 h. When the rejuvenator is properly applied, the rough and cracking surface can be repaired, resulting in better covered aggregates. The micro-closure analysis visually indicates that rejuvenators can be used to repair the RAP aggregates during hot recycling. PMID:28772935

Monitoring of pre-release cracks in prestressed concrete using fiber optic sensors

NASA Astrophysics Data System (ADS)

Abdel-Jaber, Hiba; Glisic, Branko

2015-04-01

Prestressed concrete experiences low to no tensile stresses, which results in limiting the occurrence of cracks in prestressed concrete structures. However, the nature of construction of these structures requires the concrete not to be subjected to the compressive force from the prestressing tendons until after it has gained sufficient compressive strength. Although the structure is not subjected to any dead or live load during this period, it is influenced by shrinkage and thermal variations. Thus, the concrete can experience tensile stresses before the required compressive strength has been attained, which can result in the occurrence of "pre-release" cracks. Such cracks are visually closed after the transfer of the prestressing force. However, structural capacity and behavior can be impacted if cracks are not sufficiently closed. This paper researches a method for the verification of the status of pre-release cracks after transfer of the prestressing force, and it is oriented towards achievement of Level IV Structural Health Monitoring (SHM). The method relies on measurements from parallel long-gauge fiber optic sensors embedded in the concrete prior to pouring. The same sensor network is used for the detection and characterization of cracks, as well as the monitoring of the prestressing force transfer and the determination of the extent of closure of pre-release cracks. This paper outlines the researched method and presents its application to a real-life structure, the southeast leg of Streicker Bridge on the Princeton University campus. The application structure is a curved continuous girder that was constructed in 2009. Its deck experienced four pre-release cracks that were closed beyond the critical limits based on the results of this study.

Delamination micromechanics analysis

NASA Technical Reports Server (NTRS)

Adams, D. F.; Mahishi, J. M.

1985-01-01

A three-dimensional finite element analysis was developed which includes elastoplastic, orthotropic material response, and fracture initiation and propagation. Energy absorption due to physical failure processes characteristic of the heterogeneous and anisotropic nature of composite materials is modeled. A local energy release rate in the presence of plasticity was defined and used as a criterion to predict the onset and growth of cracks in both micromechanics and macromechanics analyses. This crack growth simulation technique is based upon a virtual crack extension method. A three-dimensional finite element micromechanics model is used to study the effects of broken fibers, cracked matrix and fiber-matrix debond on the fracture toughness of the unidirectional composite. The energy release rates at the onset of unstable crack growth in the micromechanics analyses are used as critical energy release rates in the macromechanics analysis. This integrated micromechanical and macromechanical fracture criterion is shown to be very effective in predicting the onset and growth of cracks in general multilayered composite laminates by applying the criterion to a single-edge notched graphite/epoxy laminate subjected to implane tension normal to the notch.

Influence of dental restorations and mastication loadings on dentine fatigue behaviour: Image-based modelling approach.

PubMed

Vukicevic, Arso M; Zelic, Ksenija; Jovicic, Gordana; Djuric, Marija; Filipovic, Nenad

2015-05-01

The aim of this study was to use Finite Element Analysis (FEA) to estimate the influence of various mastication loads and different tooth treatments (composite restoration and endodontic treatment) on dentine fatigue. The analysis of fatigue behaviour of human dentine in intact and composite restored teeth with root-canal-treatment using FEA and fatigue theory was performed. Dentine fatigue behaviour was analysed in three virtual models: intact, composite-restored and endodontically-treated tooth. Volumetric change during the polymerization of composite was modelled by thermal expansion in a heat transfer analysis. Low and high shrinkage stresses were obtained by varying the linear shrinkage of composite. Mastication forces were applied occlusally with the load of 100, 150 and 200N. Assuming one million cycles, Fatigue Failure Index (FFI) was determined using Goodman's criterion while residual fatigue lifetime assessment was performed using Paris-power law. The analysis of the Goodman diagram gave both maximal allowed crack size and maximal number of cycles for the given stress ratio. The size of cracks was measured on virtual models. For the given conditions, fatigue-failure is not likely to happen neither in the intact tooth nor in treated teeth with low shrinkage stress. In the cases of high shrinkage stress, crack length was much larger than the maximal allowed crack and failure occurred with 150 and 200N loads. The maximal allowed crack size was slightly lower in the tooth with root canal treatment which induced somewhat higher FFI than in the case of tooth with only composite restoration. Main factors that lead to dentine fatigue are levels of occlusal load and polymerization stress. However, root canal treatment has small influence on dentine fatigue. The methodology proposed in this study provides a new insight into the fatigue behaviour of teeth after dental treatments. Furthermore, it estimates maximal allowed crack size and maximal number of cycles for a specific case. Copyright © 2015 Elsevier Ltd. All rights reserved.

Technical reference for the use of the slow crack growth test for modeling and predicting the long-term performance of polyethylene gas pipes. Final report, March 1987-May 1992. Volume 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kanninen, M.F.; O'Donoghue, P.E.; Popelar, C.F.

1993-02-01

The project was undertaken for the purposes of quantifying the Battelle slow crack growth (SCG) test for predicting long-term performance of polyethylene gas distribution pipes, and of demonstrating the applicability of the methodology for use by the gas industry for accelerated characterization testing, thereby bringing the SCG test development effort to a closure. The work has revealed that the Battelle SCG test, and the linear fracture mechanics interpretation that it currently utilizes, is valid for a class of PE materials. The long-term performance of these materials in various operating conditions can therefore be effectively predicted.

NOWCASTING AND FORECASTING BEACH BACTERIA CONCENTRATION USING THE EPA VIRTUAL BEACH SOFTWARE

EPA Science Inventory

Beaches are subject to closure when bacterial counts exceed water quality criteria. Many authorities base these decisions on sample counts, which typically require a day or more to analyze. Sometimes called the persistence model, because conditions are assumed to persist, experie...

Measures against transmission of pandemic H1N1 influenza in Japan in 2009: simulation model.

PubMed

Yasuda, H; Suzuki, K

2009-11-05

The first outbreak of pandemic H1N1 influenza in Japan was contained in the Kansai region in May 2009 by social distancing measures. Modelling methods are needed to estimate the validity of these measures before their implementation on a large scale. We estimated the transmission coefficient from outbreaks of pandemic H1N1 influenza among school children in Japan in summer 2009; using this transmission coefficient, we simulated the spread of pandemic H1N1 influenza in a virtual community called the virtual Chuo Line which models an area to the west of metropolitan Tokyo. Measures evaluated in our simulation included: isolation at home, school closure, post-exposure prophylaxis and mass vaccinations of school children. We showed that post-exposure prophylaxis combined with isolation at home and school closure significantly decreases the total number of cases in the community and can mitigate the spread of pandemic H1N1 influenza, even when there is a delay in the availability of vaccine.

Evidence of shallow fault zone strengthening after the 1992 M7.5 Landers, California, earthquake

USGS Publications Warehouse

Li, Y.-G.; Vidale, J.E.; Aki, K.; Xu, Fei; Burdette, T.

1998-01-01

Repeated seismic surveys of the Landers, California, fault zone that ruptured in the magnitude (M) 7.5 earthquake of 1992 reveal an increase in seismic velocity with time. P, S, and fault zone trapped waves were excited by near-surface explosions in two locations in 1994 and 1996, and were recorded on two linear, three-component seismic arrays deployed across the Johnson Valley fault trace. The travel times of P and S waves for identical shot-receiver pairs decreased by 0.5 to 1.5 percent from 1994 to 1996, with the larger changes at stations located within the fault zone. These observations indicate that the shallow Johnson Valley fault is strengthening after the main shock, most likely because of closure of cracks that were opened by the 1992 earthquake. The increase in velocity is consistent with the prevalence of dry over wet cracks and with a reduction in the apparent crack density near the fault zone by approximately 1.0 percent from 1994 to 1996.

Use of Several Thermal Analysis Techniques to Study the Cracking of a Nitrile Butadiene Rubber (NBR) Insulator on the Booster Separation Motor (BSM) of the Space Shuttle

NASA Technical Reports Server (NTRS)

Wingard, Charles D.

1999-01-01

Two different vendor rubber formulations have been used to produce the silica-filled NBR insulators for the BSM of each of the two Solid Rocket Boosters (SRBs) on the Space Shuttle. Each cured insulator is bonded to the BSM aluminum aft closure with an epoxy adhesive, and some of the curved areas in the rubber may have significant residual stresses. A number of recently bonded NBR insulators have shown fine surface cracks, and stressed insulator areas may be aging at a faster rate than unstressed areas, thus hastening the surface cracking. Thermal analysis data on both vendor insulators by Dynamic Mechanical Analysis (DMA) through a temperature/frequency sweep from 24 to 74 C have shown a higher flexural storage modulus and Arrhenius activation energy for the stressed area than for the unstressed area. Other thermal analysis techniques are being used to study the insulator surface vs. bulk interior for better understanding this anomaly.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Langton, C.

Concrete containment structures and cement-based fills and waste forms are used at the Savannah River Site to enhance the performance of shallow land disposal systems designed for containment of low-level radioactive waste. Understanding and measuring transport through cracked concrete is important for describing the initial condition of radioactive waste containment structures at the Savannah River Site (SRS) and for predicting performance of these structures over time. This report transmits the results of a literature review on transport through cracked concrete which was performed by Professor Jason Weiss, Purdue University per SRR0000678 (RFP-RQ00001029-WY). This review complements the NRC-sponsored literature review andmore » assessment of factors relevant to performance of grouted systems for radioactive waste disposal. This review was performed by The Center for Nuclear Waste Regulatory Analyses, San Antonio, TX, and The University of Aberdeen, Aberdeen Scotland and was focused on tank closure. The objective of the literature review on transport through cracked concrete was to identify information in the open literature which can be applied to SRS transport models for cementitious containment structures, fills, and waste forms. In addition, the literature review was intended to: (1) Provide a framework for describing and classifying cracks in containment structures and cementitious materials used in radioactive waste disposal, (2) Document the state of knowledge and research related to transport through cracks in concrete for various exposure conditions, (3) Provide information or methodology for answering several specific questions related to cracking and transport in concrete, and (4) Provide information that can be used to design experiments on transport through cracked samples and actual structures.« less

The Role of Organic Proteins on the Crack Growth Resistance of Human Enamel

PubMed Central

Yahyazadehfar, Mobin; Arola, Dwayne

2015-01-01

With only 1% protein by weight, tooth enamel is the most highly mineralized tissue in mammals. The focus of this study was to evaluate contributions of the proteins on the fracture resistance of this unique structural material. Sections of enamel were obtained from the cusps of human molars and the crack growth resistance was quantified using a conventional fracture mechanics approach with complementary finite element analysis. In selected specimens the proteins were extracted using a potassium hydroxide treatment. Removal of the proteins resulted in approximately 40% decrease in the fracture toughness with respect to the fully proteinized control. The loss of organic content was most detrimental to the extrinsic toughening mechanisms, causing over 80% reduction in their contribution to the total energy to fracture. This degradation occurred by embrittlement of the unbroken bridging ligaments and consequent reduction in the crack closure stress. Although the organic content of tooth enamel is very small, it is essential to crack growth toughening by facilitating the formation of unbroken ligaments and in fortifying their potency. Replicating functions of the organic content will be critical to the successful development of bio-inspired materials that are designed for fracture resistance. PMID:25805107

Development of the NASA/FLAGRO computer program for analysis of airframe structures

NASA Technical Reports Server (NTRS)

Forman, R. G.; Shivakumar, V.; Newman, J. C., Jr.

1994-01-01

The NASA/FLAGRO (NASGRO) computer program was developed for fracture control analysis of space hardware and is currently the standard computer code in NASA, the U.S. Air Force, and the European Agency (ESA) for this purpose. The significant attributes of the NASGRO program are the numerous crack case solutions, the large materials file, the improved growth rate equation based on crack closure theory, and the user-friendly promptive input features. In support of the National Aging Aircraft Research Program (NAARP); NASGRO is being further developed to provide advanced state-of-the-art capability for damage tolerance and crack growth analysis of aircraft structural problems, including mechanical systems and engines. The project currently involves a cooperative development effort by NASA, FAA, and ESA. The primary tasks underway are the incorporation of advanced methodology for crack growth rate retardation resulting from spectrum loading and improved analysis for determining crack instability. Also, the current weight function solutions in NASGRO or nonlinear stress gradient problems are being extended to more crack cases, and the 2-d boundary integral routine for stress analysis and stress-intensity factor solutions is being extended to 3-d problems. Lastly, effort is underway to enhance the program to operate on personal computers and work stations in a Windows environment. Because of the increasing and already wide usage of NASGRO, the code offers an excellent mechanism for technology transfer for new fatigue and fracture mechanics capabilities developed within NAARP.

NOWCASTING AND FORECASTING BEACH BACTERIA CONCENTRATIONS USING EPA VIRTUAL BEACH SOFTWARE

EPA Science Inventory

Evidence shows that traditional persistence-based beach closure decision making is inadequate, beaches are closed when they could be open and kept open when they should be closed. Intense interest is now focused on efforts to nowcast beach conditions using surrogate variables, su...

Analytical model for effects of capsule shape on the healing efficiency in self-healing materials

PubMed Central

Li, Songpeng; Chen, Huisu

2017-01-01

The fundamental requirement for the autonomous capsule-based self-healing process to work is that cracks need to reach the capsules and break them such that the healing agent can be released. Ignoring all other aspects, the amount of healing agents released into the crack is essential to obtain a good healing. Meanwhile, from the perspective of the capsule shapes, spherical or elongated capsules (hollow tubes/fibres) are the main morphologies used in capsule-based self-healing materials. The focus of this contribution is the description of the effects of capsule shape on the efficiency of healing agent released in capsule-based self-healing material within the framework of the theory of geometrical probability and integral geometry. Analytical models are developed to characterize the amount of healing agent released per crack area from capsules for an arbitrary crack intersecting with capsules of various shapes in a virtual capsule-based self-healing material. The average crack opening distance is chosen to be a key parameter in defining the healing potential of individual cracks in the models. Furthermore, the accuracy of the developed models was verified by comparison to the data from a published numerical simulation study. PMID:29095862

MEETING IN MEXICO: NOWCASTING AND FORECASTING BEACH BACTERIA CONCENTRATION USING EPA'S VIRTUAL BEACH SOFTWARE

EPA Science Inventory

Beaches in the United States of (North) America are subject to closure when bacterial counts exceed water quality criteria. Many authorities base these decisions on water samples that typically require at least 18 hours to analyze. This persistence approach, or model, often leads...

Validity evidence for procedural competency in virtual reality robotic simulation, establishing a credible pass/fail standard for the vaginal cuff closure procedure.

PubMed

Hovgaard, Lisette Hvid; Andersen, Steven Arild Wuyts; Konge, Lars; Dalsgaard, Torur; Larsen, Christian Rifbjerg

2018-03-30

The use of robotic surgery for minimally invasive procedures has increased considerably over the last decade. Robotic surgery has potential advantages compared to laparoscopic surgery but also requires new skills. Using virtual reality (VR) simulation to facilitate the acquisition of these new skills could potentially benefit training of robotic surgical skills and also be a crucial step in developing a robotic surgical training curriculum. The study's objective was to establish validity evidence for a simulation-based test for procedural competency for the vaginal cuff closure procedure that can be used in a future simulation-based, mastery learning training curriculum. Eleven novice gynaecological surgeons without prior robotic experience and 11 experienced gynaecological robotic surgeons (> 30 robotic procedures) were recruited. After familiarization with the VR simulator, participants completed the module 'Guided Vaginal Cuff Closure' six times. Validity evidence was investigated for 18 preselected simulator metrics. The internal consistency was assessed using Cronbach's alpha and a composite score was calculated based on metrics with significant discriminative ability between the two groups. Finally, a pass/fail standard was established using the contrasting groups' method. The experienced surgeons significantly outperformed the novice surgeons on 6 of the 18 metrics. The internal consistency was 0.58 (Cronbach's alpha). The experienced surgeons' mean composite score for all six repetitions were significantly better than the novice surgeons' (76.1 vs. 63.0, respectively, p < 0.001). A pass/fail standard of 75/100 was established. Four novice surgeons passed this standard (false positives) and three experienced surgeons failed (false negatives). Our study has gathered validity evidence for a simulation-based test for procedural robotic surgical competency in the vaginal cuff closure procedure and established a credible pass/fail standard for future proficiency-based training.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bouchard, P.J.

A forthcoming revision to the R6 Leak-before-Break Assessment Procedure is briefly described. Practical application of the LbB concepts to safety-critical nuclear plant is illustrated by examples covering both low temperature and high temperature (>450{degrees}C) operating regimes. The examples highlight a number of issues which can make the development of a satisfactory LbB case problematic: for example, coping with highly loaded components, methodology assumptions and the definition of margins, the effect of crack closure owing to weld residual stresses, complex thermal stress fields or primary bending fields, the treatment of locally high stresses at crack intersections with free surfaces, the choicemore » of local limit load solution when predicting ligament breakthrough, and the scope of calculations required to support even a simplified LbB case for high temperature steam pipe-work systems.« less

Virtual Crack Extension Technique

DTIC Science & Technology

2008-12-01

the Contractor and the contents do not necessarily have the approval or endorsement of Defence R& D Canada. Contract Report DRDC Atlantic CR 2008-274...the responsibility of the Contractor and the contents do not necessarily have the approval or endorsement of Defence R& D Canada. Defence R& D

A Fruit of Yucca Mountain: The Remote Waste Package Closure System

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kevin Skinner; Greg Housley; Colleen Shelton-Davis

2011-11-01

Was the death of the Yucca Mountain repository the fate of a technical lemon or a political lemon? Without caution, this debate could lure us away from capitalizing on the fruits of the project. In March 2009, Idaho National Laboratory (INL) successfully demonstrated the Waste Package Closure System, a full-scale prototype system for closing waste packages that were to be entombed in the now abandoned Yucca Mountain repository. This article describes the system, which INL designed and built, to weld the closure lids on the waste packages, nondestructively examine the welds using four different techniques, repair the welds if necessary,more » mitigate crack initiating stresses in the surfaces of the welds, evacuate and backfill the packages with an inert gas, and perform all of these tasks remotely. As a nation, we now have a proven method for securely sealing nuclear waste packages for long term storage—regardless of whether or not the future destination for these packages will be an underground repository. Additionally, many of the system’s features and concepts may benefit other remote nuclear applications.« less

Fatigue Crack Growth Characteristics of Thin Sheet Titanium Alloy Ti 6-2-2-2-2

NASA Technical Reports Server (NTRS)

Smith, Stephen W.; Piascik, Robert S.

2001-01-01

Fatigue crack growth rates of Ti 6-2-2-2-2 as a function of stress ratio, temperature (24 or 177 C), tensile orientation and environment (laboratory air or ultrahigh vacuum) are presented. Fatigue crack growth rates of Ti 6-2-2-2-2 are also compared with two more widely used titanium alloys (Timetal 21S and Ti 6Al-4V). The fatigue crack growth rate (da/dN) of Ti 6-2-2-2-2 in laboratory air is dependent upon stress ratio (R), particularly in the near-threshold and lower-Paris regimes. For low R (less than approximately 0.5), da/dN is influenced by crack closure behavior. At higher R (> 0.5), a maximum stress-intensity factor (K(sub max)) dependence is observed. Fatigue crack growth behavior is affected by test temperature between 24 and 177 C. For moderate to high applied cyclic-stress-intensity factors (delta-K), the slope of the log da/dN versus log delta-K curve is lower in 177 C laboratory air than 24 C laboratory air. The difference in slope results in lower values of da/dN for exposure to 177 C laboratory air compared to room temperature laboratory air. The onset of this temperature effect is dependent upon the applied R. This temperature effect has not been observed in ultrahigh vacuum. Specimen orientation has been shown to affect the slope of the log da/dN versus log delta-K curve in the Paris regime.

Proceedings of the DICE THROW Symposium 21-23 June 1977. Volume 3

DTIC Science & Technology

1977-07-01

30-gal. steel oil drum. Rubber -tire hingts and rubber -tire seals wade a snug closure between the door and the upper part of the vertical entry. The...crack or rupture locations which implies that considerable strength degradation Lisociated with the presence of a fastener hole may lead to premature...occur can be partially attributed to the strength degradation of theaircraft structure, resulting from extensive in-service use, as we.l as to the

The role of organic proteins on the crack growth resistance of human enamel.

PubMed

Yahyazadehfar, Mobin; Arola, Dwayne

2015-06-01

With only 1% protein by weight, tooth enamel is the most highly mineralized tissue in mammals. The focus of this study was to evaluate contributions of the proteins on the fracture resistance of this unique structural material. Sections of enamel were obtained from the cusps of human molars and the crack growth resistance was quantified using a conventional fracture mechanics approach with complementary finite element analysis. In selected specimens the proteins were extracted using a potassium hydroxide treatment. Removal of the proteins resulted in approximately 40% decrease in the fracture toughness with respect to the fully proteinized control. The loss of organic content was most detrimental to the extrinsic toughening mechanisms, causing over 80% reduction in their contribution to the total energy to fracture. This degradation occurred by embrittlement of the unbroken bridging ligaments and consequent reduction in the crack closure stress. Although the organic content of tooth enamel is very small, it is essential to crack growth toughening by facilitating the formation of unbroken ligaments and in fortifying their potency. Replicating functions of the organic content will be critical to the successful development of bio-inspired materials that are designed for fracture resistance. Copyright © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Advances in structural damage assessment using strain measurements and invariant shape descriptors

NASA Astrophysics Data System (ADS)

Patki, Amol Suhas

Energy conservation has become one of the most important topic of engineering research over the last couple of decades all around the world and implies reduced energy consumption in order to preserve rapidly depleting natural resources. Along with development of fuel-efficient power plants and technology utilizing alternate fuel to traditional fossil fuels, the design and manufacturing of light-weight energy-efficient structures plays a major role in energy conservation. However this reduction in material and/or weight cannot be achieved at the expense of safety. Thus it is essential to either increase the confidence in the analysis of mechanics of traditional isotropic materials to reduce safety factors or develop new structural materials, such as fiber-reinforced (FRP) polymer matrix composites, which tend to have a higher strength to weight ratio. This doctoral research work will focus on two problems faced by the structural mechanics community viz. effects of closure and overloads on fatigue cracks and structural health monitoring of composites. Fatigue life prediction is largely empirical which in recent years has been shown to be a conservative design model. Investigation of crack growth mechanisms, such as crack closure can lead to design optimization. However, the lack of understanding and accepted theories introduces a degree of uncertainty in such models. Many of the complexity and uncertainty arise from the lack of an experimental technique to quantify crack closure. In this context, this research work offers the most compelling evidence to date of the effects of overload retardation and a confirmation of the Wheeler model using direct experimental observations of the stress field and crack tip plastic zone with the aid of thermoelastic stress analysis. On the other hand, the uncertainties in the post-damage behavior of energy saving FRP-composite materials increase their capital cost and maintenance cost. Damage in isotropic materials tends to be local to the area surrounding the damage, while damage in orthotropic materials tends to have more global repercussions. This calls for analysis of full-field strain distributions adding to the complexity of post-damage life estimation. This study explores shape descriptors used in the field of medical imagery, military targeting and biometric recognition for obtaining a qualitative and quantitative comparison between full-field strain data recorded from damaged composite panels using sophisticated experimental techniques. These descriptors are capable of decomposing images with 103 to 106 pixels into a feature vector with only a few hundred elements. This ability of shape descriptors to achieve enormous reduction in strain data, while providing unique representation, makes them a practical choice for the purpose of structural damage assessment. Consequently, it is relatively easy to statistically compare the shape descriptors of the full-field strain maps using similarity measures rather than the strain maps themselves. However, the wide range of geometric and design features in engineering components pose difficulties in the application of traditional shape description techniques. Thus a new shape descriptor is developed which is applicable to a wide range of specimen geometries. This work also illustrates how shape description techniques can be applied to full-field finite element model validations and updating.

Post-Closure Inspection Report for the Tonopah Test Range, Nevada. For Calendar Year 2015, Revision 0

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matthews, Patrick; Petrello, Jaclyn

This report provides the results of the annual post-closure inspections conducted at the closed corrective action units (CAUs) located on the Tonopah Test Range (TTR), Nevada. This report covers calendar year 2015 and includes inspection and repair activities completed at the following CAUs; CAU 400: Bomblet Pit and Five Points Landfill (TTR); CAU 407: Roller Coaster RadSafe Area (TTR); CAU 424: Area 3 Landfill Complexes (TTR); CAU 453: Area 9 UXO Landfill (TTR); and CAU 487: Thunderwell Site (TTR) Inspections were conducted according to the post-closure plans in the approved closure reports and subsequent correspondence with the Nevada Division ofmore » Environmental Protection. The post-closure inspection plans and subsequent correspondence modifying the requirements for each CAU are included in Appendix B. The inspection checklists are included in Appendix C. Field notes are included in Appendix D. The annual post-closure inspections were conducted on May 12, 2015. Maintenance was required at CAU 453. Cracking along the north trench was repaired. One monument is missing at CAU 424; it will be replaced in 2016. Postings at CAUs 407, 424, 453, and 487 contain contact information for TTR Security. It was noted that protocols may not be in place to ensure that the U.S. Department of Energy, National Nuclear Security Administration Nevada Field Office (NNSA/NFO) is notified if access is needed at these sites. NNSA/NFO is working with the U.S. Air Force and Sandia to determine whether more appropriate contact information or new protocols are warranted for each CAU. Based on these inspections, there has not been a significant change in vegetation, and vegetation monitoring was not recommended at CAU 400 or CAU 407 in 2015.« less

Method for computing energy release rate using the elastic work factor approach

NASA Astrophysics Data System (ADS)

Rhee, K. Y.; Ernst, H. A.

1992-01-01

The elastic work factor eta(el) concept was applied to composite structures for the calculation of total energy release rate by using a single specimen. Cracked lap shear specimens with four different unidirectional fiber orientation were used to examine the dependence of eta(el) on the material properties. Also, three different thickness ratios (lap/strap) were used to determine how geometric conditions affect eta(el). The eta(el) values were calculated in two different ways: compliance method and crack closure method. The results show that the two methods produce comparable eta(el) values and, while eta(el) is affected significantly by geometric conditions, it is reasonably independent of material properties for the given geometry. The results also showed that the elastic work factor can be used to calculate total energy release rate using a single specimen.

A three-dimensional `Kaiser damage-memory' effect through true-triaxial loading

NASA Astrophysics Data System (ADS)

Meredith, P. G.; Browning, J.; Harland, S. R.; Healy, D.; Stuart, C.; Mitchell, T. M.

2017-12-01

Microcrack damage leading to failure in rocks evolves in response to differential loading. The vast majority of experimental studies investigate damage evolution, the `Kaiser damage-memory' effect, and rock failure using conventional triaxial stress states (σ1 > σ2 = σ3). Such stress states develop a crack population that displays cylindrical transverse isotropy. However, in nature the stress state is in general truly triaxial (σ1 > σ2 > σ3) and experiments that utilise such loading conditions can generate crack populations that display planar transverse isotropy which in turn influences properties such as permeability and strength. We investigate the evolution of crack damage under both conventional and true triaxial stress conditions using results from measurements made on cubic samples of sandstone deformed in three orthogonal directions with independently controlled stress paths. We have measured, simultaneously with stress and strain, the changes in ultrasonic compressional and shear wave velocities in the three principal directions, together with the bulk acoustic emission (AE) output. Changes in acoustic wave velocities are associated with both elastic closure and opening of pre-existing cracks, and the inelastic formation of new cracks. By contrast, AE is only associated with the inelastic growth of new crack damage and as such, we use the onset of AE to determine the initiation of new crack damage. By mapping these damage onsets under both conventional triaxial and true triaxial sequential cyclic loading, we have shown that `damage envelopes' evolve dynamically and can be pushed closer to the failure envelope. Whether a stress state has been `visited' before is key to determining and understanding damage states. Crack damage populations can be generated with multiple orientations depending on the arrangement of loading directions and hence principal stress directions. The sequential cyclic loading tests show that further damage in any one population commences only when the previous maximum differential stress `seen' by that population is exceeded. Understanding anisotropic damage is important for applying the results of true-triaxial tests and hence better replicating natural fractured systems.

Review of fatigue and fracture research at NASA Langley Research Center

NASA Technical Reports Server (NTRS)

Everett, Richard A., Jr.

1988-01-01

Most dynamic components in helicopters are designed with a safe-life constant-amplitude testing approach that has not changed in many years. In contrast, the fatigue methodology in other industries has advanced significantly in the last two decades. Recent research at the NASA Langley Research Center and the U.S. Army Aerostructures Directorate at Langley are reviewed relative to fatigue and fracture design methodology for metallic components. Most of the Langley research was directed towards the damage tolerance design approach, but some work was done that is applicable to the safe-life approach. In the areas of testing, damage tolerance concepts are concentrating on the small-crack effect in crack growth and measurement of crack opening stresses. Tests were conducted to determine the effects of a machining scratch on the fatigue life of a high strength steel. In the area of analysis, work was concentrated on developing a crack closure model that will predict fatigue life under spectrum loading for several different metal alloys including a high strength steel that is often used in the dynamic components of helicopters. Work is also continuing in developing a three-dimensional, finite-element stress analysis for cracked and uncracked isotropic and anisotropic structures. A numerical technique for solving simultaneous equations called the multigrid method is being pursued to enhance the solution schemes in both the finite-element analysis and the boundary element analysis. Finally, a fracture mechanics project involving an elastic-plastic finite element analysis of J-resistance curve is also being pursued.

A thermoplastic/thermoset blend exhibiting thermal mending and reversible adhesion.

PubMed

Luo, Xiaofan; Ou, Runqing; Eberly, Daniel E; Singhal, Amit; Viratyaporn, Wantinee; Mather, Patrick T

2009-03-01

In this paper, we report on the development of a new and broadly applicable strategy to produce thermally mendable polymeric materials, demonstrated with an epoxy/poly(-caprolactone) (PCL) phase-separated blend. The initially miscible blend composed of 15.5 wt % PCL undergoes polymerization-induced phase separation during cross-linking of the epoxy, yielding a "bricks and mortar" morphology wherein the epoxy phase exists as interconnected spheres (bricks) interpenetrated with a percolating PCL matrix (mortar). The fully cured material is stiff, strong, and durable. A heating-induced "bleeding" behavior was witnessed in the form of spontaneous wetting of all free surfaces by the molten PCL phase, and this bleeding is capable of repairing damage by crack-wicking and subsequent recrystallization with only minor concomitant softening during that process. The observed bleeding is attributed to volumetric thermal expansion of PCL above its melting point in excess of epoxy brick expansion, which we term differential expansive bleeding (DEB). In controlled thermal-mending experiments, heating of a cracked specimen led to PCL extrusion from the bulk to yield a liquid layer bridging the crack gap. Upon cooling, a "scar" composed of PCL crystals formed at the site of the crack, restoring a significant portion of the mechanical strength. When a moderate force was applied to assist crack closure, thermal-mending efficiencies exceeded 100%. We further observed that the DEB phenomenon enables strong and facile adhesion of the same material to itself and to a variety of materials, without any requirement for macroscopic softening or flow.

Simulation of Complex Cracking in Plain Weave C/SiC Composite under Biaxial Loading

NASA Technical Reports Server (NTRS)

Cheng, Ron-Bin; Hsu, Su-Yuen

2012-01-01

Finite element analysis is performed on a mesh, based on computed geometry of a plain weave C/SiC composite with assumed internal stacking, to reveal the pattern of internal damage due to biaxial normal cyclic loading. The simulation encompasses intertow matrix cracking, matrix cracking inside the tows, and separation at the tow-intertow matrix and tow-tow interfaces. All these dissipative behaviors are represented by traction-separation cohesive laws. Not aimed at quantitatively predicting the overall stress-strain relation, the simulation, however, does not take the actual process of fiber debonding into account. The fiber tows are represented by a simple rule-of-mixture model where the reinforcing phase is a hypothetical one-dimensional material. Numerical results indicate that for the plain weave C/SiC composite, 1) matrix-crack initiation sites are primarily determined by large intertow matrix voids and interlayer tow-tow contacts, 2) the pattern of internal damage strongly depends on the loading path and initial stress, 3) compressive loading inflicts virtually no damage evolution. KEY WORDS: ceramic matrix composite, plain weave, cohesive model, brittle failure, smeared crack model, progressive damage, meso-mechanical analysis, finite element.

Trends in State Charter School Laws: Authorizers, Caps, Performance-Based Closures and Virtual Schools

ERIC Educational Resources Information Center

Christie, Kathy; Millard, Maria; Thomsen, Jennifer; Wixom, Micah

2014-01-01

Forty-two states and the District of Columbia have enacted charter school legislation. The Education Commission of the States (ECS) analysts reviewed laws in the 50 states in creating an online database that highlights how state charter school laws vary, particularly in how states establish standards and accountability for charter school…

A Review of Crack Closure

DTIC Science & Technology

1984-04-01

first by Paris and Erdogan [16]. Paris- Erdogan law ia represented by the equation Sda AAKn1 1 dN where A and n depend on the material and if these are...of I acelate tape , evaporation of gold on it, and the final support of the replica using electrodeposited copper [8,9]. Almost a similar procedure but...AKff for AK ýn the Paris- Erdogan law - see Equations (1) and (2). The applicability of these equations and their ability to account for the effects

Numerical modelling of orthogonal cutting: application to woodworking with a bench plane.

PubMed

Nairn, John A

2016-06-06

A numerical model for orthogonal cutting using the material point method was applied to woodcutting using a bench plane. The cutting process was modelled by accounting for surface energy associated with wood fracture toughness for crack growth parallel to the grain. By using damping to deal with dynamic crack propagation and modelling all contact between wood and the plane, simulations could initiate chip formation and proceed into steady-state chip propagation including chip curling. Once steady-state conditions were achieved, the cutting forces became constant and could be determined as a function of various simulation variables. The modelling details included a cutting tool, the tool's rake and grinding angles, a chip breaker, a base plate and a mouth opening between the base plate and the tool. The wood was modelled as an anisotropic elastic-plastic material. The simulations were verified by comparison to an analytical model and then used to conduct virtual experiments on wood planing. The virtual experiments showed interactions between depth of cut, chip breaker location and mouth opening. Additional simulations investigated the role of tool grinding angle, tool sharpness and friction.

Mechanical weathering and rock erosion by climate-dependent subcritical cracking

NASA Astrophysics Data System (ADS)

Eppes, Martha-Cary; Keanini, Russell

2017-06-01

This work constructs a fracture mechanics framework for conceptualizing mechanical rock breakdown and consequent regolith production and erosion on the surface of Earth and other terrestrial bodies. Here our analysis of fracture mechanics literature explicitly establishes for the first time that all mechanical weathering in most rock types likely progresses by climate-dependent subcritical cracking under virtually all Earth surface and near-surface environmental conditions. We substantiate and quantify this finding through development of physically based subcritical cracking and rock erosion models founded in well-vetted fracture mechanics and mechanical weathering, theory, and observation. The models show that subcritical cracking can culminate in significant rock fracture and erosion under commonly experienced environmental stress magnitudes that are significantly lower than rock critical strength. Our calculations also indicate that climate strongly influences subcritical cracking—and thus rock weathering rates—irrespective of the source of the stress (e.g., freezing, thermal cycling, and unloading). The climate dependence of subcritical cracking rates is due to the chemophysical processes acting to break bonds at crack tips experiencing these low stresses. We find that for any stress or combination of stresses lower than a rock's critical strength, linear increases in humidity lead to exponential acceleration of subcritical cracking and associated rock erosion. Our modeling also shows that these rates are sensitive to numerous other environment, rock, and mineral properties that are currently not well characterized. We propose that confining pressure from overlying soil or rock may serve to suppress subcritical cracking in near-surface environments. These results are applicable to all weathering processes.

A study of Laser Shock Peening on Fatigue behavior of IN718Plus Superalloy: Simulations and Experiments

NASA Astrophysics Data System (ADS)

Chaswal, Vibhor

Laser shock peening (LSP) for improving fatigue life of IN718Plus superalloy is investigated. Fatigue geometry and LSP parameters were optimized using finite element method (FEM). Residual stress distributions estimated by FEM were validated using Synchrotron XRD and line focus lab XRD, and correlated with microhardness. An eigenstrain analysis of LSP induced edge deflections (measured with optical interferometry) was also conducted. Transmission electron microscopy (TEM) of single-spot LSP coupons shows sudden increase in dislocation density under LSP treated region. Total life fatigue was conducted at R=0.1 at 298K and 923K, with and without LSP. S-N curve endurance limit increases at both temperatures with FEM optimized LSP samples. Based on TEM of fatigue microstructure and LSP coupons, a mechanistic description of observed fatigue improvement is attempted. Often need arises to weld components, and weld heat-affected-zone reaches near-solvus temperatures. To simulate this treatment, sub-solvus hot-rolled IN718Plus is aged at 923K. We observe precipitation of thin eta-Ni3(Al, Ti) plates after 1000 hours, making the material susceptible to cracks, and lowering fatigue life. Effect of LSP on fatigue crack growth (FCG) is studied following ASTM guidelines on M(T) geometry at R=0.1. Acceleration in FCG rate with LSP is observed for this geometry and LSP condition. Prior FEM optimization was not conducted for FCG tests, and may account for lower FCG resistance after LSP. FCG results were corroborated with COD compliance based analysis. Crack measurements were done using potential drop method, and crack closure was analyzed. Effect of LSP on overload FCG was investigated by single-cycle 100% overload followed by single-spot LSP on the crack-tip plastic zone. Crack retardation occurs after application of overload+LSP. Effective contribution of overload+LSP to crack retardation is estimated. Fractographic analysis of LSP treated fatigue samples suggests sub-surface crack nucleation, and is analyzed based on stress concentration behavior of small cracks.

Local approach to fatigue based on energy considerations

DOE Office of Scientific and Technical Information (OSTI.GOV)

Milella, P.P.

1996-12-01

The paper presents a development of a fatigue crack growth theory published by the author in 1981 based on an energy approach. In an ideally elastic material containing a crack the only mechanism through which energy can be absorbed during a virtual crack extension is that associated to the creation of new free surface. It is an in-out situation in that a crack of a given length 2a under a stress state {sigma} either becomes unstable or stays like it is. In a real elastic-plastic material the energy absorption rate R comes mainly from the energy stored ahead of themore » crack tip as plastic strain energy. The resistance R is no longer represented by a constant term, but becomes a rather complex function of crack length increasing the crack grows. The consequence is that there is sufficient energy in the system to drive the crack to a point where the driving force G is equal to the resistance R and the crack stops. Unloading the system and reloading it, the crack grows by fatigue indicating that the previous condition G = R is no longer satisfied. If this happens it is because the volume that yields ahead of the crack tip is not capable during the reloading to absorb energy with the same rate as before. This causes the crack to grow further to regain the loss through the yielding of new material and establishes again the equilibrium between G and R. The author has related this lack of capability to develop the same energy absorption rate in any of the following cycles to a shake-down effect that takes place in the plastic enclave. The theory and the equation explain why short cracks shall grow faster than large ones. It also explains why the fatigue crack growth rate depends on the ratio between the minimum and maximum stress and is practically the same in any material independently of the yield stress and toughness that the material may have.« less

Development of Benchmark Examples for Quasi-Static Delamination Propagation and Fatigue Growth Predictions

NASA Technical Reports Server (NTRS)

Krueger, Ronald

2012-01-01

The development of benchmark examples for quasi-static delamination propagation and cyclic delamination onset and growth prediction is presented and demonstrated for Abaqus/Standard. The example is based on a finite element model of a Double-Cantilever Beam specimen. The example is independent of the analysis software used and allows the assessment of the automated delamination propagation, onset and growth prediction capabilities in commercial finite element codes based on the virtual crack closure technique (VCCT). First, a quasi-static benchmark example was created for the specimen. Second, based on the static results, benchmark examples for cyclic delamination growth were created. Third, the load-displacement relationship from a propagation analysis and the benchmark results were compared, and good agreement could be achieved by selecting the appropriate input parameters. Fourth, starting from an initially straight front, the delamination was allowed to grow under cyclic loading. The number of cycles to delamination onset and the number of cycles during delamination growth for each growth increment were obtained from the automated analysis and compared to the benchmark examples. Again, good agreement between the results obtained from the growth analysis and the benchmark results could be achieved by selecting the appropriate input parameters. The benchmarking procedure proved valuable by highlighting the issues associated with choosing the input parameters of the particular implementation. Selecting the appropriate input parameters, however, was not straightforward and often required an iterative procedure. Overall the results are encouraging, but further assessment for mixed-mode delamination is required.

Development of Benchmark Examples for Static Delamination Propagation and Fatigue Growth Predictions

NASA Technical Reports Server (NTRS)

Kruger, Ronald

2011-01-01

The development of benchmark examples for static delamination propagation and cyclic delamination onset and growth prediction is presented and demonstrated for a commercial code. The example is based on a finite element model of an End-Notched Flexure (ENF) specimen. The example is independent of the analysis software used and allows the assessment of the automated delamination propagation, onset and growth prediction capabilities in commercial finite element codes based on the virtual crack closure technique (VCCT). First, static benchmark examples were created for the specimen. Second, based on the static results, benchmark examples for cyclic delamination growth were created. Third, the load-displacement relationship from a propagation analysis and the benchmark results were compared, and good agreement could be achieved by selecting the appropriate input parameters. Fourth, starting from an initially straight front, the delamination was allowed to grow under cyclic loading. The number of cycles to delamination onset and the number of cycles during stable delamination growth for each growth increment were obtained from the automated analysis and compared to the benchmark examples. Again, good agreement between the results obtained from the growth analysis and the benchmark results could be achieved by selecting the appropriate input parameters. The benchmarking procedure proved valuable by highlighting the issues associated with the input parameters of the particular implementation. Selecting the appropriate input parameters, however, was not straightforward and often required an iterative procedure. Overall, the results are encouraging but further assessment for mixed-mode delamination is required.

Evaluation of the split cantilever beam for Mode 3 delamination testing

NASA Technical Reports Server (NTRS)

Martin, Roderick H.

1989-01-01

A test rig for testing a thick split cantilever beam for scissoring delamination (mode 3) fracture toughness was developed. A 3-D finite element analysis was conducted on the test specimen to determine the strain energy release rate, G, distribution along the delamination front. The virtual crack closure technique was used to calculate the G components resulting from interlaminar tension, GI, interlaminar sliding shear, GII, and interlaminar tearing shear, GIII. The finite element analysis showed that at the delamination front no GI component existed, but a GII component was present in addition to a GIII component. Furthermore, near the free edges, the GII component was significantly higher than the GIII component. The GII/GIII ratio was found to increase with delamination length but was insensitive to the beam depth. The presence of GII at the delamination front was verified experimentally by examination of the failure surfaces. At the center of the beam, where the failure was in mode 3, there was significant fiber bridging. However, at the edges of the beam where the failure was in mode 3, there was no fiber bridging and mode 2 shear hackles were observed. Therefore, it was concluded that the split cantilever beam configuration does not represent a pure mode 3 test. The experimental work showed that the mode 2 fracture toughness, GIIc, must be less than the mode 3 fracture toughness, GIIIc. Therefore, a conservative approach to characterizing mode 3 delamination is to equate GIIIc to GIIc.

Skin-Stiffener Debond Prediction Based on Computational Fracture Analysis

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Minguet, Pierre J.; Gates, Tom (Technical Monitor)

2005-01-01

Interlaminar fracture mechanics has proven useful for characterizing the onset of delaminations in composites and has been used with limited success primarily to investigate onset in fracture toughness specimens and laboratory size coupon type specimens. Future acceptance of the methodology by industry and certification authorities however, requires the successful demonstration of the methodology on structural level. For this purpose a panel was selected that is reinforced with stringers. Shear loading causes the panel to buckle and the resulting out-of-plane deformations initiate skin/stringer separation at the location of an embedded defect. For finite element analysis, the panel and surrounding load fixture were modeled with shell elements. A small section of the stringer foot and the panel in the vicinity of the embedded defect were modeled with a local 3D solid model. Across the width of the stringer foot the mixed-mode strain energy release rates were calculated using the virtual crack closure technique. A failure index was calculated by correlating the results with the mixed-mode failure criterion of the graphite/epoxy material. For small applied loads the failure index is well below one across the entire width. With increasing load the failure index approaches one first near the edge of the stringer foot from which delamination is expected to grow. With increasing delamination lengths the buckling pattern of the panel changes and the failure index increases which suggests that rapid delamination growth from the initial defect is to be expected.

Development and Application of Benchmark Examples for Mode II Static Delamination Propagation and Fatigue Growth Predictions

NASA Technical Reports Server (NTRS)

Krueger, Ronald

2011-01-01

The development of benchmark examples for static delamination propagation and cyclic delamination onset and growth prediction is presented and demonstrated for a commercial code. The example is based on a finite element model of an End-Notched Flexure (ENF) specimen. The example is independent of the analysis software used and allows the assessment of the automated delamination propagation, onset and growth prediction capabilities in commercial finite element codes based on the virtual crack closure technique (VCCT). First, static benchmark examples were created for the specimen. Second, based on the static results, benchmark examples for cyclic delamination growth were created. Third, the load-displacement relationship from a propagation analysis and the benchmark results were compared, and good agreement could be achieved by selecting the appropriate input parameters. Fourth, starting from an initially straight front, the delamination was allowed to grow under cyclic loading. The number of cycles to delamination onset and the number of cycles during delamination growth for each growth increment were obtained from the automated analysis and compared to the benchmark examples. Again, good agreement between the results obtained from the growth analysis and the benchmark results could be achieved by selecting the appropriate input parameters. The benchmarking procedure proved valuable by highlighting the issues associated with choosing the input parameters of the particular implementation. Selecting the appropriate input parameters, however, was not straightforward and often required an iterative procedure. Overall the results are encouraging, but further assessment for mixed-mode delamination is required.

A Thermodynamically Consistent Damage Model for Advanced Composites

NASA Technical Reports Server (NTRS)

Maimi, Pere; Camanho, Pedro P.; Mayugo, Joan-Andreu; Davila, Carlos G.

2006-01-01

A continuum damage model for the prediction of damage onset and structural collapse of structures manufactured in fiber-reinforced plastic laminates is proposed. The principal damage mechanisms occurring in the longitudinal and transverse directions of a ply are represented by a damage tensor that is fixed in space. Crack closure under load reversal effects are taken into account using damage variables established as a function of the sign of the components of the stress tensor. Damage activation functions based on the LaRC04 failure criteria are used to predict the different damage mechanisms occurring at the ply level. The constitutive damage model is implemented in a finite element code. The objectivity of the numerical model is assured by regularizing the dissipated energy at a material point using Bazant's Crack Band Model. To verify the accuracy of the approach, analyses of coupon specimens were performed, and the numerical predictions were compared with experimental data.

Ridges on Europa: Origin by Incremental Ice-Wedging

NASA Technical Reports Server (NTRS)

Melosh, H. J.; Turtle, E. P.

2004-01-01

The surface of Europa is covered by ridges that display a variety of morphologies . The most common type is characterized by a double ridge divided by an axial trough. These ridges are, in general, narrow (typically only a few km across) and remarkably linear. They are up to a few hundred meters high and the inner and outer slopes appear to stand at the angle of repose . A number of diverse mechanisms have been proposed to explain the formation of these ubiquitous features , although none can fully account for all of their observed characteristics. We propose a different formation theory in which accumulation of material within cracks that open during the extensional phase of the tidal cycle prevents complete closure of the cracks during the tidal cycle s compressional phase. This accumulation deforms the surrounding ice and, in time, results in the growth of a landform remarkably similar to the ridges observed on Europa.

Prediction Of Formability In Sheet Metal Forming Processes Using A Local Damage Model

NASA Astrophysics Data System (ADS)

Teixeira, P.; Santos, Abel; César Sá, J.; Andrade Pires, F.; Barata da Rocha, A.

2007-05-01

The formability in sheet metal forming processes is mainly conditioned by ductile fracture resulting from geometric instabilities due to necking and strain localization. The macroscopic collapse associated with ductile failure is a result of internal degradation described throughout metallographic observations by the nucleation, growth and coalescence of voids and micro-cracks. Damage influences and is influenced by plastic deformation and therefore these two dissipative phenomena should be coupled at the constitutive level. In this contribution, Lemaitre's ductile damage model is coupled with Hill's orthotropic plasticity criterion. The coupling between damaging and material behavior is accounted for within the framework of Continuum Damage Mechanics (CDM). The resulting constitutive equations are implemented in the Abaqus/Explicit code, for the prediction of fracture onset in sheet metal forming processes. The damage evolution law takes into account the important effect of micro-crack closure, which dramatically decreases the rate of damage growth under compressive paths.

A flexural crack model for damage detection in reinforced concrete structures

NASA Astrophysics Data System (ADS)

Hamad, W. I.; Owen, J. S.; Hussein, M. F. M.

2011-07-01

The use of changes in vibration data for damage detection of reinforced concrete structures faces many challenges that obstruct its transition from a research topic to field applications. Among these is the lack of appropriate damage models that can be deployed in the damage detection methods. In this paper, a model of a simply supported reinforced concrete beam with multiple cracks is developed to examine its use for damage detection and structural health monitoring. The cracks are simulated by a model that accounts for crack formation, propagation and closure. The beam model is studied under different dynamic excitations, including sine sweep and single excitation frequency, for various damage levels. The changes in resonant frequency with increasing loads are examined along with the nonlinear vibration characteristics. The model demonstrates that the resonant frequency reduces by about 10% at the application of 30% of the ultimate load and then drops gradually by about 25% at 70% of the ultimate load. The model also illustrates some nonlinearity in the dynamic response of damaged beams. The appearance of super-harmonics shows that the nonlinearity is higher when the damage level is about 35% and then decreases with increasing damage. The restoring force-displacement relationship predicted the reduction in the overall stiffness of the damaged beam. The model quantitatively predicts the experimental vibration behaviour of damaged RC beams and also shows the damage dependency of nonlinear vibration behaviour.

The Microstructural Evolution of Fatigue Cracks in FCC Metals

NASA Astrophysics Data System (ADS)

Gross, David William

The microstructural evolution during fatigue crack propagation was investigated in a variety of planar and wavy slip FCC metals. The planar materials included Haynes 230, Nitronic 40, and 316 stainless steel, and the wavy materials included pure nickel and pure copper. Three different sets of experiments were performed to fully characterize the microstructural evolution. The first, performed on Haynes 230, mapped the strain field ahead a crack tip using digital image correlation and electron backscatter diffraction techniques. Focused ion beam (FIB) lift-out techniques were then utilized to extract transmission electron microscopy (TEM) samples at specific distances from the crack tip. TEM investigations compared the measured strain to the microstructure. Overall, the strain measured via DIC and EBSD was only weakly correlated to the density of planar slip bands in the microstructure. The second set of experiments concerned the dislocation structure around crack tips. This set of experiments was performed on all the materials. The microstructure at arrested fatigue cracks on the free surface was compared to the microstructure found beneath striations on the fracture surfaces by utilizing FIB micromachining to create site-specific TEM samples. The evolved microstructure depended on the slip type. Strong agreement was found between the crack tip microstructure at the free surface and the fracture surface. In the planar materials, the microstructure in the plastic zone consisted of bands of dislocations or deformation twins, before transitioning to a refined sub-grain microstructure near the crack flank. The sub-grain structure extended 300-500 nm away from the crack flank in all the planar slip materials studied. In contrast, the bulk structure in the wavy slip material consisted of dislocation cells and did not transition to a different microstructure as the crack tip was approached. The strain in wavy slip was highest near the crack tip, as the misorientations between the dislocation cells increased and the cell size decreased as the crack flank was approached. The final set of experiments involved reloading the arrested crack tips in monotonic tension. This was performed on both the Haynes 230 and 316 stainless steel. This technique exposed the fracture surface and location of the arrested crack tip away from the free surface, allowing for a sample to be extracted via FIB micromachining and TEM evaluation of the microstructure. This permitted the crack tip microstructure to be investigated without exposing the microstructure to crack closure or free surface effects. These experiments confirmed what was inferred from the earlier experiments, namely that the banded structure was a product of the crack tip plastic zone and the refined structure was a product of the strain associated with crack advance. Overall the microstructural complexity presented in this work was much higher than would be predicted by current models of fatigue crack propagation. It is recommended that future models attempt to simulate interactions between the dislocations emitted during fatigue crack growth and the pre-existing microstructure to more accurately simulate the processes occurring at the crack tip during crack growth.

A methodology to predict damage initiation, damage growth and residual strength in titanium matrix composites

NASA Technical Reports Server (NTRS)

Bakuckas, J. G., Jr.; Johnson, W. S.

1994-01-01

In this research, a methodology to predict damage initiation, damage growth, fatigue life, and residual strength in titanium matrix composites (TMC) is outlined. Emphasis was placed on micromechanics-based engineering approaches. Damage initiation was predicted using a local effective strain approach. A finite element analysis verified the prevailing assumptions made in the formulation of this model. Damage growth, namely, fiber-bridged matrix crack growth, was evaluated using a fiber bridging (FB) model which accounts for thermal residual stresses. This model combines continuum fracture mechanics and micromechanics analyses yielding stress-intensity factor solutions for fiber-bridged matrix cracks. It is assumed in the FB model that fibers in the wake of the matrix crack are idealized as a closure pressure, and an unknown constant frictional shear stress is assumed to act along the debond length of the bridging fibers. This frictional shear stress was used as a curve fitting parameter to the available experimental data. Fatigue life and post-fatigue residual strength were predicted based on the axial stress in the first intact 0 degree fiber calculated using the FB model and a three-dimensional finite element analysis.

A Specimen Size Effect on the Fatigue Crack Growth Rate Threshold of IN 718

NASA Technical Reports Server (NTRS)

Garr, K. R.; Hresko, G. C., III

1998-01-01

Fatigue crack growth rate (FCGR) tests were conducted on IN 718 in the solution annealed and aged condition at room temperature in accordance with E647-87. As part of each test, the FCGR threshold was measured using the decreasing Delta K method. A new heat of material was being tested and some of this material was sent to a different laboratory which wanted to use a specimen with a 127 mm width. Threshold data previously had been established on specimens with a width of 50.8 mm. As a check of the laboratory, tests were conducted at room temperature and R equal to 0.1 for comparison with the earlier data. The results were a threshold significantly higher than previously observed. Interchanging of specimen sizes and laboratories showed that the results were not due to a heat-to-heat or lab-to-lab variation. The results to be presented here are those obtained at the original laboratory. Growth rates were measured using the electric potential drop technique at R values of 0.1, 0.7, and 0.9. Compact tension specimen sizes with planer dimensions of 25.4 mm, 50.8 mm, and 127 mm were used. Crack growth rates at threshold were generally below 2.5 X 10(exp -8) mm / cycle. Closure measurements were made on some of the specimens by a manual procedure using a clip gage. When the crack growth rate data for the specimens tested at R equal to 0.1 were plotted as a function of applied Delta K, the thresholds varied with specimen width. The larger the width, the higher the threshold. The thresholds varied from 6.5 MPa-m(exp 1/2) for the 25.4 mm specimen to 15.4 MPa-m(exp 1/2) for the 127 mm specimen. At R equal to 0.7, the 25.4 mm and 50.8 mm specimens had essentially the same threshold, about 2.9 MPa-m(exp 1/2)while the 127 mm specimen had a threshold of 4.5 MPa-m(exp 1/2). When plotted as a function of effective Delta K, the R equal to 0.1 data are essentially normalized. Various aspects of the test procedure will be discussed as well as the results of analysis of the data using some different closure models.

High Resolution Integrated Hohlraum-Capsule Simulations for Virtual NIF Ignition Campaign

NASA Astrophysics Data System (ADS)

Jones, O. S.; Marinak, M. M.; Cerjan, C. J.; Clark, D. S.; Edwards, M. J.; Haan, S. W.; Langer, S. H.; Salmonson, J. D.

2009-11-01

We have undertaken a virtual campaign to assess the viability of the sequence of NIF experiments planned for 2010 that will experimentally tune the shock timing, symmetry, and ablator thickness of a cryogenic ignition capsule prior to the first ignition attempt. The virtual campaign consists of two teams. The ``red team'' creates realistic simulated diagnostic data for a given experiment from the output of a detailed radiation hydrodynamics calculation that has physics models that have been altered in a way that is consistent with probable physics uncertainties. The ``blue team'' executes a series of virtual experiments and interprets the simulated diagnostic data from those virtual experiments. To support this effort we have developed a capability to do very high spatial resolution integrated hohlraum-capsule simulations using the Hydra code. Surface perturbations for all ablator layer surfaces and the DT ice layer are calculated explicitly through mode 30. The effects of the fill tube, cracks in the ice layer, and defects in the ablator are included in models extracted from higher resolution calculations. Very high wave number mix is included through a mix model. We will show results from these calculations in the context of the ongoing virtual campaign.

Sensing sheets based on large area electronics for fatigue crack detection

NASA Astrophysics Data System (ADS)

Yao, Yao; Glisic, Branko

2015-03-01

Reliable early-stage damage detection requires continuous structural health monitoring (SHM) over large areas of structure, and with high spatial resolution of sensors. This paper presents the development stage of prototype strain sensing sheets based on Large Area Electronics (LAE), in which thin-film strain gauges and control circuits are integrated on the flexible electronics and deposited on a polyimide sheet that can cover large areas. These sensing sheets were applied for fatigue crack detection on small-scale steel plates. Two types of sensing-sheet interconnects were designed and manufactured, and dense arrays of strain gauge sensors were assembled onto the interconnects. In total, four (two for each design type) strain sensing sheets were created and tested, which were sensitive to strain at virtually every point over the whole sensing sheet area. The sensing sheets were bonded to small-scale steel plates, which had a notch on the boundary so that fatigue cracks could be generated under cyclic loading. The fatigue tests were carried out at the Carleton Laboratory of Columbia University, and the steel plates were attached through a fixture to the loading machine that applied cyclic fatigue load. Fatigue cracks then occurred and propagated across the steel plates, leading to the failure of these test samples. The strain sensor that was close to the notch successfully detected the initialization of fatigue crack and localized the damage on the plate. The strain sensor that was away from the crack successfully detected the propagation of fatigue crack based on the time history of measured strain. Overall, the results of the fatigue tests validated general principles of the strain sensing sheets for crack detection.

The effect of microstructure on the tensile and fatigue behavior of Ti-22Al-23Nb in air and vacuum

NASA Astrophysics Data System (ADS)

Luetjering, Stephanie

Titanium aluminide alloys containing the ordered orthorhombic (O) phase, based on Ti2AlNb, exhibit high specific strengths at elevated temperature along with good room temperature tensile ductility and fracture toughness values. They are thus considered as potential materials for aerospace applications both in their monolithic form and as matrices in metal matrix composites. Microstructure/property relationships have been studied to a great extend with regard to tensile and creep properties. However, only little is known in the key areas of fatigue crack initiation, fatigue crack propagation and fatigue life. The main objective of this work therefore is to get a comprehensive understanding of the effects of microstructural parameters (such as volume fraction of the individual phases, their size and distribution) on the cyclic properties of O-based titanium aluminides. Furthermore, the performance of these alloys in aggressive environments, a critical issue for this alloy class, is being addressed. Tensile, isothermal fatigue, and fatigue crack growth (FCG) tests were conducted at 20°C and 540°C both in lab air and vacuum (pressure ≤ 1 x 10-6 torr) on three microstructural conditions of a representative O-based titanium alloy, Ti-22Al-23Nb. Results indicate a strong effect of microstructure on tensile and FCG properties, whereas only a slight influence of microstructure on the fatigue life is evident. The O phase contributes mainly to the material's yield stress. The tensile elongation is predominantly influenced by the beta phase volume fraction. The observed effect of microstructure on the FCG behavior is attributed to crack closure, crack front geometry and crystallographic texture. Environmental effects on the fatigue life are pronounced at elevated temperature and high applied stress amplitudes only. These conditions lead to premature crack initiation at the specimen's surface for testing in air, whereas testing in vacuum results in subsurface crack nucleation and an extended fatigue life of about two orders of magnitude. The FCG behavior is influenced by the environment at both 20°C and 540°C, proposing fatigue crack growth mechanisms enhanced by hydrogen embrittlement.

Virtual and Traditional Slides for Teaching Cellular Morphology to Medical Laboratory Science Undergraduates: A Comparative Study of Performance Outcomes, Retention, and Self-Efficacy Beliefs

ERIC Educational Resources Information Center

Solberg, Brooke L.

2011-01-01

As a result of massive retirement and educational program expense and closure, the field of Medical Laboratory Science (MLS) is facing a critical workforce shortage. Combatting this issue by increasing undergraduate class size is a difficult proposition due to the intense psychomotor curricular requirements of MLS programs. Technological advances…

SRNL SHELF LIFE STUDIES - SCC STUDIES AT ROOM TEMPERTURE [stress corrosion cracking

DOE Office of Scientific and Technical Information (OSTI.GOV)

Mickalonis, J.; Duffey, J.

Phase II, Series 2 corrosion testing performed by the Savannah River National Laboratory (SRNL) for the Department of Energy 3013 container has been completed. The corrosion tests are part of an integrated plan conducted jointly by Los Alamos National Laboratory and the Savannah River Site. SRNL was responsible for conducting corrosion studies in small-scale vessels to address the influence of salt composition, water loading, and type of oxide/salt contact on the relative humidity inside a 3013 container and on the resulting corrosion of Type 304L and 316L stainless steel (304L and 316L). This testing was conducted in two phases: Phasemore » I evaluated a broad spectrum of salt compositions and initial water loadings on the salt mixtures exposed to 304L and 316L and the resulting corrosion; Phase II evaluated the corrosion of 304L at specific water loadings and a single salt composition. During Phase I testing at high initial moisture levels (0.35 to 1.24 wt%)a, the roomtemperature corrosion of 304L exposed to a series of plutonium oxide/chloride salt mixtures ranged from superficial staining to pitting and stress corrosion cracking (SCC). 304L teardrop coupons that exhibited SCC were directly exposed to a mixture composed of 98 wt % PuO2, 0.9 wt % NaCl, 0.9 wt % KCl, and 0.2 wt % CaCl2. Cracking was not observed in a 316L teardrop coupon. Pitting was also observed in this environment for both 304L and 316L with depths ranging from 20 to 100 μm. Neither pitting nor SCC was observed in mixtures with a greater chloride salt concentration (5 and 28 wt%). These results demonstrated that for a corrosive solution to form a balance existed between the water loading and the salt chloride concentration. This chloride solution results from the interaction of loaded water with the hydrating CaCl2 salt. In Phase II, Series 1 tests, the SCC results were shown to be reproducible with cracking occurring in as little as 85 days. The approximate 0.5 wt% moisture level was found to result in an initial relative humidity of ~55% within the small-scale vessels. Pits were found to be associated with cracks and appeared to act as initiators for the cracking. In a vapor-space only exposure, the weld oxide, which results from the TIG closure weld used to fabricate the teardrop coupon, was also shown to be more susceptible to pitting corrosion than a surface free from weld oxide. This result has important implications for the closure weld of the 3013 inner can since the weld oxide on the can internal surface cannot be removed. The results from the Phase II, Series 2 tests further demonstrated the significance of forming a solution with a critical chloride concentration for corrosion to proceed. 304L teardrop coupons were found to corrode only by pitting with a similar oxide/salt mixture as used in Series 1 testing but with a lower water loading of 0.2 wt%, which resulted in an initial relative humidity of 35-38%. These tests ran twice as long as those for Series 1 testing. The exposure condition was also found to impact the corrosion with salt-exposed surfaces showing lower corrosion resistance. Additional analyses of the Series 2 coupons are recommended especially for determining if cracks emanate from the bottom of pits. Data generated under the 2009 3013 corrosion test plan, as was presented here, increased the understanding of the corrosion process within a sealed 3013 container. Along with the corrosion data from destructive evaluations of 3013 containers, the inner can closure weld region (ICCWR) has been identified as the most vulnerable area of the inner can where corrosion may lead to corrosive species leaking to the interior surface of the outer container, thereby jeopardizing the integrity of the 3013 container. A new corrosion plan has been designed that will characterize the corrosion at the ICCWR of 3013 DEs as well as parameters affecting this corrosion.« less

Initiation and growth of multiple-site damage in the riveted lap joint of a curved stiffened fuselage panel: An experimental and analytical study

NASA Astrophysics Data System (ADS)

Ahmed, Abubaker Ali

As part of the structural integrity research of the National Aging Aircraft Research Program, a comprehensive study on multiple-site damage (MSD) initiation and growth in a pristine lap-joint fuselage panel has been conducted. The curved stiffened fuselage panel was tested at the Full-Scale Aircraft Structural Test Evaluation and Research (FASTER) facility located at the Federal Aviation Administration William J. Hughes Technical Center. A strain survey test was conducted to verify proper load application. The panel was then subjected to a fatigue test with constant-amplitude cyclic loading. The applied loading spectrum included underload marker cycles so that crack growth history could be reconstructed from post-test fractographic examinations. Crack formation and growth were monitored via nondestructive and high-magnification visual inspections. Strain gage measurements recorded during the strain survey tests indicated that the inner surface of the skin along the upper rivet row of the lap joint experienced high tensile stresses due to local bending. During the fatigue loading, cracks were detected by eddy-current inspections at multiple rivet holes along the upper rivet row. Through-thickness cracks were detected visually after about 80% of the fatigue life. Once MSD cracks from two adjacent rivet holes linked up, there was a quick deterioration in the structural integrity of the lap joint. The linkup resulted in a 2.87" (72.9-mm) lead fatigue crack that rapidly propagated across 12 rivet holes and crossed over into the next skin bay, at which stage the fatigue test was terminated. A post-fatigue residual strength test was then conducted by loading the panel quasi-statically up to final failure. The panel failed catastrophically when the crack extended instantaneously across three additional bays. Post-test fractographic examinations of the fracture surfaces in the lap joint of the fuselage panel were conducted to characterize subsurface crack initiation and growth. Results showed evidence of fretting damage and crack initiation at multiple locations near the rivet holes along the faying surface of the skin. The subsurface cracks grew significantly along the faying surface before reaching the outer surface of the skin, forming elliptical crack fronts. A finite element model (FE) of the panel was constructed and geometrically-nonlinear analyses conducted to determine strain distribution under the applied loads. The FE model was validated by comparing the analysis results with the strain gage measurements recorded during the strain survey test. The validated FE model was then used to determine stress-intensity factors at the crack tips. Stress-intensity factor results indicated that crack growth in the lap joint was under mixed-mode; however, the opening-mode stress intensity factor was dominant. The stress-intensity factors computed from the FE analysis were used to conduct cycle-by-cycle integration of fatigue crack growth. In the cycle-by-cycle integration, the NASGRO crack growth model was used with its parameters selected to account for the effects of plasticity-induced crack closure and the test environment on crack growth rate. Fatigue crack growth predictions from cycle-by-cycle computation were in good agreement with the experimental measured crack growth data. The results of the study provide key insights into the natural development and growth of MSD cracks in the pristine lap joint. The data provided by the study represent a valuable source for the evaluation and validation of analytical methodologies used for predicting MSD crack initiation and growth.

Comparing crack damage evolution in rocks deformed under conventional and true triaxial loading

NASA Astrophysics Data System (ADS)

Browning, J.; Meredith, P. G.; Stuart, C.; Healy, D.; Harland, S. R.; Mitchell, T. M.

2017-12-01

The vast majority of experimental studies investigate damage evolution using conventional triaxial stress states (σ1 > σ2 = σ3, CTA), whereas in nature the stress state is generally truly triaxial (σ1 > σ2 > σ3, TTA). We present a comparative study of crack damage evolution during CTA vs. TTA stress conditions using results from measurements made on cubic samples of sandstone deformed in three orthogonal directions with independently controlled stress paths. We have measured, simultaneously with stress and strain, changes in wave velocities in the three principal directions, together with acoustic emission (AE) output. Changes in wave velocities are associated with both elastic closure and opening of pre-existing cracks, and the inelastic formation of new cracks. By contrast, AE is associated only with the inelastic growth of new crack damage. The onset of new damage is shown to be a function of differential stress regardless of the magnitude of mean stress. Hence, we show that damage can form due to a decrease in the minimum principal stress, which reduces mean stress but increases the differential stress. We find an approximately fivefold decrease in the number of AE events in the TTA case in comparison to the CTA case. In essence, we create two end-member crack distributions; one displaying cylindrical transverse isotropy and the other planar transverse isotropy. Taken together, the AE data, the velocities and the crack densities indicate that the intermediate principal stress plays a key role in suppressing the total amount of crack growth and concentrating it in planes sub-parallel to the minimum stress. However, the size of individual cracks remains constant. Hence, the differential stress at which rocks fail (i.e. strength) will be significantly higher under TTA stress (where σ2 > σ3) than under CTA stress (where σ2 = σ3). Cyclic loading tests show that while individual stress states are important, the stress path by which these stress states are reached is equally important. Whether the stress state has been `visited' before is key to determining and understanding damage states. Further damage commences only when the previous maximum differential stress is exceeded, regardless of whether this is achieved by increasing the maximum principal stress or by decreasing the minimum principal stress.

Damage identification via asymmetric active magnetic bearing acceleration feedback control

NASA Astrophysics Data System (ADS)

Zhao, Jie; DeSmidt, Hans; Yao, Wei

2015-04-01

A Floquet-based damage detection methodology for cracked rotor systems is developed and demonstrated on a shaft-disk system. This approach utilizes measured changes in the system natural frequencies to estimate the severity and location of shaft structural cracks during operation. The damage detection algorithms are developed with the initial guess solved by least square method and iterative damage parameter vector by updating the eigenvector updating. Active Magnetic Bearing is introduced to break the symmetric structure of rotor system and the tuning range of proper stiffness/virtual mass gains is studied. The system model is built based on energy method and the equations of motion are derived by applying assumed modes method and Lagrange Principle. In addition, the crack model is based on the Strain Energy Release Rate (SERR) concept in fracture mechanics. Finally, the method is synthesized via harmonic balance and numerical examples for a shaft/disk system demonstrate the effectiveness in detecting both location and severity of the structural damage.

Virtual knotting in proteins and other open curves

NASA Astrophysics Data System (ADS)

Alexander, Keith; Taylor, Alexander; Dennis, Mark

Long filaments naturally knot, from string to long-chain molecules. Knotting in a filament affects its properties, and may be very stable or disappear under slight manipulation. Knotting has been identified in protein backbones for which these mechanical constraints are of fundamental importance to their function, although they are open curves in which knots are not mathematically well defined; knotting can only be identified by closing the ends of the chain. We introduce a new method for resolving knotting in open curves using virtual knots, a wider class of topological objects that do not use a classical closure, capturing the topological ambiguity of open curves. Having analysed all proteins in the Protein Data Bank by this new scheme, we recover and extend previous knotting results, and identify topological interest in some new cases. The statistics of virtual knots in proteins are compared with those of Hamiltonian subchains on cubic lattices, identifying a regime of open curves in which the virtual knotting description is likely to be important. This work was supported by the Leverhulme Trust Programme Grant ``Scientific Properties of Complex Knots'' and the EPSRC.

A quantitative acoustic emission study on fracture processes in ceramics based on wavelet packet decomposition

DOE Office of Scientific and Technical Information (OSTI.GOV)

Ning, J. G.; Chu, L.; Ren, H. L., E-mail: huilanren@bit.edu.cn

2014-08-28

We base a quantitative acoustic emission (AE) study on fracture processes in alumina ceramics on wavelet packet decomposition and AE source location. According to the frequency characteristics, as well as energy and ringdown counts of AE, the fracture process is divided into four stages: crack closure, nucleation, development, and critical failure. Each of the AE signals is decomposed by a 2-level wavelet package decomposition into four different (from-low-to-high) frequency bands (AA{sub 2}, AD{sub 2}, DA{sub 2}, and DD{sub 2}). The energy eigenvalues P{sub 0}, P{sub 1}, P{sub 2}, and P{sub 3} corresponding to these four frequency bands are calculated. Bymore » analyzing changes in P{sub 0} and P{sub 3} in the four stages, we determine the inverse relationship between AE frequency and the crack source size during ceramic fracture. AE signals with regard to crack nucleation can be expressed when P{sub 0} is less than 5 and P{sub 3} more than 60; whereas AE signals with regard to dangerous crack propagation can be expressed when more than 92% of P{sub 0} is greater than 4, and more than 95% of P{sub 3} is less than 45. Geiger location algorithm is used to locate AE sources and cracks in the sample. The results of this location algorithm are consistent with the positions of fractures in the sample when observed under a scanning electronic microscope; thus the locations of fractures located with Geiger's method can reflect the fracture process. The stage division by location results is in a good agreement with the division based on AE frequency characteristics. We find that both wavelet package decomposition and Geiger's AE source locations are suitable for the identification of the evolutionary process of cracks in alumina ceramics.« less

Cracking the egg: virtual embryogenesis of real robots.

PubMed

Cussat-Blanc, Sylvain; Pollack, Jordan

2014-01-01

All multicellular living beings are created from a single cell. A developmental process, called embryogenesis, takes this first fertilized cell down a complex path of reproduction, migration, and specialization into a complex organism adapted to its environment. In most cases, the first steps of the embryogenesis take place in a protected environment such as in an egg or in utero. Starting from this observation, we propose a new approach to the generation of real robots, strongly inspired by living systems. Our robots are composed of tens of specialized cells, grown from a single cell using a bio-inspired virtual developmental process. Virtual cells, controlled by gene regulatory networks, divide, migrate, and specialize to produce the robot's body plan (morphology), and then the robot is manually built from this plan. Because the robot is as easy to assemble as Lego, the building process could be easily automated.

Fracture Capabilities in Grizzly with the extended Finite Element Method (X-FEM)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dolbow, John; Zhang, Ziyu; Spencer, Benjamin

Efforts are underway to develop fracture mechanics capabilities in the Grizzly code to enable it to be used to perform deterministic fracture assessments of degraded reactor pressure vessels (RPVs). A capability was previously developed to calculate three-dimensional interaction- integrals to extract mixed-mode stress-intensity factors. This capability requires the use of a finite element mesh that conforms to the crack geometry. The eXtended Finite Element Method (X-FEM) provides a means to represent a crack geometry without explicitly fitting the finite element mesh to it. This is effected by enhancing the element kinematics to represent jump discontinuities at arbitrary locations inside ofmore » the element, as well as the incorporation of asymptotic near-tip fields to better capture crack singularities. In this work, use of only the discontinuous enrichment functions was examined to see how accurate stress intensity factors could still be calculated. This report documents the following work to enhance Grizzly’s engineering fracture capabilities by introducing arbitrary jump discontinuities for prescribed crack geometries; X-FEM Mesh Cutting in 3D: to enhance the kinematics of elements that are intersected by arbitrary crack geometries, a mesh cutting algorithm was implemented in Grizzly. The algorithm introduces new virtual nodes and creates partial elements, and then creates a new mesh connectivity; Interaction Integral Modifications: the existing code for evaluating the interaction integral in Grizzly was based on the assumption of a mesh that was fitted to the crack geometry. Modifications were made to allow for the possibility of a crack front that passes arbitrarily through the mesh; and Benchmarking for 3D Fracture: the new capabilities were benchmarked against mixed-mode three-dimensional fracture problems with known analytical solutions.« less

Brief communication: Testing the usefulness of the basilar suture as a means to determine age in great ape skeletons.

PubMed

Poe, Demelza J

2011-01-01

A fused/closed basilar suture is usually treated as an indication of old age in great apes. A sample, drawn from a variety of sources, of known-aged captive great ape skeletons was analyzed to test the usefulness of using the basilar suture to categorize adult skeletons as either "adult" or "old adult". The state of closure of the basilar suture was examined in 30 chimpanzees, 19 gorillas, and 15 orangutans, all of known age. The results show that the basilar suture demonstrates a high level of uniformity in rate of closure and is closed at an early age in virtually all known-aged individuals. Thus, an old adult category most likely includes individuals who are, in fact, relatively young. This indicates that using the basilar suture as a means to classify individual skeletons as adult or old adult is very imprecise. The homogenous nature of basilar suture closure appears to prevent meaningful application of suture status for categorizing adult ape skeletons by age groups.

Stress analysis of the cracked-lap-shear specimen - An ASTM round-robin

NASA Technical Reports Server (NTRS)

Johnson, W. S.

1987-01-01

This ASTM Round Robin was conducted to evaluate the state of the art in stress analysis of adhesively bonded joint specimens. Specifically, the participants were asked to calculate the strain-energy-release rate for two different geometry cracked lap shear (CLS) specimens at four different debond lengths. The various analytical techniques consisted of 2- and 3-dimensional finite element analysis, beam theory, plate theory, and a combination of beam theory and finite element analysis. The results were examined in terms of the total strain-energy-release rate and the mode I to mode II ratio as a function of debond length for each specimen geometry. These results basically clustered into two groups: geometric linear or geometric nonlinear analysis. The geometric nonlinear analysis is required to properly analyze the CLS specimens. The 3-D finite element analysis gave indications of edge closure plus some mode III loading. Each participant described his analytical technique and results. Nine laboratories participated.

Stress analysis of the cracked lap shear specimens: An ASTM round robin

NASA Technical Reports Server (NTRS)

Johnson, W. S.

1986-01-01

This ASTM Round Robin was conducted to evaluate the state of the art in stress analysis of adhesively bonded joint specimens. Specifically, the participants were asked to calculate the strain-energy-release rate for two different geometry cracked lap shear (CLS) specimens at four different debond lengths. The various analytical techniques consisted of 2- and 3-dimensional finite element analysis, beam theory, plate theory, and a combination of beam theory and finite element analysis. The results were examined in terms of the total strain-energy-release rate and the mode I to mode II ratio as a function of debond length for each specimen geometry. These results basically clustered into two groups: geometric linear or geometric nonlinear analysis. The geometric nonlinear analysis is required to properly analyze the CLS specimens. The 3-D finite element analysis gave indications of edge closure plus some mode III loading. Each participant described his analytical technique and results. Nine laboratories participated.

Impact of virtual reality simulation on learning barriers of phacoemulsification perceived by residents.

PubMed

Ng, Danny Siu-Chun; Sun, Zihan; Young, Alvin Lerrmann; Ko, Simon Tak-Chuen; Lok, Jerry Ka-Hing; Lai, Timothy Yuk-Yau; Sikder, Shameema; Tham, Clement C

2018-01-01

To identify residents' perceived barriers to learning phacoemulsification surgical procedures and to evaluate whether virtual reality simulation training changed these perceptions. The ophthalmology residents undertook a simulation phacoemulsification course and proficiency assessment on the Eyesi system using the previously validated training modules of intracapsular navigation, anti-tremor, capsulorrhexis, and cracking/chopping. A cross-sectional, multicenter survey on the perceived difficulties in performing phacoemulsification tasks on patients, based on the validated International Council of Ophthalmology's Ophthalmology Surgical Competency Assessment Rubric (ICO-OSCAR), using a 5-point Likert scale (1 = least and 5 = most difficulty), was conducted among residents with or without prior simulation training. Mann-Whitney U tests were carried out to compare the mean scores, and multivariate regression analyses were performed to evaluate the association of lower scores with the following potential predictors: 1) higher level trainee, 2) can complete phacoemulsification most of the time (>90%) without supervisor's intervention, and 3) prior simulation training. The study was conducted in ophthalmology residency training programs in five regional hospitals in Hong Kong. Of the 22 residents, 19 responded (86.3%), of which 13 (68.4%) had completed simulation training. Nucleus cracking/chopping was ranked highest in difficulty by all respondents followed by capsulorrhexis completion and nucleus rotation/manipulation. Respondents with prior simulation training had significantly lower difficulty scores on these three tasks (nucleus cracking/chopping 3.85 vs 4.75, P = 0.03; capsulorrhexis completion 3.31 vs 4.40, P = 0.02; and nucleus rotation/manipulation 3.00 vs 4.75, P = 0.01). In multivariate analyses, simulation training was significantly associated with lower difficulty scores on these three tasks. Residents who had completed Eyesi simulation training had higher confidence in performing the most difficult tasks perceived during phacoemulsification.

Impact of virtual reality simulation on learning barriers of phacoemulsification perceived by residents

PubMed Central

Ng, Danny Siu-Chun; Sun, Zihan; Young, Alvin Lerrmann; Ko, Simon Tak-Chuen; Lok, Jerry Ka-Hing; Lai, Timothy Yuk-Yau; Sikder, Shameema; Tham, Clement C

2018-01-01

Objective To identify residents’ perceived barriers to learning phacoemulsification surgical procedures and to evaluate whether virtual reality simulation training changed these perceptions. Design The ophthalmology residents undertook a simulation phacoemulsification course and proficiency assessment on the Eyesi system using the previously validated training modules of intracapsular navigation, anti-tremor, capsulorrhexis, and cracking/chopping. A cross-sectional, multicenter survey on the perceived difficulties in performing phacoemulsification tasks on patients, based on the validated International Council of Ophthalmology’s Ophthalmology Surgical Competency Assessment Rubric (ICO-OSCAR), using a 5-point Likert scale (1 = least and 5 = most difficulty), was conducted among residents with or without prior simulation training. Mann–Whitney U tests were carried out to compare the mean scores, and multivariate regression analyses were performed to evaluate the association of lower scores with the following potential predictors: 1) higher level trainee, 2) can complete phacoemulsification most of the time (>90%) without supervisor’s intervention, and 3) prior simulation training. Setting The study was conducted in ophthalmology residency training programs in five regional hospitals in Hong Kong. Results Of the 22 residents, 19 responded (86.3%), of which 13 (68.4%) had completed simulation training. Nucleus cracking/chopping was ranked highest in difficulty by all respondents followed by capsulorrhexis completion and nucleus rotation/manipulation. Respondents with prior simulation training had significantly lower difficulty scores on these three tasks (nucleus cracking/chopping 3.85 vs 4.75, P = 0.03; capsulorrhexis completion 3.31 vs 4.40, P = 0.02; and nucleus rotation/manipulation 3.00 vs 4.75, P = 0.01). In multivariate analyses, simulation training was significantly associated with lower difficulty scores on these three tasks. Conclusion Residents who had completed Eyesi simulation training had higher confidence in performing the most difficult tasks perceived during phacoemulsification. PMID:29785084

Face Sheet/Core Disbond Growth in Honeycomb Sandwich Panels Subjected to Ground-Air-Ground Pressurization and In-Plane Loading

NASA Technical Reports Server (NTRS)

Chen, Zhi M.; Krueger, Ronald; Rinker, Martin

2015-01-01

Typical damage modes in light honeycomb sandwich structures include face sheet/core disbonding and core fracture, both of which can pose a threat to the structural integrity of a component. These damage modes are of particular interest to aviation certification authorities since several in-service occurrences, such as rudder structural failure and other control surface malfunctions, have been attributed to face sheet/core disbonding. Extensive studies have shown that face sheet/core disbonding and core fracture can lead to damage propagation caused by internal pressure changes in the core. The increasing use of composite sandwich construction in aircraft applications makes it vitally important to understand the effect of ground-air-ground (GAG) cycles and conditions such as maneuver and gust loads on face sheet/core disbonding. The objective of the present study was to use a fracture mechanics based approach developed earlier to evaluate the loading at the disbond front caused by ground-air-ground pressurization and in-plane loading. A honeycomb sandwich panel containing a circular disbond at one face sheet/core interface was modeled with three-dimensional (3D) solid finite elements. The disbond was modeled as a discrete discontinuity and the strain energy release rate along the disbond front was computed using the Virtual Crack Closure Technique (VCCT). Special attention was paid to the pressure-deformation coupling which can decrease the pressure load within the disbonded sandwich section significantly when the structure is highly deformed. The commercial finite element analysis software, Abaqus/Standard, was used for the analyses. The recursive pressure-deformation coupling problem was solved by representing the entrapped air in the honeycomb cells as filled cavities in Abaqus/Standard. The results show that disbond size, face sheet thickness and core thickness are important parameters that determine crack tip loading at the disbond front. Further, the pressure-deformation coupling was found to have an important load decreasing effect [6]. In this paper, a detailed problem description is provided first. Second, the analysis methodology is presented. The fracture mechanics approach used is described and the specifics of the finite element model, including the fluid-filled cavities, are introduced. Third, the initial model verification and validation are discussed. Fourth, the findings from a closely related earlier study [6] are summarized. These findings provided the basis for the current investigation. Fifth, an aircraft ascent scenario from 0 to 12192 m (0 to 40000 ft) is considered and the resulting crack tip loading at the disbond front is determined. In-plane loading to simulate maneuvers and gust conditions are also considered. Sixth, the results are shown for a curved panel, which was used to simulate potential fuselage applications. Finally, a brief summary of observations is presented and recommendations for improvement are provided.

Proteins analysed as virtual knots

NASA Astrophysics Data System (ADS)

Alexander, Keith; Taylor, Alexander J.; Dennis, Mark R.

2017-02-01

Long, flexible physical filaments are naturally tangled and knotted, from macroscopic string down to long-chain molecules. The existence of knotting in a filament naturally affects its configuration and properties, and may be very stable or disappear rapidly under manipulation and interaction. Knotting has been previously identified in protein backbone chains, for which these mechanical constraints are of fundamental importance to their molecular functionality, despite their being open curves in which the knots are not mathematically well defined; knotting can only be identified by closing the termini of the chain somehow. We introduce a new method for resolving knotting in open curves using virtual knots, which are a wider class of topological objects that do not require a classical closure and so naturally capture the topological ambiguity inherent in open curves. We describe the results of analysing proteins in the Protein Data Bank by this new scheme, recovering and extending previous knotting results, and identifying topological interest in some new cases. The statistics of virtual knots in protein chains are compared with those of open random walks and Hamiltonian subchains on cubic lattices, identifying a regime of open curves in which the virtual knotting description is likely to be important.

Proteins analysed as virtual knots

PubMed Central

Alexander, Keith; Taylor, Alexander J.; Dennis, Mark R.

2017-01-01

Long, flexible physical filaments are naturally tangled and knotted, from macroscopic string down to long-chain molecules. The existence of knotting in a filament naturally affects its configuration and properties, and may be very stable or disappear rapidly under manipulation and interaction. Knotting has been previously identified in protein backbone chains, for which these mechanical constraints are of fundamental importance to their molecular functionality, despite their being open curves in which the knots are not mathematically well defined; knotting can only be identified by closing the termini of the chain somehow. We introduce a new method for resolving knotting in open curves using virtual knots, which are a wider class of topological objects that do not require a classical closure and so naturally capture the topological ambiguity inherent in open curves. We describe the results of analysing proteins in the Protein Data Bank by this new scheme, recovering and extending previous knotting results, and identifying topological interest in some new cases. The statistics of virtual knots in protein chains are compared with those of open random walks and Hamiltonian subchains on cubic lattices, identifying a regime of open curves in which the virtual knotting description is likely to be important. PMID:28205562

Translation of First North American 50 and 70 cc Total Artificial Heart Virtual and Clinical Implantations: Utility of 3D Computed Tomography to Test Fit Devices.

PubMed

Ferng, Alice S; Oliva, Isabel; Jokerst, Clinton; Avery, Ryan; Connell, Alana M; Tran, Phat L; Smith, Richard G; Khalpey, Zain

2017-08-01

Since the creation of SynCardia's 50 cc Total Artificial Hearts (TAHs), patients with irreversible biventricular failure now have two sizing options. Herein, a case series of three patients who have undergone successful 50 and 70 cc TAH implantation with complete closure of the chest cavity utilizing preoperative "virtual implantation" of different sized devices for surgical planning are presented. Computed tomography (CT) images were used for preoperative planning prior to TAH implantation. Three-dimensional (3D) reconstructions of preoperative chest CT images were generated and both 50 and 70 cc TAHs were virtually implanted into patients' thoracic cavities. During the simulation, the TAHs were projected over the native hearts in a similar position to the actual implantation, and the relationship between the devices and the atria, ventricles, chest wall, and diaphragm were assessed. The 3D reconstructed images and virtual modeling were used to simulate and determine for each patient if the 50 or 70 cc TAH would have a higher likelihood of successful implantation without complications. Subsequently, all three patients received clinical implants of the properly sized TAH based on virtual modeling, and their chest cavities were fully closed. This virtual implantation increases our confidence that the selected TAH will better fit within the thoracic cavity allowing for improved surgical outcome. Clinical implantation of the TAHs showed that our virtual modeling was an effective method for determining the correct fit and sizing of 50 and 70 cc TAHs. © 2016 International Center for Artificial Organs and Transplantation and Wiley Periodicals, Inc.

Effect of NaCl Solution Spraying on Fatigue Lives of Smooth and Slit Specimens of 0.37% Carbon Steel

NASA Astrophysics Data System (ADS)

Makabe, Chobin; Ferdous, Md. Shafiul; Shimabukuro, Akimichi; Murdani, Anggit

2017-07-01

The fatigue crack initiation life and growth rate are affected by experimental conditions. A corrosive environment can be created in a laboratory by means of dropping salt water onto the specimen surface, spraying chloride mist into the experimental chamber, etc. In the case of smooth specimens of some metals, fatigue life is shortened and the fatigue limit disappears under such corrosive experimental conditions. In this study, the effects of intermittent spraying of 3% NaCl solution-mist on corrosion fatigue behavior were investigated. The material used was 0.37% carbon steel. This is called JIS S35C in Japan. Spraying of 3% NaCl solution-mist attacked the surface layer of the specimen. It is well known that the pitting, oxidation-reduction reaction, etc. affect the fatigue strength of metals in a corrosive environment. We carried out corrosion fatigue tests with smooth specimens, holed specimens and slit specimens. Then the effects of such specimen geometry on the fatigue strength were investigated when the NaCl solution-mist was sprayed onto the specimen surface. In the case of lower stress amplitude application in slit specimens, the fatigue life in a corrosive atmosphere was longer than that in the open air. It is discussed that the behavior is related to the crack closure which happens when the oxide builds up and clogs the crack or slit.

Strain-energy-release rate analysis of the end-notched flexure specimen using the finite-element method

NASA Technical Reports Server (NTRS)

Salpekar, S. A.; Raju, I. S.; Obrien, T. K.

1987-01-01

Two-dimensional finite-element analysis of the end-notched flexure specimen was performed using 8-node isoparametric, parabolic elements to evaluate compliance and mode II strain energy release rates, G sub II. The G sub II values were computed using two different techniques: the virtural crack-closure technique (VCCT) and the rate of change of compliance with crack length (compliance derivative method). The analysis was performed for various crack-length-to-semi-span (a/L) ratios ranging from 0.2 to 0.9. Three material systems representing a wide range of material properties were analyzed. The compliance and strain energy release rates of the specimen calculated with the present finite-element analysis agree very well with beam theory equations including transverse shear. The G sub II values calculated using the compliance derivative method compared extremely well with those calculated using the VCCT. The G sub II values obtained by the compliance derivative method using the top or bottom beam deflections agreed closely with each other. The strain energy release rates from a plane-stress analysis were higher than the plane-strain values by only a small percentage, indicating that either assumption may be used in the analysis. The G sub II values for one material system calculated from the finite-element analysis agreed with one solution in the literature and disagreed with the other solution in the literature.

Moles: Tool-Assisted Environment Isolation with Closures

NASA Astrophysics Data System (ADS)

de Halleux, Jonathan; Tillmann, Nikolai

Isolating test cases from environment dependencies is often desirable, as it increases test reliability and reduces test execution time. However, code that calls non-virtual methods or consumes sealed classes is often impossible to test in isolation. Moles is a new lightweight framework which addresses this problem. For any .NET method, Moles allows test-code to provide alternative implementations, given as .NET delegates, for which C# provides very concise syntax while capturing local variables in a closure object. Using code instrumentation, the Moles framework will redirect calls to provided delegates instead of the original methods. The Moles framework is designed to work together with the dynamic symbolic execution tool Pex to enable automated test generation. In a case study, testing code programmed against the Microsoft SharePoint Foundation API, we achieved full code coverage while running tests in isolation without an actual SharePoint server. The Moles framework integrates with .NET and Visual Studio.

Exploring sex differences in drug use, health and service use characteristics among young urban crack users in Brazil.

PubMed

Bertoni, Neilane; Burnett, Chantal; Cruz, Marcelo Santos; Andrade, Tarcisio; Bastos, Francisco I; Leal, Erotildes; Fischer, Benedikt

2014-08-28

Studies have shown important gender differences among drug (including crack) users related to: drug use patterns; health risks and consequences; criminal involvement; and service needs/use. Crack use is prevalent in Brazil; however, few comparative data by sex exist. We examined and compared by sex key drug use, health, socio-economic indicators and service use in a bi-city sample of young (18-24 years), regular and marginalized crack users in Brazil. Study participants (total n = 159; n = 124 males and n = 35 females) were recruited by community-based methods from impoverished neighborhoods in Rio de Janeiro and Salvador. Assessments occurred by an anonymous interviewer-administered questionnaire and serum collection for blood-borne virus testing between November 2010 and June 2011. Descriptive statistics and differences for key variables by sex were computed; in addition, a 'chi-squared automatic interaction detector' ('CHAID') analysis explored potential primary factors differentiating male and female participants. Most participants were non-white, and had low education and multiple income sources. More women had unstable housing and income from sex work and/or panhandling/begging, whereas more men were employed. Both groups indicated multi-year histories of and frequent daily crack use, but virtually no drug injection histories. Men reported more co-use of other drugs. More women were: involved in sex-for-drug exchanges; blood-borne virus (BBV) tested and HIV+. Both groups reported similar physical and mental health patterns; however women more commonly utilized social or health services. The CHAID analysis identified sex work; paid work; begging/panhandling; as well as physical and mental health status (all at p < 0.05) as primary differentiating factors by sex. Crack users in our study showed notable differences by sex, including socio-economic indicators, drug co-use patterns, sex risks/work, BBV testing and status, and service utilization. Results emphasize the need for targeted special interventions and services for males and female crack users in Brazil.

Virtual Habitat -a Dynamic Simulation of Closed Life Support Systems -Overall Status and Outlook

NASA Astrophysics Data System (ADS)

Zhukov, Anton; Schnaitmann, Jonas; Mecsaci, Ahmad; Bickel, Thomas; Markus Czupalla, M. Sc.

In order to optimize Life Support Systems (LSS) on a system level, stability questions and closure grade must be investigated. To do so the exploration group of the Technical University of Munich (TUM) is developing the "Virtual Habitat" (V-HAB) dynamic LSS simulation software. The main advantages of the dynamic simulation of LSS within V-HAB are the possibilities to compose different LSS configurations from the LSS subsystems and conduct dynamic simulation of it to test its stability in different mission scenarios inclusive emergency events and define the closure grade of the LSS. Additional the optimization of LSS based on different criteria will be possible. The Virtual Habitat simulation tool consists of four main modules: • Closed Environment Module (CEM) -monitoring of compounds in a closed environment • Crew Module (CM) -dynamic human simulation • P/C Systems Module (PCSM) -dynamic P/C subsystems • Plant Module (PM) -dynamic plant simulation Since the first idea and version, the V-HAB simulation has been significantly updated increasing its capabilities and maturity significantly. The updates which shall be introduced concern all modules of V-HAB. In particular: Significant progress has been made in development of the human model. In addition to the exist-ing human sub-models three newly developed ones (thermal regulation, digestion and schedule controller) have been introduced and shall be presented. Regarding the Plant Module a wheat plant model has been integrated in the V-HAB and is being correlated against test data. Ad-ditionally a first version of the algae bioreactor model has been developed and integrated. In terms of the P/C System module, an innovative approach for the P/C subsystem modelling has been developed and applied. The capabilities and features of the improved V-HAB models and the overall functionality of the V-HAB are demonstrated in form of meaningful test cases. In addition to the presentation of the results, the correlation strategy for the Virtual Habitat simulation shall be introduced assessing the models current confidence level and giving an outlook on the future correlation strategy.

Pressure ulcer: an unreported complication of the Safeguard® hemostasis device. No need to crack under pressure

PubMed Central

Moretti, Claudio; Quadri, Giorgio; Gaita, Fiorenzo; Sheiban, Imad

2011-01-01

Diagnostic cardiac catheterizations are predominantly performed using the femoral artery access. Several devices have been developed to aid in the closure of femoral arteriotomy. Safeguard® is a new pneumatic compression device that has been developed for compression of the femoral artery after brief manual compression. We hereby report the case of an elderly patient who underwent a percutaneous coronary intervention via the femoral artery in whom a Safeguard™ device, left overnight because of persistent oozing, provoked an extensive pressure ulcer. Knowledge of this potential complication is important to minimize its occurance and provide appropriate treatment. PMID:21977303

Pressure ulcer: an unreported complication of the Safeguard® hemostasis device. No need to crack under pressure.

PubMed

Moretti, Claudio; Quadri, Giorgio; Gaita, Fiorenzo; Sheiban, Imad

2011-01-01

Diagnostic cardiac catheterizations are predominantly performed using the femoral artery access. Several devices have been developed to aid in the closure of femoral arteriotomy.Safeguard® is a new pneumatic compression device that has been developed for compression of the femoral artery after brief manual compression. We hereby report the case of an elderly patient who underwent a percutaneous coronary intervention via the femoral artery in whom a Safeguard™ device, left overnight because of persistent oozing, provoked an extensive pressure ulcer. Knowledge of this potential complication is important to minimize its occurance and provide appropriate treatment.

The surface crack problem for a functionally graded coating bonded to a homogeneous layer

NASA Astrophysics Data System (ADS)

Kasmalkar, Maheendra B.

In the continuing search for materials which can withstand the grueling requirements of modern day applications, Functionally Graded Materials (FGMs) seem to be a promising alternative to conventional materials. These nonhomogeneous materials offer better interfacial properties by improving bond strength and reducing thermal mismatch. Before putting these materials into application, an important step in the design of FGMs is the stress analysis and fracture characterization. The fracture performance of FGM coatings on homogeneous substrates is the focus of this study. In this study, various internal and surface crack configurations in the coating and the substrate are subjected to mechanical and thermal loads. The analysis is linear elastic. The thermo-mechanical properties of the FGM coating are assumed to vary exponentially with the spatial coordinate. The equilibrium equations are solved using integral transforms. The resulting singular integral equations are solved using numerical integration. The results of interest for this mode I formulation are the stress intensity factors and the crack opening displacements. The effects of the nonhomogeneity parameter and various dimensionless length parameters are studied. One of the most important outcomes of this study is the theoretical proof that "kink" in material property at the interface does not introduce any singularity. In the numerical results it is observed that generally the stress intensity factors tend to increase with material nonhomogeneity. Also, it is observed that the substrate thickness tends to suppress cracking in the coating. In pure thermal loading, the surface cracks may either be arrested or there might be crack closure. The stress intensity factors from different loadings can be added up to obtain the resultant stress intensity factor for multiple loading. Results in this study have wide-ranging applications. They can be applied to thermal barrier coatings on turbine components, combustion chambers, parts of the airframe for the "Space Plane", soil mechanics, bone fractures and many more applications where the material is macroscopically nonhomogeneous. Thus this study solves a basic problem common to a variety of applications in diverse fields.

The Time Variability of Individual Geysers in the Plume of Enceladus

NASA Astrophysics Data System (ADS)

Trumbo, S. K.; Ewald, S. P.; Ingersoll, A. P.

2016-12-01

Porco et al. (2014) [1] published the locations of 100 jets along the so-called "tiger stripes" that feed the massive plume of Enceladus. Hedman et al. (2013) [2] observed fluctuations in integrated plume brightness in response to periodic tidal forcing on the orbital timescale of Enceladus, in which the plume is brightest near apocenter and dimmest near pericenter. The thin crack models of Hurford et al. (2007, 2012) [3, 4] suggest that individual jets should respond to the same forces on similar timescales. However, if the jets are produced via vapor and liquid propagation through thin subterranean cracks, then they may also be controlled thermodynamically and dependent on the timescale of ice buildup on the conduit walls. Ingersoll and Ewald (2016) [5] demonstrate that the plume also varies on decadal timescales, perhaps as a result of an eleven-year tide or long-term ice accumulation within source cracks. We examine Cassini ISS Narrow Angle Camera images spanning 2005 - 2012 in order to assess the temporal variability of individual geysers and regional emission in the plume. We observe both the appearance and disappearance of individual jets, as well as visible changes in regional emission. Our observations suggest localized variations on timescales of months to years that are not easily tied to mean anomaly, but that may be indicative of subsurface processes. Theoretical models of the geyser mechanisms and subsurface plumbing predict closure timescales of individual cracks that are dependent on model parameters, such as crack width, crack tortuosity, and water table depth [6, 7, 8]. Thus, we discuss possible implications of these observations for both the mechanism and anatomy of an Enceladus geyser. [1] Porco et al. (2014), AJ, 148, 3. [2] Hedman et al. (2013), Nature, 500, 182 - 184. [3] Hurford et al. (2007), Nature, 447, 292 - 294. [4] Hurford et al. (2012), Icarus, 220, 896 - 903. [5] Ingersoll and Ewald (2016), Icarus, in review. [6] Ingersoll and Pankine (2010), Icarus, 206, 594 - 607. [7] Nakajima and Ingersoll (2016), Icarus, 272, 309 - 318. [8] Ingersoll and Nakajima (2016), Icarus, 272, 319 - 326.

Public outreach supports the entire life-cycle of the Ketzin pilot site, Germany

NASA Astrophysics Data System (ADS)

Martens, Sonja; Kollersberger, Tanja; Möller, Fabian; Liebscher, Axel

2017-04-01

Interdisciplinary research at the Ketzin pilot site in Germany contributes to the understanding of the geological CO2 storage since 2004. In addition to the research activities, public outreach has been a key element through the entire life-cycle of the project including site assessment, characterization, development as well as operation (2008-2013) and post-closure. From the very beginning of the project, the research activities were accompanied by an open dialogue with the general public including locals and interested people from all over Germany and the world. The visitor centre at the Ketzin site is run by GFZ and the most important contact point to inform about first-hand experiences from the project. Up to now, about 3,000 visitors came to the Ketzin site for guided tours and the annual open house days. In addition, project status and progress are disseminated in brochures and on the public website www.co2ketzin.de. The Ketzin project is also presented in short films, e.g. on monitoring, drilling and well closure. As the post-closure and pre-transfer phase started after the cease of CO2 injection in August 2013 and the injection facility was dismantled in December 2013, we were looking for a tool to further inform about the previous operation and site infrastructure. A virtual tour was set up for the Ketzin site which is accessible via the website. This tour includes several videos which virtually guide you on site and provide information on the (former) facilities. Public acceptance is a key issue for the Ketzin project as it is for any other CO2 storage project. For example, an open communication with the local residents helped to conduct large-scale seismic campaigns without severe restrictions. The experience from the Ketzin pilot site shows that honest communication and a diverse dissemination program is able to overcome critical public perception even for highly debated technologies.

Strength Restoration of Cracked Sandstone and Coal under a Uniaxial Compression Test and Correlated Damage Source Location Based on Acoustic Emissions.

PubMed

Feng, Xiaowei; Zhang, Nong; Zheng, Xigui; Pan, Dongjiang

2015-01-01

Underground rock masses have shown a general trend of natural balance over billions of years of ground movement. Nonetheless, man-made underground constructions disturb this balance and cause rock stability failure. Fractured rock masses are frequently encountered in underground constructions, and this study aims to restore the strength of rock masses that have experienced considerable fracturing under uniaxial compression. Coal and sandstone from a deep-buried coal mine were chosen as experimental subjects; they were crushed by uniaxial compression and then carefully restored by a chemical adhesive called MEYCO 364 with an innovative self-made device. Finally, the restored specimens were crushed once again by uniaxial compression. Axial stress, axial strain, circumferential strain, and volumetric strain data for the entire process were fully captured and are discussed here. An acoustic emission (AE) testing system was adopted to cooperate with the uniaxial compression system to provide better definitions for crack closure thresholds, crack initiation thresholds, crack damage thresholds, and three-dimensional damage source locations in intact and restored specimens. Several remarkable findings were obtained. The restoration effects of coal are considerably better than those of sandstone because the strength recovery coefficient of the former is 1.20, whereas that of the latter is 0.33, which indicates that MEYCO 364 is particularly valid for fractured rocks whose initial intact peak stress is less than that of MEYCO 364. Secondary cracked traces of restored sandstone almost follow the cracked traces of the initial intact sandstone, and the final failure is mainly caused by decoupling between the adhesive and the rock mass. However, cracked traces of restored coal only partially follow the traces of intact coal, with the final failure of the restored coal being caused by both bonding interface decoupling and self-breakage in coal. Three-dimensional damage source locations manifest such that AE events are highly correlated with a strength recovery coefficient; the AE events show a decreasing tendency when the coefficient is larger than 1, and vice versa. This study provides a feasible scheme for the reinforcement of fractured rock masses in underground constructions and reveals an internal mechanism of the crushing process for restored rock masses, which has certain instructive significance.

Strength Restoration of Cracked Sandstone and Coal under a Uniaxial Compression Test and Correlated Damage Source Location Based on Acoustic Emissions

PubMed Central

Feng, Xiaowei; Zhang, Nong; Zheng, Xigui; Pan, Dongjiang

2015-01-01

Underground rock masses have shown a general trend of natural balance over billions of years of ground movement. Nonetheless, man-made underground constructions disturb this balance and cause rock stability failure. Fractured rock masses are frequently encountered in underground constructions, and this study aims to restore the strength of rock masses that have experienced considerable fracturing under uniaxial compression. Coal and sandstone from a deep-buried coal mine were chosen as experimental subjects; they were crushed by uniaxial compression and then carefully restored by a chemical adhesive called MEYCO 364 with an innovative self-made device. Finally, the restored specimens were crushed once again by uniaxial compression. Axial stress, axial strain, circumferential strain, and volumetric strain data for the entire process were fully captured and are discussed here. An acoustic emission (AE) testing system was adopted to cooperate with the uniaxial compression system to provide better definitions for crack closure thresholds, crack initiation thresholds, crack damage thresholds, and three-dimensional damage source locations in intact and restored specimens. Several remarkable findings were obtained. The restoration effects of coal are considerably better than those of sandstone because the strength recovery coefficient of the former is 1.20, whereas that of the latter is 0.33, which indicates that MEYCO 364 is particularly valid for fractured rocks whose initial intact peak stress is less than that of MEYCO 364. Secondary cracked traces of restored sandstone almost follow the cracked traces of the initial intact sandstone, and the final failure is mainly caused by decoupling between the adhesive and the rock mass. However, cracked traces of restored coal only partially follow the traces of intact coal, with the final failure of the restored coal being caused by both bonding interface decoupling and self-breakage in coal. Three-dimensional damage source locations manifest such that AE events are highly correlated with a strength recovery coefficient; the AE events show a decreasing tendency when the coefficient is larger than 1, and vice versa. This study provides a feasible scheme for the reinforcement of fractured rock masses in underground constructions and reveals an internal mechanism of the crushing process for restored rock masses, which has certain instructive significance. PMID:26714324

Characterization and modeling of three-dimensional self-healing shape memory alloy-reinforced metal-matrix composites

DOE Office of Scientific and Technical Information (OSTI.GOV)

Zhu, Pingping; Cui, Zhiwei; Kesler, Michael S.

In this paper, three-dimensional metal-matrix composites (MMCs) reinforced by shape memory alloy (SMA) wires are modeled and simulated, by adopting an SMA constitutive model accounting for elastic deformation, phase transformation and plastic behavior. A modeling method to create composites with pre-strained SMA wires is also proposed to improve the self-healing ability. Experimental validation is provided with a composite under three-point bending. This modeling method is applied in a series of finite element simulations to investigate the self-healing effects in pre-cracked composites, especially the role of the SMA reinforcement, the softening property of the matrix, and the effect of pre-strain inmore » the SMA. The results demonstrate that SMA reinforcements provide stronger shape recovery ability than other, non-transforming materials. The softening property of the metallic matrix and the pre-strain in SMA are also beneficial to help crack closure and healing. This modeling approach can serve as an efficient tool to design SMA-reinforced MMCs with optimal self-healing properties that have potential applications in components needing a high level of reliability.« less

Analysis of Composite Skin-Stiffener Debond Specimens Using Volume Elements and a Shell/3D Modeling Technique

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Minguet, Pierre J.; Bushnell, Dennis M. (Technical Monitor)

2002-01-01

The debonding of a skin/stringer specimen subjected to tension was studied using three-dimensional volume element modeling and computational fracture mechanics. Mixed mode strain energy release rates were calculated from finite element results using the virtual crack closure technique. The simulations revealed an increase in total energy release rate in the immediate vicinity of the free edges of the specimen. Correlation of the computed mixed-mode strain energy release rates along the delamination front contour with a two-dimensional mixed-mode interlaminar fracture criterion suggested that in spite of peak total energy release rates at the free edge the delamination would not advance at the edges first. The qualitative prediction of the shape of the delamination front was confirmed by X-ray photographs of a specimen taken during testing. The good correlation between prediction based on analysis and experiment demonstrated the efficiency of a mixed-mode failure analysis for the investigation of skin/stiffener separation due to delamination in the adherents. The application of a shell/3D modeling technique for the simulation of skin/stringer debond in a specimen subjected to three-point bending is also demonstrated. The global structure was modeled with shell elements. A local three-dimensional model, extending to about three specimen thicknesses on either side of the delamination front was used to capture the details of the damaged section. Computed total strain energy release rates and mixed-mode ratios obtained from shell/3D simulations were in good agreement with results obtained from full solid models. The good correlations of the results demonstrated the effectiveness of the shell/3D modeling technique for the investigation of skin/stiffener separation due to delamination in the adherents.

A morphologically preserved multi-resolution TIN surface modeling and visualization method for virtual globes

NASA Astrophysics Data System (ADS)

Zheng, Xianwei; Xiong, Hanjiang; Gong, Jianya; Yue, Linwei

2017-07-01

Virtual globes play an important role in representing three-dimensional models of the Earth. To extend the functioning of a virtual globe beyond that of a "geobrowser", the accuracy of the geospatial data in the processing and representation should be of special concern for the scientific analysis and evaluation. In this study, we propose a method for the processing of large-scale terrain data for virtual globe visualization and analysis. The proposed method aims to construct a morphologically preserved multi-resolution triangulated irregular network (TIN) pyramid for virtual globes to accurately represent the landscape surface and simultaneously satisfy the demands of applications at different scales. By introducing cartographic principles, the TIN model in each layer is controlled with a data quality standard to formulize its level of detail generation. A point-additive algorithm is used to iteratively construct the multi-resolution TIN pyramid. The extracted landscape features are also incorporated to constrain the TIN structure, thus preserving the basic morphological shapes of the terrain surface at different levels. During the iterative construction process, the TIN in each layer is seamlessly partitioned based on a virtual node structure, and tiled with a global quadtree structure. Finally, an adaptive tessellation approach is adopted to eliminate terrain cracks in the real-time out-of-core spherical terrain rendering. The experiments undertaken in this study confirmed that the proposed method performs well in multi-resolution terrain representation, and produces high-quality underlying data that satisfy the demands of scientific analysis and evaluation.

Nanowire-templated lateral epitaxial growth of non-polar group III nitrides

DOEpatents

Wang, George T [Albuquerque, NM; Li, Qiming [Albuquerque, NM; Creighton, J Randall [Albuquerque, NM

2010-03-02

A method for growing high quality, nonpolar Group III nitrides using lateral growth from Group III nitride nanowires. The method of nanowire-templated lateral epitaxial growth (NTLEG) employs crystallographically aligned, substantially vertical Group III nitride nanowire arrays grown by metal-catalyzed metal-organic chemical vapor deposition (MOCVD) as templates for the lateral growth and coalescence of virtually crack-free Group III nitride films. This method requires no patterning or separate nitride growth step.

Wettability Investigations and Wet Transfer Enhancement of Large-Area CVD-Graphene on Aluminum Nitride

PubMed Central

Knapp, Marius; Hoffmann, René; Cimalla, Volker; Ambacher, Oliver

2017-01-01

The two-dimensional and virtually massless character of graphene attracts great interest for radio frequency devices, such as surface and bulk acoustic wave resonators. Due to its good electric conductivity, graphene might be an alternative as a virtually massless electrode by improving resonator performance regarding mass-loading effects. We report on an optimization of the commonly used wet transfer technique for large-area graphene, grown via chemical vapor deposition, onto aluminum nitride (AlN), which is mainly used as an active, piezoelectric material for acoustic devices. Today, graphene wet transfer is well-engineered for silicon dioxide (SiO2). Investigations on AlN substrates reveal highly different surface properties compared to SiO2 regarding wettability, which strongly influences the quality of transferred graphene monolayers. Both physical and chemical effects of a plasma treatment of AlN surfaces change wettability and avoid large-scale cracks in the transferred graphene sheet during desiccation. Spatially-resolved Raman spectroscopy reveals a strong strain and doping dependence on AlN plasma pretreatments correlating with the electrical conductivity of graphene. In our work, we achieved transferred crack-free large-area (40 × 40 mm2) graphene monolayers with sheet resistances down to 350 Ω/sq. These achievements make graphene more powerful as an eco-friendly and cheaper replacement for conventional electrode materials used in radio frequency resonator devices. PMID:28820462

Judging the 'passability' of dynamic gaps in a virtual rugby environment.

PubMed

Watson, Gareth; Brault, Sebastien; Kulpa, Richard; Bideau, Benoit; Butterfield, Joe; Craig, Cathy

2011-10-01

Affordances have recently been proposed as a guiding principle in perception-action research in sport (Fajen, Riley, & Turvey, 2009). In the present study, perception of the 'passability' affordance of a gap between two approaching defenders in rugby is explored. A simplified rugby gap closure scenario was created using immersive, interactive virtual reality technology where 14 novice participants (attacker) judged the passability of the gap between two virtual defenders via a perceptual judgment (button press) task. The scenario was modeled according to tau theory (Lee, 1976) and a psychophysical function was fitted to the response data. Results revealed that a tau-based informational quantity could account for 82% of the variance in the data. Findings suggest that the passability affordance in this case, is defined by this variable and participants were able to use it in order to inform prospective judgments as to passability. These findings contribute to our understanding of affordances and how they may be defined in this particular sporting scenario; however, some limitations regarding methodology, such as decoupling perception and action are also acknowledged. Copyright © 2010 Elsevier B.V. All rights reserved.

Damage and cracking of synthetic and natural glasses subjected to triaxial deformation

NASA Astrophysics Data System (ADS)

Ougier-Simonin, Audrey; Fortin, Jérôme; Guéguen, Yves; Schubnel, Alexandre; Bouyer, Frédéric

2010-05-01

Glass is an ideal elastic-brittle material. Although cracking in glass has been much investigated, going back to the pioneer work of Griffith, investigations under confining pressure have not been done so far. Besides, as glass results of the solidification of variable fused silicate mix, the impact of thermal cracking in this material cannot be neglected. Our study aims at investigating thermo-mechanical cracking effects on elastic wave velocities and mechanical strength, both under pressure, to document damage evolution on glass. We performed the experiments on a triaxial cell at room temperature, with and without pore fluid pressure, on borosilicate glass. The crack evolution has been monitored with: (i) elastic wave velocity measurements and (ii) acoustic emissions (MiniRichter system). We also measured the global mechanical behavior of our synthetic glass samples with strain gages. The original glass, produced in ideal conditions of slow cooling that prevent from any crack formation, exhibits a linear and reversible mechanical behavior and isotropic elastic velocities, as expected. It also presents a high strength as it fails at about 700 MPa of deviatoric stress for a confining pressure of 15 MPa. The damage develops progressively, with increasing acoustic emission rate, parallel to the deviatoric stress orientation and probably starts on the rare air bubbles trapped in the amorphous matrix. We choose to apply to some original glass samples a reproducible method (thermal treatment with a thermal shock of ?T = 100, 200 and 300°C) which creates cracks with a homogeneous distribution. The impact of the thermal treatment is clearly visible through the elastic wave velocity measurements as we observe crack closure under hydrostatic conditions (at about 30 MPa). Anisotropy is also observed for increasing deviatoric stress. For ?T higher than 200°C, the glass mechanical behavior becomes non linear and records an irreversible damage. The total damage observed with the acoustic emissions in these samples underlines the combination of the thermal and the mechanical cracks which drive to the sample failure. The preliminary results obtained with pore fluid pressure show a very small permeability even for a high damage level (10-21 ≤ φ ≤ 10-17). However, the glass amorphous structure makes it very different from any rock structure. In order to quantify these differences and to compare glass to rock, we managed to find a micro-crystallized basalt (Seljadur basalt, Iceland) with very low porosity (k ≤ 2%) and close chemical composition, and studied its behavior in the same experimental conditions. We show that a micro-crystallized rock remains different from a glass in terms of mechanical behavior but exhibits dynamical elastic parameters close from the glass ones.

The urethral closure function in continent and stress urinary incontinent women assessed by urethral pressure reflectometry.

PubMed

Saaby, Marie-Louise

2014-02-01

Stress urinary incontinence (SUI) occurs when the bladder pressure exceeds the urethral pressure in connection with physical effort or exertion or when sneezing or coughing and depends both on the strength of the urethral closure function and the abdominal pressure to which it is subjected. The urethral closure function in continent women and the dysfunction causing SUI are not known in details. The currently accepted view is based on the concept of a sphincteric unit and a support system. Our incomplete knowledge relates to the complexity of the closure apparatus and to inadequate assessment methods which so far have not provided robust urodynamic diagnostic tools, severity measures, or parameters to assess outcome after intervention. Urethral Pressure Reflectometry (UPR) is a novel method that measures the urethral pressure and cross-sectional area (by use of sound waves) simultaneously. The technique involves insertion of only a small, light and flexible polyurethane bag in the urethra and therefore avoids the common artifacts encountered with conventional methods. The UPR parameters can be obtained at a specific site of the urethra, e.g. the high pressure zone, and during various circumstances, i.e. resting and squeezing. During the study period, we advanced the UPR technique to enable faster measurement (within 7 seconds by the continuous technique) which allowed assessment during increased intra-abdominal pressure induced by physical straining. We investigated the urethral closure function in continent and SUI women during resting and straining by the "fast" UPR technique. Thereby new promising urethral parameters were provided that allowed characterization of the closure function based on the permanent closure forces (primarily generated by the sphincteric unit, measured by the Po-rest) and the adjunctive closure forces (primarily generated by the support system, measured by the abdominal to urethral pressure impact ratio (APIR)). The new parameters enabled a more detailed description of the efficiency of the closure function and the extent and nature of a possible dysfunction in the individual woman. The urethral closure equation (UCE) and urethral opening pressure at an abdominal pressure of 50 cm H2O (Po-Abd 50), respectively, which combine the permanent and the adjunctive closure forces, could separate continent and SUI women and thus appear to be excellent diagnostic tests. Moreover, the parameters showed highly significant negative correlation with ICIQ-SF, pad test and the number of incontinence episodes per week and are therefore valid as urodynamic severity measures. UPR in SUI women before and after TVT demonstrated a more efficient urethral closure function after the operation. The Po-rest was unchanged suggesting that the sphincteric unit was virtually unaltered and hence the permanent closure forces unchanged. However, the resting opening elastance increased by 18% indicating that at the resting state the TVT somewhat improves the closure function by providing increased resistance against the dilation of the urethra, which probably explains the decreased maximum urine flow rate found after TVT in this and previous studies. The APIR increased in all patients after TVT suggesting that the support system was re-established and thus the adjunctive closure forces improved, regardless of the type of pre-operative dysfunction. The new UPR parameters may be used as outcome measures after treatment.

Permeability of compacting porous lavas

NASA Astrophysics Data System (ADS)

Ashwell, P. A.; Kendrick, J. E.; Lavallée, Y.; Kennedy, B. M.; Hess, K.-U.; von Aulock, F. W.; Wadsworth, F. B.; Vasseur, J.; Dingwell, D. B.

2015-03-01

The highly transient nature of outgassing commonly observed at volcanoes is in part controlled by the permeability of lava domes and shallow conduits. Lava domes generally consist of a porous outer carapace surrounding a denser lava core with internal shear zones of variable porosity. Here we examine densification using uniaxial compression experiments on variably crystalline and porous rhyolitic dome lavas from the Taupo Volcanic Zone. Experiments were conducted at 900°C and an applied stress of 3 MPa to 60% strain, while monitoring acoustic emissions to track cracking. The evolution of the porous network was assessed via X-ray computed tomography, He-pycnometry, and relative gas permeability. High starting connected porosities led to low apparent viscosities and high strain rates, initially accompanied by abundant acoustic emissions. As compaction ensued, the lavas evolved; apparent viscosity increased and strain rate decreased due to strain hardening of the suspensions. Permeability fluctuations resulted from the interplay between viscous flow and brittle failure. Where phenocrysts were abundant, cracks had limited spatial extent, and pore closure decreased axial and radial permeability proportionally, maintaining the initial anisotropy. In crystal-poor lavas, axial cracks had a more profound effect, and permeability anisotropy switched to favor axial flow. Irrespective of porosity, both crystalline samples compacted to a threshold minimum porosity of 17-19%, whereas the crystal-poor sample did not achieve its compaction limit. This indicates that unconfined loading of porous dome lavas does not necessarily form an impermeable plug and may be hindered, in part by the presence of crystals.

New approaches to virtual environment surgery

NASA Technical Reports Server (NTRS)

Ross, M. D.; Twombly, A.; Lee, A. W.; Cheng, R.; Senger, S.

1999-01-01

This research focused on two main problems: 1) low cost, high fidelity stereoscopic imaging of complex tissues and organs; and 2) virtual cutting of tissue. A further objective was to develop these images and virtual tissue cutting methods for use in a telemedicine project that would connect remote sites using the Next Generation Internet. For goal one we used a CT scan of a human heart, a desktop PC with an OpenGL graphics accelerator card, and LCD stereoscopic glasses. Use of multiresolution meshes ranging from approximately 1,000,000 to 20,000 polygons speeded interactive rendering rates enormously while retaining general topography of the dataset. For goal two, we used a CT scan of an infant skull with premature closure of the right coronal suture, a Silicon Graphics Onyx workstation, a Fakespace Immersive WorkBench and CrystalEyes LCD glasses. The high fidelity mesh of the skull was reduced from one million to 50,000 polygons. The cut path was automatically calculated as the shortest distance along the mesh between a small number of hand selected vertices. The region outlined by the cut path was then separated from the skull and translated/rotated to assume a new position. The results indicate that widespread high fidelity imaging in virtual environment is possible using ordinary PC capabilities if appropriate mesh reduction methods are employed. The software cutting tool is applicable to heart and other organs for surgery planning, for training surgeons in a virtual environment, and for telemedicine purposes.

Crack Detection in Fibre Reinforced Plastic Structures Using Embedded Fibre Bragg Grating Sensors: Theory, Model Development and Experimental Validation

PubMed Central

Pereira, G. F.; Mikkelsen, L. P.; McGugan, M.

2015-01-01

In a fibre-reinforced polymer (FRP) structure designed using the emerging damage tolerance and structural health monitoring philosophy, sensors and models that describe crack propagation will enable a structure to operate despite the presence of damage by fully exploiting the material’s mechanical properties. When applying this concept to different structures, sensor systems and damage types, a combination of damage mechanics, monitoring technology, and modelling is required. The primary objective of this article is to demonstrate such a combination. This article is divided in three main topics: the damage mechanism (delamination of FRP), the structural health monitoring technology (fibre Bragg gratings to detect delamination), and the finite element method model of the structure that incorporates these concepts into a final and integrated damage-monitoring concept. A novel method for assessing a crack growth/damage event in fibre-reinforced polymer or structural adhesive-bonded structures using embedded fibre Bragg grating (FBG) sensors is presented by combining conventional measured parameters, such as wavelength shift, with parameters associated with measurement errors, typically ignored by the end-user. Conjointly, a novel model for sensor output prediction (virtual sensor) was developed using this FBG sensor crack monitoring concept and implemented in a finite element method code. The monitoring method was demonstrated and validated using glass fibre double cantilever beam specimens instrumented with an array of FBG sensors embedded in the material and tested using an experimental fracture procedure. The digital image correlation technique was used to validate the model prediction by correlating the specific sensor response caused by the crack with the developed model. PMID:26513653

Final summary report for 1989 inservice inspection (ISI) of SRS (Savannah River Site) 100-P Reactor tank

DOE Office of Scientific and Technical Information (OSTI.GOV)

Morrison, J.M.; Loibl, M.W.

1989-12-15

The integrity of the SRS reactor tanks is a key factor affecting their suitability for continued service since, unlike the external piping system and components, the tanks are virtually irreplaceable. Cracking in various areas of the process water piping systems has occurred beginning in 1960 as a result of several degradation mechanisms, chiefly intergranular stress corrosion cracking (IGSCC) and chloride-induced transgranular cracking. IGSCC, currently the primary degradation mechanism, also occurred in the knuckle'' region (tank wall-to-bottom tube sheet transition piece) unique to C Reactor and was eventually responsible for that reactor being deactivated in 1985. A program of visual examinationsmore » of the SRS reactor tanks was initiated in 1968, which used a specially designed immersible periscope. Under that program the condition of the accessible tank welds and associated heat affected zones (HAZ) was evaluated on a five-year frequency. Prior to 1986, the scope of these inspections comprised approximately 20 percent of the accessible weld area. In late 1986 and early 1987 the scope of the inspections was expanded and a 100 percent visual inspection of accessible welds was performed of the P-, L-, and K-Reactor tanks. Supplemental dye penetrant examinations were performed in L Reactor on selected areas which showed visual indications. No evidence of cracking was detected in any of these inspections of the P-, L-, and K-Reactor tanks. 17 refs., 7 figs.« less

Huygen-Fresnel Diffraction Model H-Fdm for the Simulation of Ultrasonic Time-Of Diffraction Technique in 2d Geometries

NASA Astrophysics Data System (ADS)

Reddy, K. Sanjeeva; Krishnamurthy, C. V.; Balasubramaniam, Krishnan; Balasubramanian, T.

2010-02-01

This paper discusses the evaluation of diffracted signals from cracks in 2D based on a new Huygen-Fresnel Diffraction Model (H-FDM). The model employs the frequency-domain far-field displacement expressions derived by Miller & Pursey [1] in 2D for a line source located on the free surface of a semi-infinite elastic medium. At each frequency in the bandwidth of a pulsed excitation, the complex diffracted field is obtained by summing over the unblocked virtual sources located in the section containing a vertical crack. The time-domain diffracted signal is obtained using standard FFT procedures. The effect of beam refraction from a wedge-based finite transducer has been modeled by treating the finite transducer as an array of line sources. The model has been used for predicting diffracted signals in time-of-flight from the crack like defect. The model allows the evaluation of back wall signal amplitude and lateral wave amplitude as well. Experiments have been carried out on 10 mm thick aluminum sample with surface breaking crack of lengths 2 mm and 4 mm using shear probe shoe. The simulated A-Scan results for the aluminum sample with 2 mm and 4 mm surface breaking lengths compare very well in relative amplitudes and time of arrivals with experiments. The H-FDM model offers a tool to evaluate diffraction and related phenomena quantitatively with modest computational resources.

POST CLOSURE INSPECTION AND MONITORING REPORT FOR CORRECTIVE ACTION UNIT 110: AREA 3 WMD U-3AX/BL CRATER, NEVADA TEST SITE, NEVADA FOR THE PERIOD JULY 2004 - JUNE 2005

DOE Office of Scientific and Technical Information (OSTI.GOV)

BECHTEL NEVADA

2005-08-01

This Post-Closure Inspection and Monitoring report provides the results of inspections and monitoring for Corrective Action Unit (CAU) 110, Area 3 Waste Management Division (WMD) U-3ax/bl Crater. This report includes an analysis and summary of the site inspections, repairs and maintenance, meteorological information, and soil moisture monitoring data obtained at CAU 110, for the annual period July 2004 through June 2005. Site inspections of the cover were performed quarterly to identify any significant changes to the site requiring action. The overall condition of the cover, cover vegetation, perimeter fence, and use restriction warning signs was good. Settling was observed thatmore » exceeded the action level as specified in Section VII.B.7 of the Hazardous Waste Permit Number NEV HW009 (Nevada Division of Environmental Protection, 2000). This permit states that cracks or settling greater than 15 centimeters (cm) (6 inches [in]) deep that extend 1.0 meter (m) (3 feet [ft]) or more on the cover will be evaluated and repaired within 60 days of detection.« less

Three-dimensional piezoelectric boundary elements

NASA Astrophysics Data System (ADS)

Hill, Lisa Renee

The strong coupling between mechanical and electrical fields in piezoelectric ceramics makes them appropriate for use as actuation devices; as a result, they are an important part of the emerging technologies of smart materials and structures. These piezoceramics are very brittle and susceptible to fracture, especially under the severe loading conditions which may occur in service. A significant portion of the applications under investigation involve dynamic loading conditions. Once a crack is initiated in the piezoelectric medium, the mechanical and electrical fields can act to drive the crack growth. Failure of the actuator can result from a catastrophic fracture event or from the cumulative effects of cyclic fatigue. The presence of these cracks, or other types of material defects, alter the mechanical and electrical fields inside the body. Specifically, concentrations of stress and electric field are present near a flaw and can lead to material yielding or localized depoling, which in turn can affect the sensor/actuator performance or cause failure. Understanding these effects is critical to the success of these smart structures. The complex coupling behavior and the anisotropy of the material makes the use of numerical methods necessary for all but the simplest problems. To this end, a three-dimensional boundary element method program is developed to evaluate the effect of flaws on these piezoelectric materials. The program is based on the linear governing equations of piezoelectricity and relies on a numerically evaluated Green's function for solution. The boundary element method was selected as the evaluation tool due to its ability to model the interior domain exactly. Thus, for piezoelectric materials the coupling between mechanical and electrical fields is not approximated inside the body. Holes in infinite and finite piezoceramics are investigated, with the localized stresses and electric fields clearly developed. The accuracy of the piezoelectric boundary element method is demonstrated with two problems: a two-dimensional circular void and a three-dimensional spherical cavity, both inside infinite solids. Application of the program to a finite body with a centered, spherical void illustrates the complex nature of the mechanical and electrical coupling. Mode I fracture is also examined, combining the linear boundary element solution with the modified crack closure integral to determine strain energy release rates. Experimental research has shown that the strain, rather than the total, energy release rate is a better predictor of crack growth in piezoelectric materials. Solutions for a two-dimensional slit-like crack and for three-dimensional penny and elliptical cracks are presented. These solutions are developed using the insulated crack face electrical boundary condition. Although this boundary condition is used by most researchers, recent discussion indicates that it may not be an accurate model for the slender crack geometry. The boundary element method is used with the penny crack problem to investigate the effect of different electrical boundary conditions on the strain energy release rate. Use of a conductive crack face boundary condition, rather than an insulated one, acts to increase the strain energy release rate for the penny crack. These conductive strain energies are closer to the values determined using a permeable electrical boundary condition than to the original conductive boundary condition ones. It is shown that conclusions about structural integrity are strongly dependent on the choice of boundary conditions.

A stretchable strain sensor based on a metal nanoparticle thin film for human motion detection

NASA Astrophysics Data System (ADS)

Lee, Jaehwan; Kim, Sanghyeok; Lee, Jinjae; Yang, Daejong; Park, Byong Chon; Ryu, Seunghwa; Park, Inkyu

2014-09-01

Wearable strain sensors for human motion detection are being highlighted in various fields such as medical, entertainment and sports industry. In this paper, we propose a new type of stretchable strain sensor that can detect both tensile and compressive strains and can be fabricated by a very simple process. A silver nanoparticle (Ag NP) thin film patterned on the polydimethylsiloxane (PDMS) stamp by a single-step direct transfer process is used as the strain sensing material. The working principle is the change in the electrical resistance caused by the opening/closure of micro-cracks under mechanical deformation. The fabricated stretchable strain sensor shows highly sensitive and durable sensing performances in various tensile/compressive strains, long-term cyclic loading and relaxation tests. We demonstrate the applications of our stretchable strain sensors such as flexible pressure sensors and wearable human motion detection devices with high sensitivity, response speed and mechanical robustness.Wearable strain sensors for human motion detection are being highlighted in various fields such as medical, entertainment and sports industry. In this paper, we propose a new type of stretchable strain sensor that can detect both tensile and compressive strains and can be fabricated by a very simple process. A silver nanoparticle (Ag NP) thin film patterned on the polydimethylsiloxane (PDMS) stamp by a single-step direct transfer process is used as the strain sensing material. The working principle is the change in the electrical resistance caused by the opening/closure of micro-cracks under mechanical deformation. The fabricated stretchable strain sensor shows highly sensitive and durable sensing performances in various tensile/compressive strains, long-term cyclic loading and relaxation tests. We demonstrate the applications of our stretchable strain sensors such as flexible pressure sensors and wearable human motion detection devices with high sensitivity, response speed and mechanical robustness. Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nr03295k

Predictors and outcome of surgical repair of obstetric fistula at a regional referral hospital, Mbarara, western Uganda

PubMed Central

2011-01-01

Background Obstetric fistula although virtually eliminated in high income countries, still remains a prevalent and debilitating condition in many parts of the developing world. It occurs in areas where access to care at childbirth is limited, or of poor quality and where few hospitals offer the necessary corrective surgery. Methods This was a prospective observational study where all women who attended Mbarara Regional Referral Hospital in western Uganda with obstetric fistula during the study period were assessed pre-operatively for social demographics, fistula characteristics, classification and outcomes after surgery. Assessment for fistula closure and stress incontinence after surgery was done using a dye test before discharge Results Of the 77 women who were recruited in this study, 60 (77.9%) had successful closure of their fistulae. Unsuccessful fistula closure was significantly associated with large fistula size (Odds Ratio 6 95% Confidential interval 1.46-24.63), circumferential fistulae (Odds ratio 9.33 95% Confidential interval 2.23-39.12) and moderate to severe vaginal scarring (Odds ratio 12.24 95% Confidential interval 1.52-98.30). Vaginal scarring was the only factor independently associated with unsuccessful fistula repair (Odds ratio 10 95% confidential interval 1.12-100.57). Residual stress incontinence after successful fistula closure was associated with type IIb fistulae (Odds ratio 5.56 95% Confidential interval 1.34-23.02), circumferential fistulae (Odds ratio 10.5 95% Confidential interval 1.39-79.13) and previous unsuccessful fistula repair (Odds ratio 4.8 95% Confidential interval 1.27-18.11). Independent predictors for residual stress incontinence after successful fistula closure were urethral involvement (Odds Ratio 4.024 95% Confidential interval 2.77-5.83) and previous unsuccessful fistula repair (Odds ratio 38.69 95% Confidential interval 2.13-703.88). Conclusions This study demonstrated that large fistula size, circumferential fistulae and marked vaginal scarring are predictors for unsuccessful fistula repair while predictors for residual stress incontinence after successful fistula closure were urethral involvement, circumferential fistulae and previous unsuccessful fistula repair. PMID:22151960

Predictors and outcome of surgical repair of obstetric fistula at a regional referral hospital, Mbarara, western Uganda.

PubMed

Kayondo, Musa; Wasswa, Ssalongo; Kabakyenga, Jerome; Mukiibi, Nozmo; Senkungu, Jude; Stenson, Amy; Mukasa, Peter

2011-12-07

Obstetric fistula although virtually eliminated in high income countries, still remains a prevalent and debilitating condition in many parts of the developing world. It occurs in areas where access to care at childbirth is limited, or of poor quality and where few hospitals offer the necessary corrective surgery. This was a prospective observational study where all women who attended Mbarara Regional Referral Hospital in western Uganda with obstetric fistula during the study period were assessed pre-operatively for social demographics, fistula characteristics, classification and outcomes after surgery. Assessment for fistula closure and stress incontinence after surgery was done using a dye test before discharge Of the 77 women who were recruited in this study, 60 (77.9%) had successful closure of their fistulae. Unsuccessful fistula closure was significantly associated with large fistula size (Odds Ratio 6 95% Confidential interval 1.46-24.63), circumferential fistulae (Odds ratio 9.33 95% Confidential interval 2.23-39.12) and moderate to severe vaginal scarring (Odds ratio 12.24 95% Confidential interval 1.52-98.30). Vaginal scarring was the only factor independently associated with unsuccessful fistula repair (Odds ratio 10 95% confidential interval 1.12-100.57). Residual stress incontinence after successful fistula closure was associated with type IIb fistulae (Odds ratio 5.56 95% Confidential interval 1.34-23.02), circumferential fistulae (Odds ratio 10.5 95% Confidential interval 1.39-79.13) and previous unsuccessful fistula repair (Odds ratio 4.8 95% Confidential interval 1.27-18.11). Independent predictors for residual stress incontinence after successful fistula closure were urethral involvement (Odds Ratio 4.024 95% Confidential interval 2.77-5.83) and previous unsuccessful fistula repair (Odds ratio 38.69 95% Confidential interval 2.13-703.88). This study demonstrated that large fistula size, circumferential fistulae and marked vaginal scarring are predictors for unsuccessful fistula repair while predictors for residual stress incontinence after successful fistula closure were urethral involvement, circumferential fistulae and previous unsuccessful fistula repair.

NASGRO 3.0: A Software for Analyzing Aging Aircraft

NASA Technical Reports Server (NTRS)

Mettu, S. R.; Shivakumar, V.; Beek, J. M.; Yeh, F.; Williams, L. C.; Forman, R. G.; McMahon, J. J.; Newman, J. C., Jr.

1999-01-01

Structural integrity analysis of aging aircraft is a critical necessity in view of the increasing numbers of such aircraft in general aviation, the airlines and the military. Efforts are in progress by NASA, the FAA and the DoD to focus attention on aging aircraft safety. The present paper describes the NASGRO software which is well-suited for effectively analyzing the behavior of defects that may be found in aging aircraft. The newly revised Version 3.0 has many features specifically implemented to suit the needs of the aircraft community. The fatigue crack growth computer program NASA/FLAGRO 2.0 was originally developed to analyze space hardware such as the Space Shuttle, the International Space Station and the associated payloads. Due to popular demand, the software was enhanced to suit the needs of the aircraft industry. Major improvements in Version 3.0 are the incorporation of the ability to read aircraft spectra of unlimited size, generation of common aircraft fatigue load blocks, and the incorporation of crack-growth models which include load-interaction effects such as retardation due to overloads and acceleration due to underloads. Five new crack-growth models, viz., generalized Willenborg, modified generalized Willenborg, constant closure model, Walker-Chang model and the deKoning-Newman strip-yield model, have been implemented. To facilitate easier input of geometry, material properties and load spectra, a Windows-style graphical user interface has been developed. Features to quickly change the input and rerun the problem as well as examine the output are incorporated. NASGRO has been organized into three modules, the crack-growth module being the primary one. The other two modules are the boundary element module and the material properties module. The boundary-element module provides the ability to model and analyze complex two-dimensional problems to obtain stresses and stress-intensity factors. The material properties module allows users to store and curve-fit fatigue-crack growth data. On-line help and documentation are provided for each of the modules. In addition to the popular PC windows version, a unix-based X-windows version of NASGRO is also available. A portable C++ class library called WxWindows was used to facilitate cross-platform availability of the software.

Constraint Embedding Technique for Multibody System Dynamics

NASA Technical Reports Server (NTRS)

Woo, Simon S.; Cheng, Michael K.

2011-01-01

Multibody dynamics play a critical role in simulation testbeds for space missions. There has been a considerable interest in the development of efficient computational algorithms for solving the dynamics of multibody systems. Mass matrix factorization and inversion techniques and the O(N) class of forward dynamics algorithms developed using a spatial operator algebra stand out as important breakthrough on this front. Techniques such as these provide the efficient algorithms and methods for the application and implementation of such multibody dynamics models. However, these methods are limited only to tree-topology multibody systems. Closed-chain topology systems require different techniques that are not as efficient or as broad as those for tree-topology systems. The closed-chain forward dynamics approach consists of treating the closed-chain topology as a tree-topology system subject to additional closure constraints. The resulting forward dynamics solution consists of: (a) ignoring the closure constraints and using the O(N) algorithm to solve for the free unconstrained accelerations for the system; (b) using the tree-topology solution to compute a correction force to enforce the closure constraints; and (c) correcting the unconstrained accelerations with correction accelerations resulting from the correction forces. This constraint-embedding technique shows how to use direct embedding to eliminate local closure-loops in the system and effectively convert the system back to a tree-topology system. At this point, standard tree-topology techniques can be brought to bear on the problem. The approach uses a spatial operator algebra approach to formulating the equations of motion. The operators are block-partitioned around the local body subgroups to convert them into aggregate bodies. Mass matrix operator factorization and inversion techniques are applied to the reformulated tree-topology system. Thus in essence, the new technique allows conversion of a system with closure-constraints into an equivalent tree-topology system, and thus allows one to take advantage of the host of techniques available to the latter class of systems. This technology is highly suitable for the class of multibody systems where the closure-constraints are local, i.e., where they are confined to small groupings of bodies within the system. Important examples of such local closure-constraints are constraints associated with four-bar linkages, geared motors, differential suspensions, etc. One can eliminate these closure-constraints and convert the system into a tree-topology system by embedding the constraints directly into the system dynamics and effectively replacing the body groupings with virtual aggregate bodies. Once eliminated, one can apply the well-known results and algorithms for tree-topology systems to solve the dynamics of such closed-chain system.

A Damage Model for the Simulation of Delamination in Advanced Composites under Variable-Mode Loading

NASA Technical Reports Server (NTRS)

Turon, A.; Camanho, P. P.; Costa, J.; Davila, C. G.

2006-01-01

A thermodynamically consistent damage model is proposed for the simulation of progressive delamination in composite materials under variable-mode ratio. The model is formulated in the context of Damage Mechanics. A novel constitutive equation is developed to model the initiation and propagation of delamination. A delamination initiation criterion is proposed to assure that the formulation can account for changes in the loading mode in a thermodynamically consistent way. The formulation accounts for crack closure effects to avoid interfacial penetration of two adjacent layers after complete decohesion. The model is implemented in a finite element formulation, and the numerical predictions are compared with experimental results obtained in both composite test specimens and structural components.

Evaluation of Tools for Protection of Interest against Hacking and Cracking

NASA Astrophysics Data System (ADS)

Jahankhani, Hossein; Antonijevic, Branko; Walcott, Terry

The internet considered a tool that effectively ensures communication globally has been hindered by hackers and crackers continuously. In so doing, a multitude of network facilitated tools such as firewalls, virtual private networks (VPN) and a variety of antivirus software packages has been enabled for dealing with such predicaments. However, more often than not these facilitated tools are marketed as perfect solutions to the ever culminating problems such as loss of data and privacy in networked and world wide intercommunications. We provide a forum for addressing these perceived problems in this paper.

Long-period tilt-induced accelerations associated with hydraulic fracturing

NASA Astrophysics Data System (ADS)

Milkereit, Claus; Dahm, Torsten; Cesca, Simone; Lopez, Jose; Nooshiri, Nima; Zang, Arno

2017-04-01

In 2015, several small scale hydrofracture experiments have been performed in situ from a horizontal borehole in a mine gallery in granitic rock. The hydrofracture experiments were monitored by a bundle of different near field sensors covering a broad range of frequencies (see Zang et al., Geophys. J. Int. (2017) 208, 790-813, doi: 10.1093/gji/ggw430). We installed broad band sensors in the gallery close to the fracture experiments, and observed clear long period transients on the horizontal components, with timing and polarity correlated with the opening and closing of the fractures. We interpret the broadband signals as tilt-induced excursions. The broadband signals have been measured independent whether high frequency acoustic emission have been observed or not during the individual fracture experiments. They are thus an independent measure of the success of a hydrofracture experiment and the parameter of the newly formed cracks. In this study we show that most tilt-induced long-period signals can be modeled by a rectangular crack with constant opening in an elastic full space, as first order approximation. From theoretical forward modeling, we proof that the tilt has a higher sensitivity to resolve the strike of the fracture than the displacement field. With this model, we retrieve the strike of the fractures from the tilt observed at a single sensor. The results indicate that the strike angles of the hydrofractures change systematically with the distance to the gallery wall, indicating a rotation of the principal stresses close to the free surface of the gallery. The rotation trend is similar to the one observed in previous hydrofracture experiments in mines. We compare the strength of the modeled tensile cracks, i.e. opening times crack area, with the volume of the injected fluid, and discuss the general resolving power of tilt signals for source parameter fractures. The temporal evolution of the opening and closure of the fractures is discussed.

Basaltic Dike Propagation at Yucca Mountain, Nevada, USA

NASA Astrophysics Data System (ADS)

Gaffney, E. S.; Damjanac, B.; Warpinski, N. R.

2004-12-01

We describe simulations of the propagation of basaltic dikes using a 2-dimensional, incompressible hydrofracture code including the effects of the free surface with specific application to potential interactions of rising magma with a nuclear waste repository at Yucca Mountain, Nevada. As the leading edge of the dike approaches the free surface, confinement at the crack tip is reduced and the tip accelerates relative to the magma front. In the absence of either excess confining stress or excess gas pressure in the tip cavity, this leads to an increase of crack-tip velocity by more than an order of magnitude. By casting the results in nondimensional form, they can be applied to a wide variety of intrusive situations. When applied to an alkali basalt intrusion at the proposed high-level nuclear waste repository at Yucca Mountain, the results provide for a description of the subsurface phenomena. For magma rising at 1 m/s and dikes wider than about 0.5 m, the tip of the fissure would already have breached the surface by the time magma arrived at the nominal 300-m repository depth. An approximation of the effect of magma expansion on dike propagation is used to show that removing the restriction of an incompressible magma would result in even greater crack-tip acceleration as the dike approached the surface. A second analysis with a distinct element code indicates that a dike could penetrate the repository even during the first 2000 years after closure during which time heating from radioactive decay of waste would raise the minimum horizontal compressive stress above the vertical stress for about 80 m above and below the repository horizon. Rather than sill formation, the analysis indicates that increased pressure and dike width below the repository cause the crack tip to penetrate the horizon, but much more slowly than under in situ stress conditions. The analysis did not address the effects of either anisotropic joints or heat loss on this result.

Multidisciplinary Optimization and Damage Tolerance of Stiffened Structures

NASA Astrophysics Data System (ADS)

Jrad, Mohamed

THE structural optimization of a cantilever aircraft wing with curvilinear spars and ribs and stiffeners is described. For the optimization of a complex wing, a common strategy is to divide the optimization procedure into two subsystems: the global wing optimization which optimizes the geometry of spars, ribs and wing skins; and the local panel optimization which optimizes the design variables of local panels bordered by spars and ribs. The stiffeners are placed on the local panels to increase the stiffness and buckling resistance. During the local panel optimization, the stress information is taken from the global model as a displacement boundary condition on the panel edges using the so-called "Global-Local Approach". Particle swarm optimization is used in the integration of global/local optimization to optimize the SpaRibs. Parallel computing approach has been developed in the Python programming language to reduce the CPU time. The license cycle-check method and memory self-adjustment method are two approaches that have been applied in the parallel framework in order to optimize the use of the resources by reducing the license and memory limitations and making the code robust. The integrated global-local optimization approach has been applied to subsonic NASA common research model (CRM) wing, which proves the methodology's application scaling with medium fidelity FEM analysis. The structural weight of the wing has been reduced by 42% and the parallel implementation allowed a reduction in the CPU time by 89%. The aforementioned Global-Local Approach is investigated and applied to a composite panel with crack at its center. Because of composite laminates' heterogeneity, an accurate analysis of these requires very high time and storage space. A possible alternative to reduce the computational complexity is the global-local analysis which involves an approximate analysis of the whole structure followed by a detailed analysis of a significantly smaller region of interest. Buckling analysis of a composite panel with attached longitudinal stiffeners under compressive loads is performed using Ritz method with trigonometric functions. Results are then compared to those from Abaqus FEA for different shell elements. The case of composite panel with one, two, and three stiffeners is investigated. The effect of the distance between the stiffeners on the buckling load is also studied. The variation of the buckling load and buckling modes with the stiffeners' height is investigated. It is shown that there is an optimum value of stiffeners' height beyond which the structural response of the stiffened panel is not improved and the buckling load does not increase. Furthermore, there exist different critical values of stiffener's height at which the buckling mode of the structure changes. Next, buckling analysis of a composite panel with two straight stiffeners and a crack at the center is performed. Finally, buckling analysis of a composite panel with curvilinear stiffeners and a crack at the center is also conducted. Results show that panels with a larger crack have a reduced buckling load and that the buckling load decreases slightly when using higher order 2D shell FEM elements. A damage tolerance framework, EBF3PanelOpt, has been developed to design and analyze curvilinearly stiffened panels. The framework is written with the scripting language Python and it interacts with the commercial software MSC. Patran (for geometry and mesh creation), MSC. Nastran (for finite element analysis), and MSC. Marc (for damage tolerance analysis). The crack location is set to the location of the maximum value of the major principal stress while its orientation is set normal to the major principal axis direction. The effective stress intensity factor is calculated using the Virtual Crack Closure Technique and compared to the fracture toughness of the material in order to decide whether the crack will expand or not. The ratio of these two quantities is used as a constraint, along with the buckling factor, Kreisselmeier and Steinhauser criteria, and crippling factor. The EBF3PanelOpt framework is integrated within a two-step Particle Swarm Optimization in order to minimize the weight of the panel while satisfying the aforementioned constraints and using all the shape and thickness parameters as design variables. The result of the PSO is used then as an initial guess for the Gradient Based Optimization using only the thickness parameters as design variables and employing VisualDOC. Stiffened panel with two curvilinear stiffeners is optimized for two load cases. In both cases, significant reduction has been made for the panel's weight.

RANS Simulation (Virtual Blade Model [VBM]) of Single Full Scale DOE RM1 MHK Turbine

DOE Data Explorer

Javaherchi, Teymour; Aliseda, Alberto

2013-04-10

Attached are the .cas and .dat files along with the required User Defined Functions (UDFs) and look-up table of lift and drag coefficients for Reynolds Averaged Navier-Stokes (RANS) simulation of a single full scale DOE RM1 turbine implemented in ANSYS FLUENT CFD-package. In this case study the flow field around and in the wake of the full scale DOE RM1 turbine is simulated using Blade Element Model (a.k.a Virtual Blade Model) by solving RANS equations coupled with k-\\omega turbulence closure model. It should be highlighted that in this simulation the actual geometry of the rotor blade is not modeled. The effect of turbine rotating blades are modeled using the Blade Element Theory. This simulation provides an accurate estimate for the performance of device and structure of it's turbulent far wake. Due to the simplifications implemented for modeling the rotating blades in this model, VBM is limited to capture details of the flow field in near wake region of the device.

Virtual suturing simulation based on commodity physics engine for medical learning.

PubMed

Choi, Kup-Sze; Chan, Sze-Ho; Pang, Wai-Man

2012-06-01

Development of virtual-reality medical applications is usually a complicated and labour intensive task. This paper explores the feasibility of using commodity physics engine to develop a suturing simulator prototype for manual skills training in the fields of nursing and medicine, so as to enjoy the benefits of rapid development and hardware-accelerated computation. In the prototype, spring-connected boxes of finite dimension are used to simulate soft tissues, whereas needle and thread are modelled with chained segments. Spherical joints are used to simulate suture's flexibility and to facilitate thread cutting. An algorithm is developed to simulate needle insertion and thread advancement through the tissue. Two-handed manipulations and force feedback are enabled with two haptic devices. Experiments on the closure of a wound show that the prototype is able to simulate suturing procedures at interactive rates. The simulator is also used to study a curvature-adaptive suture modelling technique. Issues and limitations of the proposed approach and future development are discussed.

A new hybrid numerical scheme for simulating fault ruptures with near-fault bulk inhomogeneities

NASA Astrophysics Data System (ADS)

Hajarolasvadi, S.; Elbanna, A. E.

2017-12-01

The Finite Difference (FD) and Boundary Integral (BI) Method have been extensively used to model spontaneously propagating shear cracks, which can serve as a useful idealization of natural earthquakes. While FD suffers from artificial dispersion and numerical dissipation and has a large computational cost as it requires the discretization of the whole volume of interest, it can be applied to a wider range of problems including ones with bulk nonlinearities and heterogeneities. On the other hand, in the BI method, the numerical consideration is confined to the crack path only, with the elastodynamic response of the bulk expressed in terms of integral relations between displacement discontinuities and tractions along the crack. Therefore, this method - its spectral boundary integral (SBI) formulation in particular - is much faster and more computationally efficient than other bulk methods such as FD. However, its application is restricted to linear elastic bulk and planar faults. This work proposes a novel hybrid numerical scheme that combines FD and the SBI to enable treating fault zone nonlinearities and heterogeneities with unprecedented resolution and in a more computationally efficient way. The main idea of the method is to enclose the inhomgeneities in a virtual strip that is introduced for computational purposes only. This strip is then discretized using a volume-based numerical method, chosen here to be the finite difference method while the virtual boundaries of the strip are handled using the SBI formulation that represents the two elastic half spaces outside the strip. Modeling the elastodynamic response in these two halfspaces needs to be carried out by an Independent Spectral Formulation before joining them to the strip with the appropriate boundary conditions. Dirichlet and Neumann boundary conditions are imposed on the strip and the two half-spaces, respectively, at each time step to propagate the solution forward. We demonstrate the validity of the approach using two examples for dynamic rupture propagation: one in the presence of a low velocity layer and the other in which off-fault plasticity is permitted. This approach is more computationally efficient than pure FD and expands the range of applications of SBI beyond the current state of the art.

Degradation Mechanisms of Electrochemically Cycled Graphite Anodes in Lithium-ion Cells

NASA Astrophysics Data System (ADS)

Bhattacharya, Sandeep

This research is aimed at developing advanced characterization methods for studying the surface and subsurface damage in Li-ion battery anodes made of polycrystalline graphite and identifying the degradation mechanisms that cause loss of electrochemical capacity. Understanding microstructural aspects of the graphite electrode degradation mechanisms during charging and discharging of Li-ion batteries is of key importance in order to design durable anodes with high capacity. An in-situ system was constructed using an electrochemical cell with an observation window, a large depth-of-field digital microscope and a micro-Raman spectrometer. It was revealed that electrode damage by removal of the surface graphite fragments of 5-10 mum size is the most intense during the first cycle that led to a drastic capacity drop. Once a solid electrolyte interphase (SEI) layer covered the electrode surface, the rate of graphite particle loss decreased. Yet, a gradual loss of capacity continued by the formation of interlayer cracks adjacent to SEI/graphite interfaces. Deposition of co-intercalation compounds, LiC6, Li2CO3 and Li2O, near the crack tips caused partial closure of propagating graphite cracks during cycling and reduced the crack growth rate. Bridging of crack faces by delaminated graphite layers also retarded crack propagation. The microstructure of the SEI layer, formed by electrochemical reduction of the ethylene carbonate based electrolyte, consisted of ˜5-20 nm sized crystalline domains (containing Li2CO3, Li2O 2 and nano-sized graphite fragments) dispersed in an amorphous matrix. During the SEI formation, two regimes of Li-ion diffusion were identified at the electrode/electrolyte interface depending on the applied voltage scan rate (dV/dt). A low Li-ion diffusion coefficient ( DLi+) at dV/dt < 0.05 mVs-1 produced a tubular SEI that uniformly covered the graphite surface and prevented damage at 25°C. At 60°C, a high D Li+ formed a Li2CO3-enriched SEI and ensued a 28% increase in the battery capacity at 25°C. On correlating the microscopic information to the electrochemical performance, novel Li2CO3-coated electrodes were fabricated that were durable. The SEI formed on pre-treated electrodes reduced the strain in the graphite lattice from 0.4% (for uncoated electrodes) to 0.1%, facilitated Li-ion diffusion and hence improved the capacity retention of Li-ion batteries during long-term cycling.

Particle-in-cell simulations for virtual cathode oscillator including foil ablation effects

DOE Office of Scientific and Technical Information (OSTI.GOV)

Singh, Gursharn; Chaturvedi, S.

2011-06-15

We have performed two- and three-dimensional, relativistic, electromagnetic, particle-in-cell simulations of an axially extracted virtual cathode oscillator (vircator). The simulations include, for the first time, self-consistent dynamics of the anode foil under the influence of the intense electron beam. This yields the variation of microwave output power as a function of time, including the role of anode ablation and anode-cathode gap closure. These simulations have been done using locally developed particle-in-cell (PIC) codes. The codes have been validated using two vircator designs available from the literature. The simulations reported in the present paper take account of foil ablation due tomore » the intense electron flux, the resulting plasma expansion and shorting of the anode-cathode gap. The variation in anode transparency due to plasma formation is automatically taken into account. We find that damage is generally higher near the axis. Also, at all radial positions, there is little damage in the early stages, followed by a period of rapid erosion, followed in turn by low damage rates. A physical explanation has been given for these trends. As a result of gap closure due to plasma formation from the foil, the output microwave power initially increases, reaches a near-flat-top and then decreases steadily, reaching a minimum around 230 ns. This is consistent with a typical plasma expansion velocity of {approx}2 cm/{mu}s reported in the literature. We also find a significant variation in the dominant output frequency, from 6.3 to 7.6 GHz. This variation is small as long as the plasma density is small, up to {approx}40 ns. As the AK gap starts filling with plasma, there is a steady increase in this frequency.« less

Role of geomechanically grown fractures on dispersive transport in heterogeneous geological formations.

PubMed

Nick, H M; Paluszny, A; Blunt, M J; Matthai, S K

2011-11-01

A second order in space accurate implicit scheme for time-dependent advection-dispersion equations and a discrete fracture propagation model are employed to model solute transport in porous media. We study the impact of the fractures on mass transport and dispersion. To model flow and transport, pressure and transport equations are integrated using a finite-element, node-centered finite-volume approach. Fracture geometries are incrementally developed from a random distributions of material flaws using an adoptive geomechanical finite-element model that also produces fracture aperture distributions. This quasistatic propagation assumes a linear elastic rock matrix, and crack propagation is governed by a subcritical crack growth failure criterion. Fracture propagation, intersection, and closure are handled geometrically. The flow and transport simulations are separately conducted for a range of fracture densities that are generated by the geomechanical finite-element model. These computations show that the most influential parameters for solute transport in fractured porous media are as follows: fracture density and fracture-matrix flux ratio that is influenced by matrix permeability. Using an equivalent fracture aperture size, computed on the basis of equivalent permeability of the system, we also obtain an acceptable prediction of the macrodispersion of poorly interconnected fracture networks. The results hold for fractures at relatively low density.

Multiscale Analysis of Nanocomposites and Their Use in Structural Level Applications

NASA Astrophysics Data System (ADS)

Hasan, Zeaid

This research focuses on the benefits of using nanocomposites in aerospace structural components to prevent or delay the onset of unique composite failure modes, such as delamination. Analytical, numerical, and experimental analyses were conducted to provide a comprehensive understanding of how carbon nanotubes (CNTs) can provide additional structural integrity when they are used in specific hot spots within a structure. A multiscale approach was implemented to determine the mechanical and thermal properties of the nanocomposites, which were used in detailed finite element models (FEMs) to analyze interlaminar failures in T and Hat section stringers. The delamination that first occurs between the tow filler and the bondline between the stringer and skin was of particular interest. Both locations are considered to be hot spots in such structural components, and failures tend to initiate from these areas. In this research, nanocomposite use was investigated as an alternative to traditional methods of suppressing delamination. The stringer was analyzed under different loading conditions and assuming different structural defects. Initial damage, defined as the first drop in the load displacement curve was considered to be a useful variable to compare the different behaviors in this study and was detected via the virtual crack closure technique (VCCT) implemented in the FE analysis. Experiments were conducted to test T section skin/stringer specimens under pull-off loading, replicating those used in composite panels as stiffeners. Two types of designs were considered: one using pure epoxy to fill the tow region and another that used nanocomposite with 5 wt. % CNTs. The response variable in the tests was the initial damage. Detailed analyses were conducted using FEMs to correlate with the experimental data. The correlation between both the experiment and model was satisfactory. Finally, the effects of thermal cure and temperature variation on nanocomposite structure behavior were studied, and both variables were determined to influence the nanocomposite structure performance.

A Method for Calculating Strain Energy Release Rates in Preliminary Design of Composite Skin/Stringer Debonding Under Multi-Axial Loading

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Minguet, Pierre J.; OBrien, T. Kevin

1999-01-01

Three simple procedures were developed to determine strain energy release rates, G, in composite skin/stringer specimens for various combinations of unaxial and biaxial (in-plane/out-of-plane) loading conditions. These procedures may be used for parametric design studies in such a way that only a few finite element computations will be necessary for a study of many load combinations. The results were compared with mixed mode strain energy release rates calculated directly from nonlinear two-dimensional plane-strain finite element analyses using the virtual crack closure technique. The first procedure involved solving three unknown parameters needed to determine the energy release rates. Good agreement was obtained when the external loads were used in the expression derived. This superposition technique was only applicable if the structure exhibits a linear load/deflection behavior. Consequently, a second technique was derived which was applicable in the case of nonlinear load/deformation behavior. The technique involved calculating six unknown parameters from a set of six simultaneous linear equations with data from six nonlinear analyses to determine the energy release rates. This procedure was not time efficient, and hence, less appealing. A third procedure was developed to calculate mixed mode energy release rates as a function of delamination lengths. This procedure required only one nonlinear finite element analysis of the specimen with a single delamination length to obtain a reference solution for the energy release rates and the scale factors. The delamination was extended in three separate linear models of the local area in the vicinity of the delamination subjected to unit loads to obtain the distribution of G with delamination lengths. This set of sub-problems was Although additional modeling effort is required to create the sub- models, this local technique is efficient for parametric studies.

Analysis of Composite Panel-Stiffener Debonding Using a Shell/3D Modeling Technique

NASA Technical Reports Server (NTRS)

Krueger, Ronald; Ratcliffe, James; Minguet, Pierre J.

2007-01-01

Interlaminar fracture mechanics has proven useful for characterizing the onset of delaminations in composites and has been used successfully primarily to investigate onset in fracture toughness specimens and laboratory size coupon type specimens. Future acceptance of the methodology by industry and certification authorities, however, requires the successful demonstration of the methodology on the structural level. For this purpose, a panel was selected that is reinforced with stiffeners. Shear loading causes the panel to buckle, and the resulting out-of-plane deformations initiate skin/stiffener separation at the location of an embedded defect. A small section of the stiffener foot, web and noodle as well as the panel skin in the vicinity of the delamination front were modeled with a local 3D solid model. Across the width of the stiffener foot, the mixedmode strain energy release rates were calculated using the virtual crack closure technique. A failure index was calculated by correlating the results with a mixed-mode failure criterion of the graphite/epoxy material. Computed failure indices were compared to corresponding results where the entire web was modeled with shell elements and only a small section of the stiffener foot and panel were modeled locally with solid elements. Including the stiffener web in the local 3D solid model increased the computed failure index. Further including the noodle and transition radius in the local 3D solid model changed the local distribution across the width. The magnitude of the failure index decreased with increasing transition radius and noodle area. For the transition radii modeled, the material properties used for the noodle area had a negligible effect on the results. The results of this study are intended to be used as a guide for conducting finite element and fracture mechanics analyses of delamination and debonding in complex structures such as integrally stiffened panels.

Damage Recovery in Carrara Marble

NASA Astrophysics Data System (ADS)

Meyer, G.; Brantut, N.; Mitchell, T. M.; Meredith, P. G.

2017-12-01

We investigate the effect of confining pressure on the recovery of elastic wave velocities following deformation episodes in Carrara Marble. Dry Carrara Marble cores were deformed in the ductile regime (Pc = 40 MPa) up to 3% axial strain. After deformation, samples were held at constant stress conditions for extended periods of time (5-8 days) whilst continuously recording volumetric strain and seismic wave velocities. The velocity data were used to invert for microcrack densities using an effective medium approach. Finally, thin sections were produced to characterise the microstructures after recovery. During deformation, elastic wave speeds decreased with increasing strain by more than 30% of the value for the intact rock due to the formation of distributed microcracks. Under constant hydrostatic pressure, wave speeds progressively recovered 12-90% of the initial drop, depending on the applied confining pressure. In contrast, the strain recovery (deformation towards the initial shape of the sample) during holding time is negligible (of the order of 10-4). Tests performed under nonhydrostatic (triaxial) stress conditions during recovery showed some time-dependent creep deformation together with very significant recovery of wave velocities. The recovery is interpreted as a progressive reduction in crack density within the sample. The process is highly dependent on confining pressure, which favours it. We propose that the driving process for wave speed recovery is the time-dependent increase of contact area between crack surfaces due to the formation and growth of asperity contacts. We develop a micromechanical model for crack closure driven by asperity creep, which shows a good fit to the experimental data. Most of the recovery is achieved in the initial few hours, implying it is the fastest recovery or healing process, and thus occurs prior to any chemical healing or mineral precipitation. Our data corroborate field observations of post-seismic fault behavior.

The effect of heat sinks in GTA microwelding

DOE Office of Scientific and Technical Information (OSTI.GOV)

Knorovsky, G.A.; Burchett, S.N.

1989-01-01

When miniature devices containing glass-to-metal seals are closure welded it is accepted practice to incorporate thermal heat sinks into the fixturing. This is intended to assure that the heat from gas tungsten arc (GTA) welding will not cause thermal stress-induced cracking of the seals and loss of hermeticity. The design of these heat sinks has never been systematically studied; instead only ''engineering horse sense'' has been applied. This practice has been successful in the past; however, the component being GTA welded have become smaller and more complex (i.e., more pins) and glass cracking problems are being encountered. The technology ofmore » producing glass seal-containing lids (called ''headers'') has benefited from finite element analyses in deciding how to optimally dimension pin-to-glass seal diameter ratios and glass-to-metal thickness ratios in order to minimize thermal stresses locked in during manufacture. It appeared likely that an analysts of the stresses generated by welding would also be beneficial. Recently, computer speed and code capabilities have increased to the point where finite element analysis of a close simulation of real hardware can be made, including the effect of external heat sinks. The work reported here involves an analysis (with some supporting experimental data) of a miniature thermal battery which encountered glass cracking problems. In the course of the analysis various heat sink practices were examined. Among other findings, through-thickness thermal gradients in a header with a heat sink were found to equal in-plane thermal gradients in a header without any heat sinking at the glass seal positions. Also noted were significant variations due to relatively minor changes in the weld preparation geometry. A summary of good practice for heat sinking will be presented. 4 refs., 6 figs., 2 tabs.« less

Shape Memory Alloy Modeling and Applications to Porous and Composite Structures

NASA Astrophysics Data System (ADS)

Zhu, Pingping

There has been a growing concern about an exciting class of advanced material -- shape memory alloys (SMAs) since their discovery several decades ago. SMAs exhibit large reversible stresses and strains owing to a thermoelastic phase transformation. They have been widely used in many engineering fields including aerospace, biomedical, and automotive engineering, especially as sensors, actuators, bone implants and deployable switches. The behavior of SMAs is very complex due to the coupling between thermal and mechanical effects. Theoretical and computational tools are used in this dissertation to investigate the mechanical behavior of SMA and its related structures for seeking better and wider application of this material. In the first part of this dissertation, we proposed an improved macroscopic phenomenological constitutive model of SMA that accounts for all major mechanical behaviors including elasticity, phase transformation, reorientation and plasticity. The model is based on some previous work developed in the Brinson group, and the current efforts are focused on plasticity, the application of a pre-defined strain, unification of notations, and other coding-related work. A user subroutine script VUMAT is developed to implement the constitutive model to the commercial finite element software Abaqus. Typical simulation results based on the model are presented, as well as verification with some experimental results. In the second part, we apply the developed constitutive model to a series of two-dimensional SMA plates with structured arrays of pores to investigate the structural response, especially the stress, strain, phase transformation, and plastic fields. Results are documented about the coupling of the elastic, transformation and plastic fields about the arrays of pores. Theoretical and experimental DIC results are also utilized to validate some simulation results. Conclusions are then drawn to provide understanding in the effect of pores and the underlying mechanism of pore interactions in the SMA foams. Additionally, the influence of geometric features including the number, size and locations of pores are studied to guide the design and optimization of porous SMAs. Thirdly, modeling and simulation are performed on a series of cracked self-healing SMA composite systems. These composites are to be applied in aeronautic structures where fatigue crack initiation and propagation is a significant safety and economic concern, based on a liquid-assisted SMA self-healing technology. We develop a modeling approach in Abaqus to create composite models with the as-is or pre-strained SMA wires. The modeling approach is validated by two simulation cases following the experiment setups. The amount of crack closure in the SMA-reinforced MMC is then focused, especially on the role of the SMA reinforcement, the softening property of the matrix, and the effect of pre-strain in the SMA. Composites with various geometric configurations of SMA are also created to study how the number, location, length and orientation of the SMA wires would affect the crack closure and self-healing behavior. These studies, from three aspects, provide deep insights to SMA and its related applications from the modeling and simulation point of view, which can further guide the development and application of this unique material.

Pore Structure Model for Predicting Elastic Wavespeeds in Fluid-Saturated Sandstones

NASA Astrophysics Data System (ADS)

Zimmerman, R. W.; David, E. C.

2011-12-01

During hydrostatic compression, in the elastic regime, ultrasonic P and S wave velocities measured on rock cores generally increase with pressure, and reach asymptotic values at high pressures. The pressure dependence of seismic velocities is generally thought to be due to the closure of compliant cracks, in which case the high-pressure velocities must reflect only the influence of the non-closable, equant "pores". Assuming that pores can be represented by spheroids, we can relate the elastic properties to the pore structure using an effective medium theory. Moreover, the closure pressure of a thin crack-like pore is directly proportional to its aspect ratio. Hence, our first aim is to use the pressure dependence of seismic velocities to invert the aspect ratio distribution. We use a simple analytical algorithm developed by Zimmerman (Compressibility of Sandstones, 1991), which can be used for any effective medium theory. Previous works have used overly restrictive assumptions, such as assuming that the stiff pores are spherical, or that the interactions between pores can be neglected. Here, we assume that the rock contains an exponential distribution of crack aspect ratios, and one family of stiff pores having an aspect ratio lying somewhere between 0.01 and 1. We develop our model in two versions, using the Differential Scheme, and the Mori-Tanaka scheme. The inversion is done using data obtained in dry experiments, since pore fluids have a strong effect on velocities and tend to mask the effect of the pore geometry. This avoids complicated joint inversion of dry and wet data, such as done by Cheng and Toksoz (JGR, 1979). Our results show that for many sets of data on sandstones, we can fit very well the dry velocities. Our second aim is to predict the saturated velocities from our pore structure model, noting that at a given differential stress, the pore structure should be the same as for a dry test. Our results show that the Biot-Gassmann predictions always underpredict the rock stiffness and that, for ultrasonic measurements performed at high frequencies (~MHz), it is more accurate to use the results from effective medium theories, which implicitly assume that the fluid is trapped in the pores. Hence, we use the aspect ratio distribution inverted from dry data, but this time introducing fluid in the pores. For a good number of experimental data on sandstones, our predictions for the saturated velocities match well the experimental data. This validates the use of a spheroidal model for pores. The results are only very weakly dependent on the choice of the effective medium theory. We conclude that our method, which remain relatively simple, is a useful tool to extract the pore aspect ratio distribution, as well as predicting the saturated velocities for sandstones.

Dynamical origins of the community structure of an online multi-layer society

NASA Astrophysics Data System (ADS)

Klimek, Peter; Diakonova, Marina; Eguíluz, Víctor M.; San Miguel, Maxi; Thurner, Stefan

2016-08-01

Social structures emerge as a result of individuals managing a variety of different social relationships. Societies can be represented as highly structured dynamic multiplex networks. Here we study the dynamical origins of the specific community structures of a large-scale social multiplex network of a human society that interacts in a virtual world of a massive multiplayer online game. There we find substantial differences in the community structures of different social actions, represented by the various layers in the multiplex network. Community sizes distributions are either fat-tailed or appear to be centered around a size of 50 individuals. To understand these observations we propose a voter model that is built around the principle of triadic closure. It explicitly models the co-evolution of node- and link-dynamics across different layers of the multiplex network. Depending on link and node fluctuation probabilities, the model exhibits an anomalous shattered fragmentation transition, where one layer fragments from one large component into many small components. The observed community size distributions are in good agreement with the predicted fragmentation in the model. This suggests that several detailed features of the fragmentation in societies can be traced back to the triadic closure processes.

Utility and Scope of Rapid Prototyping in Patients with Complex Muscular Ventricular Septal Defects or Double-Outlet Right Ventricle: Does it Alter Management Decisions?

PubMed

Bhatla, Puneet; Tretter, Justin T; Ludomirsky, Achi; Argilla, Michael; Latson, Larry A; Chakravarti, Sujata; Barker, Piers C; Yoo, Shi-Joon; McElhinney, Doff B; Wake, Nicole; Mosca, Ralph S

2017-01-01

Rapid prototyping facilitates comprehension of complex cardiac anatomy. However, determining when this additional information proves instrumental in patient management remains a challenge. We describe our experience with patient-specific anatomic models created using rapid prototyping from various imaging modalities, suggesting their utility in surgical and interventional planning in congenital heart disease (CHD). Virtual and physical 3-dimensional (3D) models were generated from CT or MRI data, using commercially available software for patients with complex muscular ventricular septal defects (CMVSD) and double-outlet right ventricle (DORV). Six patients with complex anatomy and uncertainty of the optimal management strategy were included in this study. The models were subsequently used to guide management decisions, and the outcomes reviewed. 3D models clearly demonstrated the complex intra-cardiac anatomy in all six patients and were utilized to guide management decisions. In the three patients with CMVSD, one underwent successful endovascular device closure following a prior failed attempt at transcatheter closure, and the other two underwent successful primary surgical closure with the aid of 3D models. In all three cases of DORV, the models provided better anatomic delineation and additional information that altered or confirmed the surgical plan. Patient-specific 3D heart models show promise in accurately defining intra-cardiac anatomy in CHD, specifically CMVSD and DORV. We believe these models improve understanding of the complex anatomical spatial relationships in these defects and provide additional insight for pre/intra-interventional management and surgical planning.

Rotorcraft fatigue life-prediction: Past, present, and future

NASA Technical Reports Server (NTRS)

Everett, Richard A., Jr.; Elber, W.

1994-01-01

In this paper the methods used for calculating the fatigue life of metallic dynamic components in rotorcraft is reviewed. In the past, rotorcraft fatigue design has combined constant amplitude tests of full-scale parts with flight loads and usage data in a conservative manner to provide 'safe life' component replacement times. This is in contrast to other industries, such as the automobile industry, where spectrum loading in fatigue testing is a part of the design procedure. Traditionally, the linear cumulative damage rule has been used in a deterministic manner using a conservative value for fatigue strength based on a one in a thousand probability of failure. Conservatism on load and usage are also often employed. This procedure will be discussed along with the current U.S. Army fatigue life specification for new rotorcraft which is the so-called 'six nines' reliability requirement. In order to achieve the six nines reliability requirement the exploration and adoption of new approaches in design and fleet management may also be necessary if this requirement is to be met with a minimum impact on structural weight. To this end a fracture mechanics approach to fatigue life design may be required in order to provide a more accurate estimate of damage progression. Also reviewed in this paper is a fracture mechanics approach for calculating total fatigue life which is based on a crack-closure small crack considerations.

Oxidation of C/SiC Composites at Reduced Oxygen Partial Pressures

NASA Technical Reports Server (NTRS)

Opila, E. J.; Serra, J. L.

2007-01-01

T-300 carbon fibers and T-300 carbon fiber reinforced silicon carbide composites (C/SiC) were oxidized in flowing reduced oxygen partial pressure environments at a total pressure of one atmosphere (0.5 atm O2, 0.05 atm O2 and 0.005 atm O2, balance argon). Experiments were conducted at four temperatures (816deg, 1149deg, 1343deg, and 1538 C). The oxidation kinetics were monitored using thermogravimetric analysis. T-300 fibers were oxidized to completion for times between 0.6 and 90 h. Results indicated that fiber oxidation kinetics were gas phase diffusion controlled. Oxidation rates had an oxygen partial pressure dependence with a power law exponent close to one. In addition, oxidation rates were only weakly dependent on temperature. The C/SiC coupon oxidation kinetics showed some variability, attributed to differences in the number and width of cracks in the SiC seal coat. In general, weight losses were observed indicating oxidation of the carbon fibers dominated the oxidation behavior. Low temperatures and high oxygen pressures resulted in the most rapid consumption of the carbon fibers. At higher temperatures, the lower oxidation rates were primarily attributed to crack closure due to SiC thermal expansion, rather than oxidation of SiC since these reduced rates were observed even at the lowest oxygen partial pressures where SiC oxidation is minimal.

Investigation on the cold rolling and structuring of cold sprayed copper-coated steel sheets

NASA Astrophysics Data System (ADS)

Bobzin, K.; Öte, M.; Wiesner, S.; Gerdt, L.; Senge, S.; Hirt, G.

2017-03-01

A current driving force of research is lightweight design. One of the approaches to reduce the weight of a component without causing an overall stiffness decrease is the use of multi-material components. One of the main challenges of this approach is the low bonding strength between different materials. Focusing on steel-aluminum multi-material components, thermally sprayed copper coatings can come into use as a bonding agent between steel sheets and high pressure die cast aluminum to improve the bonding strength. This paper presents a combination of cold gas spraying of copper coatings and their subsequent structuring by rolling as surface pretreatment method of the steel inserts. Therefore, flat rolling experiments are performed with samples in “as sprayed” and heat treated conditions to determine the influence of the rolling process on the bond strength and the formability of the coating. Furthermore, the influence of the rolling on the roughness and the hardness of the coating was examined. In the next step, the coated surface was structured, to create a surface topology suited for a form closure connection in a subsequent high-pressure die casting process. No cracks were observed after the cold rolling process with a thickness reduction of up to ε = 14 % for heat treated samples. Structuring of heat treated samples could be realized without delamination and cracking.

Linear Instability Analysis of non-uniform Bubbly Mixing layer with Two-Fluid model

NASA Astrophysics Data System (ADS)

Sharma, Subash; Chetty, Krishna; Lopez de Bertodano, Martin

We examine the inviscid instability of a non-uniform adiabatic bubbly shear layer with a Two-Fluid model. The Two-Fluid model is made well-posed with the closure relations for interfacial forces. First, a characteristic analysis is carried out to study the well posedness of the model over range of void fraction with interfacial forces for virtual mass, interfacial drag, interfacial pressure. A dispersion analysis then allow us to obtain growth rate and wavelength. Then, the well-posed two-fluid model is solved using CFD to validate the results obtained with the linear stability analysis. The effect of the void fraction and the distribution profile on stability is analyzed.

Characterization of the deformation and thermal behavior of granitic exfoliation sheets with LiDAR and infrared thermography (Yosemite Valley, USA)

NASA Astrophysics Data System (ADS)

Guerin, Antoine; Derron, Marc-Henri; Jaboyedoff, Michel; Collins, Brian D.; Stock, Greg M.

2017-04-01

Yosemite Valley is a long (11 km) and deep ( 1 km) glacier-carved valley, bounded by steep granitic cliffs cutting the western slope of the central Sierra Nevada mountain range (California, USA). These cliffs produce numerous rockfalls every year (925 events reported between 1857 and 2011) and this rockfall activity is often linked to the presence of sheeting joints (Stock et al., 2013), also called exfoliation joints, formed in response to stress changes associated with changes in the topography (Martel, 2011). Furthermore, the historical rockfall inventory indicates that many events occurred without recognized triggers (Austin et al., 2014), in summer time, and on sunny days in particular. This suggests that thermal stress changes are involved in triggering of rockfalls (Collins and Stock, 2016). To further characterize the relationship between thermal stresses and rock face deformation, we carried out three experiments in Yosemite Valley during October 2015: (i) monitoring of a sub-vertical granodiorite exfoliation sheet on the Rhombus Wall for 24 consecutive hours (from 8:00 p.m. to 8:00 p.m.) using terrestrial LiDAR, crackmeters and infrared thermal sensors; (ii) monitoring the El Capitan rockwall composed of tens of exfoliation sheets for 8 consecutive hours (from 5:30 p.m. to 1:30 a.m.) with terrestrial LiDAR and thermal imaging; (iii) collecting several sequences of thermal GigaPan panoramas during periods of rock cooling on both cliffs (Rhombus Wall and El Capitan). In parallel to these experiments, we also developed a method for calibrating and correcting the raw apparent temperature measured by our thermal imager (a FLIR T660 infrared camera) from thermoresistances, reflective and black papers and by using some information given by the LiDAR point clouds (range, dip and dip direction). LiDAR monitoring of experiments (i) and (ii) allowed us to detect millimetric deformations for the exfoliations sheets whose crack aperture is persistent, deep and greater than 9 cm, confirming the results of Collins and Stock (2016). Then, the LiDAR - infrared thermography coupling allowed us to establish a link between the contraction - expansion cycles observed and daily thermal variations: the cycles of contraction (crack closure) occur between 3:00 p.m. and 8:00 a.m. and are associated with cooling, whereas the opposite is true for the expansion cycles (crack opening). In addition, in the case of experiment (i), we observe a delay of about 40 minutes between the time when surface temperatures are minimum and the maximum closure of the crack (-5.33 +/- 0.01 mm), which occurs a little before 8:00 a.m. Concerning the thermal behavior of the exfoliation sheets, the experiments (i) and (ii) show that the exfoliation sheets are almost always colder than surrounding stable areas, except during the hottest hours of the day when the temperatures are similar. At the end of the night, the temperature deviation between an exfoliation sheet and a stable part can reach 5 to 6 Celsius degrees (values valid for October) and this thermal contrast makes it possible to remotely detect the presence of exfoliation sheets in a rockwall. This result was then confirmed by the experiment (iii) which shows that a whole series of exfoliation sheets could be detected at a distance of 1 km, by means of thermal comparisons. Coupled to the LiDAR, infrared thermography can thus be useful for drawing a 3D map of exfoliation sheets in a cliff of several hundred meters high.

Fatigue behavior and encrustation characteristics of nanocrystalline metals

NASA Astrophysics Data System (ADS)

Lai, Li-Chung

The nanocrstalline (NC) metals have been reported to have high mechanical performance owing to it's small grain interior and a large volume fraction of grain boundary (GB) atoms. Small grain leads to the forbidden dislocation activities in grain interior while GB activities become dominant due to a higher volume fraction of GB atoms. Regarding the fatigue response to nanocrstalline metals, it has been reported that decreasing grain led to both significantly improvement on the fatigue-endurance limit and deleterious effect on the resistance to subcritical fatigue crack propagation. The increases endurance limit has been attributed to the greater resistance to fatigue crack initiation at near-surface regions. On the other hand, the less resistance to fatigue crack growth were resulted from less tortuous fatigue crack profiles supported by the deflection/closure theory. However, it has never been studied the influence of proceeding and pre-existing defects on the fatigue performance considering the difference response of NC structure from than coarse grain (CG) structure. In the present work, the influence of electrical discharge machining (EDM) and surface defects on the fatigue behavior of both conventional cold-rolled CG and electro-deposited (ED) NC Ni were investigated. The experimental results revealed considerable influence by EDM on the fatigue strength of NC Ni, while it has little or no affect on that for CG Ni. Specifically, EDM led to a 50 to 75% reduction in fatigue strength for NC Ni despite a relatively small depth of EDM affected material (˜ 1% of width). Rationale for this effect can be attributed to grain growth, microcracks, and a higher sulfur content at the GBs in the EDM affected zone. In addition, the pre-existing surface defects that appear to be due to impurity segregation near the electro-deposition substrate significantly reduced the fatigue resistance of ED NC Ni. In order to understand the fatigued behavior in NC Ni, crack tip grain structures were investigated using transmission electron microscope (TEM). Crack tip grain growth was observed at early state of crack propagation with low stress intensity factor (K ˜ 6 MPa m 1/2). As K increased, the size of grain growth zone increased exponentially in width and crack propagation behavior transmitted from interganular to transgranular. It appears that this transmission is associated with grain growth. The coalesced grains due to grain rotation/GB diffusion created larger paths for more extended dislocation movement. Dislocation activities become less forbidden and the dislocation-slip mechanism can be dominant leading to a more plastically transgranular fracture. In addition to fatigue study of ED NC Ni, encrustation on ED NC Ti was investigated. The use of materials for medical applications in the urinary tract is hampered by the formation of calcium-based crystalline deposits, generally referred to as encrustation, that act as precursors to urinary stones. Anecdotal evidence suggests that titanium can possess encrustation-resistant properties in vivo and may be useful in urologic applications. To test the utility of coating surfaces with nanostructured titanium, several forms of materials were submersed in artificial urine with saturating concentrations of calcium for a period of 14 days. The specimens were then analyzed using scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) to determine the extent of encrustation on the surface of the various samples. Our observations indicate that nanostructured titanium offers superior resistance to encrustation when compared to polyurethane, polyvinyl chloride or conventional coarser grained titanium. Further studies investigating the use of nanostructured titanium in urologic applications are warranted.

Development of high integrity, maximum durability concrete structures for LLW disposal facilities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Taylor, W.P.

1992-05-01

A number of disposal facilities for Low-Level Radioactive Wastes have been planned for the Savannah River Site. Design has been completed for disposal vaults for several waste classifications and construction is nearly complete or well underway on some facilities. Specific design criteria varies somewhat for each waste classification. All disposal units have been designed as below-grade concrete vaults, although the majority will be above ground for many years before being encapsulated with earth at final closure. Some classes of vaults have a minimum required service life of 100 years. All vaults utilize a unique blend of cement, blast furnace slagmore » and pozzolan. The design synthesizes the properties of the concrete mix with carefully planned design details and construction methodologies to (1) eliminate uncontrolled cracking; (2) minimize leakage potential; and (3) maximize durability. The first of these vaults will become operational in 1992. 9 refs.« less

Development of high integrity, maximum durability concrete structures for LLW disposal facilities

DOE Office of Scientific and Technical Information (OSTI.GOV)

Taylor, W.P.

1992-01-01

A number of disposal facilities for Low-Level Radioactive Wastes have been planned for the Savannah River Site. Design has been completed for disposal vaults for several waste classifications and construction is nearly complete or well underway on some facilities. Specific design criteria varies somewhat for each waste classification. All disposal units have been designed as below-grade concrete vaults, although the majority will be above ground for many years before being encapsulated with earth at final closure. Some classes of vaults have a minimum required service life of 100 years. All vaults utilize a unique blend of cement, blast furnace slagmore » and pozzolan. The design synthesizes the properties of the concrete mix with carefully planned design details and construction methodologies to (1) eliminate uncontrolled cracking; (2) minimize leakage potential; and (3) maximize durability. The first of these vaults will become operational in 1992. 9 refs.« less

The role of collagen on the structural response of dermal layers in mammals and fish

NASA Astrophysics Data System (ADS)

Sherman, Vincent Robert

We study in depth the role of collagen in the protective layers of mammals (skin) and fish (scales) in depth to reveal its contribution to their mechanical performance. In order to gain an understanding of the structure property relations, we investigate its hierarchical arrangement and how it results in a specialized response. For rabbit skin, chosen as a model material for the dermis of vertebrates, deformation is expressed in terms of four mechanisms of collagen fibril activity that virtually eliminate the possibility of tearing in notched samples: fibril straightening, fibril reorientation towards the tensile direction, elastic stretching, and interfibrillar sliding. A model reflecting the in vivo shape of collagen is derived. The model incorporates the effects of its elasticity, viscoelasticity, and orientation. For arapaima and alligator gar scales, we investigate their protective function and identify key features which result in their resistance to failure. For the elasmoid scales of the arapaima, we show that the scale has a Bouligand-like arrangement of collagen layers which stretch, rotate, and delaminate to dissipate energy and arrest cracking prior to catastrophic failure. Atop the foundation are mineral ridges; this arrangement provides high toughness and resistance to penetration by predator teeth. We show that the ganoid scales of the alligator gar have a boney composite foundation of collagen and hydroxyapatite as well as an external surface of pure hydroxyapatite. Failure averting features of the gar scale include: crack inhibiting mineral decussation in the external ganoine layer; mineral crystals and tubules which deflect cracks in the bony region; and saw-tooth ridges along the interface between the two scale layers which direct cracks away from the weak interface. Furthermore, the scale's geometry is optimized to provide full coverage while accommodating physiological motion. Key features of the scale morphology are replicated in a bioinspired model which retains protection and flexibility.

Impurity-doped optical shock, detonation and damage location sensor

DOEpatents

Weiss, J.D.

1995-02-07

A shock, detonation, and damage location sensor providing continuous fiber-optic means of measuring shock speed and damage location, and could be designed through proper cabling to have virtually any desired crush pressure. The sensor has one or a plurality of parallel multimode optical fibers, or a singlemode fiber core, surrounded by an elongated cladding, doped along their entire length with impurities to fluoresce in response to light at a different wavelength entering one end of the fiber(s). The length of a fiber would be continuously shorted as it is progressively destroyed by a shock wave traveling parallel to its axis. The resulting backscattered and shifted light would eventually enter a detector and be converted into a proportional electrical signals which would be evaluated to determine shock velocity and damage location. The corresponding reduction in output, because of the shortening of the optical fibers, is used as it is received to determine the velocity and position of the shock front as a function of time. As a damage location sensor the sensor fiber cracks along with the structure to which it is mounted. The size of the resulting drop in detector output is indicative of the location of the crack. 8 figs.

Impurity-doped optical shock, detonation and damage location sensor

DOEpatents

Weiss, Jonathan D.

1995-01-01

A shock, detonation, and damage location sensor providing continuous fiber-optic means of measuring shock speed and damage location, and could be designed through proper cabling to have virtually any desired crush pressure. The sensor has one or a plurality of parallel multimode optical fibers, or a singlemode fiber core, surrounded by an elongated cladding, doped along their entire length with impurities to fluoresce in response to light at a different wavelength entering one end of the fiber(s). The length of a fiber would be continuously shorted as it is progressively destroyed by a shock wave traveling parallel to its axis. The resulting backscattered and shifted light would eventually enter a detector and be converted into a proportional electrical signals which would be evaluated to determine shock velocity and damage location. The corresponding reduction in output, because of the shortening of the optical fibers, is used as it is received to determine the velocity and position of the shock front as a function of time. As a damage location sensor the sensor fiber cracks along with the structure to which it is mounted. The size of the resulting drop in detector output is indicative of the location of the crack.

On the Mechanical Properties and Microstructure of Nitinol forBiomedical Stent Applications

DOE Office of Scientific and Technical Information (OSTI.GOV)

Robertson, Scott W.

2006-01-01

This dissertation was motivated by the alarming number of biomedical device failures reported in the literature, coupled with the growing trend towards the use of Nitinol for endovascular stents. The research is aimed at addressing two of the primary failure modes in Nitinol endovascular stents: fatigue-crack growth and overload fracture. The small dimensions of stents, coupled with their complex geometries and variability among manufacturers, make it virtually impossible to determine generic material constants associated with specific devices. Instead, the research utilizes a hybrid of standard test techniques (fracture mechanics and x-ray micro-diffraction) and custom-designed testing apparatus for the determination ofmore » the fracture properties of specimens that are suitable representations of self-expanding Nitinol stents. Specifically, the role of texture (crystallographic alignment of atoms) and the austenite-to-martensite phase transformation on the propagation of cracks in Nitinol was evaluated under simulated body conditions and over a multitude of stresses and strains. The results determined through this research were then used to create conservative safe operating and inspection criteria to be used by the biomedical community for the determination of specific device vulnerability to failure by fracture and/or fatigue.« less

Reach-to-grasp movement as a minimization process.

PubMed

Yang, Fang; Feldman, Anatol G

2010-02-01

It is known that hand transport and grasping are functionally different but spatially coordinated components of reach-to-grasp (RTG) movements. As an extension of this notion, we suggested that body segments involved in RTG movements are controlled as a coherent ensemble by a global minimization process associated with the necessity for the hand to reach the motor goal. Different RTG components emerge following this process without pre-programming. Specifically, the minimization process may result from the tendency of neuromuscular elements to diminish the spatial gap between the actual arm-hand configuration and its virtual (referent) configuration specified by the brain. The referent configuration is specified depending on the object shape, localization, and orientation. Since the minimization process is gradual, it can be interrupted and resumed following mechanical perturbations, at any phase during RTG movements, including hand closure. To test this prediction of the minimization hypothesis, we asked subjects to reach and grasp a cube placed within the reach of the arm. Vision was prevented during movement until the hand returned to its initial position. As predicted, by arresting wrist motion at different points of hand transport in randomly selected trials, it was possible to halt changes in hand aperture at any phase, not only during hand opening but also during hand closure. Aperture changes resumed soon after the wrist was released. Another test of the minimization hypothesis was made in RTG movements to an object placed beyond the reach of the arm. It has previously been shown (Rossi et al. in J Physiol 538:659-671, 2002) that in such movements, the trunk motion begins to contribute to hand transport only after a critical phase when the shifts in the referent arm configuration have finished (at about the time when hand velocity is maximal). The minimization rule suggests that when the virtual contribution of the arm to hand transport is completed, guidance of hand aperture switches from the arm to the trunk control system. As a consequence, hand aperture changes can be halted by trunk arrests but only if they are prolonged beyond a critical phase. As predicted, hand transport and hand aperture in RTG movements beyond the reach of the arm were halted by trunk arrests only if they were prolonged beyond the time of peak hand velocity. Hand motion and aperture changes resumed only when the trunk was released. While supporting the minimization hypothesis, our findings imply that not only spatial but also temporal characteristics of each component, including the shortest, hand closure component of RTG movements, are controlled in a flexible, task-specific way.

Peridynamic theory for modeling three-dimensional damage growth in metallic and composite structures

NASA Astrophysics Data System (ADS)

Ochoa-Ricoux, Juan Pedro

A recently introduced nonlocal peridynamic theory removes the obstacles present in classical continuum mechanics that limit the prediction of crack initiation and growth in materials. It is also applicable at different length scales. This study presents an alternative approach for the derivation of peridynamic equations of motion based on the principle of virtual work. It also presents solutions for the longitudinal vibration of a bar subjected to an initial stretch, propagation of a pre-existing crack in a plate subjected to velocity boundary conditions, and crack initiation and growth in a plate with a circular cutout. Furthermore, damage growth in composites involves complex and progressive failure modes. Current computational tools are incapable of predicting failure in composite materials mainly due to their mathematical structure. However, the peridynamic theory removes these obstacles by taking into account non-local interactions between material points. Hence, an application of the peridynamic theory to predict how damage propagates in fiber reinforced composite materials subjected to mechanical and thermal loading conditions is presented. Finally, an analysis approach based on a merger of the finite element method and the peridynamic theory is proposed. Its validity is established through qualitative and quantitative comparisons against the test results for a stiffened composite curved panel with a central slot under combined internal pressure and axial tension. The predicted initial and final failure loads, as well as the final failure modes, are in close agreement with the experimental observations. This proposed approach demonstrates the capability of the PD approach to assess the durability of complex composite structures.

Large-Strain Monitoring Above a Longwall Coal Mine With GPS and Seismic Measurements

NASA Astrophysics Data System (ADS)

Swanson, P. L.; Andreatta, V.; Meertens, C. M.; Krahenbuhl, T.; Kenner, B.

2001-12-01

As part of an effort to evaluate continuous GPS measurements for use in mine safety studies, a joint GPS-seismic experiment was conducted at an underground longwall coal mine near Paonia, Colorado in June, 2001. Seismic and deformation signals were measured using prototype low-cost monitoring systems as a longwall panel was excavated 150 m beneath the site. Data from both seismic and GPS instruments were logged onto low-power PC-104 Linux computers which were networked using a wireless LAN. The seismic system under development at NIOSH/SRL is based on multiple distributed 8-channel 24-bit A/D converters. The GPS system uses a serial single-frequency (L1) receiver and UNAVCO's "Jstream" Java data logging software. For this experiment, a continuously operating dual-frequency GPS receiver was installed 2.4 km away to serve as a reference site. In addition to the continuously operating sites, 10 benchmarks were surveyed daily with short "rapid-static" occupations in order to provide greater spatial sampling. Two single-frequency sites were located 35 meters apart on a relatively steep north-facing slope. As mining progressed from the east, net displacements of 1.2 meters to the north and 1.65 meters of subsidence were observed over a period of 6 days. The east component exhibited up to 0.45 meters of eastward displacement (toward the excavation) followed by reverse movement to the west. This cycle, observed approximately two days earlier at the eastern L1 site, is consistent with a change in surface strain from tension to compression as the excavation front passed underneath. As this strain "wave" propagated across the field site, surface deformation underwent a cycle of tension crack nucleation, crack opening (up to 15 cm normal displacements), subsequent crack closure, and production of low-angle-thrust compressional deformation features. Analysis of seismic results, surface deformation, and additional survey results are presented.

Decline of radionuclides in the nearshore environment following nuclear reactor closure: A U.K. case study

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cundy, A.B.; Croudace, I.W.; Warwick, P.E.

1999-09-01

Radioactive discharges from nuclear facilities are frequently made into the marine environment and their fate during and after cessation of discharges is a matter of interest and concern. This study examines the decline of the radionuclides {sup 60} and {sup 65}Zn along the southern UK. coast, over the per 1988--1998, following the closure of the steam-generating heavy water (SGHW) reactor at AEA Winfrith, Dorset, UK. {sup 60}Co and {sup 65}Zn (and other activation products such as {sup 63}Ni and {sup 55}Fe) were widely dispersed in the marine environment off the central south coast of England, due to authorized releases frommore » AEA Winfrith. Significant interaction occurred with clay-rich sediments and biota. A general exponential decline in {sup 60}Co activities (and in {sup 65}Zn activity) is found in intertidal mudflat sediments, seaweed and marine fauna in different areas along the south coast following closure of the reactor in 1990. Effective half-lives are determined which vary from 1 to 4 years in surface sediments ({sup 60}Co only), 1--4 years in seaweed and 0.5--2.5 years in crustaceans, bivalves and molluscs. Physical mixing and bioturbation largely control the rate at which {sup 60}Co declines in surface sediments. Both {sup 60}Co and {sup 65}Zn show a relatively slow rate of decline in seaweed and in marine fauna, showing that even after the virtual cessation of discharge from nuclear facilities, contamination of these organisms may persist for a number of years, albeit at reduced activities. Reasons for this persistence are likely to include absorption of radionuclides from sediment, and release and recycling of radionuclides via breakdown of contaminated organic material.« less

Time-dependent recovery of microcrack damage and seismic wave speeds in deformed limestone

NASA Astrophysics Data System (ADS)

Brantut, Nicolas

2015-12-01

Limestone samples were deformed up to 5% inelastic axial strain at an effective confining pressure Peff=50 MPa in the cataclastic flow regime and subsequently maintained under constant static stress conditions (either isostatic of triaxial) for extended periods of time while elastic wave speeds and permeability were continuously monitored. During deformation, both seismic wave speeds and permeability decrease with increasing strain, due to the growth of subvertical microcracks and inelastic porosity reduction. During the static hold period under water-saturated conditions, the seismic wave speeds recovered gradually, typically by around 5% (relative to their initial value) after 2 days, while permeability remained constant. The recovery in wave speed increases with increasing confining pressure but decreases with increasing applied differential stress. The recovery is markedly lower when the samples are saturated with an inert fluid as opposed to water. The evolution in wave speed is interpreted quantitatively in terms of microcrack density, which shows that the post-deformation recovery is associated with a decrease in effective microcrack length, typically of the order to 10% after 2 days. The proposed mechanism for the observed damage recovery is microcrack closure due to a combination of backsliding on wing cracks driven by time-dependent friction and closure due to pressure solution at contacts between propping particles or asperities and microcrack walls. The recovery rates observed in the experiments, and the proposed underlying mechanisms, are compatible with seismological observations of seismic wave speed recovery along faults following earthquakes.

The role of water in the recovery of microcrack damage, permeability and seismic wave speeds in limestone

NASA Astrophysics Data System (ADS)

Brantut, N.

2015-12-01

Limestone samples were deformed up to 5% inelastic axial strain at an effective confining pressure P_{eff}=50 MPa, in the cataclastic flow regime, and subsequently maintained under constant static stress conditions for extended periods of time while elastic wave speeds and permeability were continously monitored. During deformation, both seismic wave speeds and permeability decrease with increasing strain, due to the growth of sub-vertical microcracks and inelastic porosity reduction. During the static hold period under water-satured conditions, the seismic wave speeds recovered gradually, typically by around 5% (relative to their initial value) after two days, while permeability remained constant. The recovery in wave speed increases with increasing confining pressure, but decreases with increasing applied differential stress. The recovery is markedly lower when the samples are saturated with an inert fluid as opposed to water. The evolution in wave speed is interpreted quantitatively in terms of microcrack density, which shows that the post-deformation recovery is associated with an decrease in effective microcrack length, typically of the order to 10% after two days. The proposed mechanism for the observed damage recovery is microcrack closure due to a combination of backsliding on wing cracks driven by time-dependent friction and closure due to pressure-solution at contacts between propping particles or asperities and microcrack walls. The recovery rates observed in the experiments, and the proposed underlying mechanisms, are compatible with seismological observations of seismic wave speed recovery along faults following earthquakes.

Fluid Production Induced Stress Analysis Surrounding an Elliptic Fracture

NASA Astrophysics Data System (ADS)

Pandit, Harshad Rajendra

Hydraulic fracturing is an effective technique used in well stimulation to increase petroleum well production. A combination of multi-stage hydraulic fracturing and horizontal drilling has led to the recent boom in shale gas production which has changed the energy landscape of North America. During the fracking process, highly pressurized mixture of water and proppants (sand and chemicals) is injected into to a crack, which fractures the surrounding rock structure and proppants help in keeping the fracture open. Over a longer period, however, these fractures tend to close due to the difference between the compressive stress exerted by the reservoir on the fracture and the fluid pressure inside the fracture. During production, fluid pressure inside the fracture is reduced further which can accelerate the closure of a fracture. In this thesis, we study the stress distribution around a hydraulic fracture caused by fluid production. It is shown that fluid flow can induce a very high hoop stress near the fracture tip. As the pressure gradient increases stress concentration increases. If a fracture is very thin, the flow induced stress along the fracture decreases, but the stress concentration at the fracture tip increases and become unbounded for an infinitely thin fracture. The result from the present study can be used for studying the fracture closure problem, and ultimately this in turn can lead to the development of better proppants so that prolific well production can be sustained for a long period of time.

Materials for Consideration in Standardized Canister Design Activities.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bryan, Charles R.; Ilgen, Anastasia Gennadyevna; Enos, David George

2014-10-01

This document identifies materials and material mitigation processes that might be used in new designs for standardized canisters for storage, transportation, and disposal of spent nuclear fuel. It also addresses potential corrosion issues with existing dual-purpose canisters (DPCs) that could be addressed in new canister designs. The major potential corrosion risk during storage is stress corrosion cracking of the weld regions on the 304 SS/316 SS canister shell due to deliquescence of chloride salts on the surface. Two approaches are proposed to alleviate this potential risk. First, the existing canister materials (304 and 316 SS) could be used, but themore » welds mitigated to relieve residual stresses and/or sensitization. Alternatively, more corrosion-resistant steels such as super-austenitic or duplex stainless steels, could be used. Experimental testing is needed to verify that these alternatives would successfully reduce the risk of stress corrosion cracking during fuel storage. For disposal in a geologic repository, the canister will be enclosed in a corrosion-resistant or corrosion-allowance overpack that will provide barrier capability and mechanical strength. The canister shell will no longer have a barrier function and its containment integrity can be ignored. The basket and neutron absorbers within the canister have the important role of limiting the possibility of post-closure criticality. The time period for corrosion is much longer in the post-closure period, and one major unanswered question is whether the basket materials will corrode slowly enough to maintain structural integrity for at least 10,000 years. Whereas there is extensive literature on stainless steels, this evaluation recommends testing of 304 and 316 SS, and more corrosion-resistant steels such as super-austenitic, duplex, and super-duplex stainless steels, at repository-relevant physical and chemical conditions. Both general and localized corrosion testing methods would be used to establish corrosion rates and component lifetimes. Finally, it is unlikely that the aluminum-based neutron absorber materials that are commonly used in existing DPCs would survive for 10,000 years in disposal environments, because the aluminum will act as a sacrificial anode for the steel. We recommend additional testing of borated and Gd-bearing stainless steels, to establish general and localized corrosion resistance in repository-relevant environmental conditions.« less

Successful bridge to transplant in a highly sensitized patient with a complicated pump pocket infection.

PubMed

McGee, Edwin C; Cotts, William; Tambur, Anat R; Friedewald, John; Kim, John; O'Connell, John; Wallace, Suzanne; McCarthy, Patrick M

2008-05-01

A 32-year-old man with doxorubicin-induced cardiomyopathy presented in cardiogenic shock. He underwent placement of a Novacor (WorldHeart, Inc., Oakland, CA) left ventricular assist device as a bridge to transplant. Post-operatively he developed a pump pocket infection and dehiscence of his abdominal wound with exposure of the pump. This was treated with irrigation and drainage, antibiotic bead placement and flap closure. Both pre- and post-operative panel-reactive antibodies (PRA) were elevated. He underwent desensitization with intravenous immune globulin (IVIg), rituximab, mycophenolate mofetil and pre-operative plasmapheresis. A donor heart was identified and found to be acceptable by virtual crossmatch. He was transplanted and is doing well with normal graft function at >1 year post-operatively.

Virtual earthquake engineering laboratory with physics-based degrading materials on parallel computers

NASA Astrophysics Data System (ADS)

Cho, In Ho

For the last few decades, we have obtained tremendous insight into underlying microscopic mechanisms of degrading quasi-brittle materials from persistent and near-saintly efforts in laboratories, and at the same time we have seen unprecedented evolution in computational technology such as massively parallel computers. Thus, time is ripe to embark on a novel approach to settle unanswered questions, especially for the earthquake engineering community, by harmoniously combining the microphysics mechanisms with advanced parallel computing technology. To begin with, it should be stressed that we placed a great deal of emphasis on preserving clear meaning and physical counterparts of all the microscopic material models proposed herein, since it is directly tied to the belief that by doing so, the more physical mechanisms we incorporate, the better prediction we can obtain. We departed from reviewing representative microscopic analysis methodologies, selecting out "fixed-type" multidirectional smeared crack model as the base framework for nonlinear quasi-brittle materials, since it is widely believed to best retain the physical nature of actual cracks. Microscopic stress functions are proposed by integrating well-received existing models to update normal stresses on the crack surfaces (three orthogonal surfaces are allowed to initiate herein) under cyclic loading. Unlike the normal stress update, special attention had to be paid to the shear stress update on the crack surfaces, due primarily to the well-known pathological nature of the fixed-type smeared crack model---spurious large stress transfer over the open crack under nonproportional loading. In hopes of exploiting physical mechanism to resolve this deleterious nature of the fixed crack model, a tribology-inspired three-dimensional (3d) interlocking mechanism has been proposed. Following the main trend of tribology (i.e., the science and engineering of interacting surfaces), we introduced the base fabric of solid particle-soft matrix to explain realistic interlocking over rough crack surfaces, and the adopted Gaussian distribution feeds random particle sizes to the entire domain. Validation against a well-documented rough crack experiment reveals promising accuracy of the proposed 3d interlocking model. A consumed energy-based damage model has been proposed for the weak correlation between the normal and shear stresses on the crack surfaces, and also for describing the nature of irrecoverable damage. Since the evaluation of the consumed energy is directly linked to the microscopic deformation, which can be efficiently tracked on the crack surfaces, the proposed damage model is believed to provide a more physical interpretation than existing damage mechanics, which fundamentally stem from mathematical derivation with few physical counterparts. Another novel point of the present work lies in the topological transition-based "smart" steel bar model, notably with evolving compressive buckling length. We presented a systematic framework of information flow between the key ingredients of composite materials (i.e., steel bar and its surrounding concrete elements). The smart steel model suggested can incorporate smooth transition during reversal loading, tensile rupture, early buckling after reversal from excessive tensile loading, and even compressive buckling. Especially, the buckling length is made to evolve according to the damage states of the surrounding elements of each bar, while all other dominant models leave the length unchanged. What lies behind all the aforementioned novel attempts is, of course, the problem-optimized parallel platform. In fact, the parallel computing in our field has been restricted to monotonic shock or blast loading with explicit algorithm which is characteristically feasible to be parallelized. In the present study, efficient parallelization strategies for the highly demanding implicit nonlinear finite element analysis (FEA) program for real-scale reinforced concrete (RC) structures under cyclic loading are proposed. Quantitative comparison of state-of-the-art parallel strategies, in terms of factorization, had been carried out, leading to the problem-optimized solver, which is successfully embracing the penalty method and banded nature. Particularly, the penalty method employed imparts considerable smoothness to the global response, which yields a practical superiority of the parallel triangular system solver over other advanced solvers such as parallel preconditioned conjugate gradient method. Other salient issues on parallelization are also addressed. The parallel platform established offers unprecedented access to simulations of real-scale structures, giving new understanding about the physics-based mechanisms adopted and probabilistic randomness at the entire system level. Particularly, the platform enables bold simulations of real-scale RC structures exposed to cyclic loading---H-shaped wall system and 4-story T-shaped wall system. The simulations show the desired capability of accurate prediction of global force-displacement responses, postpeak softening behavior, and compressive buckling of longitudinal steel bars. It is fascinating to see that intrinsic randomness of the 3d interlocking model appears to cause "localized" damage of the real-scale structures, which is consistent with reported observations in different fields such as granular media. Equipped with accuracy, stability and scalability as demonstrated so far, the parallel platform is believed to serve as a fertile ground for the introducing of further physical mechanisms into various research fields as well as the earthquake engineering community. In the near future, it can be further expanded to run in concert with reliable FEA programs such as FRAME3d or OPENSEES. Following the central notion of "multiscale" analysis technique, actual infrastructures exposed to extreme natural hazard can be successfully tackled by this next generation analysis tool---the harmonious union of the parallel platform and a general FEA program. At the same time, any type of experiments can be easily conducted by this "virtual laboratory."

The Lead Crack Fatigue Lifting Framework

DTIC Science & Technology

2010-04-01

15000 20000 25000 30000 35000 40000 F-WELD Test Hours C ra ck d ep th (m m ) IPP (1) IPP (2) IPP (3) IPP ( 4 ) IPP (5) IPP ( 6 ) IPP (6_A) FASS 226 (2...FASS 226 ( 4 ) FASS 226 (5) FASS 226 (5L) FASS 226 ( 6 ) FASS 226 (6R) SIH_OBD_SPLICE SIH_FS238/239 (1) SIH_FS238/239 (2) SIH_FS238/239 (3) SIH_FS238/239...conservative. 0.001 0.01 0.1 1 10 0 1 2 3 4 5 6 CB9 RHS CB8 LHS CB12 LHS CB12 RHS CB1 LHS FT55 LHS FT55 RHS ST16 LHS ST16 RHS Log average FLEI Virtual test

Oxygen diffusion barrier coating

NASA Technical Reports Server (NTRS)

Unnam, Jalaiah (Inventor); Clark, Ronald K. (Inventor)

1987-01-01

A method for coating a titanium panel or foil with aluminum and amorphous silicon to provide an oxygen barrier abrogating oxidation of the substrate metal is developed. The process is accomplished with known inexpensive procedures common in materials research laboratories, i.e., electron beam deposition and sputtering. The procedures are conductive to treating foil gage titanium and result in submicron layers which virtually add no weight to the titanium. There are no costly heating steps. The coatings blend with the substrate titanium until separate mechanical properties are subsumed by those of the substrate without cracking or spallation. This method appreciably increases the ability of titanium to mechanically perform in high thermal environments such as those witnessed on structures of space vehicles during re-entry

Using a Virtual Reality Social Network During Awake Craniotomy to Map Social Cognition: Prospective Trial.

PubMed

Bernard, Florian; Lemée, Jean-Michel; Aubin, Ghislaine; Ter Minassian, Aram; Menei, Philippe

2018-06-26

In awake craniotomy, it is possible to temporarily inactivate regions of the brain using direct electrical stimulation, while the patient performs neuropsychological tasks. If the patient shows decreased performance in a given task, the neurosurgeon will not remove these regions, so as to maintain all brain functions. The objective of our study was to describe our experience of using a virtual reality (VR) social network during awake craniotomy and discuss its future applications for perioperative mapping of nonverbal language, empathy, and theory of mind. This was a single-center, prospective, unblinded trial. During wound closure, different VR experiences with a VR headset were proposed to the patient. This project sought to explore interactions with the neuropsychologist's avatar in virtual locations using a VR social network as an available experience. Three patients experienced VR. Despite some limitations due to patient positioning during the operation and the limitation of nonverbal cues inherent to the app, the neuropsychologist, as an avatar, could communicate with the patient and explore gesture communication while wearing a VR headset. With some improvements, VR social networks can be used in the near future to map social cognition during awake craniotomy. ClinicalTrials.gov NCT03010943; https://clinicaltrials.gov/ct2/show/NCT03010943 (Archived at WebCite at http://www.webcitation.org/70CYDil0P). ©Florian Bernard, Jean-Michel Lemée, Ghislaine Aubin, Aram Ter Minassian, Philippe Menei. Originally published in the Journal of Medical Internet Research (http://www.jmir.org), 26.06.2018.

Guided Immediate Implant Placement with Wound Closure by Computer-Aided Design/Computer-Assisted Manufacture Sealing Socket Abutment: Case Report.

PubMed

Finelle, Gary; Lee, Sang J

Digital technology has been widely used in the field of implant dentistry. From a surgical standpoint, computer-guided surgery can be utilized to enhance primary implant stability and to improve the precision of implant placement. From a prosthetic standpoint, computer-aided design/computer-assisted manufacture (CAD/CAM) technology has brought about various restorative options, including the fabrication of customized abutments through a virtual design based on computer-guided surgical planning. This case report describes a novel technique combining the use of a three-dimensional (3D) printed surgical template for the immediate placement of an implant, with CAD/CAM technology to optimize hard and soft tissue healing after bone grafting with the use of a socket sealing abutment.

Hydrologic data on channel adjustments, 1970 to 1975, on the Rio Grande downstream from Cochiti Dam, New Mexico before and after closure

USGS Publications Warehouse

Dewey, Jack D.; Roybal, F.E.; Funderburg, D.E.

1979-01-01

Cross-section channel profiles, sediment transport and hydrologic data have been observed and computed for a series of pre-dam and post-dam investigations from 1970 to 1975 at 37 cross sections established along a 59-mile study reach from Cochiti Dam to Isleta Diversion Dam, New Mexico. Cochiti Dam began impounding water in November 1973. Because the dam will trap virtually all of the sediment load originating upstream and water discharge will be controlled, it is expected that equilibrium values of channel width, depth, slope and sediment-transport capability in the existing main stem of the Rio Grande will change. Changes in cross sections with time and space and changes in size distribution of sediments are documented. (Woodard-USGS).

How does pea architecture influence light sharing in virtual wheat–pea mixtures? A simulation study based on pea genotypes with contrasting architectures

PubMed Central

Barillot, Romain; Combes, Didier; Chevalier, Valérie; Fournier, Christian; Escobar-Gutiérrez, Abraham J.

2012-01-01

Background and aims Light interception is a key factor driving the functioning of wheat–pea intercrops. The sharing of light is related to the canopy structure, which results from the architectural parameters of the mixed species. In the present study, we characterized six contrasting pea genotypes and identified architectural parameters whose range of variability leads to various levels of light sharing within virtual wheat–pea mixtures. Methodology Virtual plants were derived from magnetic digitizations performed during the growing cycle in a greenhouse experiment. Plant mock-ups were used as inputs of a radiative transfer model in order to estimate light interception in virtual wheat–pea mixtures. The turbid medium approach, extended to well-mixed canopies, was used as a framework for assessing the effects of leaf area index (LAI) and mean leaf inclination on light sharing. Principal results Three groups of pea genotypes were distinguished: (i) early and leafy cultivars, (ii) late semi-leafless cultivars and (iii) low-development semi-leafless cultivars. Within open canopies, light sharing was well described by the turbid medium approach and was therefore determined by the architectural parameters that composed LAI and foliage inclination. When canopy closure started, the turbid medium approach was unable to properly infer light partitioning because of the vertical structure of the canopy. This was related to the architectural parameters that determine the height of pea genotypes. Light capture was therefore affected by the development of leaflets, number of branches and phytomers, as well as internode length. Conclusions This study provides information on pea architecture and identifies parameters whose variability can be used to drive light sharing within wheat–pea mixtures. These results could be used to build up the architecture of pea ideotypes adapted to multi-specific stands towards light competition. PMID:23240074

Multi-fault clustering and diagnosis of gear system mined by spectrum entropy clustering based on higher order cumulants

NASA Astrophysics Data System (ADS)

Shao, Renping; Li, Jing; Hu, Wentao; Dong, Feifei

2013-02-01

Higher order cumulants (HOC) is a new kind of modern signal analysis of theory and technology. Spectrum entropy clustering (SEC) is a data mining method of statistics, extracting useful characteristics from a mass of nonlinear and non-stationary data. Following a discussion on the characteristics of HOC theory and SEC method in this paper, the study of signal processing techniques and the unique merits of nonlinear coupling characteristic analysis in processing random and non-stationary signals are introduced. Also, a new clustering analysis and diagnosis method is proposed for detecting multi-damage on gear by introducing the combination of HOC and SEC into the damage-detection and diagnosis of the gear system. The noise is restrained by HOC and by extracting coupling features and separating the characteristic signal at different speeds and frequency bands. Under such circumstances, the weak signal characteristics in the system are emphasized and the characteristic of multi-fault is extracted. Adopting a data-mining method of SEC conducts an analysis and diagnosis at various running states, such as the speed of 300 r/min, 900 r/min, 1200 r/min, and 1500 r/min of the following six signals: no-fault, short crack-fault in tooth root, long crack-fault in tooth root, short crack-fault in pitch circle, long crack-fault in pitch circle, and wear-fault on tooth. Research shows that this combined method of detection and diagnosis can also identify the degree of damage of some faults. On this basis, the virtual instrument of the gear system which detects damage and diagnoses faults is developed by combining with advantages of MATLAB and VC++, employing component object module technology, adopting mixed programming methods, and calling the program transformed from an *.m file under VC++. This software system possesses functions of collecting and introducing vibration signals of gear, analyzing and processing signals, extracting features, visualizing graphics, detecting and diagnosing faults, detecting and monitoring, etc. Finally, the results of testing and verifying show that the developed system can effectively be used to detect and diagnose faults in an actual operating gear transmission system.

Multi-fault clustering and diagnosis of gear system mined by spectrum entropy clustering based on higher order cumulants.

PubMed

Shao, Renping; Li, Jing; Hu, Wentao; Dong, Feifei

2013-02-01

Higher order cumulants (HOC) is a new kind of modern signal analysis of theory and technology. Spectrum entropy clustering (SEC) is a data mining method of statistics, extracting useful characteristics from a mass of nonlinear and non-stationary data. Following a discussion on the characteristics of HOC theory and SEC method in this paper, the study of signal processing techniques and the unique merits of nonlinear coupling characteristic analysis in processing random and non-stationary signals are introduced. Also, a new clustering analysis and diagnosis method is proposed for detecting multi-damage on gear by introducing the combination of HOC and SEC into the damage-detection and diagnosis of the gear system. The noise is restrained by HOC and by extracting coupling features and separating the characteristic signal at different speeds and frequency bands. Under such circumstances, the weak signal characteristics in the system are emphasized and the characteristic of multi-fault is extracted. Adopting a data-mining method of SEC conducts an analysis and diagnosis at various running states, such as the speed of 300 r/min, 900 r/min, 1200 r/min, and 1500 r/min of the following six signals: no-fault, short crack-fault in tooth root, long crack-fault in tooth root, short crack-fault in pitch circle, long crack-fault in pitch circle, and wear-fault on tooth. Research shows that this combined method of detection and diagnosis can also identify the degree of damage of some faults. On this basis, the virtual instrument of the gear system which detects damage and diagnoses faults is developed by combining with advantages of MATLAB and VC++, employing component object module technology, adopting mixed programming methods, and calling the program transformed from an *.m file under VC++. This software system possesses functions of collecting and introducing vibration signals of gear, analyzing and processing signals, extracting features, visualizing graphics, detecting and diagnosing faults, detecting and monitoring, etc. Finally, the results of testing and verifying show that the developed system can effectively be used to detect and diagnose faults in an actual operating gear transmission system.

Detection of postseismic fault-zone collapse following the Landers earthquake

USGS Publications Warehouse

Massonnet, D.; Thatcher, W.; Vadon, H.

1996-01-01

Stress changes caused by fault movement in an earthquake induce transient aseismic crustal movements in the earthquake source region that continue for months to decades following large events. These motions reflect aseismic adjustments of the fault zone and/or bulk deformation of the surroundings in response to applied stresses, and supply information regarding the inelastic behaviour of the Earth's crust. These processes are imperfectly understood because it is difficult to infer what occurs at depth using only surface measurements, which are in general poorly sampled. Here we push satellite radar interferometry to near its typical artefact level, to obtain a map of the postseismic deformation field in the three years following the 28 June 1992 Landers, California earthquake. From the map, we deduce two distinct types of deformation: afterslip at depth on the fault that ruptured in the earthquake, and shortening normal to the fault zone. The latter movement may reflect the closure of dilatant cracks and fluid expulsion from a transiently over-pressured fault zone.

Laser Glass Frit Sealing for Encapsulation of Vacuum Insulation Glasses

NASA Astrophysics Data System (ADS)

Kind, H.; Gehlen, E.; Aden, M.; Olowinsky, A.; Gillner, A.

Laser glass frit sealing is a joining method predestined in electronics for the sealing of engineered materials housings in dimensions of some 1 mm2 to several 10 mm2. The application field ranges from encapsulation of display panels to sensor housings. Laser glass frit sealing enables a hermetical closure excluding humidity and gas penetration. But the seam quality is also interesting for other applications requiring a hermetical sealing. One application is the encapsulation of vacuum insulation glass. The gap between two panes must be evacuated for reducing the thermal conductivity. Only an efficient encapsulating technique ensures durable tight joints of two panes for years. Laser glass frit sealing is an alternative joining method even though the material properties of soda lime glass like sensitivity to thermal stresses are much higher as known from engineered materials. An adapted thermal management of the process is necessary to prevent the thermal stresses within the pane to achieve crack free and tight glass frit seams.

RCRA, superfund and EPCRA hotline training module. Introduction to: Closure/post-closure (40 cfr parts 264/265, subpart g) updated July 1996

DOE Office of Scientific and Technical Information (OSTI.GOV)

NONE

1996-07-01

The module explains the difference between closure and post-closure. It lists the types of facilities that are subject to closure/post-closures and defines the difference between partial and final closure. It specifies who submits a closure plan and when a closure plan must be submitted, lists the steps in the process, and states the time frame for submittal. It identifies when and how a closure must be amended. It explains the time frame for notification of closure and the deadlines for beginning and completing closure. It specifies which facilities need contingent post-closure plans and lists and the elements of post-closure andmore » cites the requirements. It specifies the conditions and timing for amending a post-closure plan and states who must certify closure/post-closure.« less

Post-Closure Inspection and Monitoring Report for Corrective Action Unit 110: Area 3 WMD U-3ax/bl Crater, Nevada Test Site, Nevada, For the Period July 2007-June 2008

DOE Office of Scientific and Technical Information (OSTI.GOV)

NSTec Environmental Restoration

2008-08-01

This Post-Closure Inspection and Monitoring Report (PCIMR) provides the results of inspections and monitoring for Corrective Action Unit (CAU) 110, Area 3 WMD [Waste Management Division] U-3ax/bl Crater. This PCIMR includes an analysis and summary of the site inspections, repairs and maintenance, meteorological information, and soil moisture monitoring data obtained at CAU 110 for the period July 2007 through June 2008. Site inspections of the cover were performed quarterly to identify any significant changes to the site requiring action. The overall condition of the cover, perimeter fence, and use restriction (UR) warning signs was good. However, settling was observed thatmore » exceeded the action level as specified in Section VII.B.7 of the Hazardous Waste Permit Number NEV HW021 (Nevada Division of Environmental Protection, 2005). This permit states that cracks or settling greater than 15 centimeters (6 inches) deep that extend 1.0 meter (m) (3 feet [ft]) or more on the cover will be evaluated and repaired within 60 days of detection. Two areas of settling and cracks were observed on the south and east edges of the cover during the September 2007 inspection that exceeded the action level and required repair. The areas were repaired in October 2007. Additional settling and cracks were observed along the east side of the cover during the December 2007 inspection that exceeded the action level, and the area was repaired in January 2008. Significant animal burrows were also observed during the March 2008 inspection, and small mammal trapping and relocation was performed in April 2008. The semiannual subsidence surveys were performed in September 2007 and March 2008. No significant subsidence was observed in the survey data. Monument 5 shows the greatest amount of subsidence (-0.02 m [-0.08 ft] compared to the baseline survey of 2000). This amount is negligible and near the resolution of the survey instruments; it does not indicate that subsidence is occurring overall on the cover. Soil moisture results obtained to date indicate that the CAU 110 cover is performing well. Time Domain Reflectometry (TDR) data show regular changes in the shallow subsurface with significant rain events; however, major changes in volumetric moisture content (VMC) appear to be limited to 1.8 m (6 ft) below ground surface or shallower, depending on the location on the cover. At 2.4 m (8 ft) below the cover surface, TDR data show soil moisture content remained between 9 and 15 percent VMC, depending on the TDR location. The west portion of the cover tends to reflect a lower moisture content and less variability in annual fluctuations in moisture content at this depth. Results of soil moisture monitoring of the cover indicate that VMC at the compliance level (at 2.4 m [8 ft] below the cover surface) is approaching a steady state. If the moisture content at this level remains consistent with recent years, then a recommendation may be made for establishing compliance levels for future monitoring.« less

The Velocity and Attenuation of Acoustic Emission Waves in SiC/SiC Composites Loaded in Tension

NASA Technical Reports Server (NTRS)

Morscher, Gregory N.; Gyekenyesi, Andrew L.; Gray, Hugh R. (Technical Monitor)

2002-01-01

The behavior of acoustic waves produced by microfracture events and from pencil lead breaks was studied for two different silicon carbide fiber-reinforced silicon carbide matrix composites. The two composite systems both consisted of Hi-Nicalon (trademark) fibers and carbon interfaces but had different matrix compositions that led to considerable differences in damage accumulation and acoustic response. This behavior was primarily due to an order of magnitude difference in the interfacial shear stress for the two composite systems. Load/unload/reload tensile tests were performed and measurements were made over the entire stress range in order to determine the stress-dependence of acoustic activity for increasing damage states. It was found that using the extensional wave velocities from acoustic emission (AE) events produced from pencil lead breaks performed outside of the transducers enabled accurate measurements of the stiffness of the composite. The extensional wave velocities changed as a function of the damage state and the stress where the measurement was taken. Attenuation for AE waveforms from the pencil lead breaks occurred only for the composite possessing the lower interfacial shear stress and only at significantly high stresses. At zero stress after unloading from a peak stress, no attenuation occurred for this composite because of crack closure. For the high interfacial stress composite no attenuation was discernable at peak or zero stress over the entire stress-range of the composite. From these observations, it is believed that attenuation of AE waveforms is dependent on the magnitude of matrix crack opening.

Molecular docking and NMR binding studies to identify novel inhibitors of human phosphomevalonate kinase

DOE Office of Scientific and Technical Information (OSTI.GOV)

Boonsri, Pornthip; Department of Chemistry, NANOTEC Center of Nanotechnology, National Nanotechnology Center, Faculty of Science, Kasetsart University, Bangkok 10900; Neumann, Terrence S.

2013-01-04

Highlights: Black-Right-Pointing-Pointer Natural and synthetic inhibitors of human phosphomevalonate kinase identified. Black-Right-Pointing-Pointer Virtual screening yielded a hit rate of 15%, with inhibitor K{sub d}'s of 10-60 {mu}M. Black-Right-Pointing-Pointer NMR studies indicate significant protein conformational changes upon binding. -- Abstract: Phosphomevalonate kinase (PMK) phosphorylates mevalonate-5-phosphate (M5P) in the mevalonate pathway, which is the sole source of isoprenoids and steroids in humans. We have identified new PMK inhibitors with virtual screening, using autodock. Promising hits were verified and their affinity measured using NMR-based {sup 1}H-{sup 15}N heteronuclear single quantum coherence (HSQC) chemical shift perturbation and fluorescence titrations. Chemical shift changes were monitored,more » plotted, and fitted to obtain dissociation constants (K{sub d}). Tight binding compounds with K{sub d}'s ranging from 6-60 {mu}M were identified. These compounds tended to have significant polarity and negative charge, similar to the natural substrates (M5P and ATP). HSQC cross peak changes suggest that binding induces a global conformational change, such as domain closure. Compounds identified in this study serve as chemical genetic probes of human PMK, to explore pharmacology of the mevalonate pathway, as well as starting points for further drug development.« less

40 CFR 265.310 - Closure and post-closure care.

Code of Federal Regulations, 2013 CFR

2013-07-01

... 40 Protection of Environment 27 2013-07-01 2013-07-01 false Closure and post-closure care. 265.310... DISPOSAL FACILITIES Landfills § 265.310 Closure and post-closure care. (a) At final closure of the landfill... subsoils present. (b) After final closure, the owner or operator must comply with all post-closure...

40 CFR 265.310 - Closure and post-closure care.

Code of Federal Regulations, 2012 CFR

2012-07-01

... 40 Protection of Environment 27 2012-07-01 2012-07-01 false Closure and post-closure care. 265.310... DISPOSAL FACILITIES Landfills § 265.310 Closure and post-closure care. (a) At final closure of the landfill... subsoils present. (b) After final closure, the owner or operator must comply with all post-closure...

40 CFR 265.310 - Closure and post-closure care.

Code of Federal Regulations, 2014 CFR

2014-07-01

... 40 Protection of Environment 26 2014-07-01 2014-07-01 false Closure and post-closure care. 265.310... DISPOSAL FACILITIES Landfills § 265.310 Closure and post-closure care. (a) At final closure of the landfill... subsoils present. (b) After final closure, the owner or operator must comply with all post-closure...

40 CFR 264.228 - Closure and post-closure care.

Code of Federal Regulations, 2010 CFR

2010-07-01

... 40 Protection of Environment 25 2010-07-01 2010-07-01 false Closure and post-closure care. 264.228... Surface Impoundments § 264.228 Closure and post-closure care. (a) At closure, the owner or operator must... materials are left in place at final closure, the owner or operator must comply with all post-closure...

40 CFR 264.228 - Closure and post-closure care.

Code of Federal Regulations, 2013 CFR

2013-07-01

... 40 Protection of Environment 27 2013-07-01 2013-07-01 false Closure and post-closure care. 264.228... Surface Impoundments § 264.228 Closure and post-closure care. (a) At closure, the owner or operator must... materials are left in place at final closure, the owner or operator must comply with all post-closure...

40 CFR 264.228 - Closure and post-closure care.

Code of Federal Regulations, 2012 CFR

2012-07-01

... 40 Protection of Environment 27 2012-07-01 2012-07-01 false Closure and post-closure care. 264.228... Surface Impoundments § 264.228 Closure and post-closure care. (a) At closure, the owner or operator must... materials are left in place at final closure, the owner or operator must comply with all post-closure...

40 CFR 264.228 - Closure and post-closure care.

Code of Federal Regulations, 2011 CFR

2011-07-01

... 40 Protection of Environment 26 2011-07-01 2011-07-01 false Closure and post-closure care. 264.228... Surface Impoundments § 264.228 Closure and post-closure care. (a) At closure, the owner or operator must... materials are left in place at final closure, the owner or operator must comply with all post-closure...

40 CFR 264.228 - Closure and post-closure care.

Code of Federal Regulations, 2014 CFR

2014-07-01

... 40 Protection of Environment 26 2014-07-01 2014-07-01 false Closure and post-closure care. 264.228... Surface Impoundments § 264.228 Closure and post-closure care. (a) At closure, the owner or operator must... materials are left in place at final closure, the owner or operator must comply with all post-closure...

40 CFR 265.228 - Closure and post-closure care.

Code of Federal Regulations, 2012 CFR

2012-07-01

... 40 Protection of Environment 27 2012-07-01 2012-07-01 false Closure and post-closure care. 265.228... DISPOSAL FACILITIES Surface Impoundments § 265.228 Closure and post-closure care. (a) At closure, the owner... impoundment and provide post-closure care for a landfill under subpart G and § 265.310, including the...

40 CFR 265.228 - Closure and post-closure care.

Code of Federal Regulations, 2011 CFR

2011-07-01

... 40 Protection of Environment 26 2011-07-01 2011-07-01 false Closure and post-closure care. 265.228... DISPOSAL FACILITIES Surface Impoundments § 265.228 Closure and post-closure care. (a) At closure, the owner... impoundment and provide post-closure care for a landfill under subpart G and § 265.310, including the...

Elastic wave velocity and acoustic emission monitoring during Gypsum dehydration under triaxial stress conditions

NASA Astrophysics Data System (ADS)

Brantut, N.; David, E. C.; Héripré, E.; Schubnel, A. J.; Zimmerman, R. W.; Gueguen, Y.

2010-12-01

Dehydration experiments were performed on natural Gypsum polycrystal samples coming from Volterra, Italy in order to study contemporaneously the evolution of P and S elastic wave velocities and acoustic emission (AE) triggering. During these experiments, temperature was slowly raised at 0.15 degrees C per minute under constant stress conditions. Two experiments were realized under quasi-hydrostatic stress (15 and 55 MPa respectively). The third experiment was realized under constant triaxial stress (σ3=45MPa, σ1=75MPa). All three were drained (10MPa constant pore pressure). In each experiments, both P and S wave velocities reduced drastically (as much as approx. 50% in the low confining pressure case) at the onset of dehydration. Importantly, the Vp/Vs ratio also decreased. Shortly after the onset of decrease in P and S wave velocities, the dehydration reaction was also accompanied by bursts of AEs. Time serie locations of the AEs show that they initiated from the pore pressure port, ie from where the pore fluid could easily be drained, and then slowly migrated within the sample. In each experiments, the AE rate could be positively correlated to the reaction rate, inferred from pore volumetry. In such a way, the AE rate reached a peak when the reaction was the fastest. Focal mechanism analysis of the largest AEs showed they had a large volumetric component in compaction, confirming that AEs were indeed related to pore closure and/or collapse. In addition, the AE rate also increased with confinement, ie when a larger amount of compaction was observed. Interestingly, when under differential stress conditions, AE focal mechanisms were mainly in shear. Additional dehydration experiments performed within an environmental scanning electron microscope under low vacuum highlight that, in drained conditions at least, the reaction seems to take place in two phases. First, cracks are being opened along cleavage planes within a single gypsum crystal, which allows for the fluid to escape. Second, the solid volume shrinks and pore collapse can occur. Such a scenario is also consistent with our in-situ analysis under pressure. Finally, a differential effective medium theory approach is used to invert crack density and crack average aspect ratio from elastic wave velocity measurements. Coupling this to Biot-Gassman equation, we can correct for some of the dispersion effects (mainly squirt flow) between the ultrasonic (MHz) and the seismic frequency (Hz) ranges. When doing so, we observe, that, under low confining pressures and in drained conditions at least, the evolution of elastic wave velocities is dominated by the effect due to nucleation of low aspect ratio crack during dehydration. Our results thus seem to point out that, because dehydration reaction are accompanied with crack nucleation, the signature of these reactions in nature, should, in fact, possibly be that of a low Vp/Vs ratio, contrarily to what has been instinctively assumed until now.

40 CFR 264.146 - Use of a mechanism for financial assurance of both closure and post-closure care.

Code of Federal Regulations, 2013 CFR

2013-07-01

... assurance of both closure and post-closure care. 264.146 Section 264.146 Protection of Environment... mechanism for financial assurance of both closure and post-closure care. An owner or operator may satisfy the requirements for financial assurance for both closure and post-closure care for one or more...

40 CFR 265.146 - Use of a mechanism for financial assurance of both closure and post-closure care.

Code of Federal Regulations, 2013 CFR

2013-07-01

... assurance of both closure and post-closure care. 265.146 Section 265.146 Protection of Environment... Use of a mechanism for financial assurance of both closure and post-closure care. An owner or operator may satisfy the requirements for financial assurance for both closure and post-closure care for one or...

Characterization and analysis of surface notches on Ti-alloy plates fabricated by additive manufacturing techniques

NASA Astrophysics Data System (ADS)

Chan, Kwai S.

2015-12-01

Rectangular plates of Ti-6Al-4V with extra low interstitial (ELI) were fabricated by layer-by-layer deposition techniques that included electron beam melting (EBM) and laser beam melting (LBM). The surface conditions of these plates were characterized using x-ray micro-computed tomography. The depth and radius of surface notch-like features on the LBM and EBM plates were measured from sectional images of individual virtual slices of the rectangular plates. The stress concentration factors of individual surface notches were computed and analyzed statistically to determine the appropriate distributions for the notch depth, notch radius, and stress concentration factor. These results were correlated with the fatigue life of the Ti-6Al-4V ELI alloys from an earlier investigation. A surface notch analysis was performed to assess the debit in the fatigue strength due to the surface notches. The assessment revealed that the fatigue lives of the additively manufactured plates with rough surface topographies and notch-like features are dominated by the fatigue crack growth of large cracks for both the LBM and EBM materials. The fatigue strength reduction due to the surface notches can be as large as 60%-75%. It is concluded that for better fatigue performance, the surface notches on EBM and LBM materials need to be removed by machining and the surface roughness be improved to a surface finish of about 1 μm.

Jumbo squid beaks: inspiration for design of robust organic composites.

PubMed

Miserez, Ali; Li, Youli; Waite, J Herbert; Zok, Frank

2007-01-01

The hard tissues found in some invertebrate marine organisms represent intriguing paradigms for robust, lightweight materials. The present study focuses on one such tissue: that comprising the beak of the jumbo squid (Dosidicus gigas). Its main constituents are chitin fibers (15-20wt.%) and histidine- and glycine-rich proteins (40-45%). Notably absent are mineral phases, metals and halogens. Despite being fully organic, beak hardness and stiffness are at least twice those of the most competitive synthetic organic materials (notably engineering polymers) and comparable to those of Glycera and Nereis jaws. Furthermore, the combination of hardness and stiffness makes the beaks more resistant to plastic deformation when in contact with blunt abrasives than virtually all metals and polymers. The 3,4-dihydroxy-l-phenylalanine and abundant histidine content in the beak proteins as well as the pigmented hydrolysis-resistant residue are suggestive of aromatic cross-linking. A high cross-linking density between the proteins and chitin may be the single most important determinant of hardness and stiffness in the beak. Beak microstructure is characterized by a lamellar arrangement of the constituents, with a weak interface that promotes crack deflection and endows the structure with high fracture toughness. The susceptibility of this microstructure to cracking along these interfaces from contact stresses at the external surface is mitigated by the presence of a protective coating.

Thermally responsive polymer systems for self-healing, reversible adhesion and shape memory applications

NASA Astrophysics Data System (ADS)

Luo, Xiaofan

Responsive polymers are "smart" materials that are capable of performing prescribed, dynamic functions under an applied stimulus. In this dissertation, we explore several novel design strategies to develop thermally responsive polymers and polymer composites for self-healing, reversible adhesion and shape memory applications. In the first case described in Chapters 2 and 3, a thermally triggered self-healing material was prepared by blending a high-temperature epoxy resin with a thermoplastic polymer, poly(epsilon-caprolactone) (PCL). The initially miscible system undergoes polymerization induced phase separation (PIPS) during the curing of epoxy and yields a variety of compositionally dependent morphologies. At a particular PCL loading, the cured blend displays a "bricks-and-mortar" morphology in which epoxy exists as interconnected spheres ("bricks") within a continuous PCL matrix ("mortar"). A heat induced "bleeding" phenomenon was observed in the form of spontaneous wetting of all free surfaces by the molten PCL, and is attributed to the volumetric thermal expansion of PCL above its melting point in excess of epoxy brick expansion, which we term differential expansive bleeding (DEB). This DEB is capable of healing damage such as cracks. In controlled self-healing experiments, heating of a cracked specimen led to PCL bleeding from the bulk that yields a liquid layer bridging the crack gap. Upon cooling, a "scar" composed of PCL crystals was formed at the site of the crack, restoring a significant portion of mechanical strength. We further utilized DEB to enable strong and thermally-reversible adhesion of the material to itself and to metallic substrates, without any requirement for macroscopic softening or flow. After that, Chapters 4--6 present a novel composite strategy for the design and fabrication of shape memory polymer composites. The basic approach involves physically combining two or more functional components into an interpenetrating fiber/matrix structure, allowing them to function in a synergistic fashion yet remain physically separated. This latter aspect is critical since it enables the control of overall composite properties and functions by separately tuning each component. Utilizing the intrinsic versatility of this approach, composites with novel properties and functions (in addition to "regular" shape memory) have been developed, including (1) shape memory elastomeric composites (SMECs; Chapter 4), (2) triple-shape polymeric composites (TSPCs; Chapter 5), and (3) electrically conductive nanocomposites (Chapter 6). Then in Chapter 7, by combining the success in both thermoplastic based self-healing and shape memory polymer composites, we demonstrate a thermally triggered self-healing coating. This coating features a unique "shape memory assisted self-healing" mechanism in which crack closure (via shape memory) and crack re-bonding (via melting and diffusion of the thermoplastic healing agent) are achieved simultaneously upon a single heating step, leading to both structural and functional (corrosion resistance) recovery. Finally, Chapter 8 presents for the first time the preparation of functionally graded shape memory polymers (SMPs) that, unlike conventional SMPs, have a range of glass transition temperatures that are spatially graded. This was achieved using a temperature gradient curing method that imposes different vitrification limits at different positions along the gradient. The resulting material is capable of responding to a wide range of thermal triggers and a good candidate for low-cost, material based temperature sensors. All the aforementioned materials and methods show great potential for practical applications due to their high performance, low cost and broad applicability. Some recommendations for future research and development are given in Chapter 9.

The influence of idealized surface heterogeneity on virtual turbulent flux measurements

NASA Astrophysics Data System (ADS)

De Roo, Frederik; Mauder, Matthias

2018-04-01

The imbalance of the surface energy budget in eddy-covariance measurements is still an unsolved problem. A possible cause is the presence of land surface heterogeneity, which affects the boundary-layer turbulence. To investigate the impact of surface variables on the partitioning of the energy budget of flux measurements in the surface layer under convective conditions, we set up a systematic parameter study by means of large-eddy simulation. For the study we use a virtual control volume approach, which allows the determination of advection by the mean flow, flux-divergence and storage terms of the energy budget at the virtual measurement site, in addition to the standard turbulent flux. We focus on the heterogeneity of the surface fluxes and keep the topography flat. The surface fluxes vary locally in intensity and these patches have different length scales. Intensity and length scales can vary for the two horizontal dimensions but follow an idealized chessboard pattern. Our main focus lies on surface heterogeneity of the kilometer scale, and one order of magnitude smaller. For these two length scales, we investigate the average response of the fluxes at a number of virtual towers, when varying the heterogeneity length within the length scale and when varying the contrast between the different patches. For each simulation, virtual measurement towers were positioned at functionally different positions (e.g., downdraft region, updraft region, at border between domains, etc.). As the storage term is always small, the non-closure is given by the sum of the advection by the mean flow and the flux-divergence. Remarkably, the missing flux can be described by either the advection by the mean flow or the flux-divergence separately, because the latter two have a high correlation with each other. For kilometer scale heterogeneity, we notice a clear dependence of the updrafts and downdrafts on the surface heterogeneity and likewise we also see a dependence of the energy partitioning on the tower location. For the hectometer scale, we do not notice such a clear dependence. Finally, we seek correlators for the energy balance ratio in the simulations. The correlation with the friction velocity is less pronounced than previously found, but this is likely due to our concentration on effectively strongly to freely convective conditions.

On sorption and swelling of CO 2 in clays

DOE PAGES

Busch, A.; Bertier, P.; Gensterblum, Y.; ...

2016-03-23

One well-studied technology is the geological storage of carbon dioxide (CO 2), and a number of demonstration projects around the world have proven its feasibility and challenges. Storage conformance and seal integrity are among the most important aspects, as they determine risk of leakage as well as limits for storage capacity and injectivity. By providing evidence for safe storage is critical for improving public acceptance. Most caprocks are composed of clays as dominant mineral type which can typically be illite, kaolinite, chlorite or smectite. A number of recent studies addressed the interaction between CO 2 and these different clays andmore » it was shown that clay minerals adsorb considerable quantities of CO 2. For smectite this uptake can lead to volumetric expansion followed by the generation of swelling pressures. On the one hand CO 2 adsorption traps CO 2, on the other hand swelling pressures can potentially change local stress regimes and in unfavourable situations shear-type failure is assumed to occur. Moreover, for storage in a reservoir having high clay contents the CO 2 uptake can add to storage capacity which is widely underestimated so far. Smectite-rich seals in direct contact with a dry CO 2 plume at the interface to the reservoir might dehydrate leading to dehydration cracks. Such dehydration cracks can provide pathways for CO 2 ingress and further accelerate dewatering and penetration of the seal by supercritical CO 2. At the same time, swelling may also lead to the closure of fractures or the reduction of fracture apertures, thereby improving seal integrity. Finally, the goal of this communication is to theoretically evaluate and discuss these scenarios in greater detail in terms of phenomenological mechanisms, but also in terms of potential risks or benefits for carbon storage.« less

Self-healing mortar with pH-sensitive superabsorbent polymers: testing of the sealing efficiency by water flow tests

NASA Astrophysics Data System (ADS)

Gruyaert, Elke; Debbaut, Brenda; Snoeck, Didier; Díaz, Pilar; Arizo, Alejandro; Tziviloglou, Eirini; Schlangen, Erik; De Belie, Nele

2016-08-01

Superabsorbent polymers (SAPs) have potential to be used as healing agent in self-healing concrete due to their property to attract moisture from the environment and their capacity to promote autogenous healing. A possible drawback, however, is their uptake of mixing water during concrete manufacturing, resulting in an increased volume of macro-pores in the hardened concrete. To limit this drawback, newly developed SAPs with a high swelling and pH-sensitiveness were developed and tested within the FP7 project HEALCON. Evaluation of their self-sealing performance occurred through a water permeability test via water flow, a test method also developed within HEALCON. Three different sizes of the newly developed SAP were compared with a commercial SAP. Swelling tests in cement filtrate solution indicated that the commercial and in-house synthesized SAPs performed quite similar, but the difference between the swelling capacity at pH 9 and pH 13 is more pronounced for the self-synthesized SAPs. Moreover, in comparison to the commercial SAPs, less macro-pores are formed in the cement matrix of mixes with self-synthesized SAPs and the effect on the mechanical properties is lower, but not negligible, when using high amounts of SAPs. Although the immediate sealing effect of cracks in mortar was the highest for the commercial SAPs, the in-house made SAPs with a particle size between 400 and 600 μm performed the best with regard to crack closure (mainly CaCO3 precipitation) and self-sealing efficiency, after exposing the specimens to 28 wet-dry cycles. Some specimens could even withstand a water pressure of 2 bar.

Management of hidradenitis suppurativa wounds with an internal vacuum-assisted closure device.

PubMed

Chen, Y Erin; Gerstle, Theodore; Verma, Kapil; Treiser, Matthew D; Kimball, Alexandra B; Orgill, Dennis P

2014-03-01

Hidradenitis suppurativa is a chronic, debilitating disease that is difficult to treat. Once medical management fails, wide local excision offers the best chance for cure. However, the resultant wound often proves too large or contaminated for immediate closure. The authors performed a retrospective chart review of hidradenitis cases managed surgically between 2005 and 2010. Data collected included patient characteristics, management method, and outcomes. Approximately half of the patients received internal vacuum-assisted closure therapy using the vacuum-assisted closure system and delayed closure and half of the patients received immediate primary closure at the time of their excision. Delayed closure consisted of closing the majority of the wound in a linear fashion following internal vacuum-assisted closure while accepting healing by means of secondary intention for small wound areas. Patients managed with internal vacuum-assisted closure had wounds on average four times larger in area than patients managed without internal vacuum-assisted closure. In both groups, all wounds were eventually closed primarily. Healing times averaged 2.2 months with internal vacuum-assisted closure and 2.7 months without. At an average follow-up time of 2.3 months, all patients with internal vacuum-assisted closure had no recurrence of their local disease. Severe hidradenitis presents a treatment challenge, as surgical excisions are often complicated by difficult closures and unsatisfactory recurrence rates. This study demonstrates that wide local excision with reasonable outcomes can be achieved using accelerated delayed primary closure. This method uses internal vacuum-assisted closure as a bridge between excision and delayed primary closure, facilitating closure without recurrence in large, heavily contaminated wounds. Therapeutic, III.

Cataract surgeons outperform medical students in Eyesi virtual reality cataract surgery: evidence for construct validity.

PubMed

Selvander, Madeleine; Asman, Peter

2013-08-01

To investigate construct validity for modules hydromaneuvers and phaco on the Eyesi surgical simulator. Seven cataract surgeons and 17 medical students performed capsulorhexis, hydromaneuvers, phaco, navigation, forceps, cracking and chopping modules in a standardized manner. Three trials were performed on each module (two on phaco) in the above order. Performance parameters as calculated by the simulator for each trial were saved. Video recordings of the second trial of the modules capsulorhexis, hydromaneuvers and phaco were evaluated with the modified Objective Structured Assessment of Surgical Skill (OSATS) and Objective Structured Assessment of Cataract Surgical Skill (OSACSS) tools. Cataract surgeons outperformed medical students with regard to overall score on capsulorhexis (p < 0.001, p = 0.035, p = 0.010 for the tree iterations, respectively), navigation (p = 0.024, p = 0.307, p = 0.007), forceps (p = 0.017, p = 0.03, p = 0.028). Less obvious differences in overall score were found for modules cracking and chopping (p = 0.266, p = 0.022, p = 0.324) and phaco (p = 0.011, p = 0.081 for the two iterations, respectively). No differences in overall score were found on hydromaneuvers (p = 0.588, p = 0.503, p = 0.773), but surgeons received better scores from the evaluations of the modified OSATS (p = 0.001) and OSACSS (capsulorhexis, p = 0.003; hydromaneuvers, p = 0.017; phaco, p = 0.001). Construct validity was found on several modules previously not investigated (phaco, hydromaneuvers, cracking and chopping, navigation), and our results confirm previously demonstrated construct validity for capsulorhexis and forceps modules. Interestingly, validation of the hydromaneuvers module required OSACSS video evaluation tool. A further development of the scoring system in the simulator for the hydromaneuvers module would be advantageous and make training and evaluation of progress more accessible and immediate. © 2012 The Authors. Acta Ophthalmologica © 2012 Acta Ophthalmologica Scandinavica Foundation.

Scanning 3D full human bodies using Kinects.

PubMed

Tong, Jing; Zhou, Jin; Liu, Ligang; Pan, Zhigeng; Yan, Hao

2012-04-01

Depth camera such as Microsoft Kinect, is much cheaper than conventional 3D scanning devices, and thus it can be acquired for everyday users easily. However, the depth data captured by Kinect over a certain distance is of extreme low quality. In this paper, we present a novel scanning system for capturing 3D full human body models by using multiple Kinects. To avoid the interference phenomena, we use two Kinects to capture the upper part and lower part of a human body respectively without overlapping region. A third Kinect is used to capture the middle part of the human body from the opposite direction. We propose a practical approach for registering the various body parts of different views under non-rigid deformation. First, a rough mesh template is constructed and used to deform successive frames pairwisely. Second, global alignment is performed to distribute errors in the deformation space, which can solve the loop closure problem efficiently. Misalignment caused by complex occlusion can also be handled reasonably by our global alignment algorithm. The experimental results have shown the efficiency and applicability of our system. Our system obtains impressive results in a few minutes with low price devices, thus is practically useful for generating personalized avatars for everyday users. Our system has been used for 3D human animation and virtual try on, and can further facilitate a range of home–oriented virtual reality (VR) applications.

RANS Simulation (Virtual Blade Model [VBM]) of Array of Three Coaxial Lab Scaled DOE RM1 MHK Turbine with 5D Spacing

DOE Data Explorer

Javaherchi, Teymour

2016-06-08

Attached are the .cas and .dat files along with the required User Defined Functions (UDFs) and look-up table of lift and drag coefficients for the Reynolds Averaged Navier-Stokes (RANS) simulation of three coaxially located lab-scaled DOE RM1 turbine implemented in ANSYS FLUENT CFD-package. The lab-scaled DOE RM1 is a re-design geometry, based of the full scale DOE RM1 design, producing same power output as the full scale model, while operating at matched Tip Speed Ratio values at reachable laboratory Reynolds number (see attached paper). In this case study the flow field around and in the wake of the lab-scaled DOE RM1 turbines in a coaxial array is simulated using Blade Element Model (a.k.a Virtual Blade Model) by solving RANS equations coupled with k-\\omega turbulence closure model. It should be highlighted that in this simulation the actual geometry of the rotor blade is not modeled. The effect of turbine rotating blades are modeled using the Blade Element Theory. This simulation provides an accurate estimate for the performance of each device and structure of their turbulent far wake. The results of these simulations were validated against the developed in-house experimental data. Simulations for other turbine configurations are available upon request.

RANS Simulation (Virtual Blade Model [VBM]) of Single Lab Scaled DOE RM1 MHK Turbine

DOE Data Explorer

Javaherchi, Teymour; Stelzenmuller, Nick; Aliseda, Alberto; Seydel, Joseph

2014-04-15

Attached are the .cas and .dat files for the Reynolds Averaged Navier-Stokes (RANS) simulation of a single lab-scaled DOE RM1 turbine implemented in ANSYS FLUENT CFD-package. The lab-scaled DOE RM1 is a re-design geometry, based of the full scale DOE RM1 design, producing same power output as the full scale model, while operating at matched Tip Speed Ratio values at reachable laboratory Reynolds number (see attached paper). In this case study the flow field around and in the wake of the lab-scaled DOE RM1 turbine is simulated using Blade Element Model (a.k.a Virtual Blade Model) by solving RANS equations coupled with k-\\omega turbulence closure model. It should be highlighted that in this simulation the actual geometry of the rotor blade is not modeled. The effect of turbine rotating blades are modeled using the Blade Element Theory. This simulation provides an accurate estimate for the performance of device and structure of it's turbulent far wake. Due to the simplifications implemented for modeling the rotating blades in this model, VBM is limited to capture details of the flow field in near wake region of the device. The required User Defined Functions (UDFs) and look-up table of lift and drag coefficients are included along with the .cas and .dat files.

Intestinal perforation in typhoid fever: a historical and state-of-the-art review.

PubMed

Bitar, R; Tarpley, J

1985-01-01

The appropriate therapy for intestinal perforation in typhoid fever has been controversial since the late 1880s. Around the turn of the century, surgery became the established mode of therapy, with a mortality of 69% based on 166 patients in the English-language medical literature, and continued to be the preferred treatment until the advent of chloramphenicol in 1948. At this time the surgical mortality was approximately 50%. Following the recovery of a few patients with perforation treated only with antimicrobial agents (six initially, then eventually 22), nonsurgical therapy became the accepted mode of treatment. This change was never justified and this review demonstrates this. Appropriate therapy is virtually always surgical, usually consisting of simple closure and irrigation. Chloramphenicol alone is inadequate antimicrobial therapy in a patient with perforation and must be supplemented by other antimicrobials directed against enteric aerobic gram-negative bacilli and enteric anaerobes.

The description of a new species of the Neotropical land crab genus Gecarcinus Leach, 1814 (Crustacea, Decapoda, Brachyura, Gecarcinidae)

PubMed Central

Perger, Robert; Wall, Adam

2014-01-01

Abstract In this contribution a new species of the land crab genus Gecarcinus Leach, 1814, from the Neotropical Pacific coast of South America is described and illustrated. In addition to its unique body color, Gecarcinus nobilii sp. n. is distinguished from congeners by a distinctly wider carapace front and differences in the shape of the infraorbital margin. The new species is not isolated from Gecarcinus populations from the Pacific coast of Central America by an insurmountable geographic barrier. Considering the closure of the Panamanian Isthmus as a calibration point for morphological divergence between the trans-isthmian mainland populations of Gecarcinus, the virtual lack of morphological differentiation (other than color) between them and the distinctness of G. nobilii sp. n. suggests that G. nobilii sp. n. evolved from a common ancestor before the Isthmus closed. PMID:25152688

Impacts of nuclear plant shutdown on coal-fired power generation and infant health in the Tennessee Valley in the 1980s

NASA Astrophysics Data System (ADS)

Severnini, Edson

2017-04-01

The Fukushima nuclear accident in March 2011 generated deep public anxiety and uncertainty about the future of nuclear energy. However, differently to fossil fuel plants, nuclear plants produce virtually no greenhouse gas emissions or air pollutants during power generation. Here we show the effect on air pollution and infant health in the context of the temporary closure of nuclear plants by the Tennessee Valley Authority (TVA) in the 1980s. After the Three Mile Island accident in 1979, the US Nuclear Regulatory Commission intensified inspections throughout the nation, leading to the shutdown of two large nuclear power plants in the TVA area. In response to that shutdown, electricity generation shifted one to one to coal-fired power plants within TVA, increasing particle pollution in counties where they were located. Consequently, infant health may have deteriorated in the most affected places, indicating deleterious effects to public health.

Natural orifice transluminal endoscopic surgery gastrotomy closure with an over-the-endoscope clip: a randomized, controlled porcine study (with videos).

PubMed

von Renteln, Daniel; Schmidt, Arthur; Vassiliou, Melina C; Gieselmann, Maria; Caca, Karel

2009-10-01

Secure endoscopic closure of transgastric natural orifice transluminal endoscopic surgery (NOTES) access is of paramount importance. The over-the-scope clip (OTSC) system has previously been shown to be effective for NOTES gastrotomy closure. To compare OTSC gastrotomy closure with surgical closure. Randomized, controlled animal study. Animal facility laboratory. Thirty-six female domestic pigs. Gastrotomies were created by using a needle-knife and an 18-mm balloon. The animals were subsequently randomized to either open surgical repair with interrupted sutures or endoscopic repair with 12-mm OTSCs. In addition, pressurized leak tests were performed in ex vivo specimens of 18-mm scalpel incisions closed with suture (n = 14) and of intact stomachs (n = 10). The mean time for endoscopic closure was 9.8 minutes (range 3-22, SD 5.5). No complications occurred during either type of gastrotomy closure. At necropsy, examination of all OTSC and surgical closures demonstrated complete sealing of gastrotomy sites without evidence of injury to adjacent organs. Pressurized leak tests showed a mean burst pressure of 83 mm Hg (range 30-140, SD 27) for OTSC closures and 67 mm Hg (range 30-130, SD 27.7) for surgical sutures. Ex vivo hand-sewn sutures of 18-mm gastrotomies (n = 14) exhibited a mean burst pressure of 65 mm Hg (range 20-140, SD 31) and intact ex vivo stomachs (n = 10) had a mean burst pressure of 126 mm Hg (range 90-170, SD 28). The burst pressure of ex vivo intact stomachs was significantly higher compared with OTSC closures (P < .01), in vivo surgical closures (P < .01), and ex vivo hand-sewn closures (P < .01). There was a trend toward higher burst pressures in the OTSC closures compared with surgical closures (P = .063) and ex vivo hand-sewn closures (P = .094). In vivo surgical closures demonstrated similar burst pressures compared with ex vivo hand-sewn closures (P = .848). Nonsurvival setting. Endoscopic closure by using the OTSC system is comparable to surgical closure in a nonsurvival porcine model. This technique is easy to perform and is suitable for NOTES gastrotomy closure.

Investigation of Cracks Found in Helicopter Longerons

NASA Technical Reports Server (NTRS)

Newman, John A.; Baughman, James M.; Wallace, Terryl A.

2009-01-01

Four cracked longerons, containing a total of eight cracks, were provided for study. Cracked regions were cut from the longerons. Load was applied to open the cracks, enabling crack surface examination. Examination revealed that crack propagation was driven by fatigue loading in all eight cases. Fatigue crack initiation appears to have occurred on the top edge of the longerons near geometric changes that affect component bending stiffness. Additionally, metallurigical analysis has revealed a local depletion in alloying elements in the crack initiation regions that may be a contributing factor. Fatigue crack propagation appeared to be initially driven by opening-mode loading, but at a crack length of approximately 0.5 inches (12.7 mm), there is evidence of mixed-mode crack loading. For the longest cracks studied, shear-mode displacements destroyed crack-surface features of interest over significant portions of the crack surfaces.

Investigation of Helicopter Longeron Cracks

NASA Technical Reports Server (NTRS)

Newman, John A.; Baughman, James; Wallace, Terryl A.

2009-01-01

Four cracked longerons, containing a total of eight cracks, were provided for study. Cracked regions were cut from the longerons. Load was applied to open the cracks, enabling crack surface examination. Examination revealed that crack propagation was driven by fatigue loading in all eight cases. Fatigue crack initiation appears to have occurred on the top edge of the longerons near geometric changes that affect component bending stiffness. Additionally, metallurgical analysis has revealed a local depletion in alloying elements in the crack initiation regions that may be a contributing factor. Fatigue crack propagation appeared to be initially driven by opening-mode loading, but at a crack length of approximately 0.5 inches (12.7 mm), there is evidence of mixed-mode crack loading. For the longest cracks studied, shear-mode displacements destroyed crack-surface features of interest over significant portions of the crack surfaces.

Evaluation of the Momentum Closure Schemes in MPAS-Ocean

NASA Astrophysics Data System (ADS)

Zhao, Shimei; Liu, Yudi; Liu, Wei

2018-04-01

In order to compare and evaluate the performances of the Laplacian viscosity closure, the biharmonic viscosity closure, and the Leith closure momentum schemes in the MPAS-Ocean model, a variety of physical quantities, such as the relative reference potential energy (RPE) change, the RPE time change rate (RPETCR), the grid Reynolds number, the root mean square (RMS) of kinetic energy, and the spectra of kinetic energy and enstrophy, are calculated on the basis of results of a 3D baroclinic periodic channel. Results indicate that: 1) The RPETCR demonstrates a saturation phenomenon in baroclinic eddy tests. The critical grid Reynolds number corresponding to RPETCR saturation differs between the three closures: the largest value is in the biharmonic viscosity closure, followed by that in the Laplacian viscosity closure, and that in the Leith closure is the smallest. 2) All three closures can effectively suppress spurious dianeutral mixing by reducing the grid Reynolds number under sub-saturation conditions of the RPETCR, but they can also damage certain physical processes. Generally, the damage to the rotation process is greater than that to the advection process. 3) The dissipation in the biharmonic viscosity closure is strongly dependent on scales. Most dissipation concentrates on small scales, and the energy of small-scale eddies is often transferred to large-scale kinetic energy. The viscous dissipation in the Laplacian viscosity closure is the strongest on various scales, followed by that in the Leith closure. Note that part of the small-scale kinetic energy is also transferred to large-scale kinetic energy in the Leith closure. 4) The characteristic length scale L and the dimensionless parameter D in the Leith closure are inherently coupled. The RPETCR is inversely proportional to the product of D and L. When the product of D and L is constant, both the simulated RPETCR and the inhibition of spurious dianeutral mixing are the same in all tests using the Leith closure. The dissipative scale in the Leith closure depends on the parameter L, and the dissipative intensity depends on the parameter D. 5) Although optimal results may not be achieved by using the optimal parameters obtained from the 2D barotropic model in the 3D baroclinic simulation, the total energies are dissipative in all three closures. Dissipation is the strongest in the biharmonic viscosity closure, followed by that in the Leith closure, and that in the Laplacian viscosity closure is the weakest. Mesoscale eddies develop the fastest in the biharmonic viscosity closure after the baroclinic adjustment process finishes, and the kinetic energy reaches its maximum, which is attributed to the smallest dissipation of enstrophy in the biharmonic viscosity closure. Mesoscale eddies develop the slowest, and the kinetic energy peak value is the smallest in the Laplacian viscosity closure. Results in the Leith closure are between that in the biharmonic viscosity closure and the Laplacian viscosity closure.

40 CFR 264.151 - Wording of the instruments.

Code of Federal Regulations, 2013 CFR

2013-07-01

... assurance for closure or post-closure care is demonstrated through the financial test specified in subpart H... parts 264 and 265. The current closure and/or post-closure cost estimates covered by such a test are... CFR parts 264 and 265. The current closure and/or post-closure cost estimate covered by the test are...

40 CFR 264.151 - Wording of the instruments.

Code of Federal Regulations, 2012 CFR

2012-07-01

... assurance for closure or post-closure care is demonstrated through the financial test specified in subpart H... parts 264 and 265. The current closure and/or post-closure cost estimates covered by such a test are... CFR parts 264 and 265. The current closure and/or post-closure cost estimate covered by the test are...

40 CFR 264.151 - Wording of the instruments.

Code of Federal Regulations, 2010 CFR

2010-07-01

... assurance for closure or post-closure care is demonstrated through the financial test specified in subpart H... parts 264 and 265. The current closure and/or post-closure cost estimates covered by such a test are... CFR parts 264 and 265. The current closure and/or post-closure cost estimate covered by the test are...

40 CFR 264.118 - Post-closure plan; amendment of plan.

Code of Federal Regulations, 2014 CFR

2014-07-01

... days after an unexpected event has occurred which has affected the post-closure plan. An owner or... 40 Protection of Environment 26 2014-07-01 2014-07-01 false Post-closure plan; amendment of plan... FACILITIES Closure and Post-Closure § 264.118 Post-closure plan; amendment of plan. (a) Written Plan. The...

40 CFR 264.118 - Post-closure plan; amendment of plan.

Code of Federal Regulations, 2011 CFR

2011-07-01

... days after an unexpected event has occurred which has affected the post-closure plan. An owner or... 40 Protection of Environment 26 2011-07-01 2011-07-01 false Post-closure plan; amendment of plan... FACILITIES Closure and Post-Closure § 264.118 Post-closure plan; amendment of plan. (a) Written Plan. The...

40 CFR 264.118 - Post-closure plan; amendment of plan.

Code of Federal Regulations, 2012 CFR

2012-07-01

... days after an unexpected event has occurred which has affected the post-closure plan. An owner or... 40 Protection of Environment 27 2012-07-01 2012-07-01 false Post-closure plan; amendment of plan... FACILITIES Closure and Post-Closure § 264.118 Post-closure plan; amendment of plan. (a) Written Plan. The...

40 CFR 264.118 - Post-closure plan; amendment of plan.

Code of Federal Regulations, 2013 CFR

2013-07-01

... days after an unexpected event has occurred which has affected the post-closure plan. An owner or... 40 Protection of Environment 27 2013-07-01 2013-07-01 false Post-closure plan; amendment of plan... FACILITIES Closure and Post-Closure § 264.118 Post-closure plan; amendment of plan. (a) Written Plan. The...

40 CFR 264.118 - Post-closure plan; amendment of plan.

Code of Federal Regulations, 2010 CFR

2010-07-01

... days after an unexpected event has occurred which has affected the post-closure plan. An owner or... 40 Protection of Environment 25 2010-07-01 2010-07-01 false Post-closure plan; amendment of plan... FACILITIES Closure and Post-Closure § 264.118 Post-closure plan; amendment of plan. (a) Written Plan. The...

Detection of postseismic fault-zone collapse following the Landers earthquake

NASA Astrophysics Data System (ADS)

Massonnet, Didier; Thatcher, Wayne; Vadon, Hélèna

1996-08-01

STRESS changes caused by fault movement in an earthquake induce transient aseismic crustal movements in the earthquake source region that continue for months to decades following large events1-4. These motions reflect aseismic adjustments of the fault zone and/or bulk deformation of the surroundings in response to applied stresses2,5-7, and supply information regarding the inelastic behaviour of the Earth's crust. These processes are imperfectly understood because it is difficult to infer what occurs at depth using only surface measurements2, which are in general poorly sampled. Here we push satellite radar interferometry to near its typical artefact level, to obtain a map of the postseismic deformation field in the three years following the 28 June 1992 Landers, California earthquake. From the map, we deduce two distinct types of deformation: afterslip at depth on the fault that ruptured in the earthquake, and shortening normal to the fault zone. The latter movement may reflect the closure of dilatant cracks and fluid expulsion from a transiently over-pressured fault zone6-8.

Delamination growth analysis in quasi-isotropic laminates under loads simulating low-velocity impact

NASA Technical Reports Server (NTRS)

Shivakumar, K. N.; Elber, W.

1984-01-01

A geometrically nonlinear finite-element analysis has been developed to calculate the strain energy released by delaminating plates during impact loading. Only the first mode of deformation, which is equivalent to static deflection, was treated. Both the impact loading and delamination in the plate were assumed to be axisymmetric. The strain energy release rate in peeling, GI, and shear sliding, GII, modes were calculated using the fracture mechanics crack closure technique. Energy release rates for various delamination sizes and locations and for various plate configurations and materials were compared. The analysis indicated that shear sliding was the primary mode of delamination growth. The analysis also indicated that the midplane (maximum transverse shear stress plane) delamination was more critical and would grow first before any other delamination of the same size near the midplane region. The delamination growth rate was higher (neutrally stable) for a low toughness (brittle) matrix and slower (stable) for high toughness matrix. The energy release rate in the peeling mode, GI, for a near-surface delamination can be as high as 0.5GII, and can contribute significantly to the delamination growth.

A Crack Growth Evaluation Method for Interacting Multiple Cracks

NASA Astrophysics Data System (ADS)

Kamaya, Masayuki

When stress corrosion cracking or corrosion fatigue occurs, multiple cracks are frequently initiated in the same area. According to section XI of the ASME Boiler and Pressure Vessel Code, multiple cracks are considered as a single combined crack in crack growth analysis, if the specified conditions are satisfied. In crack growth processes, however, no prescription for the interference between multiple cracks is given in this code. The JSME Post-Construction Code, issued in May 2000, prescribes the conditions of crack coalescence in the crack growth process. This study aimed to extend this prescription to more general cases. A simulation model was applied, to simulate the crack growth process, taking into account the interference between two cracks. This model made it possible to analyze multiple crack growth behaviors for many cases (e. g. different relative position and length) that could not be studied by experiment only. Based on these analyses, a new crack growth analysis method was suggested for taking into account the interference between multiple cracks.

Advanced reactors and novel reactions for the conversion of triglyceride based oils into high quality renewable transportation fuels

NASA Astrophysics Data System (ADS)

Linnen, Michael James

Sustainable energy continues to grow more important to all societies, leading to the research and development of a variety of alternative and renewable energy technologies. Of these, renewable liquid transportation fuels may be the most visible to consumers, and this visibility is further magnified by the long-term trend of increasingly expensive petroleum fuels that the public consumes. While first-generation biofuels such as biodiesel and fuel ethanol have been integrated into the existing fuel infrastructures of several countries, the chemical differences between them and their petroleum counterparts reduce their effectiveness. This gives rise to the development and commercialization of second generation biofuels, many of which are intended to have equivalent properties to those of their petroleum counterparts. In this dissertation, the primary reactions for a second-generation biofuel process, known herein as the University of North Dakota noncatalytic cracking process (NCP), have been studied at the fundamental level and improved. The NCP is capable of producing renewable fuels and chemicals that are virtually the same as their petroleum counterparts in performance and quality (i.e., petroleum-equivalent). In addition, a novel analytical method, FIMSDIST was developed which, within certain limitations, can increase the elution capabilities of GC analysis and decrease sample processing times compared to other high resolution methods. These advances are particularly useful for studies of highly heterogeneous fuel and/or organic chemical intermediates, such as those studied for the NCP. However the data from FIMSDIST must be supplemented with data from other methods such as for certain carboxylic acid, to provide accurate, comprehensive results, From a series of TAG cracking experiments that were performed, it was found that coke formation during cracking is most likely the result of excessive temperature and/or residence time in a cracking reactor. Based on this, a tubular cracking reactor was developed that could operate continuously without coke formation. The design also was proven to be scalable. Yields from the reactor were determined under a variety of conditions in order to predict the outputs from the NCP and to establish relationships/correlations between operating parameters and the product distribution. These studies led to the conclusion that the most severe operating conditions which do not induce coking are optimal over the experimental domain. In order to develop economical deoxygenation catalysts for use within the NCP, a series of experiments were performed using nickel catalysts, demonstrating that nickel catalysts could outperform their predecessor, a high cost palladium-based catalyst. A nickel catalyst was then tested in a packed bed reactor in order to determine suitable operating conditions for its commercial utilization in packed bed reactors.

40 CFR 261.151 - Wording of the instruments.

Code of Federal Regulations, 2013 CFR

2013-07-01

... assurance for closure or post-closure care is demonstrated through the financial test specified in subpart H... parts 264 and 265. The current closure and/or post-closure cost estimates covered by such a test are... financial test specified in subpart H of 40 CFR parts 264 and 265. The current closure and/or post-closure...

50 CFR 660.360 - Recreational fishery-management measures.

Code of Federal Regulations, 2011 CFR

2011-10-01

... authorized for recreational fishing are hook-and-line and spear. Spears may be propelled by hand or by... when the closure is in effect. The closure is not in effect at this time. This closure may be imposed... when the closure is in effect. The closure is not in effect at this time. This closure may be imposed...

50 CFR 660.360 - Recreational fishery-management measures.

Code of Federal Regulations, 2012 CFR

2012-10-01

... authorized for recreational fishing are hook-and-line and spear. Spears may be propelled by hand or by... when the closure is in effect. The closure is not in effect at this time. This closure may be imposed... when the closure is in effect. The closure is not in effect at this time. This closure may be imposed...

Evaluation of Container Closure System Integrity for Frozen Storage Drug Products.

PubMed

Nieto, Alejandra; Roehl, Holger; Brown, Helen; Nikoloff, Jonas; Adler, Michael; Mahler, Hanns-Christian

2016-01-01

Sometimes, drug product for parenteral administration is stored in a frozen state (e.g., -20 °C or -80 °C), particularly during early stages of development of some biotech molecules in order to provide sufficient stability. Shipment of frozen product could potentially be performed in the frozen state, yet possibly at different temperatures, for example, using dry ice (-80 °C). Container closure systems of drug products usually consist of a glass vial, rubber stopper, and an aluminum crimped cap. In the frozen state, the glass transition temperature (Tg) of commonly used rubber stoppers is between -55 and -65 °C. Below their Tg, rubber stoppers are known to lose their elastic properties and become brittle, and thus potentially fail to maintain container closure integrity in the frozen state. Leaks during frozen temperature storage and transportation are likely to be transient, yet, can possibly risk container closure integrity and lead to microbial contamination. After thawing, the rubber stopper is supposed to re-seal the container closure system. Given the transient nature of the possible impact on container closure integrity in the frozen state, typical container closure integrity testing methods (used at room temperature conditions) are unable to evaluate and thus confirm container closure integrity in the frozen state. Here we present the development of a novel method (thermal physical container closure integrity) for direct assessment of container closure integrity by a physical method (physical container closure integrity) at frozen conditions, using a modified He leakage test. In this study, different container closure systems were evaluated with regard to physical container closure integrity in the frozen state to assess the suitability of vial/stopper combinations and were compared to a gas headspace method. In summary, the thermal physical container closure integrity He leakage method was more sensitive in detecting physical container closure integrity impact than gas headspace and aided identification of an unsuitable container closure system. Sometimes, drug product for parenteral administration is stored in a frozen state (e.g., -20 °C or -80 °C), particularly during early stages of development of some biotech molecules in order to provide sufficient stability. Container closure systems for drug products usually consist of a glass vial, rubber stopper, and an aluminum crimped cap. In the frozen state, the glass transition temperature (Tg) of commonly used rubber stoppers is between -55 and -65 °C. Leaks during frozen temperature storage and transportation are likely to be transient, yet they can possibly risk container closure integrity and lead to microbial contamination and sterility breach. After thawing, the rubber stopper is expected to re-seal the container closure system. Given the transient nature of the possible impact on container closure integrity in the frozen state, typical container closure integrity testing methods (used at room temperature conditions) are unable to evaluate and thus confirm container closure integrity in the frozen state. Here we present the development of a novel method (thermal container closure integrity) for direct measurement of container closure integrity by a physical method (physical container closure integrity) at frozen conditions, using a modified He leakage test. In this study, we found that the thermal container closure integrity He leakage method was more sensitive in detecting physical container closure integrity impact than gas headspace and aided identification of an unsuitable container closure system. © PDA, Inc. 2016.

Unified risk analysis of fatigue failure in ductile alloy components during all three stages of fatigue crack evolution process.

PubMed

Patankar, Ravindra

2003-10-01

Statistical fatigue life of a ductile alloy specimen is traditionally divided into three stages, namely, crack nucleation, small crack growth, and large crack growth. Crack nucleation and small crack growth show a wide variation and hence a big spread on cycles versus crack length graph. Relatively, large crack growth shows a lesser variation. Therefore, different models are fitted to the different stages of the fatigue evolution process, thus treating different stages as different phenomena. With these independent models, it is impossible to predict one phenomenon based on the information available about the other phenomenon. Experimentally, it is easier to carry out crack length measurements of large cracks compared to nucleating cracks and small cracks. Thus, it is easier to collect statistical data for large crack growth compared to the painstaking effort it would take to collect statistical data for crack nucleation and small crack growth. This article presents a fracture mechanics-based stochastic model of fatigue crack growth in ductile alloys that are commonly encountered in mechanical structures and machine components. The model has been validated by Ray (1998) for crack propagation by various statistical fatigue data. Based on the model, this article proposes a technique to predict statistical information of fatigue crack nucleation and small crack growth properties that uses the statistical properties of large crack growth under constant amplitude stress excitation. The statistical properties of large crack growth under constant amplitude stress excitation can be obtained via experiments.

Resource Conservation and Recovery Act closure report: Area 2 Bitcutter and Postshot Containment Shops Injection Wells, Correction Action Unit 90

DOE Office of Scientific and Technical Information (OSTI.GOV)

NONE

This Closure Report provides documentation of the activities conducted during the Resource Conservation and Recovery Act (RCRA) closure of the Bitcutter and Postshot Containment Shops Injection Wells located in Area 2 of the Nevada Test Site (NTS), Oak Spring Quadrangle (USGS, 1986), Township 10 South, Range 53 East, Nye County, Nevada. This report discusses the Bitcutter Shop Inside Injection Well (CAU 90-A) closure-in-place and the Bitcutter Shop Outside Injection Well (CAU 90-B) and Postshot Containment Shop Injection Well (CAU 90-C) clean closures. This Closure Report provides background information about the unit, the results of the characterization activities and actions conductedmore » to determine the closure design. It also provides a discussion of the drainage analysis, preliminary closure activities, final closure activities, waste management activities, and the Post-Closure Care requirements.« less

Linearly exact parallel closures for slab geometry

NASA Astrophysics Data System (ADS)

Ji, Jeong-Young; Held, Eric D.; Jhang, Hogun

2013-08-01

Parallel closures are obtained by solving a linearized kinetic equation with a model collision operator using the Fourier transform method. The closures expressed in wave number space are exact for time-dependent linear problems to within the limits of the model collision operator. In the adiabatic, collisionless limit, an inverse Fourier transform is performed to obtain integral (nonlocal) parallel closures in real space; parallel heat flow and viscosity closures for density, temperature, and flow velocity equations replace Braginskii's parallel closure relations, and parallel flow velocity and heat flow closures for density and temperature equations replace Spitzer's parallel transport relations. It is verified that the closures reproduce the exact linear response function of Hammett and Perkins [Phys. Rev. Lett. 64, 3019 (1990)] for Landau damping given a temperature gradient. In contrast to their approximate closures where the vanishing viscosity coefficient numerically gives an exact response, our closures relate the heat flow and nonvanishing viscosity to temperature and flow velocity (gradients).

Crack-tip-opening angle measurements and crack tunneling under stable tearing in thin sheet 2024-T3 aluminum alloy

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Sutton, M. A.

1993-01-01

The stable tearing behavior of thin sheets 2024-T3 aluminum alloy was studied for middle crack tension specimens having initial cracks that were: flat cracks (low fatigue stress) and 45 degrees through-thickness slant cracks (high fatigue stress). The critical crack-tip-opening angle (CTOA) values during stable tearing were measured by two independent methods, optical microscopy and digital image correlation. Results from the two methods agreed well. The CTOA measurements and observations of the fracture surfaces showed that the initial stable tearing behavior of low and high fatigue stress tests is significantly different. The cracks in the low fatigue stress tests underwent a transition from flat-to-slant crack growth, during which the CTOA values were high and significant crack tunneling occurred. After crack growth equal to about the thickness, CTOA reached a constant value of 6 deg and after crack growth equal to about twice the thickness, crack tunneling stabilized. The initial high CTOA values, in the low fatigue crack tests, coincided with large three-dimensional crack front shape changes due to a variation in the through-thickness crack tip constraint. The cracks in the high fatigue stress tests reach the same constant CTOA value after crack growth equal to about the thickness, but produced only a slightly higher CTOA value during initial crack growth. For crack growth on the 45 degree slant, the crack front and local field variables are still highly three-dimensional. However, the constant CTOA values and stable crack front shape may allow the process to be approximated with two-dimensional models.

Observations on early and delayed colostomy closure.

PubMed

Tade, A O; Salami, B A; Ayoade, B A

2011-06-01

Traditional treatment of a variety of colorectal pathologies had included a diverting colostomy that was closed eight or more weeks later during a readmission. The aim of this retrospective study was to determine the outcomes of early colostomy closure and delayed colostomy closure in patients with temporary colostomies following traumatic and non-traumatic colorectal pathologies. In this study early colostomy closure was the closure of a colostomy within three weeks of its construction, while delayed colostomy closure referred to closure after 3 weeks. Complete records of the 37 adult patients who had temporary colostomy constructed and closed between Jan. 1997 December 2003 for various colorectal pathologies were studied. Fourteen patients had early colostomy closure while 23 had delayed closure. In the early colostomy closure group there were 10 men and 4 women. The mean age of the patients was 28yr with a range of 18-65yr. Colostomies were closed 9-18 days after initial colostomy construction. There was no mortality. Morbidity rate 28.6% (4 out of 14). There were two faecal fistulas (14.3%). Twenty-three patients had delayed colostomy closure 8 weeks to 18 months after initial colostomy construction. These were patients unfit for early surgery after initial colostomy construction because of carcinoma, significant weight loss, or sepsis. There was no mortality. Morbidity rate was 26.1%. There were 3 faecal fistulas (13.2%). Outcomes following early colostomy closure and delayed closure were comparable. Patients fit for surgery should have early closure whilst patients who may have compromised health should have delayed closure.

System for closure of a physical anomaly

DOEpatents

Bearinger, Jane P; Maitland, Duncan J; Schumann, Daniel L; Wilson, Thomas S

2014-11-11

Systems for closure of a physical anomaly. Closure is accomplished by a closure body with an exterior surface. The exterior surface contacts the opening of the anomaly and closes the anomaly. The closure body has a primary shape for closing the anomaly and a secondary shape for being positioned in the physical anomaly. The closure body preferably comprises a shape memory polymer.

Stapler vs suture closure of pancreatic remnant after distal pancreatectomy: a meta-analysis.

PubMed

Zhou, Wei; Lv, Ran; Wang, Xianfa; Mou, Yiping; Cai, Xiujun; Herr, Ingrid

2010-10-01

Suture closure and stapler closure of the pancreatic remnant after distal pancreatectomy are the techniques used most often. The ideal choice remains a matter of debate. Five bibliographic databases covering 1970 to July 2009 were searched. Sixteen articles met the inclusion criteria. Stapler closure was performed in 671 patients, while suture closure was conducted in 1,615 patients. The pancreatic fistula rate ranged from 0% to 40.0% for stapler closure of the pancreatic stump and from 9.3% to 45.7% for the suture closure technique. There were no significant difference between the stapler and suture closure groups with respect to the pancreatic fistula formation rate (22.1% vs 31.2%; odds ratio, .85; 95% confidence interval, .66-1.08), although there was a trend toward favoring stapler closure. In 4 studies including 437 patients, stapler closure was associated with a trend (not statistically significant) toward a reduction in intra-abdominal abscess (odds ratio, .53; 95% confidence interval, .24-1.15). No significant differences occur between suture and stapler closure with respect to the pancreatic fistula or intra-abdominal abscess after distal pancreatectomy, though there is a trend favoring stapler closure. Copyright © 2010 Elsevier Inc. All rights reserved.

Influence of Pore-Fluid Pressure on Elastic Wave Velocity and Electrical Conductivity in Water-Saturated Rocks

NASA Astrophysics Data System (ADS)

Higuchi, A.; Watanabe, T.

2013-12-01

Pore-fluid pressure in seismogenic zones can play a key role in the occurrence of earthquakes (e.g., Sibson, 2009). Its evaluation via geophysical observations can lead to a good understanding of seismic activities. The evaluation requires a thorough understanding of the influence of the pore-fluid pressure on geophysical observables like seismic velocity and electrical conductivity. We have studied the influence of pore-fluid pressure on elastic wave velocity and electrical conductivity in water-saturated rocks. Fine grained (100-500μm) biotite granite (Aji, Kagawa pref., Japan) was used as rock samples. The density is 2.658-2.668 g/cm3, and the porosity 0.68-0.87%. The sample is composed of 52.8% plagioclase, 36.0% Quartz, 3.0% K-feldspar, 8.2% biotite. SEM images show that a lot of grain boundaries are open. Few intracrystalline cracks were observed. Following the method proposed by David and Zimmerman (2012), the distribution function of crack aspect ratio was evaluated from the pressure dependence of compressional and shear wave velocities in a dry sample. Cylindrical sample has dimensions of 25 mm in diameter and 30 mm in length, and saturated with 0.01 mol/l KCl aqueous solution. Compressional and shear wave velocities were measured with the pulse transmission technique (PZT transducers, f=2 MHz), and electrical conductivity the two-electrode method (Ag-AgCl electrodes, f=1 Hz-100 kHz). Simultaneous measurements of velocities and conductivity were made using a 200 MPa hydrostatic pressure vessel, in which confining and pore-fluid pressures can be separately controlled. The pore-fluid is electrically insulated from the metal work of the pressure vessel by using a newly designed plastic device (Watanabe and Higuchi, 2013). The confining pressure was progressively increased up to 25 MPa, while the pore-fluid pressure was kept at 0.1 MPa. It took five days or longer for the electrical conductivity to become stationary after increasing the confining pressure. Elastic wave velocities and electrical conductivity showed reproducibly contrasting changes for a small increase in the confining pressure. The elastic wave velocities increased only by 5% as the confining pressure increased from 0.1 MPa to 25 MPa, while the electrical conductivity decreased by an order of magnitude. Based on the SEM examinations, open grain boundaries work as cracks. The changes in elastic wave velocities and electrical conductivity must be caused by the closure of open grain boundaries. Most (˜80%) of the decrease in electrical conductivity occurred below the confining pressure of 5 MPa. As the confining pressure increased from 0.1 MPa to 5 MPa, cracks with the aspect ratio smaller than 7.5×10-5 were closed. The decrease in porosity was only 0.0005%. Such a small change in porosity caused a large change in electrical conductivity. The connectivity of fluid was maintained at the confining pressure of 25 MPa by cracks with the aspect ratio larger than 3.7×10-4. Simultaneous measurements have provided us a lot of information on the microstructure of fluid-bearing rocks.

Quantity Effect of Radial Cracks on the Cracking Propagation Behavior and the Crack Morphology

PubMed Central

Chen, Jingjing; Xu, Jun; Liu, Bohan; Yao, Xuefeng; Li, Yibing

2014-01-01

In this letter, the quantity effect of radial cracks on the cracking propagation behavior as well as the circular crack generation on the impacted glass plate within the sandwiched glass sheets are experimentally investigated via high-speed photography system. Results show that the radial crack velocity on the backing glass layer decreases with the crack number under the same impact conditions during large quantities of repeated experiments. Thus, the “energy conversion factor” is suggested to elucidate the physical relation between the cracking number and the crack propagation speed. Besides, the number of radial crack also takes the determinative effect in the crack morphology of the impacted glass plate. This study may shed lights on understanding the cracking and propagation mechanism in laminated glass structures and provide useful tool to explore the impact information on the cracking debris. PMID:25048684

Comparing the floquet stability of open and breathing fatigue cracks in an overhung rotordynamic system

NASA Astrophysics Data System (ADS)

Varney, Philip; Green, Itzhak

2017-11-01

Rotor cracks represent an uncommon but serious threat to rotating machines and must be detected early to avoid catastrophic machine failure. An important aspect of analyzing rotor cracks is understanding their influence on the rotor stability. It is well-known that the extent of rotor instability versus shaft speed is exacerbated by deeper cracks. Consequently, crack propagation can eventually result in an unstable response even if the shaft speed remains constant. Most previous investigations of crack-induced rotor instability concern simple Jeffcott rotors. This work advances the state-of-the-art by (a) providing a novel inertial-frame model of an overhung rotor, and (b) assessing the stability of the cracked overhung rotor using Floquet stability analysis. The rotor Floquet stability analysis is performed for both an open crack and a breathing crack, and conclusions are drawn regarding the importance of appropriately selecting the crack model. The rotor stability is analyzed versus crack depth, external viscous damping ratio, and rotor inertia. In general, this work concludes that the onset of instability occurs at lower shaft speeds for thick rotors, lower viscous damping ratios, and deeper cracks. In addition, when comparing commensurate cracks, the breathing crack is shown to induce more regions of instability than the open crack, though the open crack generally predicts an unstable response for shallower cracks than the breathing crack. Keywords: rotordynamics, stability, rotor cracks.

Mass exchange in an experimental new-generation life support system model based on biological regeneration of environment.

PubMed

Tikhomirov, A A; Ushakova, S A; Manukovsky, N S; Lisovsky, G M; Kudenko, Yu A; Kovalev, V S; Gubanov, V G; Barkhatov, Yu V; Gribovskaya, I V; Zolotukhin, I G; Gros, J B; Lasseur, Ch

2003-01-01

An experimental model of a biological life support system was used to evaluate qualitative and quantitative parameters of the internal mass exchange. The photosynthesizing unit included the higher plant component (wheat and radish), and the heterotrophic unit consisted of a soil-like substrate, California worms, mushrooms and microbial microflora. The gas mass exchange involved evolution of oxygen by the photosynthesizing component and its uptake by the heterotroph component along with the formation and maintaining of the SLS structure, growth of mushrooms and California worms, human respiration, and some other processes. Human presence in the system in the form of "virtual human" that at regular intervals took part in the respirative gas exchange during the experiment. Experimental data demonstrated good oxygen/carbon dioxide balance, and the closure of the cycles of these gases was almost complete. The water cycle was nearly 100% closed. The main components in the water mass exchange were transpiration water and the watering solution with mineral elements. Human consumption of the edible plant biomass (grains and roots) was simulated by processing these products by a unique physicochemical method of oxidizing them to inorganic mineral compounds, which were then returned into the system and fully assimilated by the plants. The oxidation was achieved by "wet combustion" of organic biomass, using hydrogen peroxide following a special procedure, which does not require high temperature and pressure. Hydrogen peroxide is produced from the water inside the system. The closure of the cycle was estimated for individual elements and compounds. Stoichiometric proportions are given for the main components included in the experimental model of the system. Approaches to the mathematical modeling of the cycling processes are discussed, using the data of the experimental model. Nitrogen, as a representative of biogenic elements, shows an almost 100% closure of the cycle inside the system. The proposed experimental model of a biological system is discussed as a candidate for potential application in the investigations aimed at creating ecosystems with largely closed cycles of the internal mass exchange. The formation and maintenance of sustainable cycling of vitally important chemical elements and compounds in biological life support systems (BLSS) is an extremely pressing problem. To attain the stable functioning of biological life support systems (BLSS) and to maintain a high degree of closure of material cycles in than, it is essential to understand the character of mass exchange processes and stoichiometnc proportions of the initial and synthesized components of the system. c2003 COSPAR. Published by Elsevier Science Ltd. All rights reserved.

A comparison of stress in cracked fibrous tissue specimens with varied crack location, loading, and orientation using finite element analysis.

PubMed

Peloquin, John M; Elliott, Dawn M

2016-04-01

Cracks in fibrous soft tissue, such as intervertebral disc annulus fibrosus and knee meniscus, cause pain and compromise joint mechanics. A crack concentrates stress at its tip, making further failure and crack extension (fracture) more likely. Ex vivo mechanical testing is an important tool for studying the loading conditions required for crack extension, but prior work has shown that it is difficult to reproduce crack extension. Most prior work used edge crack specimens in uniaxial tension, with the crack 90° to the edge of the specimen. This configuration does not necessarily represent the loading conditions that cause in vivo crack extension. To find a potentially better choice for experiments aiming to reproduce crack extension, we used finite element analysis to compare, in factorial combination, (1) center crack vs. edge crack location, (2) biaxial vs. uniaxial loading, and (3) crack-fiber angles ranging from 0° to 90°. The simulated material was annulus fibrosus fibrocartilage with a single fiber family. We hypothesized that one of the simulated test cases would produce a stronger stress concentration than the commonly used uniaxially loaded 90° crack-fiber angle edge crack case. Stress concentrations were compared between cases in terms of fiber-parallel stress (representing risk of fiber rupture), fiber-perpendicular stress (representing risk of matrix rupture), and fiber shear stress (representing risk of fiber sliding). Fiber-perpendicular stress and fiber shear stress concentrations were greatest in edge crack specimens (of any crack-fiber angle) and center crack specimens with a 90° crack-fiber angle. However, unless the crack is parallel to the fiber direction, these stress components alone are insufficient to cause crack opening and extension. Fiber-parallel stress concentrations were greatest in center crack specimens with a 45° crack-fiber angle, either biaxially or uniaxially loaded. We therefore recommend that the 45° center crack case be tried in future experiments intended to study crack extension by fiber rupture. Copyright © 2015 Elsevier Ltd. All rights reserved.

Development of a chromium-free consumable for joining stainless steels

NASA Astrophysics Data System (ADS)

Sowards, Jeffrey William

Government regulations in the United States (OSHA Standards: 1910; 1915; 1917; 1918; 1926) and abroad are decreasing allowable exposure levels of hexavalent chromium to welding related personnel. The latest OSHA ruling in 2006 reduced the permissible exposure limit of airborne hexavalent chromium from 52 to 5 mug m-3. Achieving the new level may not be practical from an engineering controls standpoint during the fabrication of tightly enclosed stainless steel components such as the inside of ship hulls and boiler vessels. One method of addressing this problem is to implement a chromium-free welding consumable that provides equivalent mechanical performance and corrosion characteristics to current stainless steel welding consumables. This project was aimed at developing such a consumable and evaluating its suitability for replacement of current stainless steel consumables such as E308L-16. A new shielded metal arc welding (SMAW) consumable based on the Ni-Cu-Ru system was developed for austenitic stainless steel welding. The focus of this work was evaluating the mechanical properties, weldability, and fume formation characteristics of the various iterations of consumables developed. Welds deposited on Type 304 stainless steel were evaluated with weldability tests including: mechanical testing, hot ductility testing, Strain-to-fracture testing, Transverse Varestraint testing, and button melting. Mechanical properties of weld deposits of each consumable were found to exceed minimum values of Type 304 stainless steel based on tensile testing. Guide bend testing showed that weld deposits met minimum weld ductility requirements for stainless steel consumables, such as E308-16. Hot ductility testing revealed a narrow crack susceptible region (33 to 54°C) indicating a low susceptibility to weld metal liquation cracking. GTA welds exhibited superior ductility when compared to SMA welds. This was attributed to a lack of slag inclusions in the weld deposit, which are effective weld strengtheners. Varestraint testing revealed that weld deposits have a higher solidification cracking susceptibility than stainless steel consumables used to join Type 304. Higher cracking susceptibility was attributed to austenitic solidification of the weld metal resulting in increased weld segregation and stabilization of a TiC eutectic reaction at the end of solidification. No solidification cracks were observed in actual weld deposits. Evaluation of weld microsegregation patterns showed higher dilutions of Type 304 increased segregation of Ti, promoting a TiC eutectic reaction at the end of solidification. Thermodynamic modeling techniques were used to describe the solidification the Ni-Cu weld deposits as a function of dilution with Type 304. Solidification cracking susceptibility was shown to increase with dilution during evaluation with the Cast Pin Tear Test indicating high dilution welds should be avoided to minimize solidification cracking during welding. The Strain-to-fracture test was used to examine DDC cracking susceptibility, and revealed that this alloy has a higher susceptibility to solid-state weld cracking than austenitic stainless alloys such as 304. Threshold strain levels necessary to initiate cracking in the weld deposits were in the range of 2 to 3%. These values are comparable to other Ni-base alloys with a moderate to high susceptibility to DDC. Fume generation rates (FGR) of the new consumable were measured and bulk fume phases were analyzed with X-ray diffraction. FGR values were found to be similar to current SMAW and flux cored arc welding consumables. No chromium bearing compounds were observed during X-ray diffraction measurements, and the bulk fume consisted primarily of halides and metallic-oxides. Fume generated by the new consumable was subjected to colorimetric testing showing hexavalent Cr content (0.02 wt-%) was reduced by two orders of magnitude compared to E308-16 (2.6 wt-%). The source of this hexavalent chromium was from evaporation of the base metal due to the welding heat source. The consumable developed in this study, having a nominal composition of Ni-7.5Cu-1Ru-0.5Al-0.5Ti-0.02C, met virtually all the design criteria that were initially established. Work performed by the Fontana Corrosion Center showed that the weld deposits met corrosion design criteria to prevent localized attack of the weld metal. Work performed in this study showed that mechanical properties were comparable to stainless steel consumables, and weld cracking susceptibility was comparable to Ni-base welding consumables. The consumable was also found to have good operability characteristics. (Abstract shortened by UMI.)

Stump closure of a thick pancreas using stapler closure increases pancreatic fistula after distal pancreatectomy.

PubMed

Kawai, Manabu; Tani, Masaji; Okada, Ken-ichi; Hirono, Seiko; Miyazawa, Motoki; Shimizu, Astusi; Kitahata, Yuji; Yamaue, Hiroki

2013-09-01

The appropriate surgical stump closure after distal pancreatectomy (DP) is still controversial. This study investigated the benefits and risks of stapler closure during DP. The risk factors of pancreatic fistulas were investigated in 122 DPs among 3 types of stump closure: hand-sewn suture (n = 32), bipolar scissors (n = 45), and stapler closure (n = 45). There was no significant difference in the incidence of pancreatic fistula between the 3 types of stump closure (hand-sewn suture [44%] vs bipolar scissors [37.7%] vs stapler closure [35.5%]). By using receiver operating characteristics curves, 12 mm was the best cutoff value of the thickness of the pancreas for pancreatic fistulas after DP using stapler closure. Three factors (ie, male sex, body mass index >25 kg/m(2), and stapler closure) were independent risk factors of pancreatic fistulas after DP with a pancreas thicker than 12 mm. A pancreas thicker than 12 mm significantly increased the incidence of pancreatic fistulas after DP using stapler closure. Copyright © 2013 Elsevier Inc. All rights reserved.

40 CFR 264.258 - Closure and post-closure care.

Code of Federal Regulations, 2010 CFR

2010-07-01

... Waste Piles § 264.258 Closure and post-closure care. (a) At closure, the owner or operator must remove... that apply to landfills (§ 264.310). (c)(1) The owner or operator of a waste pile that does not comply...(c) or § 264.251(b), must: (i) Include in the closure plan for the pile under § 264.112 both a plan...

The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

NASA Technical Reports Server (NTRS)

Piascik, Robert S.

2015-01-01

The corrosion fatigue crack growth characteristics of small (greater than 35 micrometers) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500×) crack length measurements in laboratory air and 1% sodium chloride (NaCl) environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.

The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

NASA Technical Reports Server (NTRS)

Piascik, R. S.

2001-01-01

The corrosion fatigue crack growth characteristics of small (less than 35 microns) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500X) crack length measurements in laboratory air and 1% NaCl environments. To quantify the "small crack effect" in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K(sub max) growth rates are similar to small crack da/dN.

The Effect of Rural Hospital Closures on Community Economic Health

PubMed Central

Holmes, George M; Slifkin, Rebecca T; Randolph, Randy K; Poley, Stephanie

2006-01-01

Objective To examine the effect of rural hospital closures on the local economy. Data Sources U.S. Census Bureau, OSCAR, Medicare Cost Reports, and surveys of individuals knowledgeable about local hospital closures. Study Design Economic data at the county level for 1990–2000 were combined with information on hospital closures. The study sample was restricted to rural counties experiencing a closure during the sample period. Longitudinal regression methods were used to estimate the effect of hospital closure on per-capita income, unemployment rate, and other community economic measures. Models included both leading and lagged closure terms allowing a preclosure economic downturn as well as time for the closure to be fully realized by the community. Data Collection Information on closures was collected by contacting every state hospital association, reconciling information gathered with that contained in the American Hospital Association file and OIG reports. Principal Findings Results indicate that the closure of the sole hospital in the community reduces per-capita income by $703 (p<0.05) or 4 percent (p<0.05) and increases the unemployment rate by 1.6 percentage points (p<0.01). Closures in communities with alternative sources of hospital care had no long-term economic impact, although income decreased for 2 years following the closure. Conclusions The local economic effects of a hospital closure should be considered when regulations that affect hospitals' financial well-being are designed or changed. PMID:16584460

APPARATUS AND METHOD FOR WELDING END CLOSURE TO CONTAINER

DOEpatents

Frantz, C.E.; Correy, T.B.

1959-08-01

A semi-automatic apparatus is described for welding a closure to the open end of a can containing a nuclear fuel slug. An arc is struck at the center of the closure and is shifted to a region near its periphery. Then the assembly of closure, can, and fuel slug is rotated so that the peripheral region of the closure is preheated. Next the arc is shifted to the periphery itself of the closure, and the assembly is rotated so that the closure is welded to the can.

Positive Catch & Economic Benefits of Periodic Octopus Fishery Closures: Do Effective, Narrowly Targeted Actions 'Catalyze' Broader Management?

PubMed

Oliver, Thomas A; Oleson, Kirsten L L; Ratsimbazafy, Hajanaina; Raberinary, Daniel; Benbow, Sophie; Harris, Alasdair

2015-01-01

Eight years of octopus fishery records from southwest Madagascar reveal significant positive impacts from 36 periodic closures on: (a) fishery catches and (b) village fishery income, such that (c) economic benefits from increased landings outweigh costs of foregone catch. Closures covered ~20% of a village's fished area and lasted 2-7 months. Octopus landings and catch per unit effort (CPUE) significantly increased in the 30 days following a closure's reopening, relative to the 30 days before a closure (landings: +718%, p<0.0001; CPUE: +87%, p<0.0001; n = 36). Open-access control sites showed no before/after change when they occurred independently of other management ("no ban", n = 17/36). On the other hand, open-access control sites showed modest catch increases when they extended a 6-week seasonal fishery shutdown ("ban", n = 19/36). The seasonal fishery shutdown affects the entire region, so confound all potential control sites. In villages implementing a closure, octopus fishery income doubled in the 30 days after a closure, relative to 30 days before (+132%, p<0.001, n = 28). Control villages not implementing a closure showed no increase in income after "no ban" closures and modest increases after "ban" closures. Villages did not show a significant decline in income during closure events. Landings in closure sites generated more revenue than simulated landings assuming continued open-access fishing at that site (27/36 show positive net earnings; mean +$305/closure; mean +57.7% monthly). Benefits accrued faster than local fishers' time preferences during 17-27 of the 36 closures. High reported rates of illegal fishing during closures correlated with poor economic performance. We discuss the implications of our findings for broader co-management arrangements, particularly for catalyzing more comprehensive management.

Lace Up for Healthy Feet: The Impact of Shoe Closure on Plantar Stress Response.

PubMed

Rahemi, Hadi; Armstrong, David G; Enriquez, Ana; Owl, Joshua; Talal, Talal K; Najafi, Bijan

2017-07-01

This study examined the impact of shoe closure on plantar thermal stress response (TSR), which is known to be a surrogate of shear stress and skin perfusion. It is aimed to explore potential impact of shoe closure on increasing risk factors associated with plantar ulcers in people with diabetic peripheral neuropathy (DPN). Fifteen eligible subjects were enrolled. The left foot was used as a reference and fitted to a self-adjusted and habitual lace-tightening method by each subject. The right foot was used as a test closure and fitted into three lace closure conditions: loose, tight, and preset optimal closure (reel clutch, BOA technology). Thermal images were taken after 5 minutes of acclimatization (pre-trial) and immediately after 200 walking steps in each shoe closure condition (post-trial). TSR was calculated from the thermal images. TSR was significantly higher in the test closure with loose (70.24%, P = .000) and tight (66.85%, P = .007) and lower (-206.53%, P = .000) in the preset optimal closure when compared to the reference closure. Only lace closure conditions affected TSR with no significant impact from age, BMI, and gender in our sample in a multivariable regression model. The results from this study suggest that shoelace closure technique can have a profound effect on TSR. It therefore stands to reason that optimal lace closure may have an impact in reducing risk of plantar ulcers in people with DPN. Interestingly, results revealed that even a self-adjusted lace closure may not be necessarily optimal and a preset closure setting like reel clutch might ultimately be recommended to minimize risk. Further study is warranted to confirm or refute these interesting results.

[Factors influencing the prognosis of patent ductus arteriosus in very low birth weight infants].

PubMed

Wang, Chen-Hong; Shi, Li-Ping; Ma, Xiao-Lu; Luo, Fang; Chen, Zheng; Lin, Hui-Jia; DU, Li-Zhong

2016-08-01

To investigate the factors influencing the prognosis of patent ductus arteriosus (PDA) in very low birth weight (VLBW) infants. A total of 194 VLBW infants who were admitted from January 2012 to December 2014 were enrolled as study subjects. According to cardiac ultrasound findings and treatment outcome, these infants were divided into non-PDA group, spontaneous closure group, pharmaceutical closure group, and surgical closure group. Their clinical and echocardiographic characteristics were analyzed. The spontaneous closure rate of PDA was 58.7%. The spontaneous closure group showed significantly higher gestational age, birth weight, and proportion of small-for-gestational-age infants than the pharmaceutical and surgical closure groups (P<0.05). The pharmaceutical and surgical closure groups had a significantly higher incidence rate of neonatal respiratory distress syndrome and a significantly higher proportion of infants who were given pulmonary surfactant (PS) than the spontaneous closure group (P<0.05). During different periods of time, the spontaneous closure group had a significantly smaller ductus arteriosus diameter than the pharmaceutical and surgical closure groups (P<0.05). The multivariate logistic regression analysis showed that gestational age, application of PS, and ductus arteriosus diameter at 48 hours were significantly associated with the prognosis of PDA. The major transductal flow pattern in the spontaneous closure group was closing pattern, while in the pharmaceutical and surgical closure groups, the main flow patterns were pulmonary hypertension and growing patterns within 48 hours and growing pattern on days 4 and 7. The VLBW infants have a high spontaneous closure rate of PDA. A decreased closure rate of PDA is associated with the lower gestational age and the application of PS. PDA with a large ductus arteriosus diameter and a growing or pulsatile flow pattern cannot easily achieve spontaneous closure.

Methodology for extracting local constants from petroleum cracking flows

DOEpatents

Chang, Shen-Lin; Lottes, Steven A.; Zhou, Chenn Q.

2000-01-01

A methodology provides for the extraction of local chemical kinetic model constants for use in a reacting flow computational fluid dynamics (CFD) computer code with chemical kinetic computations to optimize the operating conditions or design of the system, including retrofit design improvements to existing systems. The coupled CFD and kinetic computer code are used in combination with data obtained from a matrix of experimental tests to extract the kinetic constants. Local fluid dynamic effects are implicitly included in the extracted local kinetic constants for each particular application system to which the methodology is applied. The extracted local kinetic model constants work well over a fairly broad range of operating conditions for specific and complex reaction sets in specific and complex reactor systems. While disclosed in terms of use in a Fluid Catalytic Cracking (FCC) riser, the inventive methodology has application in virtually any reaction set to extract constants for any particular application and reaction set formulation. The methodology includes the step of: (1) selecting the test data sets for various conditions; (2) establishing the general trend of the parametric effect on the measured product yields; (3) calculating product yields for the selected test conditions using coupled computational fluid dynamics and chemical kinetics; (4) adjusting the local kinetic constants to match calculated product yields with experimental data; and (5) validating the determined set of local kinetic constants by comparing the calculated results with experimental data from additional test runs at different operating conditions.

DDR-type zeolite membrane synthesis, modification and gas permeation studies

DOE PAGES

Yang, Shaowei; Cao, Zishu; Arvanitis, Antonios; ...

2016-01-22

DDR-type zeolite membrane was synthesized on porous α-alumina substrate by hydrothermal treatment of a ball-milled Sigmal-1 crystal seed layer in an aluminum-free precursor solution containing 1-Adamantylamine as the structure directing agent (SDA). The as-synthesized DDR zeolite membranes were defect-free but the supported zeolite layers were susceptible to crack development during the subsequent high-temperature SDA removal process. The cracks were effectively eliminated by the liquid phase chemical deposition method using tetramethoxysilane as the precursor for silica deposits. The modified membrane was extensively studied for H 2, He, O 2, N 2, CO 2, CH 4, and i-C 4H 10 pure gasmore » permeation and CO 2/CH 4 mixture separation. At 297 K and 2-bar feed gas pressure, the membrane achieved a CO 2/CH 4 separation factor of ~92 for a feed containing 90% CO 2, which decreased to 62 for a feed containing 10% CO 2 with the CO 2 permeance virtually unchanged at ~1.8×10 –7 mol/m• sup>2 s • Pa regardless of the feed composition. It also exhibited an O 2/N 2 permselectivity of 1.8 at 297 K. Furthermore, the gas permeation behaviors of the current aluminum-containing DDR type zeolite membrane are generally in good agreement with the findings in both experimental and theoretical studies on the pure-silica DDR membranes in recent literature.« less

40 CFR 264.280 - Closure and post-closure care.

Code of Federal Regulations, 2013 CFR

2013-07-01

... Section 264.280 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED) STANDARDS FOR OWNERS AND OPERATORS OF HAZARDOUS WASTE TREATMENT, STORAGE, AND DISPOSAL FACILITIES Land Treatment § 264.280 Closure and post-closure care. (a) During the closure period the owner or...

40 CFR 264.280 - Closure and post-closure care.

Code of Federal Regulations, 2011 CFR

2011-07-01

... Section 264.280 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED) STANDARDS FOR OWNERS AND OPERATORS OF HAZARDOUS WASTE TREATMENT, STORAGE, AND DISPOSAL FACILITIES Land Treatment § 264.280 Closure and post-closure care. (a) During the closure period the owner or...

40 CFR 264.280 - Closure and post-closure care.

Code of Federal Regulations, 2012 CFR

2012-07-01

... Section 264.280 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED) STANDARDS FOR OWNERS AND OPERATORS OF HAZARDOUS WASTE TREATMENT, STORAGE, AND DISPOSAL FACILITIES Land Treatment § 264.280 Closure and post-closure care. (a) During the closure period the owner or...

40 CFR 264.280 - Closure and post-closure care.

Code of Federal Regulations, 2014 CFR

2014-07-01

... Section 264.280 Protection of Environment ENVIRONMENTAL PROTECTION AGENCY (CONTINUED) SOLID WASTES (CONTINUED) STANDARDS FOR OWNERS AND OPERATORS OF HAZARDOUS WASTE TREATMENT, STORAGE, AND DISPOSAL FACILITIES Land Treatment § 264.280 Closure and post-closure care. (a) During the closure period the owner or...

Radial Cracks Would Signal Wearout Of Turbine Blades

NASA Technical Reports Server (NTRS)

Paulus, Donald E.

1990-01-01

Nonfatal defects made to appear before fatal ones. Proposed to design turbine blades to crack radially before they crack chordwise. Advance radial cracking promoted in design by adjusting thermal stresses and net bending stresses. Prior appearance of radial crack or cracks in used blade serves as warning that more-threatening chordwise crack or cracks may subsequently appear. Blade replaced before it fails.

Edge-Cracking Behavior of CoCrFeMnNi High-Entropy Alloy During Hot Rolling

NASA Astrophysics Data System (ADS)

Won, Jong Woo; Kang, Minju; Kwon, Heoun-Jun; Lim, Ka Ram; Seo, Seong Moon; Na, Young Sang

2018-05-01

This work investigated edge-cracking behavior of equiatomic CoCrFeMnNi high-entropy alloy during hot rolling at rolling temperatures 500 ≤ T R ≤ 1000 °C. Edge cracks did not form in the material rolled at 500 °C, but widened and deepened into the inside of plate as T R increased from 500 °C. Edge cracks were most severe in the material rolled at 1000 °C. Mn-Cr-O type non-metallic inclusion and oxidation were identified as major factors that caused edge cracking. The inclusions near edge region acted as preferential sites for crack formation. Connection between inclusion cracks and surface cracks induced edge cracking. Rolling at T R ≥ 600 °C generated distinct inclusion cracks whereas they were not serious at T R = 500 °C, so noticeable edge cracks formed at T R ≥ 600 °C. At T R = 1000 °C, significant oxidation occurred at the crack surface. This accelerated edge crack penetration by embrittling the crack tip, so severe edge cracking occurred at T R = 1000 °C.

Analysis of unclad and sub-clad semi-elliptical flaws in pressure vessel steels

DOE Office of Scientific and Technical Information (OSTI.GOV)

Irizarry-Quinones, H.; Macdonald, B.D.; McAfee, W.J.

This study was conducted to support warm prestressing experiments on unclad and sub-clad flawed beams loaded in pure bending. Two cladding yield strengths were investigated: 0.6 Sy and 0.8 Sy, where Sy is the yield strength of the base metal. Cladding and base metal were assumed to be stress free at the stress relief temperature for the 3D elastic-plastic finite element analysis used to model the experiments. The model results indicated that when cooled from the stress relief temperature, the cladding was put in tension due to its greater coefficient of thermal expansion. When cooled, the cladding exhibited various amountsmore » of tensile yielding. The degree of yielding depended on the amount of cooling and the strength of the cladding relative to that of the base metal. When subjected to tensile bending stress, the sub-clad flaw elastic-plastic stress intensity factor, K{sub I}(J), was at first dominated by crack closing force due to tensile yielding in the cladding. Thus, imposed loads initially caused no increase in K{sub I}(J) near the clad-base interface. However, K{sub I}(J) at the flaw depth was little affected. When the cladding residual stress was overcome, K{sub I}(J) gradually increased until the cladding began to flow. Thereafter, the rate at which K{sub I}(J) increased with load was the same as that of an unclad beam. A plastic zone corrected K{sub I} approximation for the unclad flaw was found by the superposition of standard Newman and Raju solutions with those due to a cladding crack closure force approximated by the Kaya and Erdogan solution. These elastic estimates of the effect of cladding in reducing the crack driving force were quite in keeping with the 3D elastic-plastic finite element solution for the sub-clad flaw. The results were also compared with the analysis of clad beam experiments by Keeney and the conclusions by Miyazaki, et al. A number of sub-clad flaw specimens not subjected to warm prestressing were thought to have suffered degraded toughness caused by locally intensified strain aging embrittlement (LISAE) due to welding over the preexisting flaw.« less

Metalloproteinase Expression is Associated with Traumatic Wound Failure

DTIC Science & Technology

2010-04-01

Traumatic amputation- no.(%) 15 Size of wound (cm3 )* Associated vascular injury- no.(%) 7 Wound closure method no.(%) Suture 29 Skin graft 9 Number...definitive closure or coverage with skin graft . Im- paired wound healing included delayed wound closure or wound dehiscence after closure or coverage...closure time period of 10 d. Dehiscence was defined as spontaneous partial or com- plete wound disruption after primary closure or > 90% skin graft loss

Novel application of polyelectrolyte multilayers as nanoscopic closures with hermetic sealing.

PubMed

Marcott, Stephanie A; Ada, Sena; Gibson, Phillip; Camesano, Terri A; Nagarajan, R

2012-03-01

Closure systems for personnel protection applications, such as protective clothing or respirator face seals, should provide effective permeation barrier to toxic gases. Currently available mechanical closure systems based on the hook and loop types (example, Velcro) do not provide adequate barrier to gas permeation. To achieve hermetic sealing, we propose a nonmechanical, nanoscopic molecular closure system based on complementary polyelectrolyte multilayers, one with a polycation outermost layer and the other with a polyanion outermost layer. The closure surfaces were prepared by depositing polyelectrolyte multilayers under a variety of deposition conditions, on conformable polymer substrates (thin films of polyethylene teraphthalate, PET or polyimide, PI). The hermetic sealing property of the closures was evaluated by measuring the air flow resistance using the dynamic moisture permeation cell (DMPC) at different humidity conditions. The DMPC measurements show that the polyelectrolyte multilayer closures provide significantly large resistance to air flow, approximately 20-800 times larger than that possible with conventional hook and loop type closure systems, at all humidity levels (from 5 to 95% relative humidity). Hence, from the point of view of providing a hermetic seal against toxic gas permeation, the polyelectrolyte multilayer closures are viable candidates for further engineering development. However, the adhesive strength of the multilayer closures measured by atomic force microscopy suggests that the magnitude of adhesion is much smaller than what is possible with mechanical closures. Therefore, we envisage the development of a composite closure system combining the mechanical closure to provide strong adhesion and the multilayer closure to provide hermetic sealing. © 2012 American Chemical Society

Shear fatigue crack growth - A literature survey

NASA Technical Reports Server (NTRS)

Liu, H. W.

1985-01-01

Recent studies of shear crack growth are reviewed, emphasizing test methods and data analyses. The combined mode I and mode II elastic crack tip stress fields are considered. The development and design of the compact shear specimen are described, and the results of fatigue crack growth tests using compact shear specimens are reviewed. The fatigue crack growth tests are discussed and the results of inclined cracks in tensile panels, center cracks in plates under biaxial loading, cracked beam specimens with combined bending and shear loading, center-cracked panels and double edge-cracked plates under cyclic shear loading are examined and analyzed in detail.

32 CFR 989.25 - Base closure and realignment.

Code of Federal Regulations, 2013 CFR

2013-07-01

... 32 National Defense 6 2013-07-01 2013-07-01 false Base closure and realignment. 989.25 Section 989... PROTECTION ENVIRONMENTAL IMPACT ANALYSIS PROCESS (EIAP) § 989.25 Base closure and realignment. Base closure or realignment may entail special requirements for environmental analysis. The permanent base closure...

32 CFR 989.25 - Base closure and realignment.

Code of Federal Regulations, 2012 CFR

2012-07-01

... 32 National Defense 6 2012-07-01 2012-07-01 false Base closure and realignment. 989.25 Section 989... PROTECTION ENVIRONMENTAL IMPACT ANALYSIS PROCESS (EIAP) § 989.25 Base closure and realignment. Base closure or realignment may entail special requirements for environmental analysis. The permanent base closure...

Advanced Composite Wind Turbine Blade Design Based on Durability and Damage Tolerance

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abumeri, Galib; Abdi, Frank

2012-02-16

The objective of the program was to demonstrate and verify Certification-by-Analysis (CBA) capability for wind turbine blades made from advanced lightweight composite materials. The approach integrated durability and damage tolerance analysis with robust design and virtual testing capabilities to deliver superior, durable, low weight, low cost, long life, and reliable wind blade design. The GENOA durability and life prediction software suite was be used as the primary simulation tool. First, a micromechanics-based computational approach was used to assess the durability of composite laminates with ply drop features commonly used in wind turbine applications. Ply drops occur in composite joints andmore » closures of wind turbine blades to reduce skin thicknesses along the blade span. They increase localized stress concentration, which may cause premature delamination failure in composite and reduced fatigue service life. Durability and damage tolerance (D&DT) were evaluated utilizing a multi-scale micro-macro progressive failure analysis (PFA) technique. PFA is finite element based and is capable of detecting all stages of material damage including initiation and propagation of delamination. It assesses multiple failure criteria and includes the effects of manufacturing anomalies (i.e., void, fiber waviness). Two different approaches have been used within PFA. The first approach is Virtual Crack Closure Technique (VCCT) PFA while the second one is strength-based. Constituent stiffness and strength properties for glass and carbon based material systems were reverse engineered for use in D&DT evaluation of coupons with ply drops under static loading. Lamina and laminate properties calculated using manufacturing and composite architecture details matched closely published test data. Similarly, resin properties were determined for fatigue life calculation. The simulation not only reproduced static strength and fatigue life as observed in the test, it also showed composite damage and fracture modes that resemble those reported in the tests. The results show that computational simulation can be relied on to enhance the design of tapered composite structures such as the ones used in turbine wind blades. A computational simulation for durability, damage tolerance (D&DT) and reliability of composite wind turbine blade structures in presence of uncertainties in material properties was performed. A composite turbine blade was first assessed with finite element based multi-scale progressive failure analysis to determine failure modes and locations as well as the fracture load. D&DT analyses were then validated with static test performed at Sandia National Laboratories. The work was followed by detailed weight analysis to identify contribution of various materials to the overall weight of the blade. The methodology ensured that certain types of failure modes, such as delamination progression, are contained to reduce risk to the structure. Probabilistic analysis indicated that composite shear strength has a great influence on the blade ultimate load under static loading. Weight was reduced by 12% with robust design without loss in reliability or D&DT. Structural benefits obtained with the use of enhanced matrix properties through nanoparticles infusion were also assessed. Thin unidirectional fiberglass layers enriched with silica nanoparticles were applied to the outer surfaces of a wind blade to improve its overall structural performance and durability. The wind blade was a 9-meter prototype structure manufactured and tested subject to three saddle static loading at Sandia National Laboratory (SNL). The blade manufacturing did not include the use of any nano-material. With silica nanoparticles in glass composite applied to the exterior surfaces of the blade, the durability and damage tolerance (D&DT) results from multi-scale PFA showed an increase in ultimate load of the blade by 9.2% as compared to baseline structural performance (without nano). The use of nanoparticles lead to a delay in the onset of delamination. Load-displacement relationships obtained from testing of the blade with baseline neat material were compared to the ones from analytical simulation using neat resin and using silica nanoparticles in the resin. Multi-scale PFA results for the neat material construction matched closely those from test for both load displacement and location and type of damage and failure. AlphaSTAR demonstrated that wind blade structures made from advanced composite materials can be certified with multi-scale progressive failure analysis by following building block verification approach.« less

Three-Dimensional Analysis of Enamel Crack Behavior Using Optical Coherence Tomography.

PubMed

Segarra, M S; Shimada, Y; Sadr, A; Sumi, Y; Tagami, J

2017-03-01

The aim of this study was to nondestructively analyze enamel crack behavior on different areas of teeth using 3D swept source-optical coherence tomography (SS-OCT). Ten freshly extracted human teeth of each type on each arch ( n = 80 teeth) were inspected for enamel crack patterns on functional, contact and nonfunctional, or noncontact areas using 3D SS-OCT. The predominant crack pattern for each location on each specimen was noted and analyzed. The OCT observations were validated by direct observations of sectioned specimens under confocal laser scanning microscopy (CLSM). Cracks appeared as bright lines with SS-OCT, with 3 crack patterns identified: Type I - superficial horizontal cracks; Type II - vertically (occluso-gingival) oriented cracks; and Type III - hybrid or complicated cracks, a combination of a Type I and Type III cracks, which may or may not be confluent with each other. Type II cracks were predominant on noncontacting surfaces of incisors and canines and nonfunctional cusps of posterior teeth. Type I and III cracks were predominant on the contacting surfaces of incisors, cusps of canines, and functional cusps of posterior teeth. Cracks originating from the dental-enamel junction and enamel tufts, crack deflections, and the initiation of new cracks within the enamel (internal cracks) were observed as bright areas. CLSM observations corroborated the SS-OCT findings. We found that crack pattern, tooth type, and the location of the crack on the tooth exhibited a strong correlation. We show that the use of 3D SS-OCT permits for the nondestructive 3D imaging and analysis of enamel crack behavior in whole human teeth in vitro. 3D SS-OCT possesses potential for use in clinical studies for the analysis of enamel crack behavior.

Household responses to school closure resulting from outbreak of influenza B, North Carolina.

PubMed

Johnson, April J; Moore, Zack S; Edelson, Paul J; Kinnane, Lynda; Davies, Megan; Shay, David K; Balish, Amanda; McCarron, Meg; Blanton, Lenee; Finelli, Lyn; Averhoff, Francisco; Bresee, Joseph; Engel, Jeffrey; Fiore, Anthony

2008-07-01

School closure is a proposed strategy for reducing influenza transmission during a pandemic. Few studies have assessed how families respond to closures, or whether other interactions during closure could reduce this strategy's effect. Questionnaires were administered to 220 households (438 adults and 355 children) with school-age children in a North Carolina county during an influenza B virus outbreak that resulted in school closure. Closure was considered appropriate by 201 (91%) households. No adults missed work to solely provide childcare, and only 22 (10%) households required special childcare arrangements; 2 households incurred additional costs. Eighty-nine percent of children visited at least 1 public location during the closure despite county recommendations to avoid large gatherings. Although behavior and attitudes might differ during a pandemic, these results suggest short-term closure did not cause substantial hardship for parents. Pandemic planning guidance should address the potential for transmission in public areas during school closure.

Seven-year follow-up of percutaneous closure of patent foramen ovale.

PubMed

Mirzada, Naqibullah; Ladenvall, Per; Hansson, Per-Olof; Johansson, Magnus Carl; Furenäs, Eva; Eriksson, Peter; Dellborg, Mikael

2013-12-01

Observational studies favor percutaneous closure of patent foramen ovale (PFO) over medical treatment to reduce recurrent stroke while randomized trials fail to demonstrate significant superiority of percutaneous PFO closure. Few long-term studies are available post PFO closure. This study reports long-term clinical outcomes after percutaneous PFO closure. Between 1997 and 2006, 86 consecutive eligible patients with cerebrovascular events, presumably related to PFO, underwent percutaneous PFO closure. All 86 patients were invited to a long-term follow-up, which was carried out during 2011 and 2012. Percutaneous PFO closure was successfully performed in 85 of 86 patients. The follow-up rate was 100%. No cardiovascular or cerebrovascular deaths occurred. Two patients (both women) died from lung cancer during follow-up. Follow-up visits were conducted for 64 patients and the remaining 20 patients were followed up by phone. The mean follow-up time was 7.3 years (5 to 12.4 years). Mean age at PFO closure was 49 years. One patient had a minor stroke one month after PFO closure and a transient ischemic attack (TIA) two years afterwards. One other patient suffered from a TIA six years after closure. No long-term device-related complications were observed. Percutaneous PFO closure was associated with very low risk of recurrent stroke and is suitable in most patients. We observed no mortality and no long-term device-related complications related to PFO closure, indicating that percutaneous PFO closure is a safe and efficient treatment even in the long term.

A fracture criterion for widespread cracking in thin-sheet aluminum alloys

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Dawicke, D. S.; Sutton, M. A.; Bigelow, C. A.

1993-01-01

An elastic-plastic finite-element analysis was used with a critical crack-tip-opening angle (CTOA) fracture criterion to model stable crack growth in thin-sheet 2024-T3 aluminum alloy panels with single and multiple-site damage (MSD) cracks. Comparisons were made between critical angles determined from the analyses and those measured with photographic methods. Calculated load against crack extension and load against crack-tip displacement on single crack specimens agreed well with test data even for large-scale plastic deformations. The analyses were also able to predict the stable tearing behavior of large lead cracks in the presence of stably tearing MSD cracks. Small MSD cracks significantly reduced the residual strength for large lead cracks.

Stress-intensity factor equations for cracks in three-dimensional finite bodies

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Raju, I. S.

1981-01-01

Empirical stress intensity factor equations are presented for embedded elliptical cracks, semi-elliptical surface cracks, quarter-elliptical corner cracks, semi-elliptical surface cracks at a hole, and quarter-elliptical corner cracks at a hole in finite plates. The plates were subjected to remote tensile loading. Equations give stress intensity factors as a function of parametric angle, crack depth, crack length, plate thickness, and where applicable, hole radius. The stress intensity factors used to develop the equations were obtained from three dimensional finite element analyses of these crack configurations.

Patent foramen ovale closure and medical treatments for secondary stroke prevention: a systematic review of observational and randomized evidence.

PubMed

Kitsios, Georgios D; Dahabreh, Issa J; Abu Dabrh, Abd Moain; Thaler, David E; Kent, David M

2012-02-01

Patients discovered to have a patent foramen ovale in the setting of a cryptogenic stroke may be treated with percutaneous closure, antiplatelet therapy, or anticoagulants. A recent randomized trial (CLOSURE I) did not detect any benefit of closure over medical treatment alone; the optimal medical therapy is also unknown. We synthesized the available evidence on secondary stroke prevention in patients with patent foramen ovale and cryptogenic stroke. A MEDLINE search was performed for finding longitudinal studies investigating medical treatment or closure, meta-analysis of incidence rates (IR), and IR ratios of recurrent cerebrovascular events. Fifty-two single-arm studies and 7 comparative nonrandomized studies and the CLOSURE I trial were reviewed. The summary IR of recurrent stroke was 0.36 events (95% confidence interval [CI], 0.24-0.56) per 100 person-years with closure versus 2.53 events (95% CI, 1.91-3.35) per 100 person-years with medical therapy. In comparative observational studies, closure was superior to medical therapy (IR ratio=0.19; 95% CI, 0.07-0.54). The IR for the closure arm of the CLOSURE I trial was higher than the summary estimate from observational studies; there was no significant benefit of closure over medical treatment (P=0.002 comparing efficacy estimates between observational studies and the trial). Observational and randomized data (9 studies) comparing medical therapies were consistent and suggested that anticoagulants are superior to antiplatelets for preventing stroke recurrence (IR ratio=0.42; 95% CI, 0.18-0.98). Although further randomized trial data are needed to precisely determine the effects of closure on stroke recurrence, the results of CLOSURE I challenge the credibility of a substantial body of observational evidence strongly favoring mechanical closure over medical therapy.

Galaxy dynamics and the mass density of the universe.

PubMed

Rubin, V C

1993-06-01

Dynamical evidence accumulated over the past 20 years has convinced astronomers that luminous matter in a spiral galaxy constitutes no more than 10% of the mass of a galaxy. An additional 90% is inferred by its gravitational effect on luminous material. Here I review recent observations concerning the distribution of luminous and nonluminous matter in the Milky Way, in galaxies, and in galaxy clusters. Observations of neutral hydrogen disks, some extending in radius several times the optical disk, confirm that a massive dark halo is a major component of virtually every spiral. A recent surprise has been the discovery that stellar and gas motions in ellipticals are enormously complex. To date, only for a few spheroidal galaxies do the velocities extend far enough to probe the outer mass distribution. But the diverse kinematics of inner cores, peripheral to deducing the overall mass distribution, offer additional evidence that ellipticals have acquired gas-rich systems after initial formation. Dynamical results are consistent with a low-density universe, in which the required dark matter could be baryonic. On smallest scales of galaxies [10 kiloparsec (kpc); Ho = 50 km.sec-1.megaparsec-1] the luminous matter constitutes only 1% of the closure density. On scales greater than binary galaxies (i.e., >/=100 kpc) all systems indicate a density approximately 10% of the closure density, a density consistent with the low baryon density in the universe. If large-scale motions in the universe require a higher mass density, these motions would constitute the first dynamical evidence for nonbaryonic matter in a universe of higher density.

Galaxy dynamics and the mass density of the universe.

PubMed Central

Rubin, V C

1993-01-01

Dynamical evidence accumulated over the past 20 years has convinced astronomers that luminous matter in a spiral galaxy constitutes no more than 10% of the mass of a galaxy. An additional 90% is inferred by its gravitational effect on luminous material. Here I review recent observations concerning the distribution of luminous and nonluminous matter in the Milky Way, in galaxies, and in galaxy clusters. Observations of neutral hydrogen disks, some extending in radius several times the optical disk, confirm that a massive dark halo is a major component of virtually every spiral. A recent surprise has been the discovery that stellar and gas motions in ellipticals are enormously complex. To date, only for a few spheroidal galaxies do the velocities extend far enough to probe the outer mass distribution. But the diverse kinematics of inner cores, peripheral to deducing the overall mass distribution, offer additional evidence that ellipticals have acquired gas-rich systems after initial formation. Dynamical results are consistent with a low-density universe, in which the required dark matter could be baryonic. On smallest scales of galaxies [10 kiloparsec (kpc); Ho = 50 km.sec-1.megaparsec-1] the luminous matter constitutes only 1% of the closure density. On scales greater than binary galaxies (i.e., >/=100 kpc) all systems indicate a density approximately 10% of the closure density, a density consistent with the low baryon density in the universe. If large-scale motions in the universe require a higher mass density, these motions would constitute the first dynamical evidence for nonbaryonic matter in a universe of higher density. Images Fig. 3 Fig. 5 PMID:11607393