Slow Growth of a Crack with Contacting Faces in a Viscoelastic Body

NASA Astrophysics Data System (ADS)

Selivanov, M. F.

2017-11-01

An algorithm for solving the problem of slow growth of a mode I crack with a zone of partial contact of the faces is proposed. The algorithm is based on a crack model with a cohesive zone, an iterative method of finding a solution for the elastic opening displacement, and elasto-viscoelastic analogy, which makes it possible to describe the time-dependent opening displacement in Boltzmann-Volterra form. A deformation criterion with a constant critical opening displacement and cohesive strength during quasistatic crack growth is used. The algorithm was numerically illustrated for tensile loading at infinity and two concentrated forces symmetric about the crack line that cause the crack faces to contact. When the crack propagates, the contact zone disappears and its dynamic growth begins.

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.

Extended analytical solutions for effective elastic moduli of cracked porous media

NASA Astrophysics Data System (ADS)

Nguyen, Sy-Tuan; To, Quy Dong; Vu, Minh Ngoc

2017-05-01

Extended solutions are derived, on the basis of the micromechanical methods, for the effective elastic moduli of porous media containing stiff pores and both open and closed cracks. Analytical formulas of the overall bulk and shear moduli are obtained as functions of the elastic moduli of the solid skeleton, porosity and the densities of open and closed cracks families. We show that the obtained results are extensions of the classical widely used Walsh's (JGR, 1965) and Budiansky-O‧Connell's (JGR, 1974) solutions. Parametric sensitivity analysis clarifies the impact of the model parameters on the effective elastic properties. An inverse analysis, using sonic and density data, is considered to quantify the density of both open and closed cracks. It is observed that the density of closed cracks depends strongly on stress condition while the dependence of open cracks on the confining stress is negligible.

Internal state variable approach for predicting stiffness reductions in fibrous laminated composites with matrix cracks

NASA Technical Reports Server (NTRS)

Lee, Jong-Won; Allen, D. H.; Harris, C. E.

1989-01-01

A mathematical model utilizing the internal state variable concept is proposed for predicting the upper bound of the reduced axial stiffnesses in cross-ply laminates with matrix cracks. The axial crack opening displacement is explicitly expressed in terms of the observable axial strain and the undamaged material properties. A crack parameter representing the effect of matrix cracks on the observable axial Young's modulus is calculated for glass/epoxy and graphite/epoxy material systems. The results show that the matrix crack opening displacement and the effective Young's modulus depend not on the crack length, but on its ratio to the crack spacing.

Three-dimensional effects in interfacial crack propagation

NASA Astrophysics Data System (ADS)

Liechti, K. M.; Chai, Y.-S.; Liang, Y.-M.

1992-09-01

The paper describes the use of crack-opening interferometry for examining the variation in normal crack-opening displacements (NCOD) along the front of an interfacial crack in an edge-cracked bimaterial strip under biaxial loading. For the glass/epoxy combination considered here, the crack front was concave in the direction of crack growth, in contrast to previous observations with a glass/polyurethane/glass sandwich specimen and cracks in homogeneous materials. The NCOD were greatest in the interior of the specimen for all mode-mixes considered and the exponents in a power-law fit of NCOD versus distance from the crack front decreased toward the free surface. The exponents varied with mode-mix, suggesting that interfacial crack-front geometries could be similarly affected.

Behavior of Fatigue Crack Tip Opening in Air and Corrosive Atmosphere

NASA Astrophysics Data System (ADS)

Hayashi, Morihito; Toeda, Kazunori

In the study, a formula for predicting fatigue crack tip opening displacement is deduced firstly. And then, due to comparing actual crack growth rate with the deduced formula, the crack tip configuration factor is defined to figure out the crack tip opening configuration that is useful to clarify the behavior of fatigue crack tip formation apparently. Applying the concept, the crack growth of 7/3 brass and 6/4 brass is predicted from the formula, by replacing material properties such as plastic flow resistance, Young modulus, the Poisson ratio, and fatigue toughness, and fatigue test conditions such as the stress intensity factor range, the load ratio, and cycle frequency. Furthermore, the theoretically expected results are verified with the fatigue tests which were carried out on CT specimens under different load conditions of load ratio, cycle frequency, and cyclic peak load, in different environments of air or corrosive ammonia atmosphere, for various brasses. And by comparing and discussing the calculated crack growth rate with attained experimental results, the apparent configuration factor at the crack tip is determined. And through the attained factor which changes along with crack growth, the behaviors of fatigue crack tip formation under different test conditions have been found out.

Eddy-current inversion in the thin-skin limit: Determination of depth and opening for a long crack

NASA Astrophysics Data System (ADS)

Burke, S. K.

1994-09-01

A method for crack size determination using eddy-current nondestructive evaluation is presented for the case of a plate containing an infinitely long crack of uniform depth and uniform crack opening. The approach is based on the approximate solution to Maxwell's equations for nonmagnetic conductors in the limit of small skin depth and relies on least-squares polynomial fits to a normalized coil impedance function as a function of skin depth. The method is straightforward to implement and is relatively insensitive to both systematic and random errors. The procedure requires the computation of two functions: a normalizing function, which depends both on the coil parameters and the skin depth, and a crack-depth function which depends only on the coil parameters in addition to the crack depth. The practical perfomance of the method was tested using a set of simulated cracks in the form of electro-discharge machined slots in aluminum alloy plates. The crack depths and crack opening deduced from the eddy-current measurements agree with the actual crack dimensions to within 10% or better. Recommendations concerning the optimum conditions for crack sizing are also made.

Crack Opening Displacement Behavior in Ceramic Matrix Composites

NASA Technical Reports Server (NTRS)

Sevener, Kathy; Tracy, Jared; Chen, Zhe; Daly, Sam; Kiser, Doug

2017-01-01

Ceramic Matrix Composites (CMC) modeling and life prediction strongly depend on oxidation, and therefore require a thorough understanding of when matrix cracks occur, the extent of cracking for given conditions (time-temperature-environment-stress), and the interactions of matrix cracks with fibers and interfaces. In this work, the evolution of matrix cracks in a melt-infiltrated Silicon Carbide/Silicon Carbide (SiC/SiC) CMC under uniaxial tension was examined using scanning electron microscopy (SEM) combined with digital image correlation (DIC) and manual crack opening displacement (COD) measurements. Strain relaxation due to matrix cracking, the relationship between COD's and applied stress, and damage evolution at stresses below the proportional limit were assessed. Direct experimental observation of strain relaxation adjacent to regions of matrix cracking is presented and discussed. Additionally, crack openings were found to increase linearly with increasing applied stress, and no crack was found to pass fully through the gage cross-section. This observation is discussed in the context of the assumption of through-cracks for all loading conditions and fiber architectures in oxidation modeling. Finally, the combination of SEM with DIC is demonstrated throughout to be a powerful means for damage identification and quantification in CMC's at stresses well below the proportional limit.

Fracture mechanics criteria for turbine engine hot section components

NASA Technical Reports Server (NTRS)

Meyers, G. J.

1982-01-01

The application of several fracture mechanics data correlation parameters to predicting the crack propagation life of turbine engine hot section components was evaluated. An engine survey was conducted to determine the locations where conventional fracture mechanics approaches may not be adequate to characterize cracking behavior. Both linear and nonlinear fracture mechanics analyses of a cracked annular combustor liner configuration were performed. Isothermal and variable temperature crack propagation tests were performed on Hastelloy X combustor liner material. The crack growth data was reduced using the stress intensity factor, the strain intensity factor, the J integral, crack opening displacement, and Tomkins' model. The parameter which showed the most effectiveness in correlation high temperature and variable temperature Hastelloy X crack growth data was crack opening displacement.

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.

Recent evaluations of crack-opening-area in circumferentially cracked pipes

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

Rahman, S.; Brust, F.; Ghadiali, N.

1997-04-01

Leak-before-break (LBB) analyses for circumferentially cracked pipes are currently being conducted in the nuclear industry to justify elimination of pipe whip restraints and jet shields which are present because of the expected dynamic effects from pipe rupture. The application of the LBB methodology frequently requires calculation of leak rates. The leak rates depend on the crack-opening area of the through-wall crack in the pipe. In addition to LBB analyses which assume a hypothetical flaw size, there is also interest in the integrity of actual leaking cracks corresponding to current leakage detection requirements in NRC Regulatory Guide 1.45, or for assessingmore » temporary repair of Class 2 and 3 pipes that have leaks as are being evaluated in ASME Section XI. The objectives of this study were to review, evaluate, and refine current predictive models for performing crack-opening-area analyses of circumferentially cracked pipes. The results from twenty-five full-scale pipe fracture experiments, conducted in the Degraded Piping Program, the International Piping Integrity Research Group Program, and the Short Cracks in Piping and Piping Welds Program, were used to verify the analytical models. Standard statistical analyses were performed to assess used to verify the analytical models. Standard statistical analyses were performed to assess quantitatively the accuracy of the predictive models. The evaluation also involved finite element analyses for determining the crack-opening profile often needed to perform leak-rate calculations.« less

Effect of crack openings on carbonation-induced corrosion

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

Ghantous, Rita Maria, E-mail: rita-maria.ghantous@yncrea.fr; LMDC, Université de Toulouse, INSA, UPS, Toulouse; Poyet, Stéphane

Reinforced concrete is widely used in the construction of buildings, historical monuments, infrastructures and nuclear power plants. For a variety of reasons, many concrete structures are subject to unavoidable cracks that accelerate the diffusion of atmospheric carbon dioxide to the steel/concrete interface. Carbonation at the interface induces steel corrosion that could cause the development of new cracks in the structure, a determining factor for its durability. The aim of this article is to study the effect of existing cracks on the development of carbonation-induced corrosion. The results indicate that, after the initiation phase, the corrosion kinetics decreases with time andmore » the free corrosion potential increases independently of the crack opening. In addition, the corroded zone matches the carbonated one. The interpretation of these results allows the authors to conclude that, during the corrosion process, corrosion products seal the crack and act as a barrier to oxygen and water diffusion. Consequently, the influence of crack opening on corrosion development is masked and the corrosion development is limited.« less

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.

Determination of leakage areas in nuclear piping

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

Keim, E.

1997-04-01

For the design and operation of nuclear power plants the Leak-Before-Break (LBB) behavior of a piping component has to be shown. This means that the length of a crack resulting in a leak is smaller than the critical crack length and that the leak is safely detectable by a suitable monitoring system. The LBB-concept of Siemens/KWU is based on computer codes for the evaluation of critical crack lengths, crack openings, leakage areas and leakage rates, developed by Siemens/KWU. In the experience with the leak rate program is described while this paper deals with the computation of crack openings and leakagemore » areas of longitudinal and circumferential cracks by means of fracture mechanics. The leakage areas are determined by the integration of the crack openings along the crack front, considering plasticity and geometrical effects. They are evaluated with respect to minimum values for the design of leak detection systems, and maximum values for controlling jet and reaction forces. By means of fracture mechanics LBB for subcritical cracks has to be shown and the calculation of leakage areas is the basis for quantitatively determining the discharge rate of leaking subcritical through-wall cracks. The analytical approach and its validation will be presented for two examples of complex structures. The first one is a pipe branch containing a circumferential crack and the second one is a pipe bend with a longitudinal crack.« less

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.

The effect of transverse crack upon parametric instability of a rotor-bearing system with an asymmetric disk

NASA Astrophysics Data System (ADS)

Han, Qinkai; Chu, Fulei

2012-12-01

It is well known that either the asymmetric disk or transverse crack brings parametric inertia (or stiffness) excitation to the rotor-bearing system. When both of them appear in a rotor system, the parametric instability behaviors have not gained sufficient attentions. Thus, the effect of transverse crack upon parametric instability of a rotor-bearing system with an asymmetric disk is studied. First, the finite element equations of motion are established for the asymmetric rotor system. Both the open and breathing transverse cracks are taken into account in the model. Then, the discrete state transition matrix (DSTM) method is introduced for numerically acquiring the instability regions. Based upon these, some computations for a practical asymmetric rotor system with open or breathing transverse crack are conducted, respectively. Variations of the primary and combination instability regions induced by the asymmetric disk with the crack depth are observed, and the effect of the orientation angle between the crack and asymmetric disk on various instability regions are discussed in detail. It is shown that for the asymmetric angle around 0, the existence of transverse (either open or breathing) crack has attenuation effect upon the instability regions. Under certain crack depth, the instability regions could be vanished by the transverse crack. When the asymmetric angle is around π/2, increasing the crack depth would enhance the instability regions.

Stress-intensity factors and crack-opening displacements for round compact specimens. [fracture toughness of metallic materials

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1979-01-01

A two dimensional, boundary collocation stress analysis was used to analyze various round compact specimens. The influence of the round external boundary and of pin-loaded holes on stress intensity factors and crack opening displacements was determined as a function of crack-length-to-specimen-width ratios. A wide-range equation for the stress intensity factors was developed. Equations for crack-surface displacements and load-point displacements were also developed. In addition, stress intensity factors were calculated from compliance methods to demonstrate that load-displacement records must be made at the loading points and not along the crack line for crack-length-to-specimen-width ratios less than about 0.4.

Microcracks, micropores, and their petrologic interpretation for 72415 and 15418

NASA Technical Reports Server (NTRS)

Richter, D.; Simmons, G.; Siegfried, R.

1976-01-01

Lunar samples 72415 and 15418 have complex microstructures that indicate a series of fracturing and healing events. Both samples contain relatively few open microcracks but many sealed and healed microcracks. Dunite 72415 contains abundant healed cracks that formed tectonically, symplectic intergrowths spatially and probably genetically related to microcracks, and a cataclastic matrix that has been extensively sintered. Metamorphosed breccia 15418 contains many post-metamorphic healed cracks, large shock induced cracks that have been sealed with glass, and a few younger, thin, open shock induced cracks.

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.

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.

InSAR analysis for detecting the route of hydrothermal fluid to the surface during the 2015 phreatic eruption of Hakone Volcano, Japan

NASA Astrophysics Data System (ADS)

Doke, Ryosuke; Harada, Masatake; Mannen, Kazutaka; Itadera, Kazuhiro; Takenaka, Jun

2018-04-01

Although the 2015 Hakone Volcano eruption was a small-scale phreatic eruption with a discharged mass of only about 100 tons, interferometric synthetic aperture radar successfully detected surface deformations related to the eruption. Inversion model of the underground hydrothermal system based on measured ground displacements by ALOS-2/PALSAR-2 images showed that a crack opened at an elevation of about 530-830 m, probably at the time of the eruption. A geomorphological analysis detected several old NW-SE trending fissures, and the open crack was located just beneath one of the fissures. Thus, the crack that opened during the 2015 eruption could have been a preexisting crack that formed during a more voluminous hydrothermal eruption. In addition, the inversion model implies that a sill deflation occurred at an elevation of about 225 m, probably at the time of the eruption. The deflation of sill-like body represents a preexisting hydrothermal reservoir at an elevation of 100-400 m, which intruded fluid in the open crack prior to eruption. The volume changes of the open crack and the sill were calculated to be 1.14 × 105 m3 (inflation) and 0.49 × 105 m3 (deflation), respectively. A very local swelling (about 200 m in diameter) was also detected at the eruption center 2 months before the eruption. The local swelling, whose rate in satellite line-of-sight was 0.7-0.9 cm/day during May 2015 and declined in June, had been monitored until the time of the eruption, when its uplift halted. This was modeled as a point pressure source at an elevation of about 900 m (at a depth of about 80-90 m from the ground surface) and is considered to be a minor hydrothermal reservoir just beneath the fumarolic field. Our analysis shows that the northernmost tip of the open crack reached within 200 m of the surface. Thus, it is reasonable to assume that the hydrothermal fluid in the open crack found a way to the surface and formed the eruption.[Figure not available: see fulltext.

Short fatigue crack behavior in notched 2024-T3 aluminum specimens

NASA Technical Reports Server (NTRS)

Lee, J. J.; Sharpe, W. N., Jr.

1986-01-01

Single-edge, semi-circular notched specimens of Al 2024-T3, 2.3 mm thick, were cyclicly loaded at R-ratios of 0.5, 0.0, -1.0, and -2.0. The notch roots were periodically inspected using a replica technique which duplicates the bore surface. The replicas were examined under an optical microscope to determine the initiation of very short cracks and to monitor the growth of short cracks ranging in length from a few tens of microns to the specimen thickness. In addition to short crack growth measurements, the crack opening displacement (COD) was measured for surface cracks as short as 0.035 mm and for through-thickness cracks using the Interferometric Strain/Displacement Gage (ISDG), a laser-based optical technique. The growth rates of short cracks were faster than the long crack growth rates for R-ratios of -1.0 and -2.0. No significant difference between short and long crack growth rates was observed for R = 0.0. Short cracks had slower growth rates than long cracks for R = 0.5. The crack opening stresses measured for short cracks were smaller than those predicted for large cracks, with little difference appearing for positive R-ratios and large differences noted for negative R-ratios.

Strip Yield Model Numerical Application to Different Geometries and Loading Conditions

NASA Technical Reports Server (NTRS)

Hatamleh, Omar; Forman, Royce; Shivakumar, Venkataraman; Lyons, Jed

2006-01-01

A new numerical method based on the strip-yield analysis approach was developed for calculating the Crack Tip Opening Displacement (CTOD). This approach can be applied for different crack configurations having infinite and finite geometries, and arbitrary applied loading conditions. The new technique adapts the boundary element / dislocation density method to obtain crack-face opening displacements at any point on a crack, and succeeds by obtaining requisite values as a series of definite integrals, the functional parts of each being evaluated exactly in a closed form.

Electrically reversible cracks in an intermetallic film controlled by an electric field

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

Liu, Z. Q.; Liu, J. H.; Biegalski, M. D.

Cracks in solid-state materials are typically irreversible. We report electrically reversible opening and closing of nanoscale cracks in an intermetallic thin film grown on a ferroelectric substrate driven by a small electric field (~0.83 kV/cm). Accordingly, a nonvolatile colossal electroresistance on-off ratio of more than 10 8 is measured across the cracks in the intermetallic film at room temperature. Cracks are easily formed with low-frequency voltage cycling and remain stable when the device is operated at high frequency, which offers intriguing potential for next-generation high-frequency memory applications. Moreover, endurance testing demonstrates that the opening and closing of such cracks canmore » reach over 10 7 cycles under 10-μs pulses, without catastrophic failure of the film.« less

Electrically reversible cracks in an intermetallic film controlled by an electric field

DOE PAGES

Liu, Z. Q.; Liu, J. H.; Biegalski, M. D.; ...

2018-01-03

Cracks in solid-state materials are typically irreversible. We report electrically reversible opening and closing of nanoscale cracks in an intermetallic thin film grown on a ferroelectric substrate driven by a small electric field (~0.83 kV/cm). Accordingly, a nonvolatile colossal electroresistance on-off ratio of more than 10 8 is measured across the cracks in the intermetallic film at room temperature. Cracks are easily formed with low-frequency voltage cycling and remain stable when the device is operated at high frequency, which offers intriguing potential for next-generation high-frequency memory applications. Moreover, endurance testing demonstrates that the opening and closing of such cracks canmore » reach over 10 7 cycles under 10-μs pulses, without catastrophic failure of the film.« less

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.

With God's Help I Can Do It: Crack Users' Formal and Informal Recovery Experiences in El Salvador

PubMed Central

Dickson-Gomez, Julia; Bodnar, Gloria; Guevara, Carmen Eugenia; Rodriguez, Karla; De Mendoza, Lorena Rivas; Corbett, A. Michelle

2013-01-01

Crack use has increased dramatically in El Salvador in the last few decades. As with other developing countries with sudden onsets of drug problems, El Salvador has few medical staff trained in addictions treatment. Little research has examined drug users' attempts to reduce or abstain from drug use in countries where government-regulated formal medical treatment for drug addiction is scarce. This paper uses qualitative and quantitative data gathered from active crack users to explore their formal and informal strategies to reduce or abstain from drugs, and compares these with components of informal and formal treatment in developed countries. PMID:20735191

Stressed Oxidation Life Prediction for C/SiC Composites

NASA Technical Reports Server (NTRS)

Levine, Stanley R.

2004-01-01

The residual strength and life of C/SiC is dominated by carbon interface and fiber oxidation if seal coat and matrix cracks are open to allow oxygen ingress. Crack opening is determined by the combination of thermal, mechanical and thermal expansion mismatch induced stresses. When cracks are open, life can be predicted by simple oxidation based models with reaction controlled kinetics at low temperature, and by gas phase diffusion controlled kinetics at high temperatures. Key life governing variables in these models include temperature, stress, initial strength, oxygen partial pressure, and total pressure. These models are described in this paper.

Plastic zone size and crack tip opening displacement of a Dugdale crack interacting with a coated circular inclusion

NASA Astrophysics Data System (ADS)

Hoh, H. J.; Xiao, Z. M.; Luo, J.

2010-09-01

An analytical investigation on the plastic zone size of a crack near a coated circular inclusion under three different loading conditions of uniaxial tension, uniform tension and pure shear was carried out. Both the crack and coated circular inclusion are embedded in an infinite matrix, with the crack oriented along the radial direction of the inclusion. In the solution procedure, the crack is simulated as a continuous distribution of edge dislocations. With the Dugdale model of small-scale yielding [J. Mech. Phys. Solids 8 (1960) p. 100], two thin strips of yielded plastic zones are introduced at both crack tips. Using the solution for a coated circular inclusion interacting with a single dislocation as the Green's function, the physical problem is formulated into a set of singular integral equations. Using the method of Erdogan and Gupta [Q. J. Appl. Math. 29 (1972) p. 525] and iterative numerical procedures, the singular integral equations are solved numerically for the plastic zone sizes and crack tip opening displacement.

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.

Fracture mechanics life analytical methods verification testing

NASA Technical Reports Server (NTRS)

Favenesi, J. A.; Clemons, T. G.; Riddell, W. T.; Ingraffea, A. R.; Wawrzynek, P. A.

1994-01-01

The objective was to evaluate NASCRAC (trademark) version 2.0, a second generation fracture analysis code, for verification and validity. NASCRAC was evaluated using a combination of comparisons to the literature, closed-form solutions, numerical analyses, and tests. Several limitations and minor errors were detected. Additionally, a number of major flaws were discovered. These major flaws were generally due to application of a specific method or theory, not due to programming logic. Results are presented for the following program capabilities: K versus a, J versus a, crack opening area, life calculation due to fatigue crack growth, tolerable crack size, proof test logic, tearing instability, creep crack growth, crack transitioning, crack retardation due to overloads, and elastic-plastic stress redistribution. It is concluded that the code is an acceptable fracture tool for K solutions of simplified geometries, for a limited number of J and crack opening area solutions, and for fatigue crack propagation with the Paris equation and constant amplitude loads when the Paris equation is applicable.

Phase-field study of grain boundary tracking behavior in crack-seal microstructures

NASA Astrophysics Data System (ADS)

Ankit, Kumar; Nestler, Britta; Selzer, Michael; Reichardt, Mathias

2013-12-01

In order to address the growth of crystals in veins, a multiphase-field model is used to capture the dynamics of crystals precipitating from a super-saturated solution. To gain a detailed understanding of the polycrystal growth phenomena in veins, we investigate the influence of various boundary conditions on crystal growth. In particular, we analyze the formation of vein microstructures resulting from the free growth of crystals as well as crack-sealing processes. We define the crystal symmetry by considering the anisotropy in surface energy to simulate crystals with flat facets and sharp corners. The resulting growth competition of crystals with different orientations is studied to deduce a consistent orientation selection rule in the free-growth regime. Using crack-sealing simulations, we correlate the grain boundary tracking behavior depending on the relative rate of crack opening, opening trajectory, initial grain size, and wall roughness. Further, we illustrate how these parameters induce the microstructural transition between blocky (crystals growing anisotropically) to fibrous morphology (isotropic) and formation of grain boundaries. The phase-field simulations of crystals in the free-growth regime (in 2D and 3D) indicate that the growth or consumption of a crystal is dependent on the orientation difference with neighboring crystals. The crack-sealing simulation results (in 2D and 3D) reveal that crystals grow isotropically and grain boundaries track the opening trajectory if the wall roughness is high, opening increments are small, and crystals touch the wall before the next crack increment starts. Further, we find that within the complete crack-seal regime, anisotropy in surface energy results in the formation of curved/oscillating grain boundaries (instead of straight) when the crack-opening velocity is increased and wall roughness is not sufficiently high. Additionally, the overall capability of phase-field method to simulate large-scale polycrystal growth in veins (in 3D) is demonstrated enumerating the main advantages of adopting the novel approach.

Infrasonic wave accompanying a crack opening during the 2015 Hakone eruption

NASA Astrophysics Data System (ADS)

Yukutake, Yohei; Ichihara, Mie; Honda, Ryou

2018-03-01

To understand the initial process of the phreatic eruption of the Hakone volcano from June 29 to July 01, 2015, we analyzed infrasound data using the cross-correlation between infrasound and vertical ground velocity and compared the results of our analysis to the crustal deformation detected by tiltmeters and broadband seismometers. An infrasound signal and vertical ground motion due to an infrasound wave coupled to the ground were detected simultaneously with the opening of a crack source beneath the Owakudani geothermal region during the 2-min time period after 07:32 JST on June 29, 2015 (JST = UTC + 8 h). Given that the upper end of the open crack was approximately 150 m beneath the surface, the time for the direct emission of highly pressurized fluid from the upper end of the open crack to the surface should have exceeded the duration of the inflation owing to the hydraulic diffusivity in the porous media. Therefore, the infrasound signal coincident with the opening of the crack may reflect a sudden emission of volcanic gas resulting from the rapid vaporization of pre-existing groundwater beneath Owakudani because of the transfer of the volumetric strain change from the deformation source. We also noticed a correlation pattern corresponding to discrete impulsive infrasound signals during vent formation, which occurred several hours to 2 days after the opening of the crack. In particular, we noted that the sudden emission of vapor coincided with the inflation of the shallow pressure source, whereas the eruptive burst events accompanied by the largest vent formation were delayed by approximately 2 days. Furthermore, we demonstrated that the correlation method is a useful tool in detecting small infrasound signals and provides important information regarding the initial processes of the eruption.[Figure not available: see fulltext.

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...

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.

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

Advanced Flaw Manufacturing and Crack Growth Control

NASA Astrophysics Data System (ADS)

Kemppainen, M.; Pitkänen, J.; Virkkunen, I.; Hänninen, H.

2004-02-01

Advanced artificial flaw manufacturing method has become available. The method produces true fatigue cracks, which are representative of most service-induced cracks. These cracks can be used to simulate behaviour of realistic cracks under service conditions. This paper introduces studies of the effects of different thermal loading cycles to crack opening and residual stress state as seen at the surface of the sample and in the ultrasonic signal. In-situ measurements were performed under dynamic thermal fatigue loading of a 20 mm long artificial crack.

A crack opening stress equation for fatigue crack growth

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1984-01-01

A general crack opening stress equation is presented which may be used to correlate crack growth rate data for various materials and thicknesses, under constant amplitude loading, once the proper constraint factor has been determined. The constraint factor, alpha, is a constraint on tensile yielding; the material yields when the stress is equal to the product of alpha and sigma. Delta-K (LEFM) is plotted against rate for 2024-T3 aluminum alloy specimens 2.3 mm thick at various stress ratios. Delta-K sub eff was plotted against rate for the same data with alpha = 1.8; the rates correlate well within a factor of two.

Spiraling Cracks in Thin Sheets

NASA Astrophysics Data System (ADS)

Romero, Victor; Roman, Benoit; Cerda, Enrique

2008-03-01

A wide kind of everyday-life industrial products come in a thin package that needs to be torn open by the user, and the opening is not always easy. We built a simple setup to study crack propagation in thin sheets coupled with large out-of-plane displacement : A cylindrical tool is inserted in a straight incision in a thin sheet, and is pushed against the sheet perpendicularly to that incision, eventually propagating a crack. When the blunt tool is continually pushed against the lip, we found that the crack follows a very robust spiraling path. Experiments may be interpreted in terms of ``Spira Mirabilis'' (logarithmic spiral). Starting with crack theory argument, we will show that the early behavior of the cut path follows a portion of a logathmic spiral, and that the path tends to another spiral with a different pitch as the crack adds more turns. Our crack experiment illustrates the fact that thin sheets mechanics is deeply connected to geometry, and finally spirals characteristics allow us to measure material crack properties of the thin layer used.

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.

Crack tip fracture toughness of base glasses for dental restoration glass-ceramics using crack opening displacements.

PubMed

Deubener, J; Höland, M; Höland, W; Janakiraman, N; Rheinberger, V M

2011-10-01

The critical stress intensity factor, also known as the crack tip toughness K(tip), was determined for three base glasses, which are used in the manufacture of glass-ceramics. The glasses included the base glass for a lithium disilicate glass-ceramic, the base glass for a fluoroapatite glass-ceramic and the base glass for a leucite glass-ceramic. These glass-ceramic are extensively used in the form of biomaterials in restorative dental medicine. The crack tip toughness was established by using crack opening displacement profiles under experimental conditions. The crack was produced by Vickers indentation. The crack tip toughness parameters determined for the three glass-ceramics differed quite significantly. The crack tip parameters of the lithium disilicate base glass and the leucite base glass were higher than that of the fluoroapatite base glass. This last material showed glass-in-glass phase separation. The discussion of the results clearly shows that the droplet glass phase is softer than the glass matrix. Therefore, the authors conclude that a direct relationship exists between the chemical nature of the glasses and the crack tip parameter. Copyright © 2011 Elsevier Ltd. All rights reserved.

Crack Monitoring of Operational Wind Turbine Foundations

PubMed Central

McAlorum, Jack; Fusiek, Grzegorz; Niewczas, Pawel; McKeeman, Iain; Rubert, Tim

2017-01-01

The degradation of onshore, reinforced-concrete wind turbine foundations is usually assessed via above-ground inspections, or through lengthy excavation campaigns that suspend wind power generation. Foundation cracks can and do occur below ground level, and while sustained measurements of crack behaviour could be used to quantify the risk of water ingress and reinforcement corrosion, these cracks have not yet been monitored during turbine operation. Here, we outline the design, fabrication and field installation of subterranean fibre-optic sensors for monitoring the opening and lateral displacements of foundation cracks during wind turbine operation. We detail methods for in situ sensor characterisation, verify sensor responses against theoretical tower strains derived from wind speed data, and then show that measured crack displacements correlate with monitored tower strains. Our results show that foundation crack opening displacements respond linearly to tower strain and do not change by more than ±5 μm. Lateral crack displacements were found to be negligible. We anticipate that the work outlined here will provide a starting point for real-time, long-term and dynamic analyses of crack displacements in future. Our findings could furthermore inform the development of cost-effective monitoring systems for ageing wind turbine foundations. PMID:28825687

Crack Monitoring of Operational Wind Turbine Foundations.

PubMed

Perry, Marcus; McAlorum, Jack; Fusiek, Grzegorz; Niewczas, Pawel; McKeeman, Iain; Rubert, Tim

2017-08-21

The degradation of onshore, reinforced-concrete wind turbine foundations is usually assessed via above-ground inspections, or through lengthy excavation campaigns that suspend wind power generation. Foundation cracks can and do occur below ground level, and while sustained measurements of crack behaviour could be used to quantify the risk of water ingress and reinforcement corrosion, these cracks have not yet been monitored during turbine operation. Here, we outline the design, fabrication and field installation of subterranean fibre-optic sensors for monitoring the opening and lateral displacements of foundation cracks during wind turbine operation. We detail methods for in situ sensor characterisation, verify sensor responses against theoretical tower strains derived from wind speed data, and then show that measured crack displacements correlate with monitored tower strains. Our results show that foundation crack opening displacements respond linearly to tower strain and do not change by more than ±5 μ m. Lateral crack displacements were found to be negligible. We anticipate that the work outlined here will provide a starting point for real-time, long-term and dynamic analyses of crack displacements in future. Our findings could furthermore inform the development of cost-effective monitoring systems for ageing wind turbine foundations.

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.

Study of the Effects of Metallurgical Factors on the Growth of Fatigue Microcracks.

DTIC Science & Technology

1987-11-25

polycrystalline) yield stress. 8. The resulting model, predicated on the notion of orientation-dependent microplastic grains, predicts quantitatively the entire...Figure 5. Predicted crack growth curves for small cracks propagating from a microplastic grain into elastic-plastic, contiguous grains; Ao is defined as...or the crack tip opening *displacement, 6. Figure 5. Predicted crack growth curves for small cracks propagating from a microplastic grain into

Investigation of eddy current examination on OD fatigue crack for steam generator tubes

NASA Astrophysics Data System (ADS)

Kong, Yuying; Ding, Boyuan; Li, Ming; Liu, Jinhong; Chen, Huaidong; Meyendorf, Norbert G.

2015-03-01

The opening width of fatigue crack was very small, and conventional Bobbin probe was very difficult to detect it in steam generator tubes. Different sizes of 8 fatigue cracks were inspected using bobbin probe rotating probe. The analysis results showed that, bobbin probe was not sensitive for fatigue crack even for small through wall crack mixed with denting signal. On the other hand, the rotating probe was easily to detect all cracks. Finally, the OD phase to depth curve for fatigue crack using rotating probe was established and the results agreed very well with the true crack size.

A Relationship Between Constraint and the Critical Crack Tip Opening Angle

NASA Technical Reports Server (NTRS)

Johnston, William M.; James, Mark A.

2009-01-01

Of the various approaches used to model and predict fracture, the Crack Tip Opening Angle (CTOA) fracture criterion has been successfully used for a wide range of two-dimensional thin-sheet and thin plate applications. As thicker structure is considered, modeling the full three-dimensional fracture process will become essential. This paper investigates relationships between the local CTOA evaluated along a three-dimensional crack front and the corresponding local constraint. Previously reported tunneling crack front shapes were measured during fracture by pausing each test and fatigue cycling the specimens to mark the crack surface. Finite element analyses were run to model the tunneling shape during fracture, with the analysis loading conditions duplicating those tests. The results show an inverse relationship between the critical fracture value and constraint which is valid both before maximum load and after maximum load.

Opening of an interface flaw in a layered elastic half-plane under compressive loading

NASA Technical Reports Server (NTRS)

Kennedy, J. M.; Fichter, W. B.; Goree, J. G.

1984-01-01

A static analysis is given of the problem of an elastic layer perfectly bonded, except for a frictionless interface crack, to a dissimilar elastic half-plane. The free surface of the layer is loaded by a finite pressure distribution directly over the crack. The problem is formulated using the two dimensional linear elasticity equations. Using Fourier transforms, the governing equations are converted to a pair of coupled singular integral equations. The integral equations are reduced to a set of simultaneous algebraic equations by expanding the unknown functions in a series of Jacobi polynomials and then evaluating the singular Cauchy-type integrals. The resulting equations are found to be ill-conditioned and, consequently, are solved in the least-squares sense. Results from the analysis show that, under a normal pressure distribution on the free surface of the layer and depending on the combination of geometric and material parameters, the ends of the crack can open. The resulting stresses at the crack-tips are singular, implying that crack growth is possible. The extent of the opening and the crack-top stress intensity factors depend on the width of the pressure distribution zone, the layer thickness, and the relative material properties of the layer and half-plane.

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.

Determination of dynamic fracture toughness using a new experimental technique

NASA Astrophysics Data System (ADS)

Cady, Carl M.; Liu, Cheng; Lovato, Manuel L.

2015-09-01

In other studies dynamic fracture toughness has been measured using Charpy impact and modified Hopkinson Bar techniques. In this paper results will be shown for the measurement of fracture toughness using a new test geometry. The crack propagation velocities range from ˜0.15 mm/s to 2.5 m/s. Digital image correlation (DIC) will be the technique used to measure both the strain and the crack growth rates. The boundary of the crack is determined using the correlation coefficient generated during image analysis and with interframe timing the crack growth rate and crack opening can be determined. A comparison of static and dynamic loading experiments will be made for brittle polymeric materials. The analysis technique presented by Sammis et al. [1] is a semi-empirical solution, however, additional Linear Elastic Fracture Mechanics analysis of the strain fields generated as part of the DIC analysis allow for the more commonly used method resembling the crack tip opening displacement (CTOD) experiment. It should be noted that this technique was developed because limited amounts of material were available and crack growth rates were to fast for a standard CTOD method.

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.

Corrosion of NiTi Wires with Cracked Oxide Layer

NASA Astrophysics Data System (ADS)

Racek, Jan; Šittner, Petr; Heller, Luděk; Pilch, Jan; Petrenec, Martin; Sedlák, Petr

2014-07-01

Corrosion behavior of superelastic NiTi shape memory alloy wires with cracked TiO2 surface oxide layers was investigated by electrochemical corrosion tests (Electrochemical Impedance Spectroscopy, Open Circuit Potential, and Potentiodynamic Polarization) on wires bent into U-shapes of various bending radii. Cracks within the oxide on the surface of the bent wires were observed by FIB-SEM and TEM methods. The density and width of the surface oxide cracks dramatically increase with decreasing bending radius. The results of electrochemical experiments consistently show that corrosion properties of NiTi wires with cracked oxide layers (static load keeps the cracks opened) are inferior compared to the corrosion properties of the straight NiTi wires covered by virgin uncracked oxides. Out of the three methods employed, the Electrochemical Impedance Spectroscopy seems to be the most appropriate test for the electrochemical characterization of the cracked oxide layers, since the impedance curves (Nyquist plot) of differently bent NiTi wires can be associated with increasing state of the surface cracking and since the NiTi wires are exposed to similar conditions as the surfaces of NiTi implants in human body. On the other hand, the potentiodynamic polarization test accelerates the corrosion processes and provides clear evidence that the corrosion resistance of bent superelastic NiTi wires degrades with oxide cracking.

Axial crack propagation and arrest in pressurized fuselage

NASA Technical Reports Server (NTRS)

Kosai, M.; Shimamoto, A.; Yu, C.-T.; Walker, S. I.; Kobayashi, A. S.; Tan, P.

1994-01-01

The crack arrest capability of a tear strap in a pressurized precracked fuselage was studied through instrumented axial rupture tests of small scale models of an idealized fuselage. Upon pressurization, rapid crack propagation initiated at an axial through crack along the stringer and immediately kinked due to the mixed modes 1 and 2 state caused by the one-sided opening of the crack flap. The diagonally running crack further turned at the tear straps. Dynamic finite element analysis of the rupturing cylinder showed that the crack kinked and also ran straight in the presence of a mixed mode state according to a modified two-parameter crack kinking criterion.

Fatigue crack tip deformation and fatigue crack propagation

NASA Technical Reports Server (NTRS)

Kang, T. S.; Liu, H. W.

1972-01-01

The effects of stress ratio, prestress cycling and plate thickness on the fatigue crack propagation rate are studied on 2024-T351 aluminum alloy. Fatigue crack propagation rate increases with the plate thickness and the stress ratio. Prestress cycling below the static yield strength has no noticeable effect on the fatigue crack propagation rate. However, prestress cycling above the static yield strength causes the material to strain harden and increases the fatigue crack propagation rate. Crack tip deformation is used to study the fatigue crack propagation. The crack tip strains and the crack opening displacements were measured from moire fringe patterns. The moire fringe patterns were obtained by a double exposure technique, using a very high density master grille (13,400 lines per inch).

Alternating method applied to edge and surface crack problems

NASA Technical Reports Server (NTRS)

Hartranft, R. J.; Sih, G. C.

1972-01-01

The Schwarz-Neumann alternating method is employed to obtain stress intensity solutions to two crack problems of practical importance: a semi-infinite elastic plate containing an edge crack which is subjected to concentrated normal and tangential forces, and an elastic half space containing a semicircular surface crack which is subjected to uniform opening pressure. The solution to the semicircular surface crack is seen to be a significant improvement over existing approximate solutions. Application of the alternating method to other crack problems of current interest is briefly discussed.

Characterizing open and non-uniform vertical heat sources: towards the identification of real vertical cracks in vibrothermography experiments

NASA Astrophysics Data System (ADS)

Castelo, A.; Mendioroz, A.; Celorrio, R.; Salazar, A.; López de Uralde, P.; Gorosmendi, I.; Gorostegui-Colinas, E.

2017-05-01

Lock-in vibrothermography is used to characterize vertical kissing and open cracks in metals. In this technique the crack heats up during ultrasound excitation due mainly to friction between the defect's faces. We have solved the inverse problem, consisting in determining the heat source distribution produced at cracks under amplitude modulated ultrasound excitation, which is an ill-posed inverse problem. As a consequence the minimization of the residual is unstable. We have stabilized the algorithm introducing a penalty term based on Total Variation functional. In the inversion, we combine amplitude and phase surface temperature data obtained at several modulation frequencies. Inversions of synthetic data with added noise indicate that compact heat sources are characterized accurately and that the particular upper contours can be retrieved for shallow heat sources. The overall shape of open and homogeneous semicircular strip-shaped heat sources representing open half-penny cracks can also be retrieved but the reconstruction of the deeper end of the heat source loses contrast. Angle-, radius- and depth-dependent inhomogeneous heat flux distributions within these semicircular strips can also be qualitatively characterized. Reconstructions of experimental data taken on samples containing calibrated heat sources confirm the predictions from reconstructions of synthetic data. We also present inversions of experimental data obtained from a real welded Inconel 718 specimen. The results are in good qualitative agreement with the results of liquids penetrants testing.

Measurement and interpretation of threshold stress intensity factors for steels in high-pressure hydrogen gas.

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

Dadfarnia, Mohsen; Nibur, Kevin A.; San Marchi, Christopher W.

2010-07-01

Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. The sustained load cracking procedures are generally consistent with those in ASME Article KD-10 of Section VIII Division 3 of the Boiler and Pressure Vessel Code, which was recently published to guide design of high-pressure hydrogen vessels. Three definitions of threshold were established for the two test methods: K{sub THi}* is the maximum applied stress intensity factor for which no crack extension was observed under constant displacement; K{sub THa} ismore » the stress intensity factor at the arrest position for a crack that extended under constant displacement; and K{sub JH} is the stress intensity factor at the onset of crack extension under rising displacement. The apparent crack initiation threshold under constant displacement, K{sub THi}*, and the crack arrest threshold, K{sub THa}, were both found to be non-conservative due to the hydrogen exposure and crack-tip deformation histories associated with typical procedures for sustained-load cracking tests under constant displacement. In contrast, K{sub JH}, which is measured under concurrent rising displacement and hydrogen gas exposure, provides a more conservative hydrogen-assisted fracture threshold that is relevant to structural components in which sub-critical crack extension is driven by internal hydrogen gas pressure.« less

Measurement and interpretation of threshold stress intensity factors for steels in high-pressure hydrogen gas.

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

Nibur, Kevin A.

2010-11-01

Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. The sustained load cracking procedures are generally consistent with those in ASME Article KD-10 of Section VIII Division 3 of the Boiler and Pressure Vessel Code, which was recently published to guide design of high-pressure hydrogen vessels. Three definitions of threshold were established for the two test methods: K{sub THi}* is the maximum applied stress intensity factor for which no crack extension was observed under constant displacement; K{sub THa} ismore » the stress intensity factor at the arrest position for a crack that extended under constant displacement; and K{sub JH} is the stress intensity factor at the onset of crack extension under rising displacement. The apparent crack initiation threshold under constant displacement, K{sub THi}*, and the crack arrest threshold, K{sub THa}, were both found to be non-conservative due to the hydrogen exposure and crack-tip deformation histories associated with typical procedures for sustained-load cracking tests under constant displacement. In contrast, K{sub JH}, which is measured under concurrent rising displacement and hydrogen gas exposure, provides a more conservative hydrogen-assisted fracture threshold that is relevant to structural components in which sub-critical crack extension is driven by internal hydrogen gas pressure.« less

Behavior of Ti-5Al-2.5Sn ELI titanium alloy sheet parent and weld metal in the presence of cracks at 20 K

NASA Technical Reports Server (NTRS)

Sullivan, T. L.

1971-01-01

Through- and surface-cracked specimens of two thicknesses were tested in uniaxial tension. Surface-cracked specimens were generally found to be stronger than through-cracked specimens with the same crack length. Apparent surface-crack fracture toughness calculated using the Anderson modified Irwin equation remained relatively constant for cracks as deep as 90 percent of the sheet thickness. Subcritical growth of surface cracks was investigated. Comparison of chamber and open air welds showed chamber welds to be slightly tougher. Both methods produced welds with toughness that compared favorably with that of the parent metal. Weld efficiencies were above 94 percent.

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.

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

Brickstad, B.; Bergman, M.

A computerized procedure has been developed that predicts the growth of an initial circumferential surface crack through a pipe and further on to failure. The crack growth mechanism can either be fatigue or stress corrosion. Consideration is taken to complex crack shapes and for the through-wall cracks, crack opening areas and leak rates are also calculated. The procedure is based on a large number of three-dimensional finite element calculations of cracked pipes. The results from these calculations are stored in a database from which the PC-program, denoted LBBPIPE, reads all necessary information. In this paper, a sensitivity analysis is presentedmore » for cracked pipes subjected to both stress corrosion and vibration fatigue.« less

Dynamic ductile fracture of a central crack

NASA Technical Reports Server (NTRS)

Tsai, Y. M.

1976-01-01

A central crack, symmetrically growing at a constant speed in a two dimensional ductile material subject to uniform tension at infinity, is investigated using the integral transform methods. The crack is assumed to be the Dugdale crack, and the finite stress condition at the crack tip is satisfied during the propagation of the crack. Exact expressions of solution are obtained for the finite stress condition at the crack tip, the crack shape, the crack opening displacement, and the energy release rate. All those expressions are written as the product of explicit dimensional quantities and a nondimensional dynamic correction function. The expressions reduce to the associated static results when the crack speed tends to zero, and the nondimensional dynamic correction functions were calculated for various values of the parameter involved.

Analysis of radially cracked ring segments subject to forces and couples

NASA Technical Reports Server (NTRS)

Gross, B.; Srawley, J. E.

1977-01-01

Results of planar boundary collocation analysis are given for ring segment (C-shaped) specimens with radial cracks, subjected to combined forces and couples. Mode I stress intensity factors and crack mouth opening displacements were determined for ratios of outer to inner radius in the range 1.1 to 2.5 and ratios of crack length to segment width in the range 0.1 to 0.8.

Analysis of radially cracked ring segments subject to forces and couples

NASA Technical Reports Server (NTRS)

Gross, B.; Strawley, J. E.

1975-01-01

Results of planar boundary collocation analysis are given for ring segment (C shaped) specimens with radial cracks, subjected to combined forces and couples. Mode I stress intensity factors and crack mouth opening displacements were determined for ratios of outer to inner radius in the range 1.1 to 2.5, and ratios of crack length to segment width in the range 0.1 to 0.8.

Interaction of Lamb Waves with Fatigue Cracks in Aluminum

DTIC Science & Technology

2011-09-01

Interaction of Lamb Waves with Fatigue Cracks in Aluminum E. D. SWENSON, C. T. OWENS and C. ALLEN ABSTRACT Elastic waves can travel across...the interaction of Lamb waves with both open and closed low-cycle fatigue cracks in aluminum plates using a three-dimensional laser Doppler vibrometer...and antisymmetric Lamb wave modes differ upon encountering fatigue cracks. INTRODUCTION The use of guided elastic waves (Lamb waves) has shown

Nondestructive estimation of depth of surface opening cracks in concrete beams

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

Arne, Kevin; In, Chiwon; Kurtis, Kimberly

Concrete is one of the most widely used construction materials and thus assessment of damage in concrete structures is of the utmost importance from both a safety point of view and a financial point of view. Of particular interest are surface opening cracks that extend through the concrete cover, as this can expose the steel reinforcement bars underneath and induce corrosion in them. This corrosion can lead to significant subsequent damage in concrete such as cracking and delamination of the cover concrete as well as rust staining on the surface of concrete. Concrete beams are designed and constructed in suchmore » a way to provide crack depths up to around 13 cm. Two different types of measurements are made in-situ to estimate depths of real surface cracks (as opposed to saw-cut notches) after unloading: one based on the impact-echo method and the other one based on the diffuse ultrasonic method. These measurements are compared to the crack depth visually observed on the sides of the beams. Discussions are given as to the advantages and disadvantages of each method.« less

[The Cagliari (Italy) Court authorizes the preimplantation genetic diagnosis].

PubMed

Jorqui Azofra, María

2007-01-01

Today, preimplantation genetic diagnosis (PGD) has been greatly accepted within the framework of positive law of many European countries. Nevertheless, in other countries, such as Italy, it is forbidden by law. The ruling of the Civil Court of Cagliari which has authorized its use to a Sardinian couple, has opened, in this way, a small crack to be able to asses possible modifications to the Italian regulation on this matter. This article analyses the ruling of the Civil Court of Cagliari (Italy) from an ethical and legal perspective. The criteria which is used to analyse the legitimacy or illegitimacy of the practice of PGD is analysed. That is, on reasons which could justify or not the transfer of embryos in vitro to the woman. With this objective in mind, the Italian and Spanish normative models which regulates this controversial subject are looked at. As a conclusion, a critical evaluation of the arguments presented is made.

Emerging patterns of crack use in Mexico City.

PubMed

Valdez, Avelardo; Kaplan, Charles; Nowotny, Kathryn M; Natera-Rey, Guillermina; Cepeda, Alice

2015-08-01

Recent studies in Mexico have documented a significant increase in crack cocaine use, indicating the potential for an emerging drug epidemic. Ethnographic observations and interviews were used describe the profiles and patterns of use among street-recruited crack users in Mexico City. The data came from an international research collaboration funded by the National Institutes of Health. A polythetic typology was developed based on five dimensions central to categorizing patterns of crack use behavior: frequency of use, duration of use, context, social networks, and social contracts. Four types of users were discovered applying these dimensions: dabblers, stable users, crack heads, and old heads. Although several similarities were documented between patterns of crack use in Mexico and those in the United States and Western Europe, several key aspects distinguished crack users in this population: (1) self-regulated use; (2) non-linear progression of crack; and (3) the influence of the dimensions pertaining to setting, social networks, and social contract as contributing to understanding of the previous two. Further, we provide a discussion of how specific contextual factors in Mexico may be giving rise to these emerging patterns. Compared to the U.S. and Europe, this study finds that the majority of crack users were able to self-regulate their use without major disruption to daily social functioning. As crack use spreads in Mexico and other Latin American countries, we need to recognize the importance of social context in developing more tailored health and social responses that are specific to these developing countries. Copyright © 2015 Elsevier B.V. All rights reserved.

Load-Differential Features for Automated Detection of Fatigue Cracks Using Guided Waves (Preprint)

DTIC Science & Technology

2011-11-01

AFRL-RX-WP-TP-2011-4363 LOAD-DIFFERENTIAL FEATURES FOR AUTOMATED DETECTION OF FATIGUE CRACKS USING GUIDED WAVES (PREPRINT) Jennifer E...AUTOMATED DETECTION OF FATIGUE CRACKS USING GUIDED WAVES (PREPRINT) 5a. CONTRACT NUMBER FA8650-09-C-5206 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER...tensile loads open fatigue cracks and thus enhance their detectability using ultrasonic methods. Here we introduce a class of load-differential methods

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.

Fracture under combined modes in 4340 steel

NASA Technical Reports Server (NTRS)

Shah, R. C.

1974-01-01

An experimental investigation was conducted to study the interaction of combined modes of loading on crack instability in the presence of the opening and sliding modes of stress intensity factors, the opening and tearing modes of stress intensity factors, and all three modes of stress intensity factors. Through-cracked and surface-cracked flat and round specimens, and round notched bar specimens fabricated from high strength 4340 steel were used for the investigation. The results are evaluated to determine fracture criteria under the combined modes of stress intensity factors for the 4340 steel. These results are compared with the results of other investigators obtained for different materials.

Buckling and postbuckling of size-dependent cracked microbeams based on a modified couple stress theory

NASA Astrophysics Data System (ADS)

Akbarzadeh Khorshidi, M.; Shariati, M.

2017-07-01

The elastic buckling analysis and the static postbuckling response of the Euler-Bernoulli microbeams containing an open edge crack are studied based on a modified couple stress theory. The cracked section is modeled by a massless elastic rotational spring. This model contains a material length scale parameter and can capture the size effect. The von Kármán nonlinearity is applied to display the postbuckling behavior. Analytical solutions of a critical buckling load and the postbuckling response are presented for simply supported cracked microbeams. This parametric study indicates the effects of the crack location, crack severity, and length scale parameter on the buckling and postbuckling behaviors of cracked microbeams.

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.

Damage Tolerant Repair Techniques for Pressurized Aircraft Fuselages

DTIC Science & Technology

1994-01-01

2 if20 20 offset 50, fatigue GST, GLARE 2 if20 20 static, no fatigue *Unidirectional SP500 carbon/epoxy tape . "* Fatigue load did not initiate a crack...Et value, so this is a reasonable assumption. It further implies zero crack opening under the patch. The Erdogan solution [51 for two collinear...Cr Figure 6. 11. Idealization of patched crack as unfailed ligament between two collinear cracks (after [5, 6)). The Erdogan solution leads to the AK

Damage Tolerant Repair Techniques for Pressurized Aircraft Fuselages

DTIC Science & Technology

1994-06-06

crack patching effectiveness, long cracks ( Erdogan ) 186 Vii Acknowledgments My three years of Ph.D. work would have been impossible without the...fatigue GST, GLARE 2 " " 20 20 static, no fatigue *Unidirectional SP500 carbon/epoxy tape . *Fatigue load did not initiate a crack. The saw cut was...assurnption It further implies zero crack opening under the pat(:r The Erdogan solutior (51 for two Coiinear Ctacks can be expressed as: %A F -. " (621 F

Simplified moment tensor analysis and unified decomposition of acoustic emission source: Application to in situ hydrofracturing test

NASA Astrophysics Data System (ADS)

Ohtsu, Masayasu

1991-04-01

An application of a moment tensor analysis to acoustic emission (AE) is studied to elucidate crack types and orientations of AE sources. In the analysis, simplified treatment is desirable, because hundreds of AE records are obtained from just one experiment and thus sophisticated treatment is realistically cumbersome. Consequently, a moment tensor inversion based on P wave amplitude is employed to determine six independent tensor components. Selecting only P wave portion from the full-space Green's function of homogeneous and isotropic material, a computer code named SiGMA (simplified Green's functions for the moment tensor analysis) is developed for the AE inversion analysis. To classify crack type and to determine crack orientation from moment tensor components, a unified decomposition of eigenvalues into a double-couple (DC) part, a compensated linear vector dipole (CLVD) part, and an isotropic part is proposed. The aim of the decomposition is to determine the proportion of shear contribution (DC) and tensile contribution (CLVD + isotropic) on AE sources and to classify cracks into a crack type of the dominant motion. Crack orientations determined from eigenvectors are presented as crack-opening vectors for tensile cracks and fault motion vectors for shear cracks, instead of stereonets. The SiGMA inversion and the unified decomposition are applied to synthetic data and AE waveforms detected during an in situ hydrofracturing test. To check the accuracy of the procedure, numerical experiments are performed on the synthetic waveforms, including cases with 10% random noise added. Results show reasonable agreement with assumed crack configurations. Although the maximum error is approximately 10% with respect to the ratios, the differences on crack orientations are less than 7°. AE waveforms detected by eight accelerometers deployed during the hydrofracturing test are analyzed. Crack types and orientations determined are in reasonable agreement with a predicted failure plane from borehole TV observation. The results suggest that tensile cracks are generated first at weak seams and then shear cracks follow on the opened joints.

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.

Growth Kinematics of Opening-Mode Fractures

NASA Astrophysics Data System (ADS)

Eichhubl, P.; Alzayer, Y.; Laubach, S.; Fall, A.

2014-12-01

Fracture aperture is a primary control on flow in fractured reservoirs of low matrix permeability including unconventional oil and gas reservoirs and most geothermal systems. Guided by principles of linear elastic fracture mechanics, fracture aperture is generally assumed to be a linear function of fracture length and elastic material properties. Natural opening-mode fractures with significant preserved aperture are observed in core and outcrop indicative of fracture opening strain accommodated by permanent solution-precipitation creep. Fracture opening may thus be decoupled from length growth if the material effectively weakens after initial elastic fracture growth by either non-elastic deformation processes or changes in elastic properties. To investigate the kinematics of fracture length and aperture growth, we reconstructed the opening history of three opening-mode fractures that are bridged by crack-seal quartz cement in Travis Peak Sandstone of the SFOT-1 well, East Texas. Similar crack-seal cement bridges had been interpreted to form by repeated incremental fracture opening and subsequent precipitation of quartz cement. We imaged crack-seal cement textures for bridges sampled at varying distance from the tips using scanning electron microscope cathodoluminescence, and determined the number and thickness of crack-seal cement increments as a function of position along the fracture length and height. Observed trends in increment number and thickness are consistent with an initial stage of fast fracture propagation relative to aperture growth, followed by a stage of slow propagation and pronounced aperture growth. Consistent with fluid inclusion observations indicative of fracture opening and propagation occurring over 30-40 m.y., we interpret the second phase of pronounced aperture growth to result from fracture opening strain accommodated by solution-precipitation creep and concurrent slow, possibly subcritical, fracture propagation. Similar deformation mechanisms are envisioned to govern fracture growth over shorter timescales in reactive chemical subsurface environments including CO2 reservoirs, organic-rich shales, and geothermal systems.

Crack opening area estimates in pressurized through-wall cracked elbows under bending

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

Franco, C.; Gilles, P.; Pignol, M.

1997-04-01

One of the most important aspects in the leak-before-break approach is the estimation of the crack opening area corresponding to potential through-wall cracks at critical locations during plant operation. In order to provide a reasonable lower bound to the leak area under such loading conditions, numerous experimental and numerical programs have been developed in USA, U.K. and FRG and widely discussed in literature. This paper aims to extend these investigations on a class of pipe elbows characteristic of PWR main coolant piping. The paper is divided in three main parts. First, a new simplified estimation scheme for leakage area ismore » described, based on the reference stress method. This approach mainly developed in U.K. and more recently in France provides a convenient way to account for the non-linear behavior of the material. Second, the method is carried out for circumferential through-wall cracks located in PWR elbows subjected to internal pressure. Finite element crack area results are presented and comparisons are made with our predictions. Finally, in the third part, the discussion is extended to elbows under combined pressure and in plane bending moment.« less

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.

Analysis and Test of Deep Flaws in Thin Sheets of Aluminum and Titanium. Volume 2: Crack Opening Displacement and Stress-Strain Data

NASA Technical Reports Server (NTRS)

Finger, R. W.

1978-01-01

Static fracture tests were performed on surface flawed specimens of aluminum and titanium alloys. A simulated proof overload cycle was applied prior to all of the cyclic tests. Variables included in each test series were flaw shapes and thickness. Additionally, test temperature was a variable for the aluminum test series. The crack opening displacement and stress-strain data obtained are presented.

The elasticity problem for a thick-walled cylinder containing a circumferential crack

NASA Technical Reports Server (NTRS)

Nied, H. F.; Erdogan, F.

1983-01-01

The elasticity problem for a long hollow circular cylinder containing an axisymmetric circumferential crack subjected to general nonaxisymmetric external loads is considered. The problem is formulated in terms of a system of singular integral equations with the Fourier coefficients of the derivative of the crack surface displacement as density functions. The stress intensity factors and the crack opening displacement are calculated for a cylinder under uniform tension, bending by end couples, and self-equilibrating residual stresses.

Crack Resistance of Welded Joints of Pipe Steels of Strength Class K60 of Different Alloying Systems

NASA Astrophysics Data System (ADS)

Tabatchikova, T. I.; Tereshchenko, N. A.; Yakovleva, I. L.; Makovetskii, A. N.; Shander, S. V.

2018-03-01

The crack resistance of welded joints of pipe steels of strength class K60 and different alloying systems is studied. The parameter of the crack tip opening displacement (CTOD) is shown to be dependent on the size of the austenite grains and on the morphology of bainite in the superheated region of the heat-affected zone of the weld. The crack resistance is shown to be controllable due to optimization of the alloying system.

The elasticity problem for a thick-walled cylinder containing a circumferential crack

NASA Technical Reports Server (NTRS)

Nied, H. F.; Erdogan, F.

1982-01-01

The elasticity problem for a long hollow circular cylinder containing an axisymmetric circumferential crack subjected to general nonaxisymmetric external loads is considered. The problem is formulated in terms of a system of singular integral equations with the Fourier coefficients of the derivative of the crack surface displacement as density functions. The stress intensity factors and the crack opening displacement are calculated for a cylinder under uniform tension, bending by end couples, and self-equilibrating residual stresses.

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

Locating and characterizing a crack in concrete with diffuse ultrasound: A four-point bending test.

PubMed

Larose, Eric; Obermann, Anne; Digulescu, Angela; Planès, Thomas; Chaix, Jean-Francois; Mazerolle, Frédéric; Moreau, Gautier

2015-07-01

This paper describes an original imaging technique, named Locadiff, that benefits from the diffuse effect of ultrasound waves in concrete to detect and locate mechanical changes associated with the opening of pre-existing cracks, and/or to the development of diffuse damage at the tip of the crack. After giving a brief overview of the theoretical model to describe the decorrelation of diffuse waveforms induced by a local change, the article introduces the inversion procedure that produces the three dimensional maps of density of changes. These maps are interpreted in terms of mechanical changes, fracture opening, and damage development. In addition, each fracture is characterized by its effective scattering cross section.

Mode I Cohesive Law Characterization of Through-Crack Propagation in a Multidirectional Laminate

NASA Technical Reports Server (NTRS)

Bergan, Andrew C.; Davila, Carlos G.; Leone, Frank A.; Awerbuch, Jonathan; Tan, Tein-Min

2014-01-01

A method is proposed and assessed for the experimental characterization of through-the-thickness crack propagation in multidirectional composite laminates with a cohesive law. The fracture toughness and crack opening displacement are measured and used to determine a cohesive law. Two methods of computing fracture toughness are assessed and compared. While previously proposed cohesive characterizations based on the R-curve exhibit size effects, the proposed approach results in a cohesive law that is a material property. The compact tension specimen configuration is used to propagate damage while load and full-field displacements are recorded. These measurements are used to compute the fracture toughness and crack opening displacement from which the cohesive law is characterized. The experimental results show that a steady-state fracture toughness is not reached. However, the proposed method extrapolates to steady-state and is demonstrated capable of predicting the structural behavior of geometrically-scaled specimens.

Eddy current probe response to open and closed surface flaws

NASA Technical Reports Server (NTRS)

Auld, B. A.; Muennemann, F.; Winslow, D. K.

1981-01-01

A general analysis of eddy current response to certain types of open and closed surface flaws is presented for both standard low-frequency and ferromagnetic-resonance (FMR) probes. It is shown analytically that for two-dimensional and three-dimensional surface flaws interrogated by a uniform probe field, the crack opening sensitivity increases with the operating frequency of the probe, this behavior being due to the Faraday induction effect. Experiments with low-frequency probes operating at or below 1 MHz and with the FMR probe operating at approximately 1000 MHz confirm this increase of the crack mouth opening displacement for practical situations where the probe field is not uniform in the vicinity of the flaw.

Modeling Transverse Cracking in Laminates With a Single Layer of Elements Per Ply

NASA Technical Reports Server (NTRS)

Van Der Meer, Frans P.; Davila, Carlos G.

2012-01-01

The objective of the present paper is to investigate the ability of mesolevel X-FEM models with a single layer of elements per ply to capture accurately all aspects of matrix cracking. In particular, we examine whether the model can predict the insitu ply thickness effect on crack initiation and propagation, the crack density as a function of strain, the strain for crack saturation, and the interaction between delamination and transverse cracks. Results reveal that the simplified model does not capture correctly the shear-lag relaxation of the stress field on either side of a crack, which leads to an overprediction of the crack density. It is also shown, however, that after onset of delamination many of the inserted matrix cracks close again, and that the density of open cracks becomes similar to the density predicted by the detailed model. The degree to which the spurious cracks affect the global response is quantified and the reliability of the mesolevel approach with a single layer of elements per ply is discussed.

Measurement and analysis of critical crack tip processes during fatigue crack growth

NASA Technical Reports Server (NTRS)

Davidson, D. L.; Hudak, S. J.; Dexter, R. J.

1985-01-01

The mechanics of fatigue crack growth under constant-amplitudes and variable-amplitude loading were examined. Critical loading histories involving relatively simple overload and overload/underload cycles were studied to provide a basic understanding of the underlying physical processes controlling crack growth. The material used for this study was 7091-T7E69, a powder metallurgy aluminum alloy. Local crack-tip parameters were measured at various times before, during, and after the overloads, these include crack-tip opening loads and displacements, and crack-tip strain fields. The latter were useed, in combination with the materials cyclic and monotonic stress-strain properties, to compute crack-tip residual stresses. The experimental results are also compared with analytical predictions obtained using the FAST-2 computer code. The sensitivity of the analytical model to constant-amplitude fatigue crack growth rate properties and to through-thickness constrain are studied.

Recent advances in the modelling of crack growth under fatigue loading conditions

NASA Technical Reports Server (NTRS)

Dekoning, A. U.; Tenhoeve, H. J.; Henriksen, T. K.

1994-01-01

Fatigue crack growth associated with cyclic (secondary) plastic flow near a crack front is modelled using an incremental formulation. A new description of threshold behaviour under small load cycles is included. Quasi-static crack extension under high load excursions is described using an incremental formulation of the R-(crack growth resistance)- curve concept. The integration of the equations is discussed. For constant amplitude load cycles the results will be compared with existing crack growth laws. It will be shown that the model also properly describes interaction effects of fatigue crack growth and quasi-static crack extension. To evaluate the more general applicability the model is included in the NASGRO computer code for damage tolerance analysis. For this purpose the NASGRO program was provided with the CORPUS and the STRIP-YIELD models for computation of the crack opening load levels. The implementation is discussed and recent results of the verification are presented.

Representing Matrix Cracks Through Decomposition of the Deformation Gradient Tensor in Continuum Damage Mechanics Methods

NASA Technical Reports Server (NTRS)

Leone, Frank A., Jr.

2015-01-01

A method is presented to represent the large-deformation kinematics of intraply matrix cracks and delaminations in continuum damage mechanics (CDM) constitutive material models. The method involves the additive decomposition of the deformation gradient tensor into 'crack' and 'bulk material' components. The response of the intact bulk material is represented by a reduced deformation gradient tensor, and the opening of an embedded cohesive interface is represented by a normalized cohesive displacement-jump vector. The rotation of the embedded interface is tracked as the material deforms and as the crack opens. The distribution of the total local deformation between the bulk material and the cohesive interface components is determined by minimizing the difference between the cohesive stress and the bulk material stress projected onto the cohesive interface. The improvements to the accuracy of CDM models that incorporate the presented method over existing approaches are demonstrated for a single element subjected to simple shear deformation and for a finite element model of a unidirectional open-hole tension specimen. The material model is implemented as a VUMAT user subroutine for the Abaqus/Explicit finite element software. The presented deformation gradient decomposition method reduces the artificial load transfer across matrix cracks subjected to large shearing deformations, and avoids the spurious secondary failure modes that often occur in analyses based on conventional progressive damage models.

Mars Odyssey All Stars: Cerberus Crack

NASA Image and Video Library

2010-12-09

Geological faulting has opened cracks in the Cerberus region that slice through flat plains and mesas alike. This image is part of an All Star set marking the occasion of NASA Mars Odyssey as the longest-working Mars spacecraft in history.

Cracking and debonding of a thin fiber reinforced concrete overlay.

DOT National Transportation Integrated Search

2017-04-01

Previous field studies suggested that macro-fibers incorporated in thin overlay pavements will result in reduced crack opening widths, vertical deflections, and debonding rates compared to that of unreinforced overlays. A simple finite element (FE) m...

Detection and monitoring of surface micro-cracks by PPP-BOTDA.

PubMed

Meng, Dewei; Ansari, Farhad; Feng, Xin

2015-06-01

Appearance of micrometer size surface cracks is common in structural elements such as welded connections, beams, and gusset plates in bridges. Brillouin scattering-based sensors are capable of making distributed strain measurements. Pre-pump-pulse Brillouin optical time domain analysis (PPP-BOTDA) provides a centimeter-level spatial resolution, which facilitates detection and monitoring of the cracks. In the work described here, in addition to the shift in Brillouin frequency (distributed strains), change in the Brillouin gain spectrum (BGS) width is investigated for the detection and monitoring of surface micro-cracks. A theoretical analysis was undertaken in order to verify the rationality of the proposed method. The theoretical approach involved simulation of strain within a segment of the optical fiber traversing a crack and use of the simulated strain distribution in the opto-mechanical relations in order to numerically obtain the change in the BGS. Simulations revealed that the increase in crack opening displacements is associated with increase in BGS width and decrease in its peak power. Experimental results also indicated that the increases in crack opening displacements are accompanied with increases in BGS widths. However, it will be difficult to use the decrease in BGS power peak as another indicator due to practical difficulties in establishing generalized power amplitude in all the experiments. The study indicated that, in combination with the shift in Brillouin frequency, the increase in BGS width will provide a strong tool for detection and monitoring of surface micro-crack growths.

Redistribution of Welding Residual Stresses of Crack Tip Opening Displacement Specimen by Local Compression.

PubMed

Kim, Young-Gon; Song, Kuk-Hyun; Lee, Dong-Hoon; Joo, Sung-Min

2018-03-01

The demand of crack tip opening displacement (CTOD) test which evaluates fracture toughness of a cracked material is very important to ensure the stability of structure under severe service environment. The validity of the CTOD test result is judged using several criterions of the specification standards. One of them is the artificially generated fatigue pre-crack length inside the specimen. For acceptable CTOD test results, fatigue pre-crack must have a reasonable sharp crack front. The propagation of fatigue crack started from the tip of the machined notch, which might have propagated irregularly due to residual stress field. To overcome this problem, test codes suggest local compression method, reversed bending method and stepwise high-R ratio method to reduce the disparity of residual stress distribution inside the specimen. In this paper, the relation between the degree of local compression and distribution of welding residual stress has been analyzed by finite element analyses in order to determine the amount of effective local compression of the test piece. Analysis results show that initial welding residual stress is dramatically varied three-dimensionally while cutting, notch machining and local compressing due to the change of internal restraint force. From the simulation result, the authors find that there is an optimum amount of local compression to modify regularly for generating fatigue pre-crack propagation. In the case of 0.5% compressions of the model width is the most effective for uniforming residual stress distribution.

Influence of crack opening and incident wave angle on second harmonic generation of Lamb waves

NASA Astrophysics Data System (ADS)

Yang, Yi; Ng, Ching-Tai; Kotousov, Andrei

2018-05-01

Techniques utilising second harmonic generation (SHG) have proven their great potential in detecting contact-type damage. However, the gap between the practical applications and laboratory studies is still quite large. The current work is aimed to bridge this gap by investigating the effects of the applied load and incident wave angle on the detectability of fatigue cracks at various lengths. Both effects are critical for practical implementations of these techniques. The present experimental study supported by three-dimensional (3D) finite element (FE) modelling has demonstrated that the applied load, which changes the crack opening and, subsequently, the contact nonlinearity, significantly affects the amplitude of the second harmonic generated by the fundamental symmetric mode (S0) of Lamb wave. This amplitude is also dependent on the length of the fatigue crack as well as the incident wave angle. The experimental and FE results correlate well, so the modelling approach can be implemented for practical design of damage monitoring systems as well as for the evaluation of the severity of the fatigue cracks.

Ballistic Impact of Braided Composites with a Soft Projectile

NASA Technical Reports Server (NTRS)

Roberts, Gary D.; Pereira, J. Michael; Revilock, Duane M., Jr.; Binienda, Wieslaw K.; Xie, Ming; Braley, Mike

2002-01-01

Impact tests using a soft gelatin projectile were performed to identify failure modes that occur at high strain energy density during impact loading. Failure modes were identified for aluminum plates and for composites plates and half-rings made from triaxial carbon fiber braid having a 0/+/- 60deg architecture. For aluminum plates, a large hole formed as a result of crack propagation from the initiation site at the center of the plate. For composite plates, fiber tensile failure occurred in the back ply at the center of the plate. Cracks then propagated from this site along the +/-60deg fiber directions until triangular flaps opened to form a hole. For composite half-rings fabricated with 0deg fibers aligned circumferentially, fiber tensile failure also occurred in the back ply. Cracks first propagated from this site perpendicular the 0deg fibers. The cracks then turned to follow the +/-60deg fibers and 0deg fibers until rectangular flaps opened to form a hole. Damage in the composites was localized near the impact site, while cracks in the aluminum extended to the boundaries.

Do chimpanzees use weight to select hammer tools?

PubMed

Schrauf, Cornelia; Call, Josep; Fuwa, Koki; Hirata, Satoshi

2012-01-01

The extent to which tool-using animals take into account relevant task parameters is poorly understood. Nut cracking is one of the most complex forms of tool use, the choice of an adequate hammer being a critical aspect in success. Several properties make a hammer suitable for nut cracking, with weight being a key factor in determining the impact of a strike; in general, the greater the weight the fewer strikes required. This study experimentally investigated whether chimpanzees are able to encode the relevance of weight as a property of hammers to crack open nuts. By presenting chimpanzees with three hammers that differed solely in weight, we assessed their ability to relate the weight of the different tools with their effectiveness and thus select the most effective one(s). Our results show that chimpanzees use weight alone in selecting tools to crack open nuts and that experience clearly affects the subjects' attentiveness to the tool properties that are relevant for the task at hand. Chimpanzees can encode the requirements that a nut-cracking tool should meet (in terms of weight) to be effective.

The effect of an overload on the rate of fatigue crack propagation under plane stress conditions

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

Bao, H.; McEvily, A.J.

1995-07-01

It has been shown that the retardation in the rate of fatigue crack growth following an overload is largely the result of surface-related, plane-stress deformation. In the present article, in order to isolate the plane-stress behavior, the effect of an overload on the subsequent rate of fatigue crack growth of 0.3-mm-thick specimens of 9Cr-1Mo steel has been investigated and compared to results obtained using 6.35-mm-thick specimens. It was found that for the 0.3-mm thickness, as with thicker specimens, two opening load levels were associated with the overload process. The upper opening load is associated with plane-stress deformation in the overloadmore » plastic zone, and this opening process is more clearly observed with thin as compared to thicker specimens. Based upon the determined level of the upper opening load, a semiempirical analysis is developed for calculating the number of delay cycles due to an overload as a function of thickness.« less

Fracture toughness of the nickel-alumina laminates by digital image-correlation technique

NASA Astrophysics Data System (ADS)

Mekky, Waleed

The purpose of this work is to implement the digital image correlation technique (DIC) in composite laminate fracture testing. The latter involves measuring the crack opening displacement (COD) during stable crack propagation and characterizing the strain development in a constrained nickel layer under applied loading. The major challenge to measure the COD of alternated metal/ceramic layers is the elastic-mismatch effect. This leads to oscillating COD measurement. Smoothing the result with built-in modules of commercial software leads to a loss of data accuracy. A least-squares fitting routine for the data output gave acceptable COD profiles. The behavior of a single Ni ligament sandwiched between two Al2O3 layers was determined for two Ni thicknesses (0.125 and 0.25mm). Modeling of the behavior via a modified Bridgman approach for rectangular cross section samples, proved limited as different mechanisms are operating. Nevertheless, the behavior is however captured to a point, but the model underestimates the results vis a vis experimental ones. The fracture-resistance curves for Nickel/Alumina laminates were developed experimentally and modeled via LEFM using the weight function approach and utilizing single-ligament-, and COD-, data. The crack-tip toughness was found to increase with Ni layer thickness due to crack-tip-shielding. The crack-initiation-toughness was estimated from the stress field and the crack-opening-displacement of the main crack.

Thermal regimes in bedrock and open fractures in the Nordnes rockslide, Norway

NASA Astrophysics Data System (ADS)

Hvidtfeldt Christiansen, Hanne; Harald Blikra, Lars

2010-05-01

The Nordnes rockslide site is located in the arctic part of the periglacial mountain landscape of Northern Norway at 69°30'N. It consists in the upper part of 1-10 m wide and 1-10 m deep open fractures. Extensive displacements measurements using GPS surveys, crackmeters, tiltmeter and lasers establish the intermunicipality monitoring programme, which shows ongoing deformation of the rockslide. In the rather special topographical setting of the open fractures we have during the International Polar Year 2007 to 2009 recorded the thermal regime of the upper part of the bedrock and of the air in the cracks for attempting to determine whether the recorded deformation can be geomorphologically controlled by bedrock surface expansion and contraction and/or by seasonal freezing or even by permafrost, or if only normal gravitational processes control the observed displacements. The upper 40 cm bedrock thermal conditions have been investigated in different exposures to identify the seasonal freezing depth and length, for determination of the influence of potential ice segregation processes causing weathering of the bedrock surfaces. The data show generally that that the bedrock surface is in the -3 to -8C freezing window for 3 to 6 months. Likewise 250 cm deep bedrock thermal monitoring have been carried out in three boreholes during one year at 900 m, 800 m and 625 m asl. extending over the area from the upper part of the unstable area and into the stable area above, for determination of the regional permafrost zone. These results in combination with thermal evidence from other deeper boreholes from the same setting in the same region show that seasonal freezing extends 5-10 m down, and that a potential active layer also is in the order of 5-10 m deep. The air temperatures in the cracks show significant cooling during winter, when the cracks have a thick snow cover, thus demonstrating the potential existence of permafrost in deeper part of the cracks and in the ground just around these. Automatic photography has been used for the last 4 years to study the seasonal snow cover duration and thickness in the open cracks. This shows that a thicker snow cover only develops in mid winter, with maximum amounts of snow in March and April, but also that not all snow melts during summer in the deeper parts of the open cracks. In addition we have found small pockets of ice in closed spaces of the bottom parts of the open crack, indicating the presence of permafrost. The combination of thermal data and the special seasonal variation in the rockslide deformations indicate that most likely refreezing of snow meltwater goes on in the open cracks for a considerable period from late summer, autumn and into the early winter, when the recorded rockslide deformation is largest. In late winter no significant deformation is recorded when the ground is constantly frozen, but there is a significant potential for ice segregation to occur where moisture is present in the rock.

Stable tearing behavior of a thin-sheet material with multiple cracks

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Newman, J. C., Jr.; Sutton, M. A.; Amstutz, B. E.

1994-01-01

Fracture tests were conducted on 2.3mm thick, 305mm wide sheets of 2024-T3 aluminum alloy with 1-5 collinear cracks. The cracks were introduced (crack history) into the specimens by three methods: (1) saw cutting; (2) fatigue precracking at a low stress range; and (3) fatigue precracking at a high stress range. For the single crack tests, the initial crack history influenced the stress required for the onset of stable crack growth and the first 10mm of crack growth. The effect on failure stress was about 4 percent or less. For the multiple crack tests, the initial crack history was shown to cause differences of more than 20 percent in the link-up stress and 13 percent in failure stress. An elastic-plastic finite element analysis employing the Crack Tip Opening Angle (CTOA) fracture criterion was used to predict the fracture behavior of the single and multiple crack tests. The numerical predictions were within 7 percent of the observed link-up and failure stress in all the tests.

Deformation mechanics of deep surface flaw cracks

NASA Technical Reports Server (NTRS)

Francis, P. H.; Nagy, A.; Beissner, R. E.

1972-01-01

A combined analytical and experimental program was conducted to determine the deformation characteristics of deep surface cracks in Mode I loading. An approximate plane finite element analysis was performed to make a parameter study on the influence of crack depth, crack geometry, and stress level on plastic zones, crack opening displacement, and back surface dimpling in Fe-3Si steel and 2219-T87 aluminum. Surface replication and profiling techniques were used to examine back surface dimple configurations in 2219-T87 aluminum. Interferometry and holography were used to evaluate the potential of various optical techniques to detect small surface dimples on large surface areas.

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

Nour, Ali, E-mail: ali.nour@polymtl.ca; Hydro Quebec, Montreal, Quebec, H2L 4P5; Massicotte, Bruno

This study is aimed at proposing a simple analytical model to investigate the post-cracking behaviour of FRC panels, using an arbitrary tension softening, stress crack opening diagram, as the input. A new relationship that links the crack opening to the panel deflection is proposed. Due to the stochastic nature of material properties, the random fibre distribution, and other uncertainties that are involved in concrete mix, this relationship is developed from the analysis of beams having the same thickness using the Monte Carlo simulation (MCS) technique. The softening diagrams obtained from direct tensile tests are used as the input for themore » calculation, in a deterministic way, of the mean load displacement response of round panels. A good agreement is found between the model predictions and the experimental results.« less

The Dugdale model for the compact specimen

NASA Technical Reports Server (NTRS)

Mall, S.; Newman, J. C., Jr.

1983-01-01

Plastic zone size and crack tip opening displacement (CTOD) equations were developed. Boundary collocation analyses were used to analyze the compact specimen subjected to various loading conditions (pin loads, concentrated forces, and uniform pressure acting on the crack surface). Stress intensity factor and crack surface displacement equations for some of these loadings were developed and used to obtain the Dugdale model. The results from the equations for plastic zone size and CTOD agreed well with numerical values calculated by Terada for crack length to width ratios greater than 0.4.

High-Temperature Intergranular Crack Growth in Martensitic 2-1/4 Cr1Mo Steel,

DTIC Science & Technology

1987-01-01

segregation of sulphur to crack-tip regions. Crack advance appears to occur by discrete jumps when a critical concentration of sulphur is achieved over the...jump-distance. At high stress intensities, reater than 48-55 HPam ,-the mo.e of fracture changes to interranular microvoid coalescence (IGMVC), and is...stze of crack opening displacement (5) at 500C. using 6 K(! - v2 )/20 E, where v - 0.3, 0 - 840 MPs and E = 160 GPa --6) ’ 27 7 Equilibriua concentration

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.

ZIP3D: An elastic and elastic-plastic finite-element analysis program for cracked bodies

NASA Technical Reports Server (NTRS)

Shivakumar, K. N.; Newman, J. C., Jr.

1990-01-01

ZIP3D is an elastic and an elastic-plastic finite element program to analyze cracks in three dimensional solids. The program may also be used to analyze uncracked bodies or multi-body problems involving contacting surfaces. For crack problems, the program has several unique features including the calculation of mixed-mode strain energy release rates using the three dimensional virtual crack closure technique, the calculation of the J integral using the equivalent domain integral method, the capability to extend the crack front under monotonic or cyclic loading, and the capability to close or open the crack surfaces during cyclic loading. The theories behind the various aspects of the program are explained briefly. Line-by-line data preparation is presented. Input data and results for an elastic analysis of a surface crack in a plate and for an elastic-plastic analysis of a single-edge-crack-tension specimen are also presented.

Matrix crack extension at a frictionally constrained fiber

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

Selvadurai, A.P.S.

1994-07-01

The paper presents the application of a boundary element scheme to the study of the behavior of a penny-shaped matrix crack which occurs at an isolated fiber which is frictionally constrained. An incremental technique is used to examine the progression of self similar extension of the matrix crack due to the axial straining of the composite region. The extension of the crack occurs at the attainment of the critical stress intensity factor in the crack opening mode. Iterative techniques are used to determine the extent to crack enlargement and the occurrence of slip and locked regions in the frictional fiber-matrixmore » interface. The studies illustrate the role of fiber-matrix interface friction on the development of stable cracks in such frictionally constrained zones. The methodologies are applied to typical isolated fiber configurations of interest to fragmentation tests.« less

Impact initiation of explosives and propellants via statistical crack mechanics

NASA Astrophysics Data System (ADS)

Dienes, J. K.; Zuo, Q. H.; Kershner, J. D.

2006-06-01

A statistical approach has been developed for modeling the dynamic response of brittle materials by superimposing the effects of a myriad of microcracks, including opening, shear, growth and coalescence, taking as a starting point the well-established theory of penny-shaped cracks. This paper discusses the general approach, but in particular an application to the sensitivity of explosives and propellants, which often contain brittle constituents. We examine the hypothesis that the intense heating by frictional sliding between the faces of a closed crack during unstable growth can form a hot spot, causing localized melting, ignition, and fast burn of the reactive material adjacent to the crack. Opening and growth of a closed crack due to the pressure of burned gases inside the crack and interactions of adjacent cracks can lead to violent reaction, with detonation as a possible consequence. This approach was used to model a multiple-shock experiment by Mulford et al. [1993. Initiation of preshocked high explosives PBX-9404, PBX-9502, PBX-9501, monitored with in-material magnetic gauging. In: Proceedings of the 10th International Detonation Symposium, pp. 459-467] involving initiation and subsequent quenching of chemical reactions in a slab of PBX 9501 impacted by a two-material flyer plate. We examine the effects of crack orientation and temperature dependence of viscosity of the melt on the response. Numerical results confirm our theoretical finding [Zuo, Q.H., Dienes, J.K., 2005. On the stability of penny-shaped cracks with friction: the five types of brittle behavior. Int. J. Solids Struct. 42, 1309-1326] that crack orientation has a significant effect on brittle behavior, especially under compressive loading where interfacial friction plays an important role. With a reasonable choice of crack orientation and a temperature-dependent viscosity obtained from molecular dynamics calculations, the calculated particle velocities compare well with those measured using embedded velocity gauges.

Stability analysis and backward whirl investigation of cracked rotors with time-varying stiffness

NASA Astrophysics Data System (ADS)

AL-Shudeifat, Mohammad A.

2015-07-01

The dynamic stability of dynamical systems with time-periodic stiffness is addressed here. Cracked rotor systems with time-periodic stiffness are well-known examples of such systems. Time-varying area moments of inertia at the cracked element cross-section of a cracked rotor have been used to formulate the time-periodic finite element stiffness matrix. The semi-infinite coefficient matrix obtained by applying the harmonic balance (HB) solution to the finite element (FE) equations of motion is employed here to study the dynamic stability of the system. Consequently, the sign of the determinant of a scaled version of a sub-matrix of this semi-infinite coefficient matrix at a finite number of harmonics in the HB solution is found to be sufficient for identifying the major unstable zones of the system in the parameter plane. Specifically, it is found that the negative determinant always corresponds to unstable zones in all of the systems considered. This approach is applied to a parametrically excited Mathieu's equation, a two degree-of-freedom linear time-periodic dynamical system, a cracked Jeffcott rotor and a finite element model of the cracked rotor system. Compared to the corresponding results obtained by Floquet's theory, the sign of the determinant of the scaled sub-matrix is found to be an efficient tool for identifying the major unstable zones of the linear time-periodic parametrically excited systems, especially large-scale FE systems. Moreover, it is found that the unstable zones for a FE cracked rotor with an open transverse crack model only appear at the backward whirl. The theoretical and experimental results have been found to agree well for verifying that the open crack model excites the backward whirl amplitudes at the critical backward whirling rotational speeds.

Characterization of crack growth under combined loading

NASA Technical Reports Server (NTRS)

Feldman, A.; Smith, F. W.; Holston, A., Jr.

1977-01-01

Room-temperature static and cyclic tests were made on 21 aluminum plates in the shape of a 91.4x91.4-cm Maltese cross with 45 deg flaws to develop crack growth and fracture toughness data under mixed-mode conditions. During cyclic testing, it was impossible to maintain a high proportion of shear-mode deformation on the crack tips. Cracks either branched or turned. Under static loading, cracks remained straight if shear stress intensity exceeded normal stress intensity. Mixed-mode crack growth rate data compared reasonably well with published single-mode data, and measured crack displacements agreed with the straight and branched crack analyses. Values of critical strain energy release rate at fracture for pure shear were approximately 50% higher than for pure normal opening, and there was a large reduction in normal stress intensity at fracture in the presence of high shear stress intensity. Net section stresses were well into the inelastic range when fracture occurred under high shear on the cracks.

Experimental and Numerical Analysis of Fracture in 41Cr4 Steel - Issues of the Stationary Cracks

NASA Astrophysics Data System (ADS)

Graba, M.

2018-02-01

This paper analyzes the process of fracture in 41Cr4 steel on the basis of experimental and numerical data obtained for non-propagating cracks. The author's previous and latest experimental results were used to determine the apparent crack initiation moment and fracture toughness for the material under plane strain conditions. Numerical simulations were carried out to assess changes in the J-integral, the crack tip opening displacement, the size of the plastic region and the distribution of stresses around the crack tip. A complex numerical analysis based on the true stress-strain curve was performed to determine the behavior of 41Cr4 steel under increasing external loads.

Elevated Temperature Crack Propagation

NASA Technical Reports Server (NTRS)

Orange, Thomas W.

1994-01-01

This paper is a summary of two NASA contracts on high temperature fatigue crack propagation in metals. The first evaluated the ability of fairly simple nonlinear fracture parameters to correlate crack propagation. Hastelloy-X specimens were tested under isothermal and thermomechanical cycling at temperatures up to 980 degrees C (1800 degrees F). The most successful correlating parameter was the crack tip opening displacement derived from the J-integral. The second evaluated the ability of several path-independent integrals to correlate crack propagation behavior. Inconel 718 specimens were tested under isothermal, thermomechanical, temperature gradient, and creep conditions at temperatures up to 650 degrees C (1200 degrees F). The integrals formulated by Blackburn and by Kishimoto correlated the data reasonably well under all test conditions.

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.

Fracture of ECAP-deformed iron and the role of extrinsic toughening mechanisms

PubMed Central

Hohenwarter, A.; Pippan, R.

2013-01-01

The fracture behaviour of pure iron deformed by equal-channel angular pressing via route A was examined. The fracture toughness was determined for different specimen orientations and measured in terms of the critical plane strain fracture toughness, KIC, the critical J integral, JIC, and the crack opening displacement for crack initiation, CODi. The results demonstrate that the crack plane orientation has a pronounced effect on the fracture toughness. Different crack plane orientations lead to either crack deflection or delamination, resulting in increased fracture resistance in comparison to one remarkably weak specimen orientation. The relation between the microstructure typical for the applied deformation route and the enormous differences in the fracture toughness depending on the crack plane orientation will be analyzed in this paper. PMID:23645995

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 ...

Study of Solidification Cracking in a Transformation-Induced Plasticity-Aided Steel

NASA Astrophysics Data System (ADS)

Agarwal, G.; Kumar, A.; Gao, H.; Amirthalingam, M.; Moon, S. C.; Dippenaar, R. J.; Richardson, I. M.; Hermans, M. J. M.

2018-04-01

In situ high-temperature laser scanning confocal microscopy is applied to study solidification cracking in a TRIP steel. Solidification cracking was observed in the interdendritic region during the last stage of solidification. Atom probe tomography revealed notable enrichment of phosphorus in the last remaining liquid. Phase field simulations also confirm phosphorus enrichment leading to severe undercooling of more than 160 K in the interdendritic region. In the presence of tensile stress, an opening at the interdendritic region is difficult to fill with the remaining liquid due to low permeability and high viscosity, resulting in solidification cracking.

Fracture analysis of stiffened panels under biaxial loading with widespread cracking

NASA Technical Reports Server (NTRS)

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

1995-01-01

An elastic-plastic finite-element analysis with a critical crack-tip-opening angle (CTOA) fracture criterion was used to model stable crack growth and fracture of 2024-T3 aluminum alloy (bare and clad) panels for several thicknesses. The panels had either single or multiple-site damage (MSD) cracks subjected to uniaxial or biaxial loading. Analyses were also conducted on cracked stiffened panels with single or MSD cracks. The critical CTOA value for each thickness was determined by matching the failure load on a middle-crack tension specimen. Comparisons were made between the critical angles determined from the finite-element analyses and those measured with photographic methods. Predicted load-against-crack extension and failure loads for panels under biaxial loading, panels with MSD cracks, and panels with various number of stiffeners were compared with test data, whenever possible. The predicted results agreed well with the test data even for large-scale plastic deformations. The analyses were also able to predict stable tearing behavior of a large lead crack in the presence of MSD cracks. The analyses were then used to study the influence of stiffeners on residual strength in the presence of widespread fatigue cracking. Small MSD cracks were found to greatly reduce the residual strength for large lead cracks even for stiffened panels.

Fracture analysis of stiffened panels under biaxial loading with widespread cracking

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.

1995-01-01

An elastic-plastic finite-element analysis with a critical crack-tip opening angle (CTOA) fracture criterion was used to model stable crack growth and fracture of 2024-T3 aluminum alloy (bare and clad) panels for several thicknesses. The panels had either single or multiple-site damage (MSD) cracks subjected to uniaxial or biaxial loading. Analyses were also conducted on cracked stiffened panels with single or MSD cracks. The critical CTOA value for each thickness was determined by matching the failure load on a middle-crack tension specimen. Comparisons were made between the critical angles determined from the finite-element analyses and those measured with photographic methods. Predicted load-against-crack extension and failure loads for panels under biaxial loading, panels with MSD cracks, and panels with various numbers of stiffeners were compared with test data whenever possible. The predicted results agreed well with the test data even for large-scale plastic deformations. The analyses were also able to predict stable tearing behavior of a large lead crack in the presence of MSD cracks. The analyses were then used to study the influence of stiffeners on residual strength in the presence of widespread fatigue cracking. Small MSD cracks were found to greatly reduce the residual strength for large lead cracks even for stiffened panels.

Continuum Damage Mechanics Models for the Analysis of Progressive Failure in Open-Hole Tension Laminates

NASA Technical Reports Server (NTRS)

Song, Kyonchan; Li, Yingyong; Rose, Cheryl A.

2011-01-01

The performance of a state-of-the-art continuum damage mechanics model for interlaminar damage, coupled with a cohesive zone model for delamination is examined for failure prediction of quasi-isotropic open-hole tension laminates. Limitations of continuum representations of intra-ply damage and the effect of mesh orientation on the analysis predictions are discussed. It is shown that accurate prediction of matrix crack paths and stress redistribution after cracking requires a mesh aligned with the fiber orientation. Based on these results, an aligned mesh is proposed for analysis of the open-hole tension specimens consisting of different meshes within the individual plies, such that the element edges are aligned with the ply fiber direction. The modeling approach is assessed by comparison of analysis predictions to experimental data for specimen configurations in which failure is dominated by complex interactions between matrix cracks and delaminations. It is shown that the different failure mechanisms observed in the tests are well predicted. In addition, the modeling approach is demonstrated to predict proper trends in the effect of scaling on strength and failure mechanisms of quasi-isotropic open-hole tension laminates.

Experimental simulation of frost wedging-induced crack propagation in alpine rockwall

NASA Astrophysics Data System (ADS)

Jia, Hailiang; Leith, Kerry; Krautblatter, Michael

2016-04-01

Frost wedging is widely presumed to be the principal mechanism responsible for shattering jointed low-porosity rocks in high alpine rockwalls. The interaction of ice and rock physics regulates the efficacy of frost wedging. In order to better understand temporal aspects of this interaction, we present results of a series of laboratory experiments monitoring crack widening as a result of ice formation in an artificial crack (4mm wide, 80mm deep) cut 20 mm from the end of a rectangular granite block. Our results indicate that i) freezing direction plays a key role in determining the magnitude of crack widening; in short-term (1 day) experiments, maximum crack widening during top-down freezing (associated with 'autumn' conditions) was around 0.11mm, while inside-out freezing (resulting from 'spring' conditions) produced only 0.02 mm of deformation; ii) neither ice, nor water pressure (direct tension and hydraulic fracturing respectively) caused measurable irreversible crack widening during short-term tests, as the calculated maximum stress intensity at the crack tip was less than the fracture toughness of our granite sample; iii) development of ice pressure is closely related to the mechanical properties of the fracture in which it forms, and as such, the interaction of ice and rock is intrinsically dynamic; iv) irreversible crack widening (about 0.03mm) was only observed following a long-term (53 day) experiment representing a simplified transition from autumn to winter conditions. We suggest this is the result of stress corrosion aided by strong opening during freezing, and to a lesser degree by ice segregation up to one week after the initial freezing period, and downward migration of liquid water during the remainder of the test. Our results suggest the fundamental assumption of frost wedging, that rapid freezing from open ends of cracks can seal water inside the crack and thus cause damage through excessive stresses induced by volumetric expansion seems questionable.

Post-cracking characteristics of high performance fiber reinforced cementitious composites

NASA Astrophysics Data System (ADS)

Suwannakarn, Supat W.

The application of high performance fiber reinforced cement composites (HPFRCC) in structural systems depends primarily on the material's tensile response, which is a direct function of fiber and matrix characteristics, the bond between them, and the fiber content or volume fraction. The objective of this dissertation is to evaluate and model the post-cracking behavior of HPFRCC. In particular, it focused on the influential parameters controlling tensile behavior and the variability associated with them. The key parameters considered include: the stress and strain at first cracking, the stress and strain at maximum post-cracking, the shape of the stress-strain or stress-elongation response, the multiple cracking process, the shape of the resistance curve after crack localization, the energy associated with the multiple cracking process, and the stress versus crack opening response of a single crack. Both steel fibers and polymeric fibers, perceived to have the greatest potential for current commercial applications, are considered. The main variables covered include fiber type (Torex, Hooked, PVA, and Spectra) and fiber volume fraction (ranging from 0.75% to 2.0%). An extensive experimental program is carried out using direct tensile tests and stress-versus crack opening displacement tests on notched tensile prisms. The key experimental results were analysed and modeled using simple prediction equations which, combined with a composite mechanics approach, allowed for predicting schematic simplified stress-strain and stress-displacement response curves for use in structural modeling. The experimental data show that specimens reinforced with Torex fibers performs best, follows by Hooked and Spectra fibers, then PVA fibers. Significant variability in key parameters was observed througout suggesting that variability must be studied further. The new information obtained can be used as input for material models for finite element analysis and can provide greater confidence in using the HPFRC composites in structural applications. It also provides a good foundation to integrate these composites in conventional structural analysis and design.

Corrosion-Fatigue Cracking in HY-80 and HY-130 Steels

DTIC Science & Technology

2015-01-22

Naval Research Laboratory Washington, DC 20375-5320 NRL/MR/6355--15-9584 Corrosion- Fatigue Cracking in HY-80 and HY-130 Steels January 22, 2015 P.S...RESPONSIBLE PERSON 19b. TELEPHONE NUMBER (include area code) b. ABSTRACT c. THIS PAGE 18. NUMBER OF PAGES 17. LIMITATION OF ABSTRACT Corrosion- Fatigue ...including [NaCl] concentration) and load ratio on fatigue crack growth kinetics of HY-80 and HY-130 steels. Fracture mechanics wedge-opening-load

Information technology: opening the box.

PubMed

Nussbaum, G M

1998-09-01

If you thought managed care was a tough nut to crack, wait until you have to start making decisions about your organization's information technology (IT). Information systems are complex and expensive, they can take years to implement, and, once installed, they need costly and regular upgrades. But for a contemporary clinical organization to function, this technology is as essential as power and water. For many years, information technology was seen as a black box, impenetrable and beyond real understanding. If done with knowledge and care, however, cracking the box opens up possibilities, not ruin.

Desiccation of a Sessile Drop of Blood: Cracks Formation and Delamination

NASA Astrophysics Data System (ADS)

Sobac, Benjamin; Brutin, David

2011-11-01

The evaporation of drops of biological fluids has been studied since few years du to several applications in medical fields such as medical tests, drug screening, biostabilization... The evaporation of a drop of whole blood leads to the formation of final typical pattern of cracks. Flow motion, adhesion, gelation and fracturation all occur during the evaporation of this complex matter. During the drying, a sol-gel transition develops. The drying kinetics is explained by a simple model of evaporation taking account of the evolution of the gelation front. The system solidifies and when stresses are too important, cracks nucleate. The cracks formation and the structure of the crack pattern are investigated. The initial crack spacing is found in good agreement with the implementation in open geometry of the model of cracks formation induced by evaporation proposed by Allain and Limat. Finally, the drop is still drying after the end of the formation of cracks which leads, like in the situation of colloid suspensions, to the observation of a delamination phenomenon.

Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy.

PubMed

Sloof, Willem G; Pei, Ruizhi; McDonald, Samuel A; Fife, Julie L; Shen, Lu; Boatemaa, Linda; Farle, Ann-Sophie; Yan, Kun; Zhang, Xun; van der Zwaag, Sybrand; Lee, Peter D; Withers, Philip J

2016-03-14

MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment. The Ti2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. For the first time, the rate and position dependence of crack repair in pristine Ti2AlC material and in previously healed cracks has been quantified. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing.

Simultaneous Measurements of Harmonic Waves at Fatigue-Cracked Interfaces

NASA Astrophysics Data System (ADS)

Hyunjo, Jeong; Dan, Barnard

2011-08-01

Nonlinear harmonic waves generated at cracked interfaces are investigated theoretically and experimentally. A compact tension specimen is fabricated and the amplitude of the transmitted wave is analyzed as a function of position along the fatigued crack surface. In order to measure as many nonlinear harmonic components as possible, broadband lithium niobate (LiNbO3) transducers are employed together with a calibration technique for making absolute amplitude measurements with fluid-coupled receiving transducers. Cracked interfaces are shown to generate high acoustic nonlinearities, which are manifested as harmonics in the power spectrum of the received signal. The first subharmonic f/2 and the second harmonic 2f waves are found to be dominant nonlinear components for an incident toneburst signal of frequency f. To explain the observed nonlinear behavior, a partially closed crack is modeled by planar half interfaces that can account for crack parameters, such as crack opening displacement and crack surface conditions. The simulation results show reasonable agreement with the experimental results.

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.

Short-crack growth behaviour in an aluminum alloy: An AGARD cooperative test program

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Edwards, P. R.

1988-01-01

An AGARD Cooperative Test Program on the growth of short fatigue cracks was conducted to define the significance of the short-crack effect, to compare test results from various laboratories, and to evaluate an existing analytical crack-growth prediction model. The initiation and growth of short fatigue cracks (5 micrometer to 2 mm) from the surface of a semi-circular notch in 2024-T3 aluminum alloy sheet material were monitored under various load histories. The cracks initiated from inclusion particle clusters or voids on the notch surface and generally grew as surface cracks. Tests were conducted under several constant-amplitude (stress ratios of -2, -1, 0, and 0.5) and spectrum (FALSTAFF and Gaussian) loading conditions at 3 stress levels each. Short crack growth was recorded using a plastic-replica technique. Over 250 edge-notched specimens were fatigue tested and nearly 950 cracks monitored by 12 participants from 9 countries. Long crack-growth rate data for cracks greater than 2 mm in length were obtained over a wide range in rates (10 to the -8 to 10 to the -1 mm/cycle) for all constant-amplitude loading conditions. Long crack-growth rate data for the FALSTAFF and Gaussian load sequences were also obtained.

Heterogeneous kinetic modeling of the catalytic conversion of cycloparaffins

NASA Astrophysics Data System (ADS)

Al-Sabawi, Mustafa N.

The limited availability of high value light hydrocarbon feedstocks along with the rise in crude prices has resulted in the international recognition of the vast potential of Canada's oil sands. With the recent expansion of Canadian bitumen production come, however, many technical challenges, one of which is the significant presence of aromatics and cycloparaffins in bitumen-derived feedstocks. In addition to their negative environmental impact, aromatics limit fluid catalytic cracking (FCC) feedstock conversion, decrease the yield and quality of valuable products such as gasoline and middle distillates, increase levels of polyaromatic hydrocarbons prone to form coke on the catalyst, and ultimately compromise the FCC unit performance. Although cycloparaffins do not have such negative impacts, they are precursors of aromatics as they frequently undergo hydrogen transfer reactions. However, cycloparaffin cracking chemistry involves other competing reactions that are complex and need much investigation. This dissertation provides insights and understanding of the fundamentals of the catalytic cracking of cycloparaffins using carefully selected model compounds such as methylcyclohexane (MCH) and decalin. Thermal and catalytic cracking of these cycloparaffins on FCC-type catalysts are carried out using the CREC Riser Simulator under operating conditions similar to those of the industrial FCC units in terms of temperature, reaction time, reactant partial pressure and catalyst-to-hydrocarbon ratio. The crystallite size of the supported zeolites is varied between 0.4 and 0.9 microns, with both activity and selectivity being monitored. Catalytic conversions ranged between 4 to 16 wt% for MCH and between 8 to 27 wt% for decalin. Reaction pathways of cycloparaffins are determined, and these include ring-opening, protolytic cracking, isomerization, hydrogen transfer and transalkylation. The yields and selectivities of over 60 and 140 products, formed during MCH and decalin catalytic conversions respectively, are reported. Using these data, heterogeneous kinetic models accounting for intracrystallite molecular transport, adsorption and thermal and catalytic cracking of both cycloparaffin reactants are established. Results show that undesirable hydrogen transfer reactions are more pronounced and selectively favoured against other reactions at lower reaction temperatures, while the desirable ring-opening and cracking reactions predominate at the higher reaction temperatures. Moreover, results of the present work show that while crystallite size may have an effect on the overall conversion in some situations, there is a definite effect on the selectivity of products obtained during the cracking of MCH and decalin and the cracking of MCH in a mixture with co-reactants such as 1,3,5-triisopropylbenzene. Keywords. cycloparaffins, naphthenes, fluid catalytic cracking, kinetic modeling, Y-zeolites, diffusion, adsorption, ring-opening, hydrogen transfer, catalyst selectivity.

An experimental study on fatigue performance of cryogenic metallic materials for IMO type B tank

NASA Astrophysics Data System (ADS)

Lee, Jin-Sung; You, Won-Hyo; Yoo, Chang-Hyuk; Kim, Kyung-Su; Kim, Yooil

2013-12-01

Three materials SUS304, 9% Ni steel and Al 5083-O alloy, which are considered possible candidate for International Maritime Organization (IMO) type B Cargo Containment System, were studied. Monotonic tensile, fatigue, fatigue crack growth rate and Crack Tip Opening Displacement tests were carried out at room, intermediate low (-100 °C) and cryogenic (-163 °C) temperatures. The initial yield and tensile strengths of all materials tended to increase with decreasing temperature, whereas the change in elastic modulus was not as remarkable. The largest and smallest improvement ratio of the initial yield strengths due to a temperature reduction were observed in the SUS304 and Al 5083- O alloy, respectively. The fatigue strengths of the three materials increased with decreasing temperature. The largest increase in fatigue strength was observed in the Al 5083-O alloy, whereas the 9% Ni steel sample showed the smallest increase. In the fatigue crack growth rate test, SUS304 and Al 5083-O alloy showed a decrease in the crack propagation rate, due to decrease in temperature, but no visible improvement in da/dN was observed in the case of 9% Ni steel. In the Crack Tip Opening Displacement (CTOD) test, CTOD values were converted to critical crack length for the comparison with different thickness specimens. The critical crack length tended to decrease in the case of SUS304 and increase for the Al 5083-O alloy with decreasing temperature. In case of 9% Ni steel, change of critical crack length was not observed due to temperature decrease. In addition, the changing material properties according to the temperature of the LNG tank were analyzed according to the international code for the construction and equipment of ships carrying liquefied gases in bulk (IGC code) and the rules of classifications.

Experimental Study on the Growth, Coalescence and Wrapping Behaviors of 3D Cross-Embedded Flaws Under Uniaxial Compression

NASA Astrophysics Data System (ADS)

Zhou, Xiao-Ping; Zhang, Jian-Zhi; Wong, Louis Ngai Yuen

2018-05-01

The crack initiation, growth, wrapping and coalescence of two 3D pre-existing cross-embedded flaws in PMMA specimens under uniaxial compression are investigated. The stress-strain curves of PMMA specimens with 3D cross-embedded flaws are obtained. The tested PMMA specimens exhibit dominant elastic deformation and eventual brittle failure. The experimental results show that four modes of crack initiation and five modes of crack coalescence are observed. The initiations of oblique secondary crack and anti-wing crack in 3D cracking behaviors are first reported as well as the coalescence of anti-wing cracks. Moreover, two types of crack wrapping are found. Substantial wrapping of petal cracks, which includes open and closed modes of wrapping, appears to be the major difference between 2D and 3D cracking behaviors of pre-existing flaws, which are also first reported. Petal crack wraps symmetrically from either the propagated wing cracks or the coalesced wing cracks. Besides, only limited growth of petal cracks is observed, and ultimate failure of specimens is induced by the further growth of the propagated wing crack. The fracture mechanism of the tested PMMA specimens is finally revealed. In addition, the initiation stress and the peak stress versus the geometry of two 3D pre-existing cross-embedded flaws are also investigated in detail.

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.

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

Residual Strength Pressure Tests and Nonlinear Analyses of Stringer- and Frame-Stiffened Aluminum Fuselage Panels with Longitudinal Cracks

NASA Technical Reports Server (NTRS)

Young, Richard D.; Rouse, Marshall; Ambur, Damodar R.; Starnes, James H., Jr.

1999-01-01

The results of residual strength pressure tests and nonlinear analyses of stringer- and frame-stiffened aluminum fuselage panels with longitudinal cracks are presented. Two types of damage are considered: a longitudinal crack located midway between stringers, and a longitudinal crack adjacent to a stringer and along a row of fasteners in a lap joint that has multiple-site damage (MSD). In both cases, the longitudinal crack is centered on a severed frame. The panels are subjected to internal pressure plus axial tension loads. The axial tension loads are equivalent to a bulkhead pressure load. Nonlinear elastic-plastic residual strength analyses of the fuselage panels are conducted using a finite element program and the crack-tip-opening-angle (CTOA) fracture criterion. Predicted crack growth and residual strength results from nonlinear analyses of the stiffened fuselage panels are compared with experimental measurements and observations. Both the test and analysis results indicate that the presence of MSD affects crack growth stability and reduces the residual strength of stiffened fuselage shells with long cracks.

Residual Strength Pressure Tests and Nonlinear Analyses of Stringer-and Frame-Stiffened Aluminum Fuselage Panels with Longitudinal Cracks

NASA Technical Reports Server (NTRS)

Young, Richard D.; Rouse, Marshall; Ambur, Damodar R.; Starnes, James H., Jr.

1998-01-01

The results of residual strength pressure tests and nonlinear analyses of stringer- and frame-stiffened aluminum fuselage panels with longitudinal cracks are presented. Two types of damage are considered: a longitudinal crack located midway between stringers, and a longitudinal crack adjacent to a stringer and along a row of fasteners in a lap joint that has multiple-site damage (MSD). In both cases, the longitudinal crack is centered on a severed frame. The panels are subjected to internal pressure plus axial tension loads. The axial tension loads are equivalent to a bulkhead pressure load. Nonlinear elastic-plastic residual strength analyses of the fuselage panels are conducted using a finite element program and the crack-tip-opening-angle (CTOA) fracture criterion. Predicted crack growth and residual strength results from nonlinear analyses of the stiffened fuselage panels are compared with experimental measurements and observations. Both the test and analysis results indicate that the presence of MSD affects crack growth stability and reduces the residual strength of stiffened fuselage shells with long cracks.

Geometrically nonlinear analysis of adhesively bonded joints

NASA Technical Reports Server (NTRS)

Dattaguru, B.; Everett, R. A., Jr.; Whitcomb, J. D.; Johnson, W. S.

1982-01-01

A geometrically nonlinear finite element analysis of cohesive failure in typical joints is presented. Cracked-lap-shear joints were chosen for analysis. Results obtained from linear and nonlinear analysis show that nonlinear effects, due to large rotations, significantly affect the calculated mode 1, crack opening, and mode 2, inplane shear, strain-energy-release rates. The ratio of the mode 1 to mode 2 strain-energy-relase rates (G1/G2) was found to be strongly affected by he adhesive modulus and the adherend thickness. The ratios between 0.2 and 0.8 can be obtained by varying adherend thickness and using either a single or double cracked-lap-shear specimen configuration. Debond growth rate data, together with the analysis, indicate that mode 1 strain-energy-release rate governs debond growth. Results from the present analysis agree well with experimentally measured joint opening displacements.

A continuous vibration theory for rotors with an open edge crack

NASA Astrophysics Data System (ADS)

Ebrahimi, Alireza; Heydari, Mahdi; Behzad, Mehdi

2014-07-01

In this paper a new continuous model for flexural vibration of rotors with an open edge crack has been developed. The cracked rotor is considered in the rotating coordinate system attached to it. Therefore, the rotor bending can be decomposed in two perpendicular directions. Two quasi-linear displacement fields are assumed for these two directions and the strain and stress fields are calculated in each direction. Then the final displacement and stress fields are obtained by composing the displacement and stress fields in the two directions. The governing equation of motion for the rotor has been obtained using the Hamilton principle and solved using a modified Galerkin method. The free vibration has been analyzed and the critical speeds have been calculated. Results are compared with the finite element results and an excellent agreement is observed.

Steam generator issues in the United States

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

Strosnider, J.R.

1997-02-01

Alloy 600 steam generator tubes in the US have exhibited degradation mechanisms similar to those observed in other countries. Effective programs have been implemented to address several degradation mechanisms including: wastage; mechanical wear; pitting; and fatigue. These degradation mechanisms are fairly well understood as indicated by the ability to effectively mitigate/manage them. Stress corrosion cracking (SCC) is the dominant degradation mechanism in the US. SCC poses significant inspection and management challenges to the industry and the regulators. The paper also addresses issues of research into SCC, inspection programs, plugging, repair strategies, water chemistry, and regulatory control. Emerging issues in themore » US include: parent tube cracking at sleeve joints; detection and repair of circumferential cracks; free span cracking; inspection and cracking of dented regions; and severe accident analysis.« less

Is there a crack epidemic among students in Brazil?: comments on media and public health issues.

PubMed

Nappo, Solange Aparecida; Sanchez, Zila M; Ribeiro, Luciana Abeid

2012-09-01

In the past year, the Brazilian Federal Government and society have reported and acted on a crack use epidemic, which has been exacerbated by the media. This study hypothesized that crack use has not increased at the rate suggested by the Brazilian media. A cross-sectional survey was carried out in 2010 using a multistage probabilistic representative sample of Brazilian middle and high school students in the country's 27 state capitals. A total of 50,890 valid questionnaires were weighted, analyzed and results compared to the 2004 national school survey dataset. Considering lifetime and past year crack use, no change in consumption was found between 2004 and 2010. Official data in Brazil on middle and high school students does not support the assertion of a crack epidemic widely publicized by the media. Government measures to treat and prevent crack use are encouraged; however, the term epidemic has been inappropriately used to represent the static prevalence of crack consumption among students.

Un Jalón, Un Volteón, y Otra Vez: High-Risk Crack Smoking Paraphernalia in México City.

PubMed

Valdez, Avelardo; Nowotny, Kathryn M; Negi, Nalini; Mora, Eduardo Zafra; Cepeda, Alice

2016-01-01

During the past decade, crack smoking has increased in Mexico among poor urban populations. Despite this increasing prevalence, little is known about the types of paraphernalia used and related sharing practices and physical harms. Data come from in-depth semi-structured interviews and observations with 156 current crack smokers in Mexico City. Findings reveal a complex, crack-smoking process in Mexico City that represents an interconnected structure of paraphernalia items and pipes that could contribute to detrimental health consequences. Specifically, we identify essential paraphernalia items that make the smoking of crack possible; describe the homemade construction of two categories of pipes; and detail the sharing practices and physical harms associated with these paraphernalia. Results point towards a smoking process that is embedded in impoverished urban neighborhoods sustained by an accessible street-level crack market. Discussed are the policy and intervention implications associated with reducing crack-related health consequences in Mexico and other Latin American countries.

Un Jalón, Un Volteón, y Otra Vez: High-Risk Crack Smoking Paraphernalia in México City

PubMed Central

Valdez, Avelardo; Cepeda, Alice; Nowotny, Kathryn M.; Mora, Eduardo Zafra; Negi, Nalini

2016-01-01

During the past decade, crack smoking has increased in Mexico among poor urban populations. Despite this increasing prevalence, little is known about the types of paraphernalia used and related sharing practices and physical harms. Data come from in-depth semi-structured interviews and observations with 156 current crack smokers in Mexico City. Findings reveal a complex, crack-smoking process in Mexico City that represents an interconnected structure of paraphernalia items and pipes that could contribute to detrimental health consequences. Specifically, we identify essential paraphernalia items that make the smoking of crack possible; describe the home- made construction of two categories of pipes; and the sharing practices and physical harms associated with these paraphernalia. Results point towards a smoking process that is embedded in impoverished urban neighborhoods sustained by an accessible street-level crack market. Discussed are the policy and intervention implications associated with reducing crack related health consequences in Mexico and other Latin American countries. PMID:27356211

Tectonic Processes on Europa: Tidal Stresses, Mechanical Response, and Visible Features

USGS Publications Warehouse

Greenberg, R.; Geissler, P.; Hoppa, G.; Tufts, B.R.; Durda, D.D.; Pappalardo, R.; Head, J.W.; Greeley, R.; Sullivan, R.; Carr, M.H.

1998-01-01

Europa's orbital eccentricity, driven by the resonance with Io and Ganymede, results in "diurnal" tides (3.5-day period) and possibly in nonsynchronous rotation. Both diurnal variation and nonsynchronous rotation can create significant stress fields on Europa's surface, and both effects may produce cracking. Patterns and time sequences of apparent tectonic features on Europa include lineaments that correlate with both sources of stress, if we take into account nonsynchronous rotation, after initial crack formation, by amounts ranging up to several tens of degrees. For example, the crosscutting time sequence of features in the Cadmus and Minos Linea region is consistent with a combined diurnal and nonsynchronous tensile-stress field, as it evolves during tens of degrees of nonsynchronous rotation. Constraints on the rotation rate from comparing Voyager and Galileo images show that significant rotation requires 104yr, but could be fast enough to have allowed significant rotation since the last global resurfacing, even if such resurfacing was as recent as a few million years ago. Once cracking is initiated, diurnal tides work cracks so that they open and close daily. Although the daily effect is small, over 105yr double ridges could plausibly be built along the cracks with sizes and morphologies consistent with observed structures, according to a model in which underlying liquid water fills the open cracks, partially freezes, and is extruded during the daily closing of the cracks. Thus, several lines of observational and theoretical evidence can be integrated if we assume nonsynchronous rotation and the existence of a liquid water layer. ?? 1998 Academic Press.

Compliance measurements of chevron notched four point bend specimen

NASA Technical Reports Server (NTRS)

Calomino, Anthony; Bubsey, Raymond; Ghosn, Louis J.

1994-01-01

The experimental stress intensity factors for various chevron notched four point bend specimens are presented. The experimental compliance is verified using the analytical solution for a straight through crack four point bend specimen and the boundary integral equation method for one chevron geometry. Excellent agreement is obtained between the experimental and analytical results. In this report, stress intensity factors, loading displacements and crack mouth opening displacements are reported for different crack lengths and different chevron geometries, under four point bend loading condition.

a 2d Model of Ultrasonic Testing for Cracks Near a Nonplanar Surface

NASA Astrophysics Data System (ADS)

Westlund, Jonathan; Boström, Anders

2010-02-01

2D P-SV elastic wave scattering by a crack near a non-planar surface is investigated. The wave scattering problem is solved in the frequency domain using a combination of the boundary element method (BEM) for the back surface displacement and a Fourier series expansion of the crack opening displacement (COD). The model accounts for the action of the transmitting and receiving ultrasonic contact probes, and the time traces are obtained by applying an inverse temporal Fourier transform.

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

Analysis of surface cracks at hole by a 3-D weight function method with stresses from finite element method

NASA Technical Reports Server (NTRS)

Zhao, W.; Newman, J. C., Jr.; Sutton, M. A.; Shivakumar, K. N.; Wu, X. R.

1995-01-01

Parallel with the work in Part-1, stress intensity factors for semi-elliptical surface cracks emanating from a circular hole are determined. The 3-D weight function method with the 3D finite element solutions for the uncracked stress distribution as in Part-1 is used for the analysis. Two different loading conditions, i.e. remote tension and wedge loading, are considered for a wide range in geometrical parameters. Both single and double surface cracks are studied and compared with other solutions available in the literature. Typical crack opening displacements are also provided.

Cracking the Code: Synchronizing Policy and Practice for Performance-Based Learning

ERIC Educational Resources Information Center

Patrick, Susan; Sturgis, Chris

2011-01-01

Performance-based learning is one of the keys to cracking open the assumptions that undergird the current educational codes, structures, and practices. By finally moving beyond the traditions of a time-based system, greater customized educational services can flourish, preparing more and more students for college and careers. This proposed policy…

78 FR 73457 - Airworthiness Directives; the Boeing Company Airplanes

Federal Register 2010, 2011, 2012, 2013, 2014

2013-12-06

... frequency eddy current inspections for cracking of the tension tie at BS 760 or 780, as applicable, and do... ties, including doing an open-hole high frequency eddy current inspection for cracks, as applicable...) and paragraph (i) of this AD, but not as AMOCs for the high frequency eddy current inspections...

Analysis of mixed-mode crack propagation using the boundary integral method

NASA Technical Reports Server (NTRS)

Mendelson, A.; Ghosn, L. J.

1986-01-01

Crack propagation in a rotating inner raceway of a high speed roller bearing is analyzed using the boundary integral equation method. The method consists of an edge crack in a plate under tension, upon which varying Hertzian stress fields are superimposed. A computer program for the boundary integral equation method was written using quadratic elements to determine the stress and displacement fields for discrete roller positions. Mode I and Mode II stress intensity factors and crack extension forces G sub 00 (energy release rate due to tensile opening mode) and G sub r0 (energy release rate due to shear displacement mode) were computed. These calculations permit determination of that crack growth angle for which the change in the crack extension forces is maximum. The crack driving force was found to be the alternating mixed-mode loading that occurs with each passage of the most heavily loaded roller. The crack is predicted to propagate in a step-like fashion alternating between radial and inclined segments, and this pattern was observed experimentally. The maximum changes DeltaG sub 00 and DeltaG sub r0 of the crack extension forces are found to be good measures of the crack propagation rate and direction.

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

Roos, E.; Maier, V.; Nagel, G.

The break preclusion concept is based on {open_quotes}KTA rules{close_quotes}, {open_quotes}RSK guidelines{close_quotes} and {open_quotes}Rahmenspeziflkation Basissicherheit{close_quotes}. These fundamental rules containing for example requirements on material, design, calculation, manufacturing and testing procedures are explained and the technical realisation is shown by means of examples. The proof of the quality of these piping systems can be executed by means of fracture mechanics calculations by showing that in every case the leakage monitoring system already detect cracks which are clearly smaller than the critical crack. Thus the leak before break behavior and the break preclusion concept is implicitly affirmed. In order to further diminish conservativitiesmore » in the fracture mechanics procedures, specific research projects are executed which are explained in this contribution.« less

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.

Time lapse photography as an approach to understanding glide avalanche activity

USGS Publications Warehouse

Hendrikx, Jordy; Peitzsch, Erich H.; Fagre, Daniel B.

2012-01-01

Avalanches resulting from glide cracks are notoriously difficult to forecast, but are a recurring problem for numerous avalanche forecasting programs. In some cases glide cracks are observed to open and then melt away in situ. In other cases, they open and then fail catastrophically as large, full-depth avalanches. Our understanding and management of these phenomena are currently limited. It is thought that an increase in the rate of snow gliding occurs prior to full-depth avalanche activity so frequent observation of glide crack movement can provide an index of instability. During spring 2011 in Glacier National Park, Montana, USA, we began an approach to track glide crack avalanche activity using a time-lapse camera focused on a southwest facing glide crack. This crack melted in-situ without failing as a glide avalanche, while other nearby glide cracks on north through southeast aspects failed. In spring 2012, a camera was aimed at a large and productive glide crack adjacent to the Going to the Sun Road. We captured three unique glide events in the field of view. Unfortunately, all of them either failed very quickly, or during periods of obscured view, so measurements of glide rate could not be obtained. However, we compared the hourly meteorological variables during the period of glide activity to the same variables prior to glide activity. The variables air temperature, relative humidity, air pressure, incoming and reflected long wave radiation, SWE, total precipitation, and snow depth were found to be statistically different for our cases examined. We propose that these are some of the potential precursors for glide avalanche activity, but do urge caution in their use, due to the simple approach and small data set size. It is hoped that by introducing a workable method to easily record glide crack movement, combined with ongoing analysis of the associated meteorological data, we will improve our understanding of when, or if, glide avalanche activity will ensue.

Stress intensity factors for part-elliptical cracks emanating from dimpled rivet holes

NASA Astrophysics Data System (ADS)

Wang, Ailun; She, Chongmin; Lin, Gang; Zhou, You; Guo, Wanlin

2014-11-01

Detailed investigations on the stress intensity factors (SIFs) for corner cracks emanated from interference fitted dimpled rivet holes are conducted using three-dimensional finite element method. The influences of the crack length a, elliptical shape factor t, far-end stress S and interference magnitude δ on the stress intensity factors are systematically studied. The SIFs for corner cracks emanated from open holes are also investigated for comparisons. An empirical formula of the normalized SIF is proposed by use of the least square method for convenience of the engineering application, which is a function of the crack length a, elliptical shape factor t, far-end stress S, interference magnitude δ and the normalized elliptical centrifugal angle φn. Based on the empirical formula, a crack growth simulation for a rivet filled hole is conducted, which shows a good agreement with the test data.

Stress analysis for structures with surface cracks

NASA Technical Reports Server (NTRS)

Bell, J. C.

1978-01-01

Two basic forms of analysis, one treating stresses around arbitrarily loaded circular cracks, the other treating stresses due to loads arbitrarily distributed on the surface of a half space, are united by a boundary-point least squares method to obtain analyses for stresses from surface cracks in places or bars. Calculations were for enough cases to show how effects from the crack vary with the depth-to-length ratio, the fractional penetration ratio, the obliquity of the load, and to some extent the fractional span ratio. The results include plots showing stress intensity factors, stress component distributions near the crack, and crack opening displacement patterns. Favorable comparisons are shown with two kinds of independent experiments, but the main method for confirming the results is by wide checking of overall satisfaction of boundary conditions, so that external confirmation is not essential. Principles involved in designing analyses which promote dependability of the results are proposed and illustrated.

Analysis of Fatigue Crack Paths in Cold Drawn Pearlitic Steel

PubMed Central

Toribio, Jesús; González, Beatriz; Matos, Juan-Carlos

2015-01-01

In this paper, a fracto-metallographic analysis was performed on the cracked specimens of cold drawn pearlitic steel subjected to fatigue tests. Fatigue cracks are transcollonial and exhibit a preference for fracturing pearlitic lamellae, with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and frequent local deflections that create microstructural roughness. At the micro-level, the cold drawn pearlitic steel exhibits higher micro-roughness than the hot rolled bar (this is a consequence of the manufacturing process by cold drawing), so that the actual fractured surface in the cold drawn wire is greater than that in the hot rolled bar, due to the fact that the crack deflection events are more frequent and with higher angle in the former (the heavily drawn prestressing steel wire). These findings show the relevant role on the manufacturing process by cold drawing in the fatigue crack propagation in pearlitic steel. PMID:28793647

Analysis of Fatigue Crack Paths in Cold Drawn Pearlitic Steel.

PubMed

Toribio, Jesús; González, Beatriz; Matos, Juan-Carlos

2015-11-04

In this paper, a fracto-metallographic analysis was performed on the cracked specimens of cold drawn pearlitic steel subjected to fatigue tests. Fatigue cracks are transcollonial and exhibit a preference for fracturing pearlitic lamellae, with non-uniform crack opening displacement values, micro-discontinuities, branchings, bifurcations and frequent local deflections that create microstructural roughness. At the micro-level, the cold drawn pearlitic steel exhibits higher micro-roughness than the hot rolled bar (this is a consequence of the manufacturing process by cold drawing), so that the actual fractured surface in the cold drawn wire is greater than that in the hot rolled bar, due to the fact that the crack deflection events are more frequent and with higher angle in the former (the heavily drawn prestressing steel wire). These findings show the relevant role on the manufacturing process by cold drawing in the fatigue crack propagation in pearlitic steel.

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.

Investigation of wing crack formation with a combined phase-field and experimental approach

NASA Astrophysics Data System (ADS)

Lee, Sanghyun; Reber, Jacqueline E.; Hayman, Nicholas W.; Wheeler, Mary F.

2016-08-01

Fractures that propagate off of weak slip planes are known as wing cracks and often play important roles in both tectonic deformation and fluid flow across reservoir seals. Previous numerical models have produced the basic kinematics of wing crack openings but generally have not been able to capture fracture geometries seen in nature. Here we present both a phase-field modeling approach and a physical experiment using gelatin for a wing crack formation. By treating the fracture surfaces as diffusive zones instead of as discontinuities, the phase-field model does not require consideration of unpredictable rock properties or stress inhomogeneities around crack tips. It is shown by benchmarking the models with physical experiments that the numerical assumptions in the phase-field approach do not affect the final model predictions of wing crack nucleation and growth. With this study, we demonstrate that it is feasible to implement the formation of wing cracks in large scale phase-field reservoir models.

Electromagnetic pulsed thermography for natural cracks inspection

NASA Astrophysics Data System (ADS)

Gao, Yunlai; Tian, Gui Yun; Wang, Ping; Wang, Haitao; Gao, Bin; Woo, Wai Lok; Li, Kongjing

2017-02-01

Emerging integrated sensing and monitoring of material degradation and cracks are increasingly required for characterizing the structural integrity and safety of infrastructure. However, most conventional nondestructive evaluation (NDE) methods are based on single modality sensing which is not adequate to evaluate structural integrity and natural cracks. This paper proposed electromagnetic pulsed thermography for fast and comprehensive defect characterization. It hybrids multiple physical phenomena i.e. magnetic flux leakage, induced eddy current and induction heating linking to physics as well as signal processing algorithms to provide abundant information of material properties and defects. New features are proposed using 1st derivation that reflects multiphysics spatial and temporal behaviors to enhance the detection of cracks with different orientations. Promising results that robust to lift-off changes and invariant features for artificial and natural cracks detection have been demonstrated that the proposed method significantly improves defect detectability. It opens up multiphysics sensing and integrated NDE with potential impact for natural understanding and better quantitative evaluation of natural cracks including stress corrosion crack (SCC) and rolling contact fatigue (RCF).

77 FR 3187 - Airworthiness Directives; the Boeing Company Airplanes

Federal Register 2010, 2011, 2012, 2013, 2014

2012-01-23

... of the hatch opening of the overwing emergency exit. This proposed AD would require repetitive... cracking on the lower main sill inner chord of the hatch opening of the overwing emergency exit, which could result in reduced structural integrity of the hatch opening of the overwing emergency exit and...

49 CFR 178.44 - Specification 3HT seamless steel cylinders for aircraft use.

Code of Federal Regulations, 2013 CFR

2013-10-01

... cylinder. (d) Manufacture. Cylinders must be manufactured using equipment and processes adequate to ensure.... (h) Openings in cylinders and connections (valves, fuse plugs, etc.) for those openings. Threads conforming to the following are required on openings: (1) Threads must be clean cut, even, without cracks...

49 CFR 178.44 - Specification 3HT seamless steel cylinders for aircraft use.

Code of Federal Regulations, 2012 CFR

2012-10-01

... cylinder. (d) Manufacture. Cylinders must be manufactured using equipment and processes adequate to ensure.... (h) Openings in cylinders and connections (valves, fuse plugs, etc.) for those openings. Threads conforming to the following are required on openings: (1) Threads must be clean cut, even, without cracks...

49 CFR 178.44 - Specification 3HT seamless steel cylinders for aircraft use.

Code of Federal Regulations, 2014 CFR

2014-10-01

... cylinder. (d) Manufacture. Cylinders must be manufactured using equipment and processes adequate to ensure.... (h) Openings in cylinders and connections (valves, fuse plugs, etc.) for those openings. Threads conforming to the following are required on openings: (1) Threads must be clean cut, even, without cracks...

Microscopic observations of self-healing products in calcareous fly ash mortars.

PubMed

Jóźwiak-Niedźwiedzka, Daria

2015-01-01

The results of microstructural characterization of mortars containing fly ash class C (High Calcium Fly Ash) from combustion of lignite are presented. The evaluation of the microstructure was performed using scanning electron microscope, optical, and confocal microscope. The tested beams were bent till the crack and microcracks opening, which were healed during the different curing time. The results showed that the replacement of cement with fly ash class C influenced the process of crack healing. The addition of HCFA, at both 30% and 60%, speeds up the self-healing process in cracks and particularly in micro-cracks. In the research, the completely filling up of the cracks by new phases has not been observed, only the beginning of such process has been noticed. © 2014 Wiley Periodicals, Inc.

Probabilistic Mesomechanical Fatigue Model

NASA Technical Reports Server (NTRS)

Tryon, Robert G.

1997-01-01

A probabilistic mesomechanical fatigue life model is proposed to link the microstructural material heterogeneities to the statistical scatter in the macrostructural response. The macrostructure is modeled as an ensemble of microelements. Cracks nucleation within the microelements and grow from the microelements to final fracture. Variations of the microelement properties are defined using statistical parameters. A micromechanical slip band decohesion model is used to determine the crack nucleation life and size. A crack tip opening displacement model is used to determine the small crack growth life and size. Paris law is used to determine the long crack growth life. The models are combined in a Monte Carlo simulation to determine the statistical distribution of total fatigue life for the macrostructure. The modeled response is compared to trends in experimental observations from the literature.

Crack identification method in beam-like structures using changes in experimentally measured frequencies and Particle Swarm Optimization

NASA Astrophysics Data System (ADS)

Khatir, Samir; Dekemele, Kevin; Loccufier, Mia; Khatir, Tawfiq; Abdel Wahab, Magd

2018-02-01

In this paper, a technique is presented for the detection and localization of an open crack in beam-like structures using experimentally measured natural frequencies and the Particle Swarm Optimization (PSO) method. The technique considers the variation in local flexibility near the crack. The natural frequencies of a cracked beam are determined experimentally and numerically using the Finite Element Method (FEM). The optimization algorithm is programmed in MATLAB. The algorithm is used to estimate the location and severity of a crack by minimizing the differences between measured and calculated frequencies. The method is verified using experimentally measured data on a cantilever steel beam. The Fourier transform is adopted to improve the frequency resolution. The results demonstrate the good accuracy of the proposed technique.

Research on anti crack mechanism of bionic coupling brake disc

NASA Astrophysics Data System (ADS)

Shi, Lifeng; Yang, Xiao; Zheng, Lingnan; Wu, Can; Ni, Jing

2017-09-01

According to the biological function of fatigue resistance possessed by biology, this study designed a Bionic Coupling Brake Disc (BCBD) which can inhibit crack propagation as the result of improving fatigue property. Thermal stress field of brake disc was calculated under emergency working condition, and circumferential and radial stress field which lead to fatigue failure of brake disc were investigated simultaneously. Results showed that the maximum temperature of surface reached 890°C and the maximum residual tensile stress was 207 Mpa when the initial velocity of vehicle was 200 km/h. Based on the theory of elastic plastic fracture mechanics, the crack opening displacement and the crack front J integrals of the BCBD and traditional brake disc (TBD) with pre-cracking were calculated, and the strength of crack front was compared. Results revealed the growth behavior of fatigue crack located on surface of brake disc, and proved the anti fatigue resistance of BCBD was better, and the strength of crack resistance of BCBD was much stronger than that of TBD. This simulation research provided significant references for optimization and manufacturing of BCBD.

The San Francisco cow; did she or didn’t she?

USGS Publications Warehouse

Hill, M.

1976-01-01

No one has suggested that Mr. Shafter's nameless cow was the cause of the 1906 earthquake, but she has been the source of as persistent a rumor as Mrs. Murphy's Chicago cow. Since 1906, "the cow that fell in the crack" has been a favorite subject of humorous speculation. large earthquakes have always produced large exaggerations, and, although it is difficult to exaggerate the terror humans feel in an earthquake, many scientists have said that much of what witnesses said they witnessed they did not witness at all. Huge, gaping cracks that legend says open and close in the earth, swallowing whole cities, are among those earthquake features that just aren't featured in earthquakes. True, soil may "snap open and shut," but most earth scientists do not think that the cracks are wide enough or deep enough to accommodate houses. 

Phase-contrast x-ray imaging of microstructure and fatigue-crack propagation in single-crystal nickel-base superalloys

NASA Astrophysics Data System (ADS)

Husseini, Naji Sami

Single-crystal nickel-base superalloys are ubiquitous in demanding turbine-blade applications, and they owe their remarkable resilience to their dendritic, hierarchical microstructure and complex composition. During normal operations, they endure rapid low-stress vibrations that may initiate fatigue cracks. This failure mode in the very high-cycle regime is poorly understood, in part due to inadequate testing and diagnostic equipment. Phase-contrast imaging with coherent synchrotron x rays, however, is an emergent technique ideally suited for dynamic processes such as crack initiation and propagation. A specially designed portable ultrasonic-fatigue apparatus, coupled with x-ray radiography, allows real-time, in situ imaging while simulating service conditions. Three contrast mechanisms - absorption, diffraction, and phase contrast - span the immense breadth of microstructural features in superalloys. Absorption contrast is sensitive to composition and crack displacements, and diffraction contrast illuminates dislocation aggregates and crystallographic misorientations. Phase contrast enhances electron-density gradients and is particularly useful for fatigue-crack studies, sensitive to internal crack tips and openings less than one micrometer. Superalloy samples were imaged without external stresses to study microstructure and mosaicity. Maps of rhenium and tungsten concentrations revealed strong segregation to the center of dendrites, as manifested by absorption contrast. Though nominally single crystals, dendrites were misoriented from the bulk by a few degrees, as revealed by diffraction contrast. For dynamic studies of cyclic fatigue, superalloys were mounted in the portable ultrasonic-fatigue apparatus, subjected to a mean tensile stress of ˜50-150 MPa, and cycled in tension to initiate and propagate fatigue cracks. Radiographs were recorded every thousand cycles over the multimillion-cycle lifetime to measure micron-scale crack growth. Crack openings were very small, as determined by absorption and phase contrast, and suggested multiple fracture modes for propagation along {111} planes at room temperature, which was verified by finite element analysis. With increasing temperature, cracks became Mode I (perpendicular to the loading axis) in character and more sensitive to the microstructure. Advancing plastic zones ahead of crack tips altered the crystallographic quality, from which diffraction contrast anticipated initiation and propagation. These studies demonstrate the extreme sensitivity of x-ray radiography for detailed studies of superalloys and crack growth processes.

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

Zhang, Ch.; Gao, X. W.; Sladek, J.

This paper reports our recent research works on crack analysis in continuously non-homogeneous and linear elastic functionally graded materials. A meshless boundary element method is developed for this purpose. Numerical examples are presented and discussed to demonstrate the efficiency and the accuracy of the present numerical method, and to show the effects of the material gradation on the crack-opening-displacements and the stress intensity factors.

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.

A robust signal processing method for quantitative high-cycle fatigue crack monitoring using soft elastomeric capacitor sensors

NASA Astrophysics Data System (ADS)

Kong, Xiangxiong; Li, Jian; Collins, William; Bennett, Caroline; Laflamme, Simon; Jo, Hongki

2017-04-01

A large-area electronics (LAE) strain sensor, termed soft elastomeric capacitor (SEC), has shown great promise in fatigue crack monitoring. The SEC is able to monitor strain changes over a mesoscale structural surface and endure large deformations without being damaged under cracking. Previous tests verified that the SEC is able to detect, localize, and monitor fatigue crack activities under low-cycle fatigue loading. In this paper, to examine the SEC's capability of monitoring high-cycle fatigue cracks, a compact specimen is tested under cyclic tension, designed to ensure realistic crack opening sizes representative of those in real steel bridges. To overcome the difficulty of low signal amplitude and relatively high noise level under high-cycle fatigue loading, a robust signal processing method is proposed to convert the measured capacitance time history from the SEC sensor to power spectral densities (PSD) in the frequency domain, such that signal's peak-to-peak amplitude can be extracted at the dominant loading frequency. A crack damage indicator is proposed as the ratio between the square root of the amplitude of PSD and load range. Results show that the crack damage indicator offers consistent indication of crack growth.

Prediction of reinforcement corrosion using corrosion induced cracks width in corroded reinforced concrete beams

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

Khan, Inamullah; François, Raoul; Castel, Arnaud

2014-02-15

This paper studies the evolution of reinforcement corrosion in comparison to corrosion crack width in a highly corroded reinforced concrete beam. Cracking and corrosion maps of the beam were drawn and steel reinforcement was recovered from the beam to observe the corrosion pattern and to measure the loss of mass of steel reinforcement. Maximum steel cross-section loss of the main reinforcement and average steel cross-section loss between stirrups were plotted against the crack width. The experimental results were compared with existing models proposed by Rodriguez et al., Vidal et al. and Zhang et al. Time prediction models for a givenmore » opening threshold are also compared to experimental results. Steel cross-section loss for stirrups was also measured and was plotted against the crack width. It was observed that steel cross-section loss in the stirrups had no relationship with the crack width of longitudinal corrosion cracks. -- Highlights: •Relationship between crack and corrosion of reinforcement was investigated. •Corrosion results of natural process and then corresponds to in-situ conditions. •Comparison with time predicting model is provided. •Prediction of load-bearing capacity from crack pattern was studied.« less

Influence of anodizing conditions on generation of internal cracks in anodic porous tin oxide films grown in NaOH electrolyte

NASA Astrophysics Data System (ADS)

Zaraska, Leszek; Gawlak, Karolina; Gurgul, Magdalena; Dziurka, Magdalena; Nowak, Marlena; Gilek, Dominika; Sulka, Grzegorz D.

2018-05-01

Nanoporous tin oxide layers were synthesized via simple one-step anodic oxidation of a low-purity Sn foil (98.8%) in sodium hydroxide electrolyte. The process of pore formation at the early stage of anodization was discussed on the basis of concepts of oxygen bubble mould effect and viscous flow of oxide. The effect of anodizing conditions on the generation of internal cracks and fractures within the anodic film was investigated in detail. It was confirmed that crack-free tin oxide films can be obtained if the anodization is carried out at the potential of 4 V independently of the electrolyte concentration. On the other hand, the porous anodic film with a totally stacked internal morphology is obtained at the potential of 5 V in 0.1 M NaOH electrolyte. The generation of internal cracks and voids can be attributed to a much lower surface porosity and local trapping of O2 inside the pores of the oxide layer. However, increasing electrolyte concentration allows for obtaining less cracked porous films due to effective and uniform liberation of oxygen bubbles from the channels through completely open pore mouths. Furthermore, it was confirmed that uniformity of the anodic tin oxide layers can be significantly improved by vigorous electrolyte stirring. Finally, we observed that the addition of ethanol to the electrolyte can reduce anodic current density and the oxide growth rate. In consequence, less cracked anodic film can be formed even at the potential of 6 V. The generation of oxygen at the pore bottoms, together with the open pore mouths were found to be critical factors responsible for the anodic formation of crack-free porous tin oxide films.

Effects of Microstructure on Tensile, Charpy Impact, and Crack Tip Opening Displacement Properties of Two API X80 Pipeline Steels

NASA Astrophysics Data System (ADS)

Shin, Sang Yong

2013-06-01

The effects of microstructure on tensile, Charpy impact, and crack tip opening displacement (CTOD) properties of two API X80 pipeline steels were investigated in this study. Two API X80 pipeline steels consisting of acicular ferrite and granular bainite, and a small amount of hard phases such as martensite and secondary phases have elongated grains along the rolling direction, so that they show different mechanical properties as the specimens' directions change. The 90 deg specimens have high tensile strength due to the low stress concentration on the fine hard phases and the high loads for the deformation of the elongated grains. In contrast, the 30 deg specimens have less elongated grains and larger hard phases such as martensite, with the size of about 3 μm, than the 90 deg specimens. Hence, the 30 deg specimens have low tensile strength because of the high stress concentration on the large hard phases and the low loads to deform grains. In the 90 deg specimen, brittle crack propagation surfaces are even since cracks propagate in a straight line along the elongated grain structure. In the 30 deg specimen, however, brittle crack propagation surfaces are uneven, and secondary cracks are observed, because of the zigzag brittle crack propagation path. In the CTOD properties, the 90 deg specimens have maximum forces of higher magnitude than the 30 deg specimens, because of the elongated grain structure. However, CTODs of the 90 deg specimens are lower than those of the 30 deg specimens because of the low plastic deformation areas by the elongated grains in the 90 deg specimens.

Stress intensity factors and COD in an orthotropic strip

NASA Technical Reports Server (NTRS)

Kaya, A. C.; Erdogan, F.

1980-01-01

The elasticity problem for an orthotropic strip or a beam with an internal or an edge crack under general loading conditions is considered. The numerical results are given for four basic loading conditions, namely, uniform tension, pure bending, three point bending, and concentrated surface shear loading. For the strip with an edge crack additional results regarding the crack opening displacements are obtained by using the plastic strip model. A critical quantity which is tabulated is the maximum compressive stress in the plane of the crack. It is shown that this stress may easily exceed the yield limit in compression and hence may severely limit the range of application of the plasticity results.

Finite element solutions for crack-tip behavior in small-scale yielding

NASA Technical Reports Server (NTRS)

Tracey, D. M.

1976-01-01

The subject considered is the stress and deformation fields in a cracked elastic-plastic power law hardening material under plane strain tensile loading. An incremental plasticity finite element formulation is developed for accurate analysis of the complete field problem including the extensively deformed near tip region, the elastic-plastic region, and the remote elastic region. The formulation has general applicability and was used to solve the small scale yielding problem for a set of material hardening exponents. Distributions of stress, strain, and crack opening displacement at the crack tip and through the elastic-plastic zone are presented as a function of the elastic stress intensity factor and material properties.

Initial Reactivity of Linkages and Monomer Rings in Lignin Pyrolysis Revealed by ReaxFF Molecular Dynamics.

PubMed

Zhang, Tingting; Li, Xiaoxia; Guo, Li

2017-10-24

The initial conversion pathways of linkages and their linked monomer units in lignin pyrolysis were investigated comprehensively by ReaxFF MD simulations facilitated by the unique VARxMD for reaction analysis. The simulated molecular model contains 15 920 atoms and was constructed on the basis of Adler's softwood lignin model. The simulations uncover the initial conversion ratio of various linkages and their linked aryl monomers. For linkages and their linked monomer aryl rings of α-O-4, β-O-4 and α-O-4 & β-5, the C α /C β ether bond cracking dominates the initial pathway accounting for at least up to 80% of their consumption. For the linkage of β-β & γ-O-α, both the C α -O ether bond cracking and its linked monomer aryl ring opening are equally important. Ring-opening reactions dominate the initial consumption of other 4-O-5, 5-5, β-1, β-2, and β-5 linkages and their linked monomers. The ether bond cracking of C α -O and C β -O occurs at low temperature, and the aryl ring-opening reactions take place at relatively high temperature. The important intermediates leading to the stable aryl ring opening are the phenoxy radicals, the bridged five-membered and three-membered rings and the bridged six-membered and three-membered rings. In addition, the reactivity of a linkage and its monomer aryl ring may be affected by other linkages. The ether bond cracking of α-O-4 and β-O-4 linkages can activate its neighboring linkage or monomer ring through the formed phenoxy radicals as intermediates. The important intermediates revealed in this article should be of help in deepening the understanding of the controlling mechanism for producing aromatic chemicals from lignin pyrolysis.

Counter-intuitive quasi-periodic motion in the autonomous vibration of cracked Timoshenko beams

NASA Astrophysics Data System (ADS)

Brandon, J. A.; Abraham, O. N. L.

1995-08-01

The time domain behaviour of a cracked Timoshenko beam is constructed by alternation of two linear models corresponding to the open and closed condition of the crack. It might be expected that a response which is composed of the alternation of two systems with different properties would extinguish the periodicities of the constituent sub-models. The numerical studies presented illustrate the perpetuation of these features without showing any evidence for the creation of periodicities based on a common assumption of the mean period of a bilinear model.

12. Detail, typical window with fireproof shutters open, northeast rear, ...

Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

12. Detail, typical window with fireproof shutters open, northeast rear, view to southwest, 135mm lens. Note cracks evidencing structural failure. - Benicia Arsenal, Powder Magazine No. 5, Junction of Interstate Highways 680 & 780, Benicia, Solano County, CA

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.

Salivary buffer capacity, pH, and stimulated flow rate of crack cocaine users.

PubMed

Woyceichoski, Iverson Ernani Cogo; Costa, Carlos Henrique; de Araújo, Cristiano Miranda; Brancher, João Armando; Resende, Luciane Grochocki; Vieira, Iran; de Lima, Antonio Adilson Soares

2013-08-01

Crack cocaine is the freebase form of cocaine that can be smoked. The use of this drug has been considered a public health problem in many countries. The aim of this study was to assess the stimulated salivary flow rate (SSFR), pH, and the buffer capacity of saliva in crack cocaine users. Stimulated whole saliva was collected from 54 selected crack cocaine users and 40 non-users. All samples were analyzed for SSFR, pH, and buffer capacity. SSFR was analyzed by gravimetric method. The buffer capacity and pH were determined using a digital pH meter. The crack cocaine users demonstrated higher buffer capacity than the control group (P > 0.05). Salivary pH was lower in crack cocaine users (P < 0.05). Mean values of the SSFR for the experimental and control groups were 1.1 and 1.3 mL/min, respectively (P > 0.05). Crack cocaine users might exhibit a significant decrease in salivary pH, but not in salivary flow rate or buffer capacity. © 2012 Blackwell Publishing Asia Pty Ltd.

Analysis of corner cracks at hole by a 3-D weight function method with stresses from finite element method

NASA Technical Reports Server (NTRS)

Zhao, W.; Newman, J. C., Jr.; Sutton, M. A.; Wu, X. R.; Shivakumar, K. N.

1995-01-01

Stress intensity factors for quarter-elliptical corner cracks emanating from a circular hole are determined using a 3-D weight function method combined with a 3-D finite element method. The 3-D finite element method is used to analyze uncracked configuration and provide stress distribution in the region where crack is to occur. Using this stress distribution as input, the 3-D weight function method is used to determine stress intensity factors. Three different loading conditions, i.e. remote tension, remote bending and wedge loading, are considered for a wide range in geometrical parameters. The significance in using 3-D uncracked stress distribution and the difference between single and double corner cracks are studied. Typical crack opening displacements are also provided. Comparisons are made with solutions available in the literature.

Characteristics of laminates with delamination control strips

NASA Technical Reports Server (NTRS)

Sun, C. T.; Goering, J. C.; Alper, J. M.; Gause, L. W.

1992-01-01

Tough resin is needed to resist delamination crack propagation. However, modulus often has to be compromised because it is difficult to retain both high modulus and toughness in a matrix material. A potential solution is to use a hybrid system in which tough resin strips are included within a conventional matrix composite. By adjusting the spacing of the tough resin strips, maximum delamination size can be controlled. Experimental results for impact damage and subsequent damage propagation in laminates containing tough resin strips are reported. Plain adhesive strips and fiber-reinforced tough resin composite strips were used in constructing the hybrid laminates. Test results indicated that size of delamination inflicted by impact was confined between the tough resin strips. As a result, significantly increased residual compressive strength was obtained. Impacted laminates containing tough resin strips were also fatigue tested. It was found that these strips reduced the growth of the impact damage area relative to the growth seen in coupons with no tough resin strips. Damage growth from an open hole under tension fatigue was evaluated using both tough resin strips and glass fiber reinforced tough resin strips. Unreinforced tough resin strips retarded delamination growth from the open hole, but did not stop matrix cracks growing in the fiber direction. Fiber reinforced tough resin strips did not contain axial delamination growth from the open hole. However, they did act as crack arresters, stopping the through-the-thickness tension crack originating from the hole.

Crack - seal veins - what we learnt since the seminal work of John Ramsay

NASA Astrophysics Data System (ADS)

Urai, Janos L.; Bons, Paul D.

2017-04-01

In the nineteen-eighties, John Ramsay and co-workers have laid the basis for much of our current understanding of tectonic veins, by proposing that tectonic veins accrete in many small increments of cracking and sealing, making the link to cyclic stress and fluid pressure cycles and the earthquake cycle, and by proposing that fibrous veins track the opening trajectory, which has created a toolbox to analyse progressive deformation in rocks. They recognised syntaxial and antitaxial veins, which grow depending on the composition of the wall rock and the vein. Following on these seminal contributions, advances in analysing the microstructure and chemical signature in crack-seal veins made a lot of progress, facilitated by the rapid increase in micro analytical tools like cathodeluminescence and EBSD. Initial modelling of crystal growth in crack- seal veins provided an explanation of how crystals become fibrous without being deformed and explained how fibres sometimes do and sometimes don't follow the opening trajectory. This was followed by numerical models of crystal growth to study the development of crystal facets after larger crack increments, and experimental study of the sealing dynamics of syntaxial veins. These models were initially kinematic, using the ELLE microdynamic simulation package, and more recently incorporating the physics of the growing interface using the Phase Field method, which now allows 3D simulation of both syntaxial and antitaxial veins and can simultaneously compute the evolving permeability of the crack using Lattice Boltzmann techniques. Parallel to these developments we developed an understanding of the strength of the vein cement, and, using Discrete Element Techniques, explored the effects of differences of the strength of the vein and its adhesion to the wall rock on fracture patterns in crack-seal systems in changing stress fields. This presentation will review these developments, showing how the ideas of John Ramsay inspired follow up work leading to a much better understanding of the complex feedback systems between fracture growth, crystal growth and fluid flow in crack-seal systems.

Interference and differentiation of the neighboring surface microcracks in distributed sensing with PPP-BOTDA.

PubMed

Meng, Dewei; Ansari, Farhad

2016-12-01

Detection of cracks while at their early stages of evolution is important in health monitoring of civil structures. Review of technical literature reveals that single or sparsely distributed multiple cracks can be detected by Brillouin-scattering-based optical fiber sensor systems. In a recent study, a pre-pump-pulse Brillouin optical time-domain analysis (PPP-BOTDA) system was employed for detection of a single microcrack. Specific characteristics of the Brillouin gain spectrum, such as Brillouin frequency shift, and Brillouin gain spectrum width, were utilized in order to detect the formation and growth of microcracks with crack opening displacements as small as 25 μm. In most situations, formations of neighboring microcracks are not detected due to inherent limitations of Brillouin-based systems. In the study reported here, the capability of PPP-BOTDA for detection of two neighboring microcracks was investigated in terms of the proximity of the microcracks with respect to each other, i.e., crack spacing distance, crack opening displacement, and the spatial resolution of the PPP-BOTDA. The extent of the study pertained both to theoretical as well as experimental investigations. The concept of shape index is introduced in order to establish an analytical method for gauging the influence of the neighboring microcracks in detection and microcrack differentiation capabilities of Brillouin-based optical fiber sensor systems.

Variations of a global constraint factor in cracked bodies under tension and bending loads

NASA Technical Reports Server (NTRS)

Newman, J. C., Jr.; Crews, J. H., Jr.; Bigelow, C. A.; Dawicke, D. S.

1994-01-01

Elastic-plastic finite-element analyses were used to calculate stresses and displacements around a crack in finite-thickness plates for an elastic-perfectly plastic material. Middle- and edge-crack specimens were analyzed under tension and bending loads. Specimens were 1.25 to 20 mm thick with various widths and crack lengths. A global constraint factor alpha(sub g), an averaged normal-stress to flow-stress ratio over the plastic region, was defined to simulate three-dimensional (3D) effects in two-dimensional (2D) models. For crack lengths and uncracked ligament lengths greater than four times the thickness, the global constraint factor was found to be nearly a unique function of a normalized stress-intensity factor (related to plastic-zone size to thickness ratio) from small- to large-scale yielding conditions for various specimen types and thickness. For crack length-to-thickness ratios less than four, the global constraint factor was specimen type, crack length and thickness dependent. Using a 2D strip-yield model and the global constraint factors, plastic-zone sizes and crack-tip displacements agreed reasonably well with the 3D analyses. For a thin sheet aluminum alloy, the critical crack-tip-opening angle during stable tearing was found to be independent of specimen type and crack length for crack length-to-thickness ratios greater than 4.

Ontogeny of Manipulative Behavior and Nut-Cracking in Young Tufted Capuchin Monkeys ("Cebus Apella"): A Perception-Action Perspective

ERIC Educational Resources Information Center

de Resende, Briseida Dogo; Ottoni, Eduardo B.; Fragaszy, Dorothy M.

2008-01-01

How do capuchin monkeys learn to use stones to crack open nuts? Perception-action theory posits that individuals explore producing varying spatial and force relations among objects and surfaces, thereby learning about affordances of such relations and how to produce them. Such learning supports the discovery of tool use. We present longitudinal…

A new exposure model to evaluate smoked illicit drugs in rodents: A study of crack cocaine.

PubMed

Hueza, Isis M; Ponce, Fernando; Garcia, Raphael C T; Marcourakis, Tânia; Yonamine, Maurício; Mantovani, Cínthia de C; Kirsten, Thiago B

2016-01-01

The use of smoked illicit drugs has spread dramatically, but few studies use proper devices to expose animals to inhalational abused drugs despite the availability of numerous smoking devices that mimic tobacco exposure in rodents. Therefore, the present study developed an inexpensive device to easily expose laboratory animals to smoked drugs. We used crack cocaine as the drug of abuse, and the cocaine plasma levels and the behaviors of animals intoxicated with the crack cocaine were evaluated to prove inhaled drug absorption and systemic activity. We developed an acrylic device with two chambers that were interconnected and separated by a hatch. Three doses of crack (100, 250, or 500 mg), which contained 63.7% cocaine, were burned in a pipe, and the rats were exposed to the smoke for 5 or 10 min (n=5/amount/period). Exposure to the 250-mg dose for 10 min achieved cocaine plasma levels that were similar to those of users (170 ng/mL). Behavioral evaluations were also performed to validate the methodology. Rats (n=10/group) for these evaluations were exposed to 250 mg of crack cocaine or air for 10 min, twice daily, for 28 consecutive days. Open-field evaluations were performed at three different periods throughout the experimental design. Exposed animals exhibited transient anorexia, increased motor activity, and shorter stays in central areas of the open field, which suggests reduced anxiety. Therefore, the developed model effectively exposed animals to crack cocaine, and this model may be useful for the investigation of other inhalational abused drugs. Copyright © 2015 Elsevier Inc. All rights reserved.

Natural glide slab avalanches, Glacier National Park, USA: A unique hazard and forecasting challenge

USGS Publications Warehouse

Reardon, Blase; Fagre, Daniel B.; Dundas, Mark; Lundy, Chris

2006-01-01

In a museum of avalanche phenomena, glide cracks and glide avalanches might be housed in the “strange but true” section. These oddities are uncommon in most snow climates and tend to be isolated to specific terrain features such as bedrock slabs. Many glide cracks never result in avalanches, and when they do, the wide range of time between crack formation and slab failure makes them highly unpredictable. Despite their relative rarity, glide cracks and glide avalanches pose a regular threat and complex forecasting challenge during the annual spring opening of the Going-to-the-Sun Road in Glacier National Park, U.S.A. During the 2006 season, a series of unusual glide cracks delayed snow removal operations by over a week and provided a unique opportunity to record detailed observations of glide avalanches and characterize their occurrence and associated weather conditions. Field observations were from snowpits, crown profiles and where possible, measurements of slab thickness, bed surface slope angle, substrate and other physical characteristics. Weather data were recorded at one SNOTEL site and two automated stations located from 0.6-10 km of observed glide slab avalanches. Nearly half (43%) of the 35 glide slab avalanches recorded were Class D2-2.5, with 15% Class D3-D3.5. The time between glide crack opening and failure ranged from 2 days to over six weeks, and the avalanches occurred in cycles associated with loss of snow water equivalent and spikes in temperature and radiation. We conclude with suggest ions for further study.

Ply cracking in composite laminates

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

Han, Youngmyong.

1989-01-01

Ply cracking behavior and accompanying stiffness changes in thermoset as well as thermoplastic matrix composites under various loading conditions are investigated. Specific topics addressed are: analytical model development for property degradations due to ply cracking under general in-plane loading; crack initiation and multiplication under static loading; and crack multiplication under cyclic loading. A model was developed to calculate the energy released due to ply cracking in a composite laminate subjected to general in-plane loading. The method is based on the use of a second order polynomial to represent the crack opening displacement and the concept of a through-the-thickness inherent flaw.more » The model is then used in conjunction with linear elastic fracture mechanics to predict the progressive ply cracking as well as first ply cracking. A resistance curve for crack multiplication is proposed as a means of characterizing the resistance to ply cracking in composite laminates. A methodology of utilizing the resistance curve to assess the crack density or overloading is also discussed. The method was applied to the graphite/thermoplastic polyimide composite to predict progressive ply cracking. However, unlike the thermoset matrix composites, a strength model is found to fit the experimental results better than the fracture mechanics based model. A set of closed form equations is also developed to calculate the accompanying stiffness changes due to the ply cracking. The effect of thermal residual stress is included in the analysis. A new method is proposed to characterize transverse ply cracking of symmetric balanced laminates under cyclic loading. The method is based on the concept of a through-the-thickness inherent flaw, the Paris law, and the resistance curve. Only two constants are needed to predict the crack density as a function of fatigue cycles.« less

Stable Tearing and Buckling Responses of Unstiffened Aluminum Shells with Long Cracks

NASA Technical Reports Server (NTRS)

Starnes, James H., Jr.; Rose, Cheryl A.

1999-01-01

The results of an analytical and experimental study of the nonlinear response of thin, unstiffened, aluminum cylindrical shells with a long longitudinal crack are presented. The shells are analyzed with a nonlinear shell analysis code that accurately accounts for global and local structural response phenomena. Results are presented for internal pressure and for axial compression loads. The effect of initial crack length on the initiation of stable crack growth and unstable crack growth in typical shells subjected to internal pressure loads is predicted using geometrically nonlinear elastic-plastic finite element analyses and the crack-tip-opening angle (CTOA) fracture criterion. The results of these analyses and of the experiments indicate that the pressure required to initiate stable crack growth and unstable crack growth in a shell subjected to internal pressure loads decreases as the initial crack length increases. The effects of crack length on the prebuckling, buckling and postbuckling responses of typical shells subjected to axial compression loads are also described. For this loading condition, the crack length was not allowed to increase as the load was increased. The results of the analyses and of the experiments indicate that the initial buckling load and collapse load for a shell subjected to axial compression loads decrease as the initial crack length increases. Initial buckling causes general instability or collapse of a shell for shorter initial crack lengths. Initial buckling is a stable local response mode for longer initial crack lengths. This stable local buckling response is followed by a stable postbuckling response, which is followed by general or overall instability of the shell.

Stable Tearing and Buckling Responses of Unstiffened Aluminum Shells with Long Cracks

NASA Technical Reports Server (NTRS)

Starnes, James H., Jr.; Rose, Cheryl A.

1998-01-01

The results of an analytical and experimental study of the nonlinear response of thin, unstiffened, aluminum cylindrical shells with a long longitudinal crack are presented. The shells are analyzed with a nonlinear shell analysis code that accurately accounts for global and local structural response phenomena. Results are presented for internal pressure and for axial compression loads. The effect of initial crack length on the initiation of stable crack growth and unstable crack growth in typical shells subjected to internal pressure loads is predicted using geometrically nonlinear elastic-plastic finite element analyses and the crack-tip-opening angle (CTOA) fracture criterion. The results of these analyses and of the experiments indicate that the pressure required to initiate stable crack growth and unstable crack growth in a shell subjected to internal pressure loads decreases as the initial crack length increases. The effects of crack length on the prebuckling, buckling and postbuckling responses of typical shells subjected to axial compression loads are also described. For this loading condition, the crack length was not allowed to increase as the load was increased. The results of the analyses and of the experiments indicate that the initial buckling load and collapse load for a shell subjected to axial compression loads decrease as the initial crack length increases. Initial buckling causes general instability or collapse of a shell for shorter initial crack lengths. Initial buckling is a stable local response mode for longer initial crack lengths. This stable local buckling response is followed by a stable postbuckling response, which is followed by general or overall instability of the shell.

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.

Kinetic studies of the stress corrosion cracking of D6AC steel

NASA Technical Reports Server (NTRS)

Noronha, P. J.

1975-01-01

The effect of load interactions on the crack growth velocity of D6AC steel under stress corrosion cracking conditions was determined. The environment was a 3.5 percent salt solution. The modified-wedge opening load specimens were fatigue precracked and subjected to a deadweight loading in creep machines. The effects of load shedding on incubation times and crack growth rates were measured using high-sensitivity compliance measurement techniques. Load shedding results in an incubation time, the length of which depends on the amount of load shed and the baseline stress intensity. The sequence of unloading the specimen also controls the subsequent incubation period. The incubation period is shorter when load shedding passes through zero load than when it does not if the specimen initially had the same baseline stress intensity. The crack growth rates following the incubation period are also different from the steady-state crack growth rate at the operating stress intensity. These data show that the susceptibility of this alloy system to stress corrosion cracking depends on the plane-strain fracture toughness and on the yield strength of the material.

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.

Electromagnetic pulsed thermography for natural cracks inspection

PubMed Central

Gao, Yunlai; Tian, Gui Yun; Wang, Ping; Wang, Haitao; Gao, Bin; Woo, Wai Lok; Li, Kongjing

2017-01-01

Emerging integrated sensing and monitoring of material degradation and cracks are increasingly required for characterizing the structural integrity and safety of infrastructure. However, most conventional nondestructive evaluation (NDE) methods are based on single modality sensing which is not adequate to evaluate structural integrity and natural cracks. This paper proposed electromagnetic pulsed thermography for fast and comprehensive defect characterization. It hybrids multiple physical phenomena i.e. magnetic flux leakage, induced eddy current and induction heating linking to physics as well as signal processing algorithms to provide abundant information of material properties and defects. New features are proposed using 1st derivation that reflects multiphysics spatial and temporal behaviors to enhance the detection of cracks with different orientations. Promising results that robust to lift-off changes and invariant features for artificial and natural cracks detection have been demonstrated that the proposed method significantly improves defect detectability. It opens up multiphysics sensing and integrated NDE with potential impact for natural understanding and better quantitative evaluation of natural cracks including stress corrosion crack (SCC) and rolling contact fatigue (RCF). PMID:28169361

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.

Analyses of Buckling and Stable Tearing in Thin-Sheet Materials

NASA Technical Reports Server (NTRS)

Seshadri, B. R.; Newman, J. C., Jr.

1998-01-01

This paper was to verify the STAGS (general shell, geometric and material nonlinear) code and the critical crack tip opening angle (CTOA) fracture criterion for predicting stable tearing in cracked panels that fail with severe out of plane buckling. Materials considered ranged from brittle to ductile behavior. Test data used in this study are reported elsewhere. The STAGS code was used to model stable tearing using a critical CTOA value that was determined from a cracked panel that was 'restrained' from buckling. ne analysis methodology was then used to predict the influence of buckling on stable tearing and failure loads. Parameters like crack length to specimen width ratio, crack configuration, thickness, and material tensile properties had a significant influence on the buckling behavior of cracked thin sheet materials. Experimental and predicted results showed a varied buckling response for different crack length to sheet thickness ratios because different buckling modes were activated. Effects of material tensile properties and fracture toughness on buckling response were presented. The STAGS code and the CTOA fracture criterion were able to predict the influence of buckling on stable tearing behavior and failure loads on a variety of materials and crack configurations.

Residual Strength Analyses of Riveted Lap-Splice Joints

NASA Technical Reports Server (NTRS)

Seshadri, B. R.; Newman, J. C., Jr.

2000-01-01

The objective of this paper was to analyze the crack-linkup behavior in riveted-stiffened lap-splice joint panels with small multiple-site damage (MSD) cracks at several adjacent rivet holes. Analyses are based on the STAGS (STructural Analysis of General Shells) code with the critical crack-tip-opening angle (CTOA) fracture criterion. To account for high constraint around a crack front, the "plane strain core" option in STAGS was used. The importance of modeling rivet flexibility with fastener elements that accurately model load transfer across the joint is discussed. Fastener holes are not modeled but rivet connectivity is accounted for by attaching rivets to the sheet on one side of the cracks that simulated both the rivet diameter and MSD cracks. Residual strength analyses made on 2024-T3 alloy (1.6-mm thick) riveted-lap-splice joints with a lead crack and various size MSD cracks were compared with test data from Boeing Airplane Company. Analyses were conducted for both restrained and unrestrained buckling conditions. Comparison of results from these analyses and results from lap-splice-joint test panels, which were partially restrained against buckling indicate that the test results were bounded by the failure loads predicted by the analyses with restrained and unrestrained conditions.

Heat Extraction from a Hydraulically Fractured Penny-Shaped Crack in Hot Dry Rock

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

Abe, H.; Mura, T.; Keer, L.M.

1976-12-01

Heat extraction from a penny-shaped crack having both inlet and outlet holes is investigated analytically by considering the hydraulic and thermal growth of the crack when fluid is injected at a constant flow rate. The rock mass is assumed to be infinitely extended, homogeneous, and isotropic. The equations for fluid flow are derived and solved to determine the flow pattern in the crack. Temperature distributions in both rock and fluid are also determined. The crack width change due to thermal contraction and the corresponding flow rate increase are discussed. Some numerical calculations of outlet temperature, thermal power extraction, and crackmore » opening displacement due to thermal contraction of rocks are presented for cracks after they attain stationary states for given inlet flow rate and outlet suction pressure. The present paper is a further development of the previous works of Bodvarsson (1969), Gringarten et al. (1975), Lowell (1976), Harlow and Pracht (1972), McFarland (1975), among others, and considers the two-dimensional rather than the one-dimensional crack. Furthermore, the crack radius and width are quantities to be determined rather than given a priori. 11 refs., 1 tab., 5 figs.« less

A novel crack healing in steels by gas nitrocarburizing

NASA Astrophysics Data System (ADS)

Li, Ai; Chen, Xing; Zhang, Chengsong; Cui, Guodong; Zhao, Hui; Yang, Chuan

2018-06-01

In this paper, the gas nitrocarburizing technique was applied for the first time to solve the challenge in crack healing of metallic materials. The crack-healing behavior of 42CrMo steel was investigated. The gas nitrocarburizing was carried out in two steps with the decrease of the healing temperature. The mechanical properties after healing were measured using the three-point blending test. X-ray diffraction, optical microscope and scanning electron microscopy were applied to characterize the phase composition and microstructure of crack healing area and analyze healing mechanisms involved. The results show that the optimal healing effect could be obtained when it is healed at 760 °C for 2 h and then at 550 °C for 4 h. The maximum healing degree reached to 63.68%. The crack healing process could be divided into two stages, i.e. healing in crack tips at high temperatures and then in crack openings at low temperatures. The volumetric expansion and filling of formed nitrides contributed to the rapid healing of the large-sized cracks. The healing efficiency could be improved by decreasing the healing temperature. Moreover, high pressure gas nitrocarburizing was considered as another potential way to improve the healing efficiency and healing degree.

A two scale analysis of tight sandstones

NASA Astrophysics Data System (ADS)

Adler, P. M.; Davy, C. A.; Song, Y.; Troadec, D.; Hauss, G.; Skoczylas, F.

2015-12-01

Tight sandstones have a low porosity and a very small permeability K. Available models for K do not compare well with measurements. These sandstones are made of SiO_2 grains, with a typical size of several hundreds of micron. These grains are separated by a network of micro-cracks, with sizes ranging between microns down to tens of nm. Therefore, the structure can be schematized by Voronoi polyhedra separated by plane and permeable polygonal micro-cracks. Our goal is to estimate K based on a two scale analysis and to compare the results to measurements. For a particular sample [2], local measurements on several scales include FIB/SEM [3], CMT and 2D SEM. FIB/SEM is selected because the peak pore size given by Mercury Intrusion Porosimetry is of 350nm. FIB/SEM imaging (with 50 nm voxel size) identifies an individual crack of 180nm average opening, whereas CMT provides a connected porosity (individual crack) for 60 nm voxel size, of 4 micron average opening. Numerical modelling is performed by combining the micro-crack network scale (given by 2D SEM) and the 3D micro-crack scale (given by either FIB/SEM or CMT). Estimates of the micro-crack density are derived from 2D SEM trace maps by counting the intersections with scanlines, the surface density of traces, and the number of fracture intersections. K is deduced by using a semi empirical formula valid for identical, isotropic and uniformly distributed fractures [1]. This value is proportional to the micro-crack transmissivity sigma. Sigma is determined by solving the Stokes equation in the micro-cracks measured by FIB/SEM or CMT. K is obtained by combining the two previous results. Good correlation with measured values on centimetric plugs is found when using sigma from CMT data. The results are discussed and further research is proposed. [1] Adler et al, Fractured porous media, Oxford Univ. Press, 2012. [2] Duan et al, Int. J. Rock Mech. Mining Sci., 65, p75, 2014. [3] Song et al, Marine and Petroleum Eng., 65, p63, 2015.

Neoproterozoic sand wedges: crack formation in frozen soils under diurnal forcing during a snowball Earth

NASA Astrophysics Data System (ADS)

Maloof, Adam C.; Kellogg, James B.; Anders, Alison M.

2002-11-01

Thermal contraction cracking of permafrost produced sand-wedge polygons at sea level on the paleo-equator during late Neoproterozoic glacial episodes. These sand wedges have been used as evidence for high (≥54°) paleo-obliquity of the Earth's ecliptic, because cracks that form wedges are hypothesized to require deep seasonal cooling so the depth of the stressed layer in the ground reaches ≥1 m, similar to the measured depths of cracks that form wedges. To test the counter hypothesis that equatorial cracks opened under a climate characterized by a strong diurnal cycle and low mean annual temperature (snowball Earth conditions), we examine crack formation in frozen ground subject to periodic temperature variations. We derive analytical expressions relating the Newtonian viscosity to the potential crack depth, concluding that cracks will form only in frozen soils with viscosities greater than ˜10 14 Pa s. We also show numerical calculations of crack growth in frozen soils with stress- and temperature-dependent rheologies and find that fractures may propagate to depths 3-25 times the depth of the thermally stressed layer in equatorial permafrost during a snowball Earth because the mean annual temperature is low enough to keep the ground cold and brittle to relatively great depths.

Crack Growth Simulation and Residual Strength Prediction in Airplane Fuselages

NASA Technical Reports Server (NTRS)

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

1999-01-01

The objectives were to create a capability to simulate curvilinear crack growth and ductile tearing in aircraft fuselages subjected to widespread fatigue damage and to validate with tests. Analysis methodology and software program (FRANC3D/STAGS) developed herein allows engineers to maintain aging aircraft economically, while insuring continuous airworthiness, and to design more damage-tolerant aircraft for the next generation. Simulations of crack growth in fuselages were described. The crack tip opening angle (CTOA) fracture criterion, obtained from laboratory tests, was used to predict fracture behavior of fuselage panel tests. Geometrically nonlinear, elastic-plastic, thin shell finite element crack growth analyses were conducted. Comparisons of stress distributions, multiple stable crack growth history, and residual strength between measured and predicted results were made to assess the validity of the methodology. Incorporation of residual plastic deformations and tear strap failure was essential for accurate residual strength predictions. Issue related to predicting crack trajectory in fuselages were also discussed. A directional criterion, including T-stress and fracture toughness orthotropy, was developed. Curvilinear crack growth was simulated in coupon and fuselage panel tests. Both T-stress and fracture toughness orthotropy were essential to predict the observed crack paths. Flapping of fuselages were predicted. Measured and predicted results agreed reasonable well.

Numerical simulations of microcrack-related damage and ignition behavior of mild-impacted polymer bonded explosives.

PubMed

Yang, Kun; Wu, Yanqing; Huang, Fenglei

2018-08-15

A physical model is developed to describe the viscoelastic-plastic deformation, cracking damage, and ignition behavior of polymer-bonded explosives (PBXs) under mild impact. This model improves on the viscoelastic-statistical crack mechanical model (Visco-SCRAM) in several respects. (i) The proposed model introduces rate-dependent plasticity into the framework which is more suitable for explosives with relatively high binder content. (ii) Damage evolution is calculated by the generalized Griffith instability criterion with the dominant (most unstable) crack size rather than the averaged crack size over all crack orientations. (iii) The fast burning of cracks following ignition and the effects of gaseous products on crack opening are considered. The predicted uniaxial and triaxial stress-strain responses of PBX9501 sample under dynamic compression loading are presented to illustrate the main features of the materials. For an uncovered cylindrical PBX charge impacted by a flat-nosed rod, the simulated results show that a triangular-shaped dead zone is formed beneath the front of the rod. The cracks in the dead zone are stable due to friction-locked stress state, whereas the cracks near the front edges of dead zone become unstable and turn into hotspots due to high-shear effects. Copyright © 2018 Elsevier B.V. All rights reserved.

Some advances/results in monitoring road cracks from 2D pavement images within the scope of the collaborative FP7 TRIMM project

NASA Astrophysics Data System (ADS)

Baltazart, Vincent; Moliard, Jean-Marc; Amhaz, Rabih; Wright, Dean; Jethwa, Manish

2015-04-01

Monitoring road surface conditions is an important issue in many countries. Several projects have looked into this issue in recent years, including TRIMM 2011-2014. The objective of such projects has been to detect surface distresses, like cracking, raveling and water ponding, in order to plan effective road maintenance and to afford a better sustainability of the pavement. The monitoring of cracking conventionally focuses on open cracks on the surface of the pavement, as opposed to reflexive cracks embedded in the pavement materials. For monitoring surface condition, in situ human visual inspection has been gradually replaced by automatic image data collection at traffic speed. Off-line image processing techniques have been developed for monitoring surface condition in support of human visual control. Full automation of crack monitoring has been approached with caution, and depends on a proper manual assessment of the performance. This work firstly presents some aspects of the current state of monitoring that have been reported so far in the literature and in previous projects: imaging technology and image processing techniques. Then, the work presents the two image processing techniques that have been developed within the scope of the TRIMM project to automatically detect pavement cracking from images. The first technique is a heuristic approach (HA) based on the search for gradient within the image. It was originally developed to process pavement images from the French imaging device, Aigle-RN. The second technique, the Minimal Path Selection (MPS) method, has been developed within an ongoing PhD work at IFSTTAR. The proposed new technique provides a fine and accurate segmentation of the crack pattern along with the estimation of the crack width. HA has been assessed against the field data collection provided by Yotta and TRL with the imaging device Tempest 2. The performance assessment has been threefold: first it was performed against the reference data set including 130 km of pavement images over UK roads, second over a few selected short sections of contiguous pavement images, and finally over a few sample images as a case study. The performance of MPS has been assessed against an older image data base. Pixel-based PGT was available to provide the most sensitive performance assessment. MPS has shown its ability to provide a very accurate cracking pattern without reducing the image resolution on the segmented images. Thus, it allows measurement of the crack width; it is found to behave more robustly against the image texture and better matched for dealing with low contrast pavement images. The benchmarking of seven automatic segmentation techniques has been provided at both the pixel and the grid levels. The performance assessment includes three minimal path selection algorithms, namely MPS, Free Form Anisotropy (FFA), one geodesic contour with automatic selection of points of interests (GC-POI), HA, and two Markov-based methods. Among others, MPS approach reached the best performance at the pixel level while it is matched to the FFA approach at the grid level. Finally, the project has emphasized the need for a reliable ground truth data collection. Owing to its accuracy, MPS may serve as a reference benchmark for other methods to provide the automatic segmentation of pavement images at the pixel level and beyond. As a counterpart, MPS requires a reduction in the computing time. Keywords: cracking, automatic segmentation, image processing, pavement, surface distress, monitoring, DICE, performance

Three-dimensional CTOA and constraint effects during stable tearing in a thin-sheet material

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Newman, J. C., Jr.; Bigelow, C. A.

1995-01-01

A small strain theory, three-dimensional elastic-plastic finite element analysis was used to simulate fracture in thin sheet 2024-T3 aluminum alloy in the T-L orientation. Both straight and tunneled cracks were modeled. The tunneled crack front shapes as a function of applied stress were obtained from the fracture surface of tested specimens. The stable crack growth behavior was measured at the specimen surface as a function of applied stress. The fracture simulation modeled the crack tunneling and extension as a function of applied stress. The results indicated that the global constraint factor, alpha(sub g), initially dropped during stable crack growth. After peak applied stress was achieved, alpha(sub g) began to increase slightly. The effect of crack front shape on alpha(sub g) was small, but the crack front shape did greatly influence the local constraint and through-thickness crack-tip opening angle (CTOA) behavior. The surface values of CTOA for the tunneled crack front model agreed well with experimental measurements, showing the same initial decrease from high values during the initial 3mm of crack growth at the specimen's surface. At the same time, the interior CTOA values increased from low angles. After the initial stable tearing region, the CTOA was constant through the thickness. The three-dimensional analysis appears to confirm the potential of CTOA as a two-dimensional fracture criterion.

78 FR 73462 - Airworthiness Directives; Bombardier, Inc. Airplanes

Federal Register 2010, 2011, 2012, 2013, 2014

2013-12-06

... fracture was attributed to stress corrosion cracking. Failure of the bolts could compromise the structural... Requirements of This Proposed AD This product has been approved by the aviation authority of another country... have recently been notified that certain laws in other countries do not allow such delegation of...

Micromechanisms of fatigue crack growth in polycarbonate polyurethane: Time dependent and hydration effects.

PubMed

Ford, Audrey C; Gramling, Hannah; Li, Samuel C; Sov, Jessica V; Srinivasan, Amrita; Pruitt, Lisa A

2018-03-01

Polycarbonate polyurethane has cartilage-like, hygroscopic, and elastomeric properties that make it an attractive material for orthopedic joint replacement application. However, little data exists on the cyclic loading and fracture behavior of polycarbonate polyurethane. This study investigates the mechanisms of fatigue crack growth in polycarbonate polyurethane with respect to time dependent effects and conditioning. We studied two commercially available polycarbonate polyurethanes, Bionate® 75D and 80A. Tension testing was performed on specimens at variable time points after being removed from hydration and variable strain rates. Fatigue crack propagation characterized three aspects of loading. Study 1 investigated the impact of continuous loading (24h/day) versus intermittent loading (8-10h/day) allowing for relaxation overnight. Study 2 evaluated the effect of frequency and study 3 examined the impact of hydration on the fatigue crack propagation in polycarbonate polyurethane. Samples loaded intermittently failed instantaneously and prematurely upon reloading while samples loaded continuously sustained longer stable cracks. Crack growth for samples tested at 2 and 5Hz was largely planar with little crack deflection. However, samples tested at 10Hz showed high degrees of crack tip deflection and multiple crack fronts. Crack growth in hydrated samples proceeded with much greater ductile crack mouth opening displacement than dry samples. An understanding of the failure mechanisms of this polymer is important to assess the long-term structural integrity of this material for use in load-bearing orthopedic implant applications. Copyright © 2018 Elsevier Ltd. All rights reserved.

Development of a relationship between external measurements and reinforcement stress

NASA Astrophysics Data System (ADS)

Brault, Andre; Hoult, Neil A.; Lees, Janet M.

2015-03-01

As many countries around the world face an aging infrastructure crisis, there is an increasing need to develop more accurate monitoring and assessment techniques for reinforced concrete structures. One of the challenges associated with assessing existing infrastructure is correlating externally measured parameters such as crack widths and surface strains with reinforcement stresses as this is dependent on a number of variables. The current research investigates how the use of distributed fiber optic sensors to measure reinforcement strain can be correlated with digital image correlation measurements of crack widths to relate external crack width measurements to reinforcement stresses. An initial set of experiments was undertaken involving a series of small-scale beam specimens tested in three-point bending with variable reinforcement properties. Relationships between crack widths and internal reinforcement strains were observed including that both the diameter and number of bars affected the measured maximum strain and crack width. A model that uses measured crack width to estimate reinforcement strain was presented and compared to the experimental results. The model was found to provide accurate estimates of load carrying capacity for a given crack width, however, the model was potentially less accurate when crack widths were used to estimate the experimental reinforcement strains. The need for more experimental data to validate the conclusions of this research was also highlighted.

Crack Growth Prediction Methodology for Multi-Site Damage: Layered Analysis and Growth During Plasticity

NASA Technical Reports Server (NTRS)

James, Mark Anthony

1999-01-01

A finite element program has been developed to perform quasi-static, elastic-plastic crack growth simulations. The model provides a general framework for mixed-mode I/II elastic-plastic fracture analysis using small strain assumptions and plane stress, plane strain, and axisymmetric finite elements. Cracks are modeled explicitly in the mesh. As the cracks propagate, automatic remeshing algorithms delete the mesh local to the crack tip, extend the crack, and build a new mesh around the new tip. State variable mapping algorithms transfer stresses and displacements from the old mesh to the new mesh. The von Mises material model is implemented in the context of a non-linear Newton solution scheme. The fracture criterion is the critical crack tip opening displacement, and crack direction is predicted by the maximum tensile stress criterion at the crack tip. The implementation can accommodate multiple curving and interacting cracks. An additional fracture algorithm based on nodal release can be used to simulate fracture along a horizontal plane of symmetry. A core of plane strain elements can be used with the nodal release algorithm to simulate the triaxial state of stress near the crack tip. Verification and validation studies compare analysis results with experimental data and published three-dimensional analysis results. Fracture predictions using nodal release for compact tension, middle-crack tension, and multi-site damage test specimens produced accurate results for residual strength and link-up loads. Curving crack predictions using remeshing/mapping were compared with experimental data for an Arcan mixed-mode specimen. Loading angles from 0 degrees to 90 degrees were analyzed. The maximum tensile stress criterion was able to predict the crack direction and path for all loading angles in which the material failed in tension. Residual strength was also accurately predicted for these cases.

On crack initiation in notched, cross-plied polymer matrix composites

NASA Astrophysics Data System (ADS)

Yang, Q. D.; Schesser, D.; Niess, M.; Wright, P.; Mavrogordato, M. N.; Sinclair, I.; Spearing, S. M.; Cox, B. N.

2015-05-01

The physics of crack initiation in a polymer matrix composite are investigated by varying the modeling choices made in simulations and comparing the resulting predictions with high-resolution in situ images of cracks. Experimental data were acquired using synchrotron-radiation computed tomography (SRCT) at a resolution on the order of 1 μm, which provides detailed measurement of the location, shape, and size of small cracks, as well as the crack opening and shear displacements. These data prove sufficient to discriminate among competing physical descriptions of crack initiation. Simulations are executed with a high-fidelity formulation, the augmented finite element method (A-FEM), which permits consideration of coupled damage mechanisms, including both discrete cracks and fine-scale continuum damage. The discrete cracks are assumed to be nonlinear fracture events, governed by reasonably general mixed-mode cohesive laws. Crack initiation is described in terms of strength parameters within the cohesive laws, so that the cohesive law provides a unified model for crack initiation and growth. Whereas the cracks investigated are typically 1 mm or less in length, the fine-scale continuum damage refers to irreversible matrix deformation occurring over gauge lengths extending down to the fiber diameter (0.007 mm). We find that the location and far-field stress for crack initiation are predicted accurately only if the variations of local stress within plies and in the presence of stress concentrators (notches, etc.) are explicitly computed and used in initiation criteria; stress redistribution due to matrix nonlinearity that occurs prior to crack initiation is accounted for; and a mixed-mode criterion is used for crack initiation. If these factors are not all considered, which is the case for commonly used failure criteria, predictions of the location and far-field stress for initiation are not accurate.

A Study of Failure in Small Pressurized Cylindrical Shells Containing a Crack

NASA Technical Reports Server (NTRS)

Barwell, Craig A.; Eber, Lorenz; Fyfe, Ian M.

1998-01-01

The deformation in the vicinity of axial cracks in thin pressurized cylinders is examined using small experimental The deformation in the vicinity of axial cracks in thin pressurized cylinders is examined using small experimental models. The loading applied was either symmetric or unsymmetric about the crack plane, the latter being caused by structural constraints such as stringers. The objective was two fold - one, to provide the experimental results which will allow computer modeling techniques to be evaluated for deformations that are significantly different from that experienced by flat plates, and the other to examine the deformations and conditions associated with the onset of crack kinking which often precedes crack curving. The stresses which control crack growth in a cylindrical geometry depend on conditions introduced by the axial bulging, which is an integral part of this type of failure. For the symmetric geometry, both the hoop and radial strain just ahead off the crack, r = a, were measured and these results compared with those obtained from a variety of structural analysis codes, in particular STAGS [1], ABAQUS and ANSYS. In addition to these measurements, the pressures at the onset of stable and unstable crack growth were obtained and the corresponding crack deformations measured as the pressures were increased to failure. For the unsymmetric cases, measurements were taken of the crack kinking angle, and the displacements in the vicinity of the crack. In general, the strains ahead of the crack showed good agreement between the three computer codes and between the codes and the experiments. In the case of crack behavior, it was determined that modeling stable tearing with a crack-tip opening displacement fracture criterion could be successfully combined with the finite-element analysis techniques as used in structural analysis codes. The analytic results obtained in this study were very compatible with the experimental observations of crack growth. Measured crack kinking angles also showed good agreement with theories based on the maximum principle stress criterion.

Simulation model of fatigue crack opening/closing phenomena for predicting RPG load under arbitrary stress distribution field

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

Toyosada, M.; Niwa, T.

1995-12-31

In this paper, Newman`s calculation model is modified to solve his neglected effect of the change of stress distribution ahead of a crack, and to leave elastic plastic materials along the crack surface because of the compatibility of Dugdale model. In addition to above treatment, the authors introduce plastic shrinkage at an immediate generation of new crack surfaces due to emancipation of internal force with the magnitude of yield stress level during unloading process in the model. Moreover, the model is expanded to arbitrary stress distribution field. By using the model, RPG load is simulated for a center notched specimenmore » under constant amplitude loading with various stress ratios and decreased maximum load while keeping minimum load.« less

On the formation of glide-snow avalanches

NASA Astrophysics Data System (ADS)

Mitterer, C.; Schweizer, J.

2012-12-01

On steep slopes the full snowpack can glide on the ground; tension cracks may open and eventually the slope may fail as a glide-snow avalanche. Due to their large mass they have considerable destructive potential. Glide-snow avalanches typically occur when the snow-soil interface is moist or wet so that basal friction is reduced. The occurrence, however, of glide cracks and their evolution to glide avalanches are still poorly understood. Consequently, glides are difficult to predict as (i) not all cracks develop into an avalanche, and (ii) for those that do, the time between crack opening and avalanche event might vary from hours to weeks - or on the other hand be so short that there is no warning at all by crack opening. To improve our understanding we monitored several slopes and related glide snow activity to meteorological data. In addition, we explored conditions that favor the formation of a thin wet basal snowpack layer with a physical-based model representing water and heat flux at the snow-soil interface. The statistical analyses revealed that glide-snow avalanche activity might be associated to an early season and a spring condition. While early season conditions tend to have warm and dry autumns followed by heavy snowfalls, spring conditions showed good agreement with increasing air temperature. The model indicates that energy (summer heat) stored in the ground might be sufficient to melt snow at the bottom of the snowpack. Due to capillary forces, water will rise for a few centimeters into the snowpack and thereby reduce friction at the interface. Alternatively, we demonstrate that also in the absence of melt water production at the bottom of the snowpack water may accumulate in the bottom layer due to an upward flux into the snowpack if a dry snowpack overlies a wet soil. The particular conditions that are obviously required at the snow-soil interface explain the strong winter-to-winter variations in snow gliding.

VLP seismicity from resonant modes of acoustic-gravity waves in a conduit-crack system filled with multiphase magma

NASA Astrophysics Data System (ADS)

Liang, C.; Prochnow, B. N.; OReilly, O. J.; Dunham, E. M.; Karlstrom, L.

2016-12-01

Oscillation of magma in volcanic conduits connected to cracks (dikes and sills) has been suggested as an explanation for very long period (VLP) seismic signals recorded at active basaltic volcanoes such as. Kilauea, Hawaii, and Erebus, Antarctica. We investigate the VLP seismicity using a linearized model for waves in and associated eigenmodes of a coupled conduit-crack system filled with multiphase magma, an extension of the Karlstrom and Dunham (2016) model for acoustic-gravity waves in volcanic conduits. We find that the long period surface displacement (as recorded on broadband seismometers) is dominated by opening/closing of the crack rather than the deformation of the conduit conduit walls. While the fundamental eigenmode is sensitive to the fluid properties and the geometry of the magma plumbing system, a closer scrutiny of various resonant modes reveals that the surface displacement is often more sensitive to higher modes. Here we present a systematic analysis of various long period acoustic-gravity wave resonant modes of a coupled conduit-crack system that the surface displacement is most sensitive to. We extend our previous work on a quasi-one-dimensional conduit model with inviscid magma to a more general axisymmetric conduit model that properly accounts for viscous boundary layers near the conduit walls, based on the numerical method developed by Prochnow et al. (submitted to Computers and Fluids, 2016). The surface displacement is dominated by either the fundamental or higher eigenmodes, depending on magma properties and the geometry of conduit and crack. An examination of the energetics of these modes reveals the complex interplay of different restoring forces (magma compressibility in the conduit, gravity, and elasticity of the crack) driving the VLP oscillations. Both nonequilibrium bubble growth and resorption and viscosity contribute to the damping of VLP signals. Our models thus provide a means to infer properties of open-vent basaltic volcanoes from seismic observations of VLP events.

A Hybrid Approach to Composite Damage and Failure Analysis Combining Synergistic Damage Mechanics and Peridynamics

DTIC Science & Technology

2017-12-31

random radial displacement a fiber is given in simulation of the manufacturing process. As seen in the figure, the crack driving force increases...will incorporate voids along with irregular fiber distributions as consequences of composite manufacturing. The crack opening displacement in the as...subjected to IMPa pressure (ANSYS does not allow the, mathematically equivalent, tensile stresses applied at both ends without any displacement constraints

Effect of Measured Welding Residual Stresses on Crack Growth

NASA Technical Reports Server (NTRS)

Hampton, Roy W.; Nelson, Drew; Doty, Laura W. (Technical Monitor)

1998-01-01

Welding residual stresses in thin plate A516-70 steel and 2219-T87 aluminum butt weldments were measured by the strain-gage hole drilling and X-ray diffraction methods. The residual stress data were used to construct 3D strain fields which were modeled as thermally induced strains. These 3D strain fields were then analyzed with the WARP31) FEM fracture analysis code in order to predict their effect on fatigue and on fracture. For analyses of fatigue crack advance and subsequent verification testing, fatigue crack growth increments were simulated by successive saw-cuts and incremental loading to generate, as a function of crack length, effects on crack growth of the interaction between residual stresses and load induced stresses. The specimen experimental response was characterized and compared to the WARM linear elastic and elastic-plastic fracture mechanics analysis predictions. To perform the fracture analysis, the plate material's crack tearing resistance was determined by tests of thin plate M(T) specimens. Fracture analyses of these specimen were performed using WARP31D to determine the critical Crack Tip Opening Angle [CTOA] of each material. These critical CTOA values were used to predict crack tearing and fracture in the weldments. To verify the fracture predictions, weldment M(T) specimen were tested in monotonic loading to fracture while characterizing the fracture process.

Combined mode I stress intensity factors of slanted cracks

NASA Astrophysics Data System (ADS)

Ismail, A. E.; Rahman, M. Q. Abdul; Ghazali, M. Z. Mohd; Zulafif Rahim, M.; Rasidi Ibrahim, M.; Fahrul Hassan, Mohd; Nor, Nik Hisyamudin Muhd; Ariffin, A. M. T.; Zaini Yunos, Muhamad

2017-08-01

The solutions of stress intensity factors (SIFs) for slanted cracks in plain strain plate are hard to find in open literature. There are some previous solutions of SIFs available, however the studies are not completed except for the case of plain stress. The slanted cracks are modelled numerically using ANSYS finite element program. There are ten slanted angles and seven relative crack depths are used and the plate contains cracks which is assumed to fulfil the plain strain condition. The plate is then stressed under tension and bending loading and the SIFs are determined according to the displacement extrapolation method. Based on the numerical analysis, both slanted angles and relative crack length, a/L played an important role in determining the modes I and II SIFs. As expected the SIFs increased when a/L is increased. Under tension force, the introduction of slanted angles increased the SIFs. Further increment of angles reduced the SIFs however they are still higher than the SIFs obtained using normal cracks. Under bending moment, the present of slanted angles are significantly reduced the SIFs compared with the normal cracks. Under similar loading, mode II SIFs increased as function of a/L and slanted angles where increasing such parameters increasing the mode II SIFs.

High-Resolution Millimeter Wave Detection of Vertical Cracks in the Space Shuttle External Tank (ET) Spray-on-Foam Insulation (SOFI)

NASA Technical Reports Server (NTRS)

Kharkovsky, S.; Zoughi, R.; Hepburn, Frank L.

2006-01-01

Space Shuttle Columbia's catastrophic failure has been attributed to a piece of spray-on-foam insulation (SOFI) that was dislodged from the external tank (ET) and struck the leading edge of the left wing. A piece of SOFI was also dislodged in the Space Shuttle Discovery's flight in 2005 and recently a crack was detected in its ET foam prior to its successful launch. Millimeter wave nondestructive testing methods have been considered as potential effective inspection tools for evaluating the integrity of the SOFI. Recently, in a specific investigation into the potential of these methods for detecting vertical cracks in SOFI was explored using a focused millimeter wave reflectometer at 150 GHz. The results showed the capability of these methods for detecting tight vertical cracks (also as a function of crack opening dimension) in exposed SOFI panels and while covered by a piece of SOFI ramp simulating a more realistic and challenging situation. Some crack-like anomalies were also detected in a blind SOFI panel. This paper presents the background for these techniques as well as representative images of the vertical crack in the SOFI panel, crack-like anomalies in the blind panel and a discussion of the practical attributes of these inspection methods.

Detecting the activation of a self-healing mechanism in concrete by acoustic emission and digital image correlation.

PubMed

Tsangouri, E; Aggelis, D G; Van Tittelboom, K; De Belie, N; Van Hemelrijck, D

2013-01-01

Autonomous crack healing in concrete is obtained when encapsulated healing agent is embedded into the material. Cracking damage in concrete elements ruptures the capsules and activates the healing process by healing agent release. Previously, the strength and stiffness recovery as well as the sealing efficiency after autonomous crack repair was well established. However, the mechanisms that trigger capsule breakage remain unknown. In parallel, the conditions under which the crack interacts with embedded capsules stay black-box. In this research, an experimental approach implementing an advanced optical and acoustic method sets up scopes to monitor and justify the crack formation and capsule breakage of concrete samples tested under three-point bending. Digital Image Correlation was used to visualize the crack opening. The optical information was the basis for an extensive and analytical study of the damage by Acoustic Emission analysis. The influence of embedding capsules on the concrete fracture process, the location of capsule damage, and the differentiation between emissions due to capsule rupture and crack formation are presented in this research. A profound observation of the capsules performance provides a clear view of the healing activation process.

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.

Dynamic behaviour of a rotating cracked beam

NASA Astrophysics Data System (ADS)

Yashar, Ahmed; Ghandchi-Tehrani, Maryam; Ferguson, Neil

2016-09-01

This paper presents a new approach to investigate and analyse the vibrational behaviour of cracked rotating cantilever beams, which can for example represent helicopter or wind turbine blades. The analytical Hamiltonian method is used in modelling the rotating beam and two numerical methods, the Rayleigh-Ritz and FEM, are used to study the natural frequencies and the mode shapes of the intact rotating beams. Subsequently, a crack is introduced into the FE model and simulations are performed to identify the modal characteristics for an open cracked rotating beam. The effect of various parameters such as non-dimensional rotating speed, hub ratio and slenderness ratio are investigated for both the intact and the cracked rotating beam, and in both directions of chordwise and flapwise motion. The veering phenomena in the natural frequencies as a function of the rotational speed and the buckling speed are considered with respect to the slenderness ratio. In addition, the mode shapes obtained for the flapwise vibration are compared using the modal assurance criterion (MAC). Finally, a new three dimensional design chart is produced, showing the effect of crack location and depth on the natural frequencies of the rotating beam. This chart will be subsequently important in identifying crack defects in rotating blades.

Detecting the Activation of a Self-Healing Mechanism in Concrete by Acoustic Emission and Digital Image Correlation

PubMed Central

Tsangouri, E.; Aggelis, D. G.; Van Tittelboom, K.; De Belie, N.; Van Hemelrijck, D.

2013-01-01

Autonomous crack healing in concrete is obtained when encapsulated healing agent is embedded into the material. Cracking damage in concrete elements ruptures the capsules and activates the healing process by healing agent release. Previously, the strength and stiffness recovery as well as the sealing efficiency after autonomous crack repair was well established. However, the mechanisms that trigger capsule breakage remain unknown. In parallel, the conditions under which the crack interacts with embedded capsules stay black-box. In this research, an experimental approach implementing an advanced optical and acoustic method sets up scopes to monitor and justify the crack formation and capsule breakage of concrete samples tested under three-point bending. Digital Image Correlation was used to visualize the crack opening. The optical information was the basis for an extensive and analytical study of the damage by Acoustic Emission analysis. The influence of embedding capsules on the concrete fracture process, the location of capsule damage, and the differentiation between emissions due to capsule rupture and crack formation are presented in this research. A profound observation of the capsules performance provides a clear view of the healing activation process. PMID:24381518

The effect of crack blunting on the competition between dislocation nucleation and cleavage

NASA Astrophysics Data System (ADS)

Fischer, Lisa L.; Beltz, Glenn E.

2001-03-01

To better understand the ductile versus brittle fracture behavior of crystalline materials, attention should be directed towards physically realistic crack geometries. Currently, continuum models of ductile versus brittle behavior are typically based on the analysis of a pre-existing sharp crack in order to use analytical solutions for the stress fields around the crack tip. This paper examines the effects of crack blunting on the competition between dislocation nucleation and atomic decohesion using continuum methods. We accomplish this by assuming that the crack geometry is elliptical, which has the primary advantage that the stress fields are available in closed form. These stress field solutions are then used to calculate the thresholds for dislocation nucleation and atomic decohesion. A Peierls-type framework is used to obtain the thresholds for dislocation nucleation, in which the region of the slip plane ahead of the crack develops a distribution of slip discontinuity prior to nucleation. This slip distribution increases as the applied load is increased until an instability is reached and the governing integral equation can no longer be solved. These calculations are carried out for various crack tip geometries to ascertain the effects of crack tip blunting. The thresholds for atomic decohesion are calculated using a cohesive zone model, in which the region of the crack front develops a distribution of opening displacement prior to atomic decohesion. Again, loading of the elliptical crack tip eventually results in an instability, which marks the onset of crack advance. These calculations are carried out for various crack tip geometries. The results of these separate calculations are presented as the critical energy release rates versus the crack tip radius of curvature for a given crack length. The two threshold curves are compared simultaneously to determine which failure mode is energetically more likely at various crack tip curvatures. From these comparisons, four possible types of material fracture behavior are identified: intrinsically brittle, quasi-brittle, intrinsically ductile, and quasi-ductile. Finally, real material examples are discussed.

Microcracking and Healing in Semibrittle Salt-Rock: Elastic and Plastic Behavior

NASA Astrophysics Data System (ADS)

Ding, J.; Chester, F. M.; Chester, J. S.; Shen, X.; Arson, C. F.

2017-12-01

Microcracking and healing during semibrittle deformation are important processes that affect physical properties such as elastic moduli and permeability. We study these processes through triaxial compression tests involving cyclic differential loading and isostatic-holds on synthetic salt-rock at room temperature and low confining pressure (Pc, 1 to 4 MPa). The salt samples are produced by uniaxial pressing of granular (300 µm dia.) halite to 75 MPa at 150˚C for 10^3 s, to create low-porosity ( 5%) aggregates of nearly equant, work-hardened grains. Alternating large- and small-load cycles are performed to track the evolution of plastic and elastic properties, respecitively, with progressive strain to 8% axial shortening. 24-hour holds are carried out at about 4% axial shortening followed by renewed cyclic loading to investigate healing. During large load cycles samples yield and exhibit distributed flow with dilatancy and small work hardening. Young's Modulus (YM) decreases and then tends to stabilize, while Poisson's Ratio (PR) increases at a reducing rate, with progressive strain. Microstructures at sequential stages show that opening-mode grain-boundary cracking, grain-boundary sliding, and some intracrystalline plasticity are the dominant deformation processes. Opening and shear occur preferentially on boundaries that are parallel and inclined to the shortening axis, respectively, leading to progressive redistribution of porosity. Opening-mode grain-boundary cracks increase in number and aperature with strain, and are linked by sliding grain-boundaries to form en echelon arrays. After a 24-hour hold, samples show yielding and flow behavior consistent with that prior to the hold, whereas YM and PR are reset to the same values documented at zero strain and subsequently evolve with additional strain similar to that documented at smaller strains prior to the hold. Open grain-boundary cracks are not closed or healed during the hold. Observations suggest that changes in elastic properties in the semibrittle salt-rock reflect weakening and healing of grain-boundaries undergoing sliding rather than progressive dilatancy or healing of opening-mode cracks. Findings are being used to inform and develop continuum damage mechanics models of semibrittle deformation in polycrystalline aggregates

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.

Energetic payoff of tool use for capuchin monkeys in the caatinga: variation by season and habitat type.

PubMed

Emidio, Ricardo Almeida; Ferreira, Renata Gonçalves

2012-04-01

In this paper, we analyze predictions from the energetic bottleneck and opportunity models to explain the use of stones to crack open encased fruit by capuchins in dry environments. The energetic bottleneck model argues that tool use derives from the need to crack open hard-encased fruits which are key resources during periods of food scarcity. The opportunity model argues that tool use by capuchins derives from simultaneous access to stones and encased fruits. The study was conducted in the Caatinga biome, northeastern Brazil, at two areas where capuchin monkeys (Sapajus libidinosus and Sapajus spp.) regularly use stones to crack open encased fruit of Syagrus cearensis and Manihot dichotoma. Energetic gains were inferred based on the number of tool-use sites used and the mass of encased fruit consumed per month, and compared across seasons and areas occupied by the two groups. For the drier habitat, a significant increase in frequency of tool use (N(dry) = 329 vs. N(wet) = 59) and in the mean monthly mass of fruits consumed in the dry season (mean(dry) = 193g vs. mean(wet) = 13.5 g) offered support for the energetic bottleneck model. However, our inference of low energetic payoffs for tool using individuals (in the drier caatinga habitat from 13 to 193 cal·ind(-1) ·month(-1) and in the wetter caatinga habitat from 805 to 1150 cal·ind(-1) ·month(-1) ) offer support for the opportunity model. Finally, our analyses indicate that consumption of six S. cearensis fruits would equal the daily requirements of capuchins for β-carotene, and the consumption of 1.22 g·day(-1) of M. dichotoma encased fruit or 1.0 g·day(-1) of S. cearensis can supply capuchin's daily requirement of vitamin C. So, specific nutritional requirements may play a role in explaining the continuous consumption of encased fruit and customary use of stones to crack open encased fruit. © 2011 Wiley Periodicals, Inc.

Research on Crack Formation in Gypsum Partitions with Doorway by Means of FEM and Fracture Mechanics

NASA Astrophysics Data System (ADS)

Kania, Tomasz; Stawiski, Bohdan

2017-10-01

Cracking damage in non-loadbearing internal partition walls is a serious problem that frequently occurs in new buildings within the short term after putting them into service or even before completion of construction. Damage in partition walls is sometimes so great that they cannot be accepted by their occupiers. This problem was illustrated by the example of damage in a gypsum partition wall with doorway attributed to deflection of the slabs beneath and above it. In searching for the deflection which causes damage in masonry walls, fracture mechanics applied to the Finite Element Method (FEM) have been used. For a description of gypsum behaviour, the smeared cracking material model has been selected, where stresses are transferred across the narrowly opened crack until its width reaches the ultimate value. Cracks in the Finite Element models overlapped the real damage observed in the buildings. In order to avoid cracks under the deflection of large floor slabs, the model of a wall with reinforcement in the doorstep zone and a 40 mm thick elastic junction between the partition and ceiling has been analysed.

Fracture mechanics life analytical methods verification testing

NASA Technical Reports Server (NTRS)

Favenesi, J. A.; Clemmons, T. G.; Lambert, T. J.

1994-01-01

Verification and validation of the basic information capabilities in NASCRAC has been completed. The basic information includes computation of K versus a, J versus a, and crack opening area versus a. These quantities represent building blocks which NASCRAC uses in its other computations such as fatigue crack life and tearing instability. Several methods were used to verify and validate the basic information capabilities. The simple configurations such as the compact tension specimen and a crack in a finite plate were verified and validated versus handbook solutions for simple loads. For general loads using weight functions, offline integration using standard FORTRAN routines was performed. For more complicated configurations such as corner cracks and semielliptical cracks, NASCRAC solutions were verified and validated versus published results and finite element analyses. A few minor problems were identified in the basic information capabilities of the simple configurations. In the more complicated configurations, significant differences between NASCRAC and reference solutions were observed because NASCRAC calculates its solutions as averaged values across the entire crack front whereas the reference solutions were computed for a single point.

An electro-mechanical impedance model of a cracked composite beam with adhesively bonded piezoelectric patches

NASA Astrophysics Data System (ADS)

Yan, Wei; Cai, J. B.; Chen, W. Q.

2011-01-01

A model of a laminated composite beam including multiple non-propagating part-through surface cracks as well as installed PZT transducers is presented based on the method of reverberation-ray matrix (MRRM) in this paper. Toward determining the local flexibility characteristics induced by the individual cracks, the concept of the massless rotational spring is applied. A Timoshenko beam theory is then used to simulate the behavior of the composite beam with open cracks. As a result, transverse shear and rotatory inertia effects are included in the model. Only one-dimensional axial vibration of the PZT wafer is considered and the imperfect interfacial bonding between PZT patches and the host beam is further investigated based on a Kelvin-type viscoelastic model. Then, an accurate electro-mechanical impedance (EMI) model can be established for crack detection in laminated beams. In this model, the effects of various parameters such as the ply-angle, fibre volume fraction, crack depth and position on the EMI signatures are highlighted. Furthermore, comparison with existent numerical results is presented to validate the present analysis.

Isolated Mesoporous Microstructures Prepared by Stress Localization-Induced Crack Manipulation.

PubMed

Wooh, Sanghyuk; Lee, Soojin; Lee, Yunchan; Ryu, Ji Ho; Lee, Won Bo; Yoon, Hyunsik; Char, Kookheon

2016-09-22

Cracks observed in brittle materials are mostly regarded as defects or failures. However, they can be a valuable tool when implemented in a controlled way. Here, we introduce a strategy to control the crack propagation of mesoporous micropatterns (prisms and pyramids), which leads to the isolation of well-defined microstructures. Mesoporous micropatterns were fabricated by the soft imprinting technique with wet TiO 2 nanoparticle (NP) pastes, followed by sintering to remove organic components. Since the volume of the paste significantly shrinks during the sintering step, stress is localized at the edge of micropatterns, in good agreement with finite element method simulations, creating well-defined cracks and their propagation. It was demonstrated that the degree of stress localization is determined by the thickness of residual layers, NP size, and heating rate. After controlled crack propagation and delamination of microparticles from the substrates, mesoporous microwires and microparticles were successfully produced and functionalized from the isolated mesoporous prisms and pyramids. The method proposed in this study for controlled crack manipulation and delamination opens a door for straightforward and economical fabrication of well-defined mesoporous microparticles.

Opening Up Without Cracking Up.

ERIC Educational Resources Information Center

Erickson, Maggie

In recognition of the fact that there are individual differences and preferences among teachers, the teachers at the Las Posas School in Camarillo, California were given the opportunity to use the methods they found most comfortable. This resulted in a variety of open classroom situations, with alternatives for parents, students, and teachers.…

Removing obstacles for pavement cost reduction by examining early age opening requirements : material properties.

DOT National Transportation Integrated Search

2015-08-01

The risk of cracking in a concrete pavement that is opened to traffic at early ages is related to the maximum tensile stress, , that : develops in the pavement and its relationship to the measured, age dependent, flexural strength of a beam, fr . T...

Three-dimensional analysis of chevron-notched specimens by boundary integral method

NASA Technical Reports Server (NTRS)

Mendelson, A.; Ghosn, L.

1983-01-01

The chevron-notched short bar and short rod specimens was analyzed by the boundary integral equations method. This method makes use of boundary surface elements in obtaining the solution. The boundary integral models were composed of linear triangular and rectangular surface segments. Results were obtained for two specimens with width to thickness ratios of 1.45 and 2.00 and for different crack length to width ratios ranging from 0.4 to 0.7. Crack opening displacement and stress intensity factors determined from displacement calculations along the crack front and compliance calculations were compared with experimental values and with finite element analysis.

An analytical approach for the calculation of stress-intensity factors in transformation-toughened ceramics

NASA Astrophysics Data System (ADS)

Müller, W. H.

1990-12-01

Stress-induced transformation toughening in Zirconia-containing ceramics is described analytically by means of a quantitative model: A Griffith crack which interacts with a transformed, circular Zirconia inclusion. Due to its volume expansion, a ZrO2-particle compresses its flanks, whereas a particle in front of the crack opens the flanks such that the crack will be attracted and finally absorbed. Erdogan's integral equation technique is applied to calculate the dislocation functions and the stress-intensity-factors which correspond to these situations. In order to derive analytical expressions, the elastic constants of the inclusion and the matrix are assumed to be equal.

Elevated temperature crack growth

NASA Technical Reports Server (NTRS)

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

1985-01-01

The objective of the Elevated Temperature Crack Growth Project is to evaluate proposed nonlinear fracture mechanics methods for application to combustor liners of aircraft gas turbine engines. During the first year of this program, proposed path-independent (P-I) integrals were reviewed for such applications. Several P-I integrals were implemented into a finite-element postprocessor which was developed and verified as part of the work. Alloy 718 was selected as the analog material for use in the forthcoming experimental work. A buttonhead, single-edge notch specimen was designed and verified for use in elevated-temperature strain control testing with significant inelastic strains. A crack mouth opening displacement measurement device was developed for further use.

Role of NSO compounds during primary cracking of a Type II kerogen and a Type III lignite

USGS Publications Warehouse

Behar, F.; Lorant, F.; Lewan, M.

2008-01-01

The aim of this work is to follow the generation of NSO compounds during the artificial maturation of an immature Type II kerogen and a Type III lignite in order to determine the different sources of the petroleum potential during primary cracking. Experiments were carried out in closed system pyrolysis in the temperature range from 225 to 350 ??C. Two types of NSOs were recovered: one is soluble in n-pentane and the second in dichloromethane. A kinetic scheme was optimised including both kerogen and NSO cracking. It was validated by complementary experiments carried out on isolated asphaltenes generated from the Type II kerogen and on the total n-pentane and DCM extracts generated from the Type III lignite. Results show that kerogen and lignite first decompose into DCM NSOs with minor generation of hydrocarbons. Then, the main source of petroleum potential originates from secondary cracking of both DCM and n-pentane NSOs through successive decomposition reactions. These results confirm the model proposed by Tissot [Tissot, B., 1969. Premie??res donne??es sur les me??canismes et la cine??tique de la formation du pe??trole dans les bassins se??dimentaires. Simulation d'un sche??ma re??actionnel sur ordinateur. Oil and Gas Science and Technology 24, 470-501] in which the main source of hydrocarbons is not the insoluble organic matter, but the NSO fraction. As secondary cracking of the NSOs largely overlaps that of the kerogen, it was demonstrated that bulk kinetics in open system is a result of both kerogen and NSO cracking. Thus, another kinetic scheme for primary cracking in open system was built as a combination of kerogen and NSO cracking. This new kinetic scheme accounts for both the rate and amounts of hydrocarbons generated in a closed pyrolysis system. Thus, the concept of successive steps for hydrocarbon generation is valid for the two types of pyrolysis system and, for the first time, a common kinetic scheme is available for extrapolating results to natural case studies. ?? 2007 Elsevier Ltd. All rights reserved.

Manufacturing and characterization of a ceramic single-use microvalve

NASA Astrophysics Data System (ADS)

Khaji, Z.; Klintberg, L.; Thornell, G.

2016-09-01

We present the manufacturing and characterization of a ceramic single-use microvalve with the potential to be integrated in lab-on-a-chip devices, and forsee its utilization in space and other demanding applications. A 3 mm diameter membrane was used as the flow barrier, and the opening mechanism was based on cracking the membrane by inducing thermal stresses on it with fast and localized resistive heating. Four manufacturing schemes based on high-temperature co-fired ceramic technology were studied. Three designs for the integrated heaters and two thicknesses of 40 and 120 μm for the membranes were considered, and the heat distribution over their membranes, the required heating energies, their opening mode, and the flows admitted through were compared. Furthermore, the effect of applying  +1 and  -1 bar pressure difference on the membrane during cracking was investigated. Thick membranes demonstrated unpromising results for low-pressure applications since the heating either resulted in microcracks or cracking of the whole chip. Because of the higher pressure tolerance of the thick membranes, the design with microcracks can be considered for high-pressure applications where flow is facilitated anyway. Thin membranes, on the other hand, showed different opening sizes depending on heater design and, consequently, heat distribution over the membranes, from microcracks to holes with sizes of 3-100% of the membrane area. For all the designs, applying  +1 bar over pressure contributed to bigger openings, whereas  -1 bar pressure difference only did so for one of the designs, resulting in smaller openings for the other two. The energy required for breaking these membranes was a few hundred mJ with no significant dependence on design and applied pressure. The maximum sustainable pressure of the valve for the current design and thin membranes was 7 bar.

Brittle Fracture Mechanics of Snow : In Situ Testing and Distinct Element Modeling

NASA Astrophysics Data System (ADS)

Faillettaz, J.; Daudon, D.; Louchet, F.

A snow slab avalanche release usually results from the rupture of the snow cover at the interface between an upper layer (slab) and an underlying substrate. Amazingly, the models proposed so far to predict this kind of rupture were only based on continuum mechanics, as they did not take into account the existing cracks or cohesion defects at the interface between the two layers, and their possible unstable propagation that eventually triggers the avalanche. This is why the present work, essentially devoted to human triggered avalanches, is based instead on Griffith's fracture approach, widely used in modelling brittle fracture of materials. The possible rupture scenario involves a propagation in a shear mode of a "basal crack" nucleated and gradually grown at the interface by the skier's weight, followed by a mode I opening and propagation of a "crown crack" at the top of the sheared zone. Different avalanche sizes are predicted according whether the basal crack propagation reaches or not the Griffith's instabil- ity size before crown crack opening (Louchet 2000). Accurate predictions therefore require a precise knowledge of snow toughness values in both modes. A theoretical estimation of toughness considering snow as an ice foam was proposed by Kirchner and Michot (2000), but the question of whether these results may be extended to an assembly of sintered grains is still open. A mode I toughness measurement of snow was also published for the first time by Kirchner and Michot on samples gathered in the Vosges range. In the present work, we developed an experimental set similar to Michot's, in order to measure mode I toughness: a vertical crack of increasing size is gradually machined from the top surface in an horizontal snow beam until failure takes place under its own weight. The toughness value is computed from the snow weight and the crack length at the onset of rapid crack propagation. A similar device was designed for mode II testing, but is still under development. The experimental cam- paign carried out in the Alps during the 2000-2001 winter on homogeneous sintered snow with a density of 200 kg/m3 (typical of a snow slab) gave results of the same or- der of magnitude as Michot's. A numerical modeling of these toughness experiments was performed using a distinct element code, considering snow as a cohesive granu- lar material. Both crack propagation and rupture patterns are in close agreement with experiments. References: Kirchner, Michot, Suzuki 2000 Fracture thoughness of snow in tension 1 Philisophical Magazine A, vol 80,N5, p1265-1272. Louchet 2001,A transition in dry snow slab avalanche triggering modes, Annales de glaciologie, vol 32,Symphosium on Snow, Avalanches and Impact of the Frest Cover, Innsbruck,Austria,22-26 may 2000, p2285-289 2

A linear least squares approach for evaluation of crack tip stress field parameters using DIC

NASA Astrophysics Data System (ADS)

Harilal, R.; Vyasarayani, C. P.; Ramji, M.

2015-12-01

In the present work, an experimental study is carried out to estimate the mixed-mode stress intensity factors (SIF) for different cracked specimen configurations using digital image correlation (DIC) technique. For the estimation of mixed-mode SIF's using DIC, a new algorithm is proposed for the extraction of crack tip location and coefficients in the multi-parameter displacement field equations. From those estimated coefficients, SIF could be extracted. The required displacement data surrounding the crack tip has been obtained using 2D-DIC technique. An open source 2D DIC software Ncorr is used for the displacement field extraction. The presented methodology has been used to extract mixed-mode SIF's for specimen configurations like single edge notch (SEN) specimen and centre slant crack (CSC) specimens made out of Al 2014-T6 alloy. The experimental results have been compared with the analytical values and they are found to be in good agreement, thereby confirming the accuracy of the algorithm being proposed.

An Effective Modal Approach to the Dynamic Evaluation of Fracture Toughness of Quasi-Brittle Materials

NASA Astrophysics Data System (ADS)

Ferreira, L. E. T.; Vareda, L. V.; Hanai, J. B.; Sousa, J. L. A. O.; Silva, A. I.

2017-05-01

A modal dynamic analysis is used as the tool to evaluate the fracture toughness of concrete from the results of notched-through beam tests. The dimensionless functions describing the relation between the frequencies and specimen geometry used for identifying the variation in the natural frequency as a function of crack depth is first determined for a 150 × 150 × 500-mm notched-through specimen. The frequency decrease resulting from the propagating crack is modeled through a modal/fracture mechanics approach, leading to determination of an effective crack length. This length, obtained numerically, is used to evaluate the fracture toughness of concrete, the critical crack mouth opening displacements, and the brittleness index proposed. The methodology is applied to tests performed on high-strength concrete specimens. The frequency response for each specimen is evaluated before and after each crack propagation step. The methodology is then validated by comparison with results from the application of other methodologies described in the literature and suggested by RILEM.

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.

High Resolution Millimeter Wave Detection of Vertical Cracks in the Space Shuttle External Tank Spray-On-Foam Insulation (SOFI)

NASA Technical Reports Server (NTRS)

Kharkovsky, S.; Zoughi, R.; Hepburn, F.

2006-01-01

Space Shuttle Columbia s catastrophic failure, the separation of a piece of spray-on-foam insulation (SOFI) from the external tank (ET) in the Space Shuttle Discovery s flight in 2005 and crack detected in its ET foam prior to its successful launch in 2006 emphasize the need for effective nondestructive methods for inspecting the shuttle ET SOFI. Millimeter wave nondestructive testing methods have been considered as potential and effective inspection tools for evaluating the integrity of the SOFI. This paper presents recent results of an investigation for the purpose of detecting vertical cracks in SOFI panels using a focused millimeter wave (150 GHz) reflectometer. The presented images of the SOFI panels show the capability of this reflectometer for detecting tight vertical cracks (also as a function of crack opening dimension) in exposed SOFI panels and while covered by a piece of SOFI ramp simulating a more realistic and challenging situation.

Advanced quantitative magnetic nondestructive evaluation methods - Theory and experiment

NASA Technical Reports Server (NTRS)

Barton, J. R.; Kusenberger, F. N.; Beissner, R. E.; Matzkanin, G. A.

1979-01-01

The paper reviews the scale of fatigue crack phenomena in relation to the size detection capabilities of nondestructive evaluation methods. An assessment of several features of fatigue in relation to the inspection of ball and roller bearings suggested the use of magnetic methods; magnetic domain phenomena including the interaction of domains and inclusions, and the influence of stress and magnetic field on domains are discussed. Experimental results indicate that simplified calculations can be used to predict many features of these results; the data predicted by analytic models which use finite element computer analysis predictions do not agree with respect to certain features. Experimental analyses obtained on rod-type fatigue specimens which show experimental magnetic measurements in relation to the crack opening displacement and volume and crack depth should provide methods for improved crack characterization in relation to fracture mechanics and life prediction.

Numerical calibration of the stable poisson loaded specimen

NASA Technical Reports Server (NTRS)

Ghosn, Louis J.; Calomino, Anthony M.; Brewer, Dave N.

1992-01-01

An analytical calibration of the Stable Poisson Loaded (SPL) specimen is presented. The specimen configuration is similar to the ASTM E-561 compact-tension specimen with displacement controlled wedge loading used for R-Curve determination. The crack mouth opening displacements (CMOD's) are produced by the diametral expansion of an axially compressed cylindrical pin located in the wake of a machined notch. Due to the unusual loading configuration, a three-dimensional finite element analysis was performed with gap elements simulating the contact between the pin and specimen. In this report, stress intensity factors, CMOD's, and crack displacement profiles are reported for different crack lengths and different contacting conditions. It was concluded that the computed stress intensity factor decreases sharply with increasing crack length, thus making the SPL specimen configuration attractive for fracture testing of brittle, high modulus materials.

Analytical stress intensity solution for the Stable Poisson Loaded specimen

NASA Technical Reports Server (NTRS)

Ghosn, Louis J.; Calomino, Anthony M.; Brewer, David N.

1993-01-01

An analytical calibration of the Stable Poisson Loaded (SPL) specimen is presented. The specimen configuration is similar to the ASTM E-561 compact-tension specimen with displacement controlled wedge loading used for R-curve determination. The crack mouth opening displacements (CMODs) are produced by the diametral expansion of an axially compressed cylindrical pin located in the wake of a machined notch. Due to the unusual loading configuration, a three-dimensional finite element analysis was performed with gap elements simulating the contact between the pin and specimen. In this report, stress intensity factors, CMODs, and crack displacement profiles, are reported for different crack lengths and different contacting conditions. It was concluded that the computed stress intensity factor decreases sharply with increasing crack length thus making the SPL specimen configuration attractive for fracture testing of brittle, high modulus materials.

Understanding and Exploiting the Effects of Loading on Ultrasonic Sensing Systems for Structural Health Monitoring

DTIC Science & Technology

2012-02-01

method to image fatigue cracks without requiring damage-free baseline measurements. Load-differential imaging maps changes in ultrasonic signals...caused by a small increase in applied load to an image, which enables detecting and locating fatigue cracks that open under load and thus distinguishing...them from other load-dependent effects. This method was successfully demonstrated in the laboratory during fatigue tests on a variety of aluminum

A Computational Efficient Physics Based Methodology for Modeling Ceramic Matrix Composites (Preprint)

DTIC Science & Technology

2011-11-01

elastic range, and with some simple forms of progressing damage . However, a general physics-based methodology to assess the initial and lifetime... damage evolution in the RVE for all possible load histories. Microstructural data on initial configuration and damage progression in CMCs were...the damaged elements will have changed, hence, a progressive damage model. The crack opening for each crack type in each element is stored as a

FUNDAMENTAL PROPERTIES OF ULTRA HIGH PERFORMANCE-STRAIN HARDENING CEMENTITIOUS COMPOSITES AND USAGE FOR REPAIR

NASA Astrophysics Data System (ADS)

Kunieda, Minoru; Shimizu, Kosuke; Eguchi, Teruyuki; Ueda, Naoshi; Nakamura, Hikaru

This paper presents the fundamental properties of Ultra High Performance-Strain Hardening Cementitious Composites (UHP-SHCC), which were depeloped for repair applications. In particular, mechanical properties such as tensile response, shrinkage and bond strength were investigated experimentally. Protective performance of the material such as air permeability, water permeability and penetration of chloride ion was also confirmed comparing to that of ordinary concrete. This paper also introduces the usage of the material in repair of concrete st ructures. Laboratory tests concerining the deterioration induced by corrosion were conducted. The UHP-SHCC that coverd the RC beam resisted not only crack opening along the rebar due to corrosion but also crack opening due to loading tests.

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...

76 FR 58098 - Airworthiness Directives; The Boeing Company Airplanes

Federal Register 2010, 2011, 2012, 2013, 2014

2011-09-20

.... ACTION: Final rule. SUMMARY: We are adopting a new airworthiness directive (AD) for all The Boeing... inspections for cracks on the area around certain fasteners of the access opening doubler on the left and... area around certain fasteners of the access opening doubler on the left and right wing center spar...

Kinematics and thermal conditions in the permafrost-affected rockwalls of the Aiguille du Midi (3842 m a.s.l., Mont Blanc massif, France)

NASA Astrophysics Data System (ADS)

Ravanel, Ludovic; Grangier, Germain; Weber, Samuel; Beutel, Jan; Magnin, Florence; Gruber, Stefan; Deline, Philip

2016-04-01

Processes that control climate-dependent rockfall from permafrost-affected rock slopes are still poorly understood. In this study, we present the results of a Wireless Sensor Network, integrated within the Swiss project PermaSense and developed in 2012, to measure rock temperature and geotechnical parameters in the steep rockwalls of the Aiguille du Midi (AdM, 3842 m a.s.l., Mont Blanc massif, France). Accessible year round by cable car, the AdM comprises two main peaks: (i) the Piton Nord with the cable car arrival station, where 4 crack-meters are placed on four major fractures, and (ii) the Piton Central with many touristic infrastructure, equipped with three 10-m-deep boreholes with 15 temperatures sensors since 2009, and where 2 crack-meters are installed along a major fracture. Three major kinematic regimes are observed: (i) opening of clefts when the rock temperature becomes positive, followed by closing during the cold period, (ii) summer opening continued by a winter opening, and (iii) closing during the warm period followed by opening in winter.

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.

Surface cracks as a long-term record of Andean plate boundary segmentation

NASA Astrophysics Data System (ADS)

Loveless, J. P.; Allmendinger, R. W.; Pritchard, M. E.

2007-12-01

Meter-scale surface cracks throughout the northern Chilean and southern Peruvian forearcs provide a long-term record of seismic segmentation along the Andean plate boundary. The cracks, mapped on high-resolution satellite imagery, show strong preferred orientations over large regions and the mean strikes of cracks vary systematically as a function of position along the margin. The spatial scale of this variation suggests that stress fields operating with similar dimensions, namely those produced by strong subduction zone earthquakes, are primarily responsible for crack evolution. The orientations of cracks are consistent with the static and dynamic coseismic stress fields calculated for several recent and historical earthquakes on distinct segments of the subduction interface. Field observations indicate that the cracks have experienced multiple episodes of opening and proximal age evidence suggests that they represent deformation as old as several hundred thousand years. We invert the crack orientation data to solve for plausible slip distributions on the Iquique, Chile segment of the margin (19°--23° S), which last ruptured in a M~8--9 event in 1877. We find that concentrations of coseismic slip resolved by the inversion coincide spatially with negative gravity anomalies, consistent with recent studies correlating subduction zone earthquake slip with forearc structure. These results suggest that distinct seismic segments or asperities on the subduction interface define characteristic earthquakes with rupture dimensions and magnitudes that are similar over many seismic cycles.

Surface cracks as a long-term record of Andean plate boundary segmentation

NASA Astrophysics Data System (ADS)

Loveless, J. P.; Allmendinger, R. W.; Pritchard, M. E.

2004-12-01

Meter-scale surface cracks throughout the northern Chilean and southern Peruvian forearcs provide a long-term record of seismic segmentation along the Andean plate boundary. The cracks, mapped on high-resolution satellite imagery, show strong preferred orientations over large regions and the mean strikes of cracks vary systematically as a function of position along the margin. The spatial scale of this variation suggests that stress fields operating with similar dimensions, namely those produced by strong subduction zone earthquakes, are primarily responsible for crack evolution. The orientations of cracks are consistent with the static and dynamic coseismic stress fields calculated for several recent and historical earthquakes on distinct segments of the subduction interface. Field observations indicate that the cracks have experienced multiple episodes of opening and proximal age evidence suggests that they represent deformation as old as several hundred thousand years. We invert the crack orientation data to solve for plausible slip distributions on the Iquique, Chile segment of the margin (19°--23° S), which last ruptured in a M~8--9 event in 1877. We find that concentrations of coseismic slip resolved by the inversion coincide spatially with negative gravity anomalies, consistent with recent studies correlating subduction zone earthquake slip with forearc structure. These results suggest that distinct seismic segments or asperities on the subduction interface define characteristic earthquakes with rupture dimensions and magnitudes that are similar over many seismic cycles.

Acrylate-endcapped polymer precursors: effect of chemical composition on the healing efficiency of active concrete cracks

NASA Astrophysics Data System (ADS)

Araújo, Maria; Van Tittelboom, Kim; Dubruel, Peter; Van Vlierberghe, Sandra; De Belie, Nele

2017-05-01

The repair of cracks in concrete is an unavoidable practice since these cracks endanger the durability of the structure. Inspired by nature, the self-healing concept has been widely investigated in concrete as a promising solution to solve the limitations of manual repair. This self-healing functionality may be realized by the incorporation of encapsulated healing agents in concrete. Depending on the nature of the cracks, different healing agents can be used. For structures subjected to repeated loads, elastic materials should be considered to cope with the crack opening and closing movement. In this study, various acrylate-endcapped polymer precursors were investigated for their suitability to heal active cracks. The strain capacity of the polymers was assessed by means of visual observation together with water flow tests after widening of the healed cracks in a stepwise manner. A strain of at least 50% could be sustained by epoxy- and siloxane-based healing agents. For polyester- and urethane/poly(propylene glycol)-based precursors, failure occurred at 50% elongation due to detachment of the polymer from the crack walls. However, for urethane/poly(propylene glycol)-based healing agent, debonding was limited to some local spots. The resistance of the polymerized healing agents against degradation in the strong alkaline environment characteristic for concrete has also been evaluated, with the urethane/poly(propylene glycol)-based precursor showing the best performance to withstand degradation.

Creep and cracking of concrete hinges: insight from centric and eccentric compression experiments.

PubMed

Schlappal, Thomas; Schweigler, Michael; Gmainer, Susanne; Peyerl, Martin; Pichler, Bernhard

2017-01-01

Existing design guidelines for concrete hinges consider bending-induced tensile cracking, but the structural behavior is oversimplified to be time-independent. This is the motivation to study creep and bending-induced tensile cracking of initially monolithic concrete hinges systematically. Material tests on plain concrete specimens and structural tests on marginally reinforced concrete hinges are performed. The experiments characterize material and structural creep under centric compression as well as bending-induced tensile cracking and the interaction between creep and cracking of concrete hinges. As for the latter two aims, three nominally identical concrete hinges are subjected to short-term and to longer-term eccentric compression tests. Obtained material and structural creep functions referring to centric compression are found to be very similar. The structural creep activity under eccentric compression is significantly larger because of the interaction between creep and cracking, i.e. bending-induced cracks progressively open and propagate under sustained eccentric loading. As for concrete hinges in frame-like integral bridge construction, it is concluded (i) that realistic simulation of variable loads requires consideration of the here-studied time-dependent behavior and (ii) that permanent compressive normal forces shall be limited by 45% of the ultimate load carrying capacity, in order to avoid damage of concrete hinges under sustained loading.

Comparison of Visual and Acoustic Emission Observations in a Four Point Bending Experiment on Barre Granite

NASA Astrophysics Data System (ADS)

Li, Bing Qiuyi; Einstein, Herbert H.

2017-09-01

We present an experimental study in which a pre-notched specimen of Barre Granite was subjected to four point bending under crack mouth opening displacement control. The experimental observations consisted of load-displacement measurements, acoustic emissions, and photography on a macroscopic ( cm) as well as microscopic ( μm) scale. These observations were compared and analysed to better understand process zone development and crack propagation. Load-displacement data showed that the load reaches its maximum at crack initiation, and the machine input work is constant while the crack propagates. AE moment magnitudes between Mw = -6 to -10 were observed, and focal mechanisms consisted of both shear and tensile components. During process zone development, AE formed a large cloud of events located near the notch tip and then tended to occur away from the notch tip as the crack propagated. Image analysis at the microscopic scale showed that microcracks formed and coalesced during process zone development; specifically, the microcracks initiated in tension and then propagated as a series of en-echelon cracks. In general, the synthesis of the three observations showed that a wider bulb of activity at lower energy tended to occur during process zone development, while crack propagation tended to be more spatially concentrated and contained higher energy.

Fracture mechanics analysis for various fiber/matrix interface loadings

NASA Technical Reports Server (NTRS)

Naik, R. A.; Crews, J. H., Jr.

1991-01-01

Fiber/matrix (F/M) cracking was analyzed to provide better understanding and guidance in developing F/M interface fracture toughness tests. Two configurations, corresponding to F/M cracking at a broken fiber and at the free edge, were investigated. The effects of mechanical loading, thermal cooldown, and friction were investigated. Each configuration was analyzed for two loadings: longitudinal and normal to the fiber. A nonlinear finite element analysis was performed to model friction and slip at the F/M interface. A new procedure for fitting a square-root singularity to calculated stresses was developed to determine stress intensity factors (K sub I and K sub II) for a bimaterial interface crack. For the case of F/M cracking at a broken fiber with longitudinal loading, crack tip conditions were strongly influenced by interface friction. As a result, an F/M interface toughness test based on this case was not recommended because nonlinear data analysis methods would be required. For the free edge crack configuration, both mechanical and thermal loading caused crack opening, thereby avoiding frictional effects. A F/M interface toughness test based on this configuration would provide data for K(sub I)/K(sub II) ratios of about 0.7 and 1.6 for fiber and radial normal loading, respectively. However, thermal effects must be accounted for in the data analysis.

Fracture mechanics analysis for various fiber/matrix interface loadings

NASA Technical Reports Server (NTRS)

Naik, Rajiv A.; Crews, John H., Jr.

1992-01-01

Fiber/matrix (F/M) cracking was analyzed to provide better understanding and guidance in developing F/M interface fracture toughness tests. Two configurations, corresponding to F/M cracking at a broken fiber and at the free edge, were investigated. The effects of mechanical loading, thermal cooldown, and friction were investigated. Each configuration was analyzed for two loadings: longitudinal and normal to the fiber. A nonlinear finite element analysis was performed to model friction and slip at the F/M interface. A new procedure for fitting a square-root singularity to calculated stresses was developed to determine stress intensity factors (K sub I and K sub II) for a bimaterial interface crack. For the case of F/M cracking at a broken fiber with longitudinal loading, crack tip conditions were strongly influenced by interface friction. As a result, an F/M interface toughness test based on this case was not recommended because nonlinear data analysis methods would be required. For the free edge crack configuration, both mechanical and thermal loading caused crack opening, theory avoiding fractional effects. A F/M interface toughness test based on this configuration would provide data for K(sub I/K(sub II) ratios of about 0.7 and 1.6 for fiber and radial normal loading, respectively. However, thermal effects must be accounted for in the data analysis.

Fracture mechanics analysis for various fiber/matrix interface loadings

NASA Technical Reports Server (NTRS)

Naik, R. A.; Crews, J. H., Jr.

1991-01-01

Fiber/matrix (F/M) cracking was analyzed to provide better understanding and guidance in developing F/M interface fracture toughness tests. Two configurations, corresponding to F/M cracking at a broken fiber and at the free edge, were investigated. The effects of mechanical loading, thermal cooldown, and friction were investigated. Each configuration was analyzed for two loadings: longitudinal and normal to the fiber. A nonlinear finite element analysis was performed to model friction and slip at the F/M interface. A new procedure for fitting a square-root singularity to calculated stresses was developed to determine stress intensity factors (K sub I and K sub II) for a bimaterial interface crack. For the case of F/M cracking at a broken fiber with longitudinal loading, crack tip conditions were strongly influenced by interface friction. As a result, an F/M interface toughness test based on this case was not recommended because nonlinear data analysis methods would be required. For the free edge crack configuration, both mechanical and thermal loading caused crack opening, thereby avoiding frictional effects. An F/M interface toughness test based on this configuration would provide data for K(sub I)/K(sub II) ratios of about 0.7 and 1.6 for fiber and radial normal loading, respectively. However, thermal effects must be accounted for in the data analysis.

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.

An integrated structural and geochemical study of fracture aperture growth in the Campito Formation of eastern California

NASA Astrophysics Data System (ADS)

Doungkaew, N.; Eichhubl, P.

2015-12-01

Processes of fracture formation control flow of fluid in the subsurface and the mechanical properties of the brittle crust. Understanding of fundamental fracture growth mechanisms is essential for understanding fracture formation and cementation in chemically reactive systems with implications for seismic and aseismic fault and fracture processes, migration of hydrocarbons, long-term CO2 storage, and geothermal energy production. A recent study on crack-seal veins in deeply buried sandstone of east Texas provided evidence for non-linear fracture growth, which is indicated by non-elliptical kinematic fracture aperture profiles. We hypothesize that similar non-linear fracture growth also occurs in other geologic settings, including under higher temperature where solution-precipitation reactions are kinetically favored. To test this hypothesis, we investigate processes of fracture growth in quartzitic sandstone of the Campito Formation, eastern California, by combining field structural observations, thin section petrography, and fluid inclusion microthermometry. Fracture aperture profile measurements of cemented opening-mode fractures show both elliptical and non-elliptical kinematic aperture profiles. In general, fractures that contain fibrous crack-seal cement have elliptical aperture profiles. Fractures filled with blocky cement have linear aperture profiles. Elliptical fracture aperture profiles are consistent with linear-elastic or plastic fracture mechanics. Linear aperture profiles may reflect aperture growth controlled by solution-precipitation creep, with the aperture distribution controlled by solution-precipitation kinetics. We hypothesize that synkinematic crack-seal cement preserves the elliptical aperture profiles of elastic fracture opening increments. Blocky cement, on the other hand, may form postkinematically relative to fracture opening, with fracture opening accommodated by continuous solution-precipitation creep.

Reinforced Concrete Modeling

DTIC Science & Technology

1982-07-01

micro- cracks within the material . This microcracking causes permanent deformation and a loss in stiffness similar to the strain hardening seen in metals...approached. Dilatation is caused by the tendency of shear stresses to open cracks in a microcracked, brittle material . 10 1.2 I , e3 3 1.0 F i ,S22 e...situation would be for a user to compromise some accuracy based on what features of a material are of the most importance for the analysis involved

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

Damage and Shaking Intensity in the M5.7 Canyondam Earthquake

NASA Astrophysics Data System (ADS)

Boatwright, J.; Chapman, K.; Gold, M. B.; Hardebeck, J. L.

2013-12-01

An M5.7 earthquake occurred southeast of Lake Almanor, CA, at 8:47 PM on May 23, 2013. Double-difference relocations of the main shock and aftershocks indicate that the earthquake nucleated at 11 km depth and ruptured up dip on a fault striking 292° and dipping 70° to the northeast. The earthquake cracked foundations, broke chimneys, and ruptured plumbing around Lake Almanor. We canvassed communities around the lake and to the south and east for earthquake damage, adding reports from our interviews to the geocoded 'Did You Feel It?' reports and to a set of damage reports collected by the Plumas County Office of Emergency Services. Three communities suffered significant damage. In Lake Almanor West, 14 km and 290° from the hypocenter, one wood-frame house was shifted on its foundation, the cripple wall of another house was racked, and water and gas pipes in five houses were ruptured. This damage indicates the shaking approached MMI 8. In Lake Almanor Country Club, 10 km and 310° from the hypocenter, more than 40 chimneys were cracked, broken, or collapsed, a coupling for the municipal water tank was ruptured, and a 200-foot long fissure opened on a slope facing the lake. This damage indicates shaking between MMI 7 and MMI 8, consistent with the accelerograph recording of PGA = 38% g and PGV = 30 cm/s at the Fire Station in Lake Almanor Country Club. This CSMIP station and a PG&E station on the crest of the Butt Valley Dam obtained the only recordings within 50 km of the epicenter. In Hamilton Branch, 10 km and 345° from the hypocenter, a foundation of a wood-frame house was damaged, and 14 chimneys and a water pipe were broken, indicative of MMI 7 shaking. All three communities are underlain by Tertiary and Quaternary basalts. The communities of Chester, Westwood, and Greenville were less damaged, suffering cracked drywall, broken windows, and objects thrown from shelves. The intensities in the three most strongly damaged communities increase as the azimuth from the source approaches the fault strike. This damage pattern could be caused by rupture directivity if the earthquake or the strongest sub-event of the earthquake ruptured up dip and to the west.

Multiaxial and thermomechanical fatigue considerations in damage tolerant design

NASA Technical Reports Server (NTRS)

Leese, G. E.; Bill, R. C.

1985-01-01

In considering damage tolerant design concepts for gas turbine hot section components, several challenging concerns arise: Complex multiaxial loading situations are encountered; Thermomechanical fatigue loading involving very wide temperature ranges is imposed on components; Some hot section materials are extremely anisotropic; and coatings and environmental interactions play an important role in crack propagation. The effects of multiaxiality and thermomechanical fatigue are considered from the standpoint of their impact on damage tolerant design concepts. Recently obtained research results as well as results from the open literature are examined and their implications for damage tolerant design are discussed. Three important needs required to advance analytical capabilities in support of damage tolerant design become readily apparent: (1) a theoretical basis to account for the effect of nonproportional loading (mechanical and mechanical/thermal); (2) the development of practical crack growth parameters that are applicable to thermomechanical fatigue situations; and (3) the development of crack growth models that address multiple crack failures.

Boundary element methods for the analysis of crack growth in the presence of residual stress fields

NASA Astrophysics Data System (ADS)

Leitao, V. M. A.; Aliabadi, M. H.; Rooke, D. P.; Cook, R.

1998-06-01

Two boundary element methods of simulating crack growth in the presence of residual stress fields are presented, and the results are compared to experimental measurements. The first method utilizes linear elastic fracture mechanics (LEFM) and superimposes the solutions due to the applied load and the residual stress field. In this method, the residual stress fields are obtained from an elastoplastic BEM analysis, and numerical weight functions are used to obtain the stress intensity factors due to the fatigue loading. The second method presented is an elastoplastic fracture mechanics (EPFM) approach for crack growth simulation. A nonlinear J-integral is used in the fatigue life calculations. The methods are shown to agree well with experimental measurements of crack growth in prestressed open hole specimens. Results are also presented for the case where the prestress is applied to specimens that have been precracked.

Simulation of anisotropic fracture behaviour of polycrystalline round blank tungsten using cohesive zone model

NASA Astrophysics Data System (ADS)

Mahler, Michael; Gaganidze, Ermile; Aktaa, Jarir

2018-04-01

The experimental observation of anisotropic fracture behaviour of round blank polycrystalline tungsten was simulated using finite element (FE) method in combination with cohesive zone model. Experiments in the past had shown that due to the anisotropic microstructure the fracture toughness varies by factor of about two for different orientations. The reason is the crack propagation direction, which is - in some orientations - not the typical crack propagation direction for mode I fracture. In some directions the crack is not growing perpendicular to the crack opening tensile load. Nevertheless, in the present paper, the microstructure is modelled by FE mesh including cohesive zone elements which mimic grain boundaries (GB). This is based on the assumption that GB's are the weakest links in the structure. The use of the correct parameters to describe the fracture process allows the description of the observed experimental orientation dependent fracture toughness.

Fracture properties of concrete specimens made from alkali activated binders

NASA Astrophysics Data System (ADS)

Šimonová, Hana; Kucharczyková, Barbara; Topolář, Libor; Bílek, Vlastimil, Jr.; Keršner, Zbyněk

2017-09-01

The aim of this paper is to quantify crack initiation and other fracture properties - effective fracture toughness and specific fracture energy - of two types of concrete with an alkali activated binder. The beam specimens with a stress concentrator were tested in a three-point bending test after 28, 90, and 365 days of maturing. Records of fracture tests in the form of load versus deflection (P-d) diagrams were evaluated using effective crack model and work-of-fracture method and load versus mouth crack opening displacement (P-CMOD) diagrams were evaluated using the Double-K fracture model. The initiation of cracks during the fracture tests for all ages was also monitored by the acoustic emission method. The higher value of monitored mechanical fracture parameters of concrete with alkali activated blast furnace slag were achieved with substitution blast furnace slag by low calcium fly ash in comparison with substitution by cement kiln dust.

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

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.

MPI Enhancements in John the Ripper

NASA Astrophysics Data System (ADS)

Sykes, Edward R.; Lin, Michael; Skoczen, Wesley

2010-11-01

John the Ripper (JtR) is an open source software package commonly used by system administrators to enforce password policy. JtR is designed to attack (i.e., crack) passwords encrypted in a wide variety of commonly used formats. While parallel implementations of JtR exist, there are several limitations to them. This research reports on two distinct algorithms that enhance this password cracking tool using the Message Passing Interface. The first algorithm is a novel approach that uses numerous processors to crack one password by using an innovative approach to workload distribution. In this algorithm the candidate password is distributed to all participating processors and the word list is divided based on probability so that each processor has the same likelihood of cracking the password while eliminating overlapping operations. The second algorithm developed in this research involves dividing the passwords within a password file equally amongst available processors while ensuring load-balanced and fault-tolerant behavior. This paper describes John the Ripper, the design of these two algorithms and preliminary results. Given the same amount of time, the original JtR can crack 29 passwords, whereas our algorithms 1 and 2 can crack an additional 35 and 45 passwords respectively.

Thermo-mechanical simulations of early-age concrete cracking with durability predictions

NASA Astrophysics Data System (ADS)

Havlásek, Petr; Šmilauer, Vít; Hájková, Karolina; Baquerizo, Luis

2017-09-01

Concrete performance is strongly affected by mix design, thermal boundary conditions, its evolving mechanical properties, and internal/external restraints with consequences to possible cracking with impaired durability. Thermo-mechanical simulations are able to capture those relevant phenomena and boundary conditions for predicting temperature, strains, stresses or cracking in reinforced concrete structures. In this paper, we propose a weakly coupled thermo-mechanical model for early age concrete with an affinity-based hydration model for thermal part, taking into account concrete mix design, cement type and thermal boundary conditions. The mechanical part uses B3/B4 model for concrete creep and shrinkage with isotropic damage model for cracking, able to predict a crack width. All models have been implemented in an open-source OOFEM software package. Validations of thermo-mechanical simulations will be presented on several massive concrete structures, showing excellent temperature predictions. Likewise, strain validation demonstrates good predictions on a restrained reinforced concrete wall and concrete beam. Durability predictions stem from induction time of reinforcement corrosion, caused by carbonation and/or chloride ingress influenced by crack width. Reinforcement corrosion in concrete struts of a bridge will serve for validation.

Slow and fast motion of cracks in inelastic solids. Part 1: Slow growth of cracks in a rate sensitive tresca solid. Part 2: Dynamic crack represented by the Dugdale model

NASA Technical Reports Server (NTRS)

Wnuk, M. P.; Sih, G. C.

1972-01-01

An extension is proposed of the classical theory of fracture to viscoelastic and elastic-plastic materials in which the plasticity effects are confined to a narrow band encompassing the crack front. It is suggested that the Griffith-Irwin criterion of fracture, which requires that the energy release rate computed for a given boundary value problem equals the critical threshold, ought to be replaced by a differential equation governing the slow growth of a crack prior to the onset of rapid propagation. A new term which enters the equation of motion in the dissipative media is proportional to the energy lost within the end sections of the crack, and thus reflects the extent of inelastic behavior of a solid. A concept of apparent surface energy is introduced to account for the geometry dependent and the rate dependent phenomena which influence toughness of an inelastic solid. Three hypotheses regarding the condition for fracture in the subcritical range of load are compared. These are: (1) constant fracture energy (Cherepanov), (2) constant opening displacement at instability (Morozov) and (3) final stretch criterion (Wnuk).

Experimental and Finite Element Modeling of Near-Threshold Fatigue Crack Growth for the K-Decreasing Test Method

NASA Technical Reports Server (NTRS)

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

2015-01-01

The experimental methods to determine near-threshold fatigue crack growth rate data are prescribed in ASTM standard E647. To produce near-threshold data at a constant stress ratio (R), the applied stress-intensity factor (K) is decreased as the crack grows based on a specified K-gradient. Consequently, as the fatigue crack growth rate threshold is approached and the crack tip opening displacement decreases, remote crack wake contact may occur due to the plastically deformed crack wake surfaces and shield the growing crack tip resulting in a reduced crack tip driving force and non-representative crack growth rate data. If such data are used to life a component, the evaluation could yield highly non-conservative predictions. Although this anomalous behavior has been shown to be affected by K-gradient, starting K level, residual stresses, environmental assisted cracking, specimen geometry, and material type, the specifications within the standard to avoid this effect are limited to a maximum fatigue crack growth rate and a suggestion for the K-gradient value. This paper provides parallel experimental and computational simulations for the K-decreasing method for two materials (an aluminum alloy, AA 2024-T3 and a titanium alloy, Ti 6-2-2-2-2) to aid in establishing clear understanding of appropriate testing requirements. These simulations investigate the effect of K-gradient, the maximum value of stress-intensity factor applied, and material type. A material independent term is developed to guide in the selection of appropriate test conditions for most engineering alloys. With the use of such a term, near-threshold fatigue crack growth rate tests can be performed at accelerated rates, near-threshold data can be acquired in days instead of weeks without having to establish testing criteria through trial and error, and these data can be acquired for most engineering materials, even those that are produced in relatively small product forms.

Main mental disorders in crack-cocaine users treated at Psychosocial Care Centers for Alcohol and Drugs in the city of Recife, Brazil.

PubMed

Castro, Antonio Gomes de; Silva, Diego César Nunes da; Figueiroa, Magda da Silva

2016-01-01

Brazil's Northeast region has the highest crack-cocaine consumption in the country. Crack-cocaine has more intense effects than cocaine powder and can cause greater chemical dependence. Psychosocial Care Centers for Alcohol and Drugs (CAPSad) are public health services that provide treatment for drug dependence. It is common for drug users, and particularly crack-cocaine users, to develop mental disorders. Objective: To evaluate the most common mental disorders in crack-cocaine dependents in treatment at CAPSad in the city of Recife, Brazil. The research database "Between rocks and shots: user profiles, consumption strategies, and social impact of crack cocaine" (CEP/CCS/UFPE no. 206/11) was consulted to establish the areas of crack cocaine consumption in the city of Recife. There were 885 patients in treatment for crack-cocaine use, with a mean age of 29.8±9.4 years. The mean duration of drug use was 6.1±4.6 years. Most of the patients were males (80.3%), had left school at some point between the 1st and 9th grades (45.6%), were unemployed and/or seeking employment (52%) and used drugs daily (56.4%). Cocaine chemical dependence was more significant when correlated with use of crack-cocaine and other drugs such as medications and hallucinogens (p = 0.01). Data from this study showed strong associations between crack-cocaine uses and development of mental disorders, particularly when abuse of multiple substances occurs. Based on these data, there is a clear need for coordination of related public policies for support and social reintegration to provide these people with comprehensive care.

Component analysis of Iranian crack; a newly abused narcotic substance in iran.

PubMed

Farhoudian, Ali; Sadeghi, Mandana; Khoddami Vishteh, Hamid Reza; Moazen, Babak; Fekri, Monir; Rahimi Movaghar, Afarin

2014-01-01

Iranian crack is a new form of narcotic substance that has found widespread prevalence in Iran in the past years. Crack only nominally resembles crack cocaine as it is widely different in its clinical signs. Thus the present study aims to quantify the chemical combination of this drug. The samples included 18 specimen of Crack collected from different zones of Tehran, Iran. All specimens were in the form of inodorous cream solid powdery substance. TLC and HPLC methods were used to perform semi-quantitative and quantitative analysis of the components, respectively. The TLC analysis showed no cocaine compound in the specimens while they all revealed to contain heroin, codeine, morphine and caffeine. All but two specimens contained thebaine. None of the specimens contained amphetamine, benzodiazepines, tricyclic antidepressants, aspirin, barbiturates, tramadol and buprenorphine. Acetaminophen was found in four specimens. HPLC revealed heroin to be the foundation substance in all specimens and most of them contained a significant amount of acetylcodeine. The present analysis of the chemical combination of Crack showed that this substance is a heroin-based narcotic which is basically different from the cocaine-based crack used in Western countries. Studies like the present one at different time points, especially when abnormal clinical signs are detected, can reveal the chemical combination of the target substance and contribute to the clinical management of its acute or chronic poisoning.

Environmentally induced crack propagation in Inconel alloys 600 and 690 under hydrogen supersaturated steam

NASA Astrophysics Data System (ADS)

Ali, Mehboob Muzzammil

Intergranular stress corrosion cracking (IGSCC) of Inconel alloys 600 and 690 was investigated by exposing them to 300--400°C in deaerated hydrogen supersaturated steam. Crack growth rates were measured in-situ for the above alloys using modified wedge-opening-loaded (M-WOL) linear elastic fracture specimens under constant displacement conditions. The applied stress intensity factors (K) used varied from 29--90 MPam1/2. An activation energy of 120 kJ/mol was found for crack growth rates as a function of temperature. This activation energy is close to the one corresponding to grain boundary self diffusion of nickel. In addition, it was found that the apparent crack growth rates (da/dt) exhibited a linear dependence with KI, given by (da/dt) = A.KIn, where n = 1 in our case. Microstructurally, crack propagation in both the alloys was predominantly along the grain boundaries. It is suggested that high fugacity hydrogen was generated at the tip of the crack, as a result of the reaction of H2O with Cr2O3 on the fracture surface leading to IGSCC. It was found that the rates of crack propagation in both alloys 600 and 690 are very similar. Moreover, under the applied KIs, both alloys 600 and 690 show a similar tendency to crack intergranularly in a direction perpendicular to the applied stress. Crack branching was also exhibited by both the alloys 600 as well as 690. The difference in the crack growth rates of alloys 600 and 690 was found to be only about 2%, which indicates that the crack growth rates in these alloys is independent of the alloy chromium content and that, possibly very similar crack growth mechanisms are active in both the alloys 600 and 690 under similar conditions of KI and temperature. HREM images at the crack tip of alloy 690 exhibit two distinct regions---a crystalline region, and an adjacent amorphous region, which is likely to be either a hydroxide or an amorphous oxide layer. This layer is expected to result from passivation reactions as the crack surface is exposed to the corrosive environment.

Fatigue and Fracture Characterization of GlasGridRTM Reinforced Asphalt Concrete Pavement

NASA Astrophysics Data System (ADS)

Safavizadeh, Seyed Amirshayan

The purpose of this research is to develop an experimental and analytical framework for describing, modeling, and predicting the reflective cracking patterns and crack growth rates in GlasGridRTM-reinforced asphalt pavements. In order to fulfill this objective, the effects of different interfacial conditions (mixture and tack coat type, and grid opening size) on reflective cracking-related failure mechanisms and the fatigue and fracture characteristics of fiberglass grid-reinforced asphalt concrete beams were studied by means of four- and threepoint bending notched beam fatigue tests (NBFTs) and cyclic and monotonic interface shear tests. The digital image correlation (DIC) technique was utilized for obtaining the displacement and strain contours of specimen surfaces during each test. The DIC analysis results were used to develop crack tip detection methods that were in turn used to determine interfacial crack lengths in the shear tests, and vertical and horizontal (interfacial) crack lengths in the notched beam fatigue tests. Linear elastic fracture mechanics (LEFM) principles were applied to the crack length data to describe the crack growth. In the case of the NBFTs, a finite element (FE) code was developed and used for modeling each beam at different stages of testing and back-calculating the stress intensity factors (SIFs) for the vertical and horizontal cracks. The local effect of reinforcement on the stiffness of the system at a vertical crack-interface intersection or the resistance of the grid system to the deflection differential at the joint/crack (hereinafter called joint stiffness) for GlasGrid-reinforced asphalt concrete beams was determined by implementing a joint stiffness parameter into the finite element code. The strain level dependency of the fatigue and fracture characteristics of the GlasGrid-reinforced beams was studied by performing four-point bending notched beam fatigue tests at strain levels of 600, 750, and 900 microstrain. These beam tests were conducted at 15°C, 20°C, and 23°C, with the main focus being to find the characteristics at 20°C. The results obtained from the tests at the different temperatures were used to investigate the effects of temperature on the reflective cracking performance of the gridreinforced beam specimens. The temperature tests were also used to investigate the validity of the time-temperature superposition (t-TS) principle in shear and the beam fatigue performance of the grid-reinforced specimens. The NBFT results suggest that different interlayer conditions do not reflect a unique failure mechanism, and thus, in order to predict and model the performance of grid-reinforced pavement, all the mechanisms involved in weakening its structural integrity, including damage within the asphalt layers and along the interface, must be considered. The shear and beam fatigue test results suggest that the grid opening size, interfacial bond quality, and mixture type play important roles in the reflective cracking performance of GlasGrid-reinforced asphalt pavements. According to the NBTF results, GlasGrid reinforcement retards reflective crack growth by stiffening the composite system and introducing a joint stiffness parameter. The results also show that the higher the bond strength and interlayer stiffness values, the higher the joint stiffness and retardation effects. The t-TS studies proved the validity of this principle in terms of the reflective crack growth of the grid-reinforced beam specimens and the shear modulus and shear strength of the grid-reinforced interfaces.

Dynamic Fracture Toughness Evaluation by Measurement of CTOD (Crack Tip Opening Displacement).

DTIC Science & Technology

1988-03-15

fracture toughness of structural steels were reported by Shoemaker and Rolfe [1]; these and similar results are also presented in the text by Rolfe and...8217 MPaV/-m/s. Following the dynamic tests of Shoemaker and Rolfe , extensions of the familiar ASTM E-399 static fracture toughness tests were examined. This...s.V.: **.4* .4 5, -~ 5 5 - 𔃿 .4.4 References [1] Shoemaker, A.K. and Rolfe , S.T., "The Static and Dynamic Low-Temperature Crack-Toughness

A Study of the Hot Workability of Al-8.5 Wt Pct Mg Alloys for Armor Plate Applications

DTIC Science & Technology

1989-01-01

427°C (500, 600, 700, and 800’F) to determine a suitable deformation temperature for the homogenization study. Later it was learned that a lower...alloys, only occurred above 5200C (.7°). Rater, according to fractographie examinaton, the decrease in e, at higher temperatures and/or higher strain...edge cracks that were several centimeters deep opened up at the drilled thermocouple hole; the remainder of the slab was almost free of edge cracks

Crack Growth of a Titanium-Aluminide Alloy under Thermal-Mechanical Fatigue

DTIC Science & Technology

1988-12-01

the elastic-plastic fracture mechanics ( EPFM ) relations such as the J-integral or crack tip opening displacement (CTOD) must be used. Much more work...has been done in the area of LEFM, using stress intensity factor range AK as a correlating factor, than in EPFM . No matter which type of analysis is...thus obvious that a simple linear summation model such as Heil’s might not be applicable to this material. Other damage mechanisms were then investigated

J-Resistance Curves of Aluminum Specimens Using Moire Interferometry

DTIC Science & Technology

1989-04-01

elastic-plastic fracture mechanics ( EPFM ) methodologies are based on the J-integral or the crack opening displacement (COD) approach. The J-resistance curve...in the HRR field [13,141. In this paper, we present further application of the approximate J-evaluation procedure in large 2024-0 and 5052-H32 aluminum...Davis, J. A. Joyce, and R. A. Hays, " Application of the J-Integral and the Modified J-Integral to Cases of Large Crack Extension and High Toughness

Dynamic J sub I-R Curve Testing of HY-130 Steel.

DTIC Science & Technology

1981-10-01

Society for Testing and Materials 0C Degrees Celsius COD Crack-opening displacement CT Compact tension CVN Charpy V-notch dia Diameter in-lb/in 2 Inch...inches per second. A key curve for HY-130 plate was developed under dynamic loading conditions using subsized compact specimens and was applied to...face grooves were machined along the crack line to a total section reduction of 20% with a standard Charpy V-notch (CVN) cutter (450 included angle

The Crack Cocaine Crisis. Joint Hearing before the Select Committee on Narcotics Abuse and Control, House of Representatives and the Select Committee on Children, Youth, and Families. House of Representatives, Ninety-Ninth Congress, Second Session (July 15, 1986).

ERIC Educational Resources Information Center

Congress of the U.S., Washington, DC. House Select Committee on Children, Youth, and Families.

This document contains witness testimonies and prepared statements from the Congressional hearing called to examine what the federal government as well as local and state governments are doing and can do to respond to the growing problem of crack cocaine, and what prevention and treatment approaches are effective. Opening statements and/or…

An Experimental Study of Penny-shaped Fluid-driven Cracks in an Elastic Matrix

NASA Astrophysics Data System (ADS)

Stone, Howard

2015-11-01

When a pressurized fluid is injected into an elastic matrix, the fluid generates a fracture that grows along a plane and forms a fluid-filled disc-like shape. For example, such problems occur in various natural and industrial applications involving the subsurface of Earth, such as hydraulic fracturing operations. We report a laboratory study of such a fluid-driven crack in a gelatin matrix, study the crack shape as a function of time, and investigate the influence of different experimental parameters such as the injection flow rate, Young's modulus of the matrix, and fluid viscosity. We find that the crack radius increases with time as a power law, which has been predicted both for the limit where viscous effects in the flow along the crack opening control the rate of crack propagation, as well as the limit where fracture toughness controls crack propagation. We vary experimental parameters to probe the physical limits and highlight that for our typical parameters both effects can be significant. Also, we measure the time evolution of crack shape, which has not been studied before. The rescaled crack shapes collapse at longer times, based on an appropriate scaling argument, and again we compare the scaling arguments in different physical limits. The gelatin system provides a useful laboratory model for further studies of fluid-driven cracks, some of which we will mention as they are inspired by the physics of hydraulic fracturing. This work is part of the PhD thesis of Ching-Yao Lai and is a collaboration with Drs. Zhong Zheng and Jason Wexler (Princeton University) and Professor Emilie Dressaire (NYU). Department of Mechanical and Aerospace Engineering.

An elasto-plastic solution for channel cracking of brittle coating on polymer substrate

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

Zhang, Chao; Chen, Fangliang; Gray, Matthew H.

In this study, an elasto-plastic channel-cracking model is presented to study the open-mode fracture of a thin layer brittle coating grown on a polymer substrate. A linear elastic shear interlayer is introduced to describe the stress transfer from the elasto-plastic substrate to the brittle coating, on basis of the shear-lag principle. The channel cracking behavior involves three stages: elastic, elasto-plastic and plastic stages, which are solved in a continuous manner based on the deformation status of the substrate. Explicit solutions are derived for the mutli-stage cracking process. Corresponding experimental tests for a titanium oxide (TiO 2) coating on a polymore » (ethylene terephthalate) substrate are conducted. The fracture toughness of the coating layer is estimated based on the crack spacing versus layer thickness relationship at certain strain levels. This method is found to be more reliable than the traditional methods using crack onset strain. Parametric studies of the fracture energy release rate for the coating and interfacial compliance of the thin film system are conducted, through which the effect of plastic deformation on the channel cracking behavior is studied extensively. The results indicate that the tangent modulus of the substrate controls the evolution curvature of crack spacing where a smaller tangent modulus corresponds to a slower saturation of crack spacing. The energy release rate also varies significantly with the properties of the interlayer. The study highlights the necessity of an elasto-plastic model for the thin film systems of brittle coating on a plastic substrate.« less

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

Luo, P.F.; Wang, J.S.; Chao, Y.J.

The stereo vision is used to study the fracture behavior in the compact tension (CT) specimen made from 304L stainless steel. During crack tip blunting, initiation, and growth in the CT specimen, both in-plane and out-of-plane displacement fields near the crack tip are measured by the stereo vision. Based on the plane stress assumption and the deformation theory of plasticity, the J integral is evaluated along several rectangular paths surrounding the crack tip by using the measured in-plane displacement field. Prior to crack growth, the J integral is path independent. For crack extension up to {Delta}a {approx} 3 mm, themore » near field J integral values are 6% to 10% lower than far field J integral values. For the crack extension of {Delta}a {approx} 4 mm, the J integral lost path independence. The far field J integral values are in good agreement with results obtained from Merkle-Corten`s formula. Both J-{Delta}a and CTOA-{Delta}a are obtained by computing the J integral value and crack tip opening angle (CTOA) at each {Delta}a. Results indicate that CTOA reached a nearly constant value at a crack extension of {Delta}a = 3 mm with a leveled resistance curve thereafter. Also, the J integral value is determined by the maximum transverse diameter of the shadow spots, which are generated by using the out-of-plane displacement field. Results indicate that for crack extension up to 0.25 mm, the J integral values evaluated by using the out-of- plane displacement are close to those obtained by using in-plane displacements and Merkle-Corten`s formula.« less

An elasto-plastic solution for channel cracking of brittle coating on polymer substrate

DOE PAGES

Zhang, Chao; Chen, Fangliang; Gray, Matthew H.; ...

2017-04-25

In this study, an elasto-plastic channel-cracking model is presented to study the open-mode fracture of a thin layer brittle coating grown on a polymer substrate. A linear elastic shear interlayer is introduced to describe the stress transfer from the elasto-plastic substrate to the brittle coating, on basis of the shear-lag principle. The channel cracking behavior involves three stages: elastic, elasto-plastic and plastic stages, which are solved in a continuous manner based on the deformation status of the substrate. Explicit solutions are derived for the mutli-stage cracking process. Corresponding experimental tests for a titanium oxide (TiO 2) coating on a polymore » (ethylene terephthalate) substrate are conducted. The fracture toughness of the coating layer is estimated based on the crack spacing versus layer thickness relationship at certain strain levels. This method is found to be more reliable than the traditional methods using crack onset strain. Parametric studies of the fracture energy release rate for the coating and interfacial compliance of the thin film system are conducted, through which the effect of plastic deformation on the channel cracking behavior is studied extensively. The results indicate that the tangent modulus of the substrate controls the evolution curvature of crack spacing where a smaller tangent modulus corresponds to a slower saturation of crack spacing. The energy release rate also varies significantly with the properties of the interlayer. The study highlights the necessity of an elasto-plastic model for the thin film systems of brittle coating on a plastic substrate.« less

Bending fracture in carbon nanotubes.

PubMed

Kuo, Wen-Shyong; Lu, Hsin-Fang

2008-12-10

A novel approach was adopted to incur bending fracture in carbon nanotubes (CNTs). Expanded graphite (EG) was made by intercalating and exfoliating natural graphite flakes. The EG was deposited with nickel particles, from which CNTs were grown by chemical vapor deposition. The CNTs were tip-grown, and their roots were fixed on the EG flakes. The EG flakes were compressed, and many CNTs on the surface were fragmented due to the compression-induced bending. Two major modes of the bending fracture were observed: cone-shaped and shear-cut. High-resolution scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to examine the crack growth within the graphene layers. The bending fracture is characterized by two-region crack growth. An opening crack first appears around the outer-tube due to the bending-induced tensile stress. The crack then branches to grow along an inclined direction toward the inner-tube due to the presence of the shear stress in between graphene layers. An inner-tube pullout with inclined side surface is formed. The onset and development of the crack in these two regions are discussed.

The Molecular Volcano Revisited: Determination of Crack Propagation and Distribution During the Crystallization of Nanoscale Amorphous Solid Water Films.

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

May, Robert A.; Smith, R. Scott; Kay, Bruce D.

2012-02-02

Temperature programmed desorption (TPD) is utilized to determine the length distribution of cracks formed through amorphous solid water (ASW) during crystallization. This distribution is determined by monitoring how the thickness of an ASW overlayer alters desorption of an underlayer of O2. As deposited the ASW overlayer prevents desorption of O2. During crystallization, cracks form through the ASW overlayer and open a path to vacuum which allows O2 to escape in a rapid episodic release known as the 'molecular volcano'. Sufficiently thick ASW overlayers further trap O2 resulting in a second O2 desorption peak commensurate with desorption of the last ofmore » the ASW overlayer. The evolution of this trapping peak with overlayer thickness is the basis for determining the distribution of crystallization induced cracks through the ASW. Reflection adsorption infrared spectroscopy (RAIRS) and TPD of multicomponent parfait structures of ASW, O2 and Kr indicate that a preponderance of these cracks propagate down from the outer surface of the ASW.« less

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

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

Subcritical crack growth of selected aerospace pressure vessel materials

NASA Technical Reports Server (NTRS)

Hall, L. R.; Bixler, W. D.

1972-01-01

This experimental program was undertaken to determine the effects of combined cyclic/sustained loads, stress level, and crack shape on the fatigue crack growth rate behavior of cracks subjected to plane strain conditions. Material/environment combinations tested included: 2219-T87 aluminum plate in gaseous helium, room air, and 3.5% NaCl solution at room temperature, liquid nitrogen, and liquid hydrogen; 5Al-2.5 Sn (ELI) titanium plate in liquid nitrogen and liquid hydrogen and 6AL-4V (ELI) STA titanium plate in gaseous helium and methanol at room temperature. Most testing was accomplished using surface flawed specimens instrumented with a clip gage to continuously monitor crack opening displacements at the specimen surface. Tapered double cantilever beam specimens were also tested. Static fracture and ten hour sustained load tests were conducted to determine fracture toughness and apparent threshold stress intensity values. Cyclic tests were performed using sinusoidal loading profiles at 333 MHz (20 cpm) and trapezoidal loading profiles at both 8.3 MHz (0.5 cpm) and 3.3 MHz (0.2 cpm). Data were evaluated using modified linear elastic fracture mechanics parameters.

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

Layer texture of hot-rolled BCC metals and its significance for stress-corrosion cracking of main gas pipelines

NASA Astrophysics Data System (ADS)

Perlovich, Yu. A.; Isaenkova, M. G.; Krymskaya, O. A.; Morozov, N. S.

2016-10-01

Based on data of X-ray texture analysis of hot-rolled BCC materials it was shown that the layerwise texture inhomogeneity of products is formed during their manufacturing. The effect can be explained by saturation with interstitial impurities of the surface layer, resulting in dynamical deformation aging (DDA). DDA prevents the dislocation slip under rolling and leads to an increase of lattice parameters in the external layer. The degree of arising inhomogeneity correlates with the tendency of hot-rolled sheets and obtained therefrom tubes to stress-corrosion cracking under exploitation, since internal layers have a compressive effect on external layers, and prevents opening of corrosion cracks at the tube surface.

An Experimental Study of a Stitched Composite with a Notch Subjected to Combined Bending and Tension Loading

NASA Technical Reports Server (NTRS)

Palmer, Susan O.; Nettles, Alan T.; Poe, C. C., Jr.

1999-01-01

A series of tests was conducted to measure the strength of stitched carbon/epoxy composites containing through-thickness damage in the form of a crack-like notch. The specimens were subjected to three types of loading: pure bending, pure tension, and combined bending and tension loads. Measurements of applied loads, strains near crack tips, and crack opening displacements (COD) were monitored in all tests. The transverse displacement at the center of the specimen was measured using a Linear Variable Differential Transformer (LVDT). The experimental data showed that the outer surface of the pure tension specimen failed at approximately 6,000 microstrain, while in combined bending and tension loads the measured tensile strains reached 10,000 microstrain.

Structures associated with strike-slip faults that bound landslide elements

USGS Publications Warehouse

Fleming, R.W.; Johnson, A.M.

1989-01-01

Large landslides are bounded on their flanks and on elements within the landslides by structures analogous to strike-slip faults. We observed the formation of thwse strike-slip faults and associated structures at two large landslides in central Utah during 1983-1985. The strike-slip faults in landslides are nearly vertical but locally may dip a few degrees toward or away from the moving ground. Fault surfaces are slickensided, and striations are subparallel to the ground surface. Displacement along strike-slip faults commonly produces scarps; scarps occur where local relief of the failure surface or ground surface is displaced and becomes adjacent to higher or lower ground, or where the landslide is thickening or thinning as a result of internal deformation. Several types of structures are formed at the ground surface as a strike-slip fault, which is fully developed at some depth below the ground surface, propagates upward in response to displacement. The simplest structure is a tension crack oriented at 45?? clockwise or counterclockwise from the trend of an underlying right- or left-lateral strike-slip fault, respectively. The tension cracks are typically arranged en echelon with the row of cracks parallel to the trace of the underlying strike-slip fault. Another common structure that forms above a developing strike-slip fault is a fault segment. Fault segments are discontinuous strike-slip faults that contain the same sense of slip but are turned clockwise or counterclockwise from a few to perhaps 20?? from the underlying strike-slip fault. The fault segments are slickensided and striated a few centimeters below the ground surface; continued displacement of the landslide causes the fault segments to open and a short tension crack propagates out of one or both ends of the fault segments. These structures, open fault segments containing a short tension crack, are termed compound cracks; and the short tension crack that propagates from the tip of the fault segment is typically oriented 45?? to the trend of the underlying fault. Fault segments are also typically arranged en echelon above the upward-propagating strike-slip fault. Continued displacement of the landslide causes the ground to buckle between the tension crack portions of the compound cracks. Still more displacement produces a thrust fault on one or both limbs of the buckle fold. These compressional structures form at right angles to the short tension cracks at the tips of the fault segments. Thus, the compressional structures are bounded on their ends by one face of a tension crack and detached from underlying material by thrusting or buckling. The tension cracks, fault segments, compound cracks, folds, and thrusts are ephemeral; they are created and destroyed with continuing displacement of the landslide. Ultimately, the structures are replaced by a throughgoing strike-slip fault. At one landslide, we observed the creation and destruction of the ephemeral structures as the landslide enlarged. Displacement of a few centimeters to about a decimeter was sufficient to produce scattered tension cracks and fault segments. Sets of compound cracks with associated folds and thrusts were produced by displacements of up to 1 m, and 1 to 2 m of displacement was required to produce a throughgoing strike-slip fault. The type of first-formed structure above an upward-propagating strike-slip fault is apparently controlled by the rheology of the material. Brittle material such as dry topsoil or the compact surface of a gravel road produces echelon tension cracks and sets of tension cracks and compressional structures, wherein the cracks and compressional structures are normal to each other and 45?? to the strike-slip fault at depth. First-formed structures in more ductile material such as moist cohesive soil are fault segments. In very ductile material such as soft clay and very wet soil in swampy areas, the first-formed structure is a throughgoing strike-slip fault. There are othe

Dynamics of a fluid-driven crack in three dimensions by the finite difference method

NASA Astrophysics Data System (ADS)

Chouet, Bernard

1986-12-01

The finite difference method is applied to the study of the dynamics of a three-dimensional fluid-filled crack excited into resonance by the sudden failure of a small barrier of area ΔS on the crack surface. The impulse response of the crack is examined for various ratios of crack width to crack length and for several values of the crack stiffness C = (b/μ)(L/d), where b is the bulk modulus of the fluid, μ is the rigidity of the solid, and L and d are the crack length and crack thickness, respectively. The motion of the crack is characterized by distinct time scales representing the duration of brittle failure and the periods of acoustic resonance in the lateral and longitudinal dimensions of the source. The rupture has a duration proportional to the area of crack expansion and is the trigger responsible for the excitation of the crack into resonance; the resonant periods are proportional to the crack stiffness and to the width and length of the crack. The crack wave sustaining the resonance is analogous to the tube wave propagating in a fluid-filled borehole. It is dispersive, showing a phase velocity that decreases with increasing wavelength. Its wave speed is always lower than the acoustic velocity of the fluid and shows a strong dependence on the crack stiffness, decreasing as the stiffness increases. The initial motion of the crack surface is an opening, and the radiated far-field compressional wave starts with a strong but brief compression which has a duration proportional to the crack stiffness and size of the rupture area; the amplitude of this pulse increases with the area of rupture but decreases with increasing stiffness. Flow into the newly created cavity triggers a pressure drop in the fluid, which produces a partial collapse of the wall propagated over the crack surface at the speed of the crack wave. The collapse of the crack surface generates a weak long-period component of dilatation following the compressional first motion in the far-field P wave train; the dilatational component is clearer in the signal from stiffer cracks when seen in the direction of the rupture. The energy loss by radiation is stronger for high frequencies, resulting in a progressive enrichment of the crack response in lower frequencies over the duration of resonance. These source characteristics translate into a far-field signature that is marked by a high-frequency content near its onset and dominated by a longer-period component in its coda. The source duration shows a strong dependence on the fluid viscosity and associated viscous damping at the crack wall.

Fracture Mechanics Testing of Titanium 6AL-4V in AF-M315E

NASA Technical Reports Server (NTRS)

Sampson, J. W.; Martinez, J.; McLean, C.

2016-01-01

The Green Propellant Infusion Mission (GPIM) will demonstrate the performance of AF-M315E monopropellant on orbit. Flight certification requires a safe-life analysis of the titanium alloy fuel tank to ensure inherent processing flaws will not cause failure during the design life of the tank. Material property inputs for this analysis require testing to determine the stress intensity factor for environment-assisted cracking (KEAC) of Ti 6Al-4V in combination with the AF-M315E monopropellant. Testing of single-edge notched, or SE(B), specimens representing the bulk tank membrane and weld material were performed in accordance with ASTM E1681. Specimens with fatigue pre-cracks were loaded into test fixtures so that the crack tips were exposed to AF-M315E at 50 C for a duration of 1,000 hours. Specimens that did not fail during exposure were opened to inspect the crack surfaces for evidence of crack growth. The threshold stress intensity value, KEAC, is the highest applied stress intensity that produced neither a failure of the specimen during the exposure nor showed evidence of crack growth. The threshold stress intensity factor for environment-assisted cracking of the Ti 6Al-4V forged tank material was found to be at least 22 ksivin and at least 31 ksivin for the weld material when exposed to AF-M315E monopropellant.

Exploring How Weathering Related Stresses and Subcritical Crack Growth May Influence the Size of Sediment Produced From Different Rock Types.

NASA Astrophysics Data System (ADS)

Eppes, M. C.; Hallet, B.; Hancock, G. S.; Mackenzie-Helnwein, P.; Keanini, R.

2016-12-01

The formation and diminution of rock debris, sediment and soil at and near Earth's surface is driven in large part by in situ, non-transport related, rock cracking. Given the relatively low magnitude stresses that arise in surface and near-surface settings, this production and diminution of granular material is likely strongly influenced and/or driven by subcritical crack growth (Eppes et al., 2016), cracking that occurs under stress loading conditions much lower than a rock's strength as typically measured in the laboratory under rapid loading. Despite a relatively sound understanding of subcritical crack growth through engineering and geophysical studies, its geomorphic and sedimentologic implications have only been minimally explored. Here, based on existing studies, we formulate several hypotheses to predict how weathering-induced stresses combined with the subcritical crack growth properties of rock may influence sediment size distribution. For example, subcritical crack growth velocity (v) can be described by v = CKIn where KI is the mode I (simple opening mode) stress intensity factor, a function of tensile stress at the crack tip and crack length; C is a rock- and environment-dependent constant; and n is material constant, the subcritical crack growth index. Fracture length and spacing in rock is strongly dependent on n, where higher n values result in fewer, more distally spaced cracks (e.g. Olsen, 1993). Thus, coarser sediment might be expected from rocks with higher n values. Weathering-related stresses such as thermal stresses and mineral hydration, however, can disproportionally stress boundaries between minerals with contrasting thermal or chemical properties and orientation, resulting in granular disintegration. Thus, rocks with properties favorable to inducing these stresses might produce sediment whose size is reflective of its constituent grains. We begin to test these hypotheses through a detailed examination of crack and rock characteristics in outcrops of granite, sandstone, and quartzite found in Shenandoah National Park, Virginia. Preliminary results reveal that many observed cracking characteristics are consistent with our hypotheses linking subcritical crack growth, weathering stresses and the production of different sized sediment from different rock types.

Rock fracture processes in chemically reactive environments

NASA Astrophysics Data System (ADS)

Eichhubl, P.

2015-12-01

Rock fracture is traditionally viewed as a brittle process involving damage nucleation and growth in a zone ahead of a larger fracture, resulting in fracture propagation once a threshold loading stress is exceeded. It is now increasingly recognized that coupled chemical-mechanical processes influence fracture growth in wide range of subsurface conditions that include igneous, metamorphic, and geothermal systems, and diagenetically reactive sedimentary systems with possible applications to hydrocarbon extraction and CO2 sequestration. Fracture processes aided or driven by chemical change can affect the onset of fracture, fracture shape and branching characteristics, and fracture network geometry, thus influencing mechanical strength and flow properties of rock systems. We are investigating two fundamental modes of chemical-mechanical interactions associated with fracture growth: 1. Fracture propagation may be aided by chemical dissolution or hydration reactions at the fracture tip allowing fracture propagation under subcritical stress loading conditions. We are evaluating effects of environmental conditions on critical (fracture toughness KIc) and subcritical (subcritical index) fracture properties using double torsion fracture mechanics tests on shale and sandstone. Depending on rock composition, the presence of reactive aqueous fluids can increase or decrease KIc and/or subcritical index. 2. Fracture may be concurrent with distributed dissolution-precipitation reactions in the hostrock beyond the immediate vicinity of the fracture tip. Reconstructing the fracture opening history recorded in crack-seal fracture cement of deeply buried sandstone we find that fracture length growth and fracture opening can be decoupled, with a phase of initial length growth followed by a phase of dominant fracture opening. This suggests that mechanical crack-tip failure processes, possibly aided by chemical crack-tip weakening, and distributed solution-precipitation creep in the hostrock can independently affect fracture opening displacement and thus fracture aperture profiles and aperture distribution.

Primary Water Stress Corrosion Cracks in Nickel Alloy Dissimilar Metal Welds: Detection and Sizing Using Established and Emerging Nondestructive Examination Techniques

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

Braatz, Brett G.; Cumblidge, Stephen E.; Doctor, Steven R.

2012-12-31

The U.S. Nuclear Regulatory Commission has established the Program to Assess the Reliability of Emerging Nondestructive Techniques (PARENT) as a follow-on to the international cooperative Program for the Inspection of Nickel Alloy Components (PINC). The goal of PINC was to evaluate the capabilities of various nondestructive evaluation (NDE) techniques to detect and characterize surface-breaking primary water stress corrosion cracks in dissimilar-metal welds (DMW) in bottom-mounted instrumentation (BMI) penetrations and small-bore (≈400-mm diameter) piping components. A series of international blind round-robin tests were conducted by commercial and university inspection teams. Results from these tests showed that a combination of conventional andmore » phased-array ultrasound techniques provided the highest performance for flaw detection and depth sizing in dissimilar metal piping welds. The effective detection of flaws in BMIs by eddy current and ultrasound shows that it may be possible to reliably inspect these components in the field. The goal of PARENT is to continue the work begun in PINC and apply the lessons learned to a series of open and blind international round-robin tests that will be conducted on a new set of piping components including large-bore (≈900-mm diameter) DMWs, small-bore DMWs, and BMIs. Open round-robin testing will engage universities and industry worldwide to investigate the reliability of emerging NDE techniques to detect and accurately size flaws having a wide range of lengths, depths, orientations, and locations. Blind round-robin testing will invite testing organizations worldwide, whose inspectors and procedures are certified by the standards for the nuclear industry in their respective countries, to investigate the ability of established NDE techniques to detect and size flaws whose characteristics range from easy to very difficult to detect and size. This paper presents highlights of PINC and reports on the plans and progress for PARENT round-robin tests.« less

InSitu SEM Investigation of Microstructural Damage Evolution and Strain Relaxation in a Melt Infiltrated SiC/SiC Composite

NASA Technical Reports Server (NTRS)

Sevener, Kathy; Chen, Zhe; Daly, Sam; Tracy, Jared; Kiser, Doug

2016-01-01

With CMC components poised to complete flight certification in turbine engines on commercial aircraft within the near future, there are many efforts within the aerospace community to model the mechanical and environmental degradation of CMCs. Direct observations of damage evolution are needed to support these modeling efforts and provide quantitative measures of damage parameters used in the various models. This study was performed to characterize the damage evolution during tensile loading of a melt infiltrated (MI) silicon carbide reinforced silicon carbide (SiC/SiC) composite. A SiC/SiC tensile coupon was loaded to a maximum global stress of 30 ksi in a tensile fixture within an SEM while observations were made at 5 ksi increments. Both traditional image analysis and DIC (digital image correlation) were used to quantify damage evolution. With the DIC analysis, microscale damage was observed at the fiber-matrix interfaces at stresses as low as 5 ksi. First matrix cracking took place between 20 and 25 ksi, accompanied by an observable relaxation in strain near matrix cracks. Matrix crack opening measurements at the maximum load ranged from 200 nm to 1.5 m. Crack opening along the fiber-matrix interface was also characterized as a function of load and angular position relative to the loading axis. This characterization was funded by NASA GRC and was performed to support NASA GRC modeling of SiC/SiC environmental degradation

Geometrically Nonlinear Field Fracture Mechanics and Crack Nucleation, Application to Strain Localization Fields in Al-Cu-Li Aerospace Alloys.

PubMed

Gupta, Satyapriya; Taupin, Vincent; Fressengeas, Claude; Jrad, Mohamad

2018-03-27

The displacement discontinuity arising between crack surfaces is assigned to smooth densities of crystal defects referred to as disconnections, through the incompatibility of the distortion tensor. In a dual way, the disconnections are defined as line defects terminating surfaces where the displacement encounters a discontinuity. A conservation statement for the crack opening displacement provides a framework for disconnection dynamics in the form of transport laws. A similar methodology applied to the discontinuity of the plastic displacement due to dislocations results in the concurrent involvement of dislocation densities in the analysis. Non-linearity of the geometrical setting is assumed for defining the elastic distortion incompatibility in the presence of both dislocations and disconnections, as well as for their transport. Crack nucleation in the presence of thermally-activated fluctuations of the atomic order is shown to derive from this nonlinearity in elastic brittle materials, without any algorithmic rule or ad hoc material parameter. Digital image correlation techniques applied to the analysis of tensile tests on ductile Al-Cu-Li samples further demonstrate the ability of the disconnection density concept to capture crack nucleation and relate strain localization bands to consistent disconnection fields and to the eventual occurrence of complex and combined crack modes in these alloys.

Laser-driven mechanical fracture in fused silica

NASA Astrophysics Data System (ADS)

Dahmani, Faiz

1999-10-01

Fused silica, widely used as optical-window material in high-fluence requirements on glass and KrF lasers, experiences optical damage. Under fatigue conditions, the damage is initiated by slow crack growth and culminates, if not arrested, with catastrophic crack growth and implosive failure when the stress intensity approaches the critical value. Since laser-induced cracks cannot be eliminated entirely, the behavior of cracked structures under service conditions must be quantified to be predicted. Systematic scientific rules must be devised to characterize laser-induced cracks and their effects, and to predict if and when it may become necessary to replace the damaged components. This thesis makes a contribution toward this end. Measurements of fatigue failure strength of laser-cracked fused silica in air at room temperature for different number of laser pulses and laser fluences are presented. The failure-strength variability is found to be due mainly to the spectrum of crack depths. Agreement with theory suggests the incorporation of a residual term into the failure-strength equation. Experiments on residual stresses induced in fused silica by the presence of a laser-induced crack are carried out using two different techniques. Theoretical modelings show that this residual stress field is of shear nature and mouth-opening. A correlation between the reduction in fracture strength of fused silica and the increase of the residual-stress field is established, providing laser systems designers and operators with guidance on the rate of crack growth as well as on the stress-related ramifications such as laser-driven cracks entail. Specifically, a hoop-stress in the immediate vicinity of a crack growing along the beam propagation direction is identified as strongly coupling to both the laser fluence and the crack growth. This coupling prompted the question of whether or not breaking the hoop stress symmetry by some external perturbation will accelerate or stymie crack growth. Experimental results on stress-inhibited laser-driven crack growth and stress-delayed-laser-damage initiation thresholds in fused silica and borosilicate glass (BK7) are presented. The results obtained show that, for very low compressive stresses (<10 psi), the damage initiation threshold is raised by as much as 78%, while the crack growth is arrested by 70%. Different loading- geometries are tested, giving different crack growth rates and raising the distinction between uniaxial and biaxial states of stresses.

De Novo Assembly and Characterization of Pericarp Transcriptome and Identification of Candidate Genes Mediating Fruit Cracking in Litchi chinensis Sonn.

PubMed Central

Li, Wei-Cai; Wu, Jian-Yang; Zhang, Hong-Na; Shi, Sheng-You; Liu, Li-Qin; Shu, Bo; Liang, Qing-Zhi; Xie, Jiang-Hui; Wei, Yong-Zan

2014-01-01

Fruit cracking has long been a topic of great concern for growers and researchers of litchi (Litchi chinensis Sonn.). To understand the molecular mechanisms underlying fruit cracking, high-throughput RNA sequencing (RNA-Seq) was first used for de novo assembly and characterization of the transcriptome of cracking pericarp of litchi. Comparative transcriptomic analyses were performed on non-cracking and cracking fruits. A total of approximately 26 million and 29 million high quality reads were obtained from the two groups of samples, and were assembled into 46,641 unigenes with an average length of 993 bp. These unigenes can be useful resources for future molecular studies of the pericarp in litchi. Furthermore, four genes (LcAQP, 1; LcPIP, 1; LcNIP, 1; LcSIP, 1) involved in water transport, five genes (LcKS, 2; LcGA2ox, 2; LcGID1, 1) involved in GA metabolism, 21 genes (LcCYP707A, 2; LcGT, 9; Lcβ-Glu, 6; LcPP2C, 2; LcABI1, 1; LcABI5, 1) involved in ABA metabolism, 13 genes (LcTPC, 1; Ca2+/H+ exchanger, 3; Ca2+-ATPase, 4; LcCDPK, 2; LcCBL, 3) involved in Ca transport and 24 genes (LcPG, 5; LcEG, 1; LcPE, 3; LcEXP, 5; Lcβ-Gal, 9; LcXET, 1) involved in cell wall metabolism were identified as genes that are differentially expressed in cracked fruits compared to non-cracked fruits. Our results open new doors to further understand the molecular mechanisms behind fruit cracking in litchi and other fruits, especially Sapindaceae plants. PMID:25272225

Analysis of Delamination Growth from Matrix Cracks in Laminates Subjected to Bending Loads

NASA Technical Reports Server (NTRS)

Murri, G. B.; Guynn, E. G.

1986-01-01

A major source of delamination damage in laminated composite materials is from low-velocity impact. In thin composite laminates under point loads, matrix cracks develop first in the plies, and delaminations then grow from these cracks at the ply interfaces. The purpose of this study was to quantify the combined effects of bending and transverse shear loads on delamination initiation from matrix cracks. Graphite-epoxy laminates with 90 deg. plies on the outside were used to provide a two-dimensional simulation of the damage due to low-velocity impact. Three plate bending problems were considered: a 4-point bending, 3-point bending, and an end-clamped center-loaded plate. Under bending, a matrix crack will form on the tension side of the laminate, through the outer 90 deg. plies and parallel to the fibers. Delaminations will then grow in the interface between the cracked 90 deg. ply and the next adjacent ply. Laminate plate theory was used to derive simple equations relating the total strain energy release rate, G, associated with the delamination growth from a 90 deg. ply crack to the applied bending load and laminate stiffness properties. Three different lay-ups were tested and results compared. Test results verified that the delamination always formed at the interface between the cracked 90 deg. ply and the next adjacent ply. Calculated values for total G sub c from the analysis showed good agreement for all configurations. The analysis was able to predict the delamination onset load for the cases considered. The result indicated that the opening mode component (Mode I) for delamination growth from a matrix crack may be much larger than the component due to interlaminar shear (Mode II).

Factors affecting cashew processing by wild bearded capuchin monkeys (Sapajus libidinosus, Kerr 1792).

PubMed

Visalberghi, Elisabetta; Albani, Alessandro; Ventricelli, Marialba; Izar, Patricia; Schino, Gabriele; Fragazsy, Dorothy

2016-08-01

Cashew nuts are very nutritious but so well defended by caustic chemicals that very few species eat them. We investigated how wild bearded capuchin monkeys (Sapajus libidinosus) living at Fazenda Boa Vista (FBV; Piauí, Brazil) process cashew nuts (Anacardium spp.) to avoid the caustic chemicals contained in the seed mesocarp. We recorded the behavior of 23 individuals toward fresh (N = 1282) and dry (N = 477) cashew nuts. Adult capuchins used different sets of behaviors to process nuts: rubbing for fresh nuts and tool use for dry nuts. Moreover, adults succeed to open dry nuts both by using teeth and tools. Age and body mass significantly affected success. Signs of discomfort (e.g., chemical burns, drooling) were rare. Young capuchins do not frequently closely observe adults processing cashew nuts, nor eat bits of nut processed by others. Thus, observing the behavior of skillful group members does not seem important for learning how to process cashew nuts, although being together with group members eating cashews is likely to facilitate interest toward nuts and their inclusion into the diet. These findings differ from those obtained when capuchins crack palm nuts, where observations of others cracking nuts and encounters with the artifacts of cracking produced by others are common and support young individuals' persistent practice at cracking. Cashew nut processing by capuchins in FBV appears to differ from that observed in a conspecific population living 320 km apart, where capuchins use tools to open both fresh and dry nuts. Moreover, in the latter population, chemical burns due to cashew caustic compounds appear to be common. The sources of these differences across populations deserve investigation, especially given that social influences on young monkeys learning to open cashew nuts at FBV seem to be nonspecific. Am. J. Primatol. 78:799-815, 2016. © 2016 Wiley Periodicals, Inc. © 2016 Wiley Periodicals, Inc.

A finite element model on effects of impact load and cavitation on fatigue crack propagation in mechanical bileaflet aortic heart valve.

PubMed

Mohammadi, H; Klassen, R J; Wan, W-K

2008-10-01

Pyrolytic carbon mechanical heart valves (MHVs) are widely used to replace dysfunctional and failed heart valves. As the human heart beats around 40 million times per year, fatigue is the prime mechanism of mechanical failure. In this study, a finite element approach is implemented to develop a model for fatigue analysis of MHVs due to the impact force between the leaflet and the stent and cavitation in the aortic position. A two-step method to predict crack propagation in the leaflets of MHVs has been developed. Stress intensity factors (SIFs) are computed at a small initiated crack located on the leaflet edge (the worst case) using the boundary element method (BEM). Static analysis of the crack is performed to analyse the stress distribution around the front crack zone when the crack is opened; this is followed by a dynamic crack analysis to consider crack propagation using the finite element approach. Two factors are taken into account in the calculation of the SIFs: first, the effect of microjet formation due to cavitation in the vicinity of leaflets, resulting in water hammer pressure; second, the effect of the impact force between the leaflet and the stent of the MHVs, both in the closing phase. The critical initial crack length, the SIFs, the water hammer pressure, and the maximum jet velocity due to cavitation have been calculated. With an initial crack length of 35 microm, the fatigue life of the heart valve is greater than 60 years (i.e. about 2.2 x 10(9) cycles) and, with an initial crack length of 170 microm, the fatigue life of the heart valve would be around 2.5 years (i.e. about 9.1 x 10(7) cycles). For an initial crack length greater than 170 microm, there is catastrophic failure and fatigue cracking no longer occurs. A finite element model of fatigue analysis using Patran command language (PCL custom code) in MSC software can be used to evaluate the useful lifespan of MHVs. Similar methodologies can be extended to other medical devices under cyclic loads.

Crack turning in integrally stiffened aircraft structures

NASA Astrophysics Data System (ADS)

Pettit, Richard Glen

Current emphasis in the aircraft industry toward reducing manufacturing cost has created a renewed interest in integrally stiffened structures. Crack turning has been identified as an approach to improve the damage tolerance and fail-safety of this class of structures. A desired behavior is for skin cracks to turn before reaching a stiffener, instead of growing straight through. A crack in a pressurized fuselage encounters high T-stress as it nears the stiffener---a condition favorable to crack turning. Also, the tear resistance of aluminum alloys typically varies with crack orientation, a form of anisotropy that can influence the crack path. The present work addresses these issues with a study of crack turning in two-dimensions, including the effects of both T-stress and fracture anisotropy. Both effects are shown to have relation to the process zone size, an interaction that is central to this study. Following an introduction to the problem, the T-stress effect is studied for a slightly curved semi-infinite crack with a cohesive process zone, yielding a closed form expression for the future crack path in an infinite medium. For a given initial crack tip curvature and tensile T-stress, the crack path instability is found to increase with process zone size. Fracture orthotropy is treated using a simple function to interpolate between the two principal fracture resistance values in two-dimensions. An extension to three-dimensions interpolates between the six principal values of fracture resistance. Also discussed is the transition between mode I and mode II fracture in metals. For isotropic materials, there is evidence that the crack seeks out a direction of either local symmetry (pure mode I) or local asymmetry (pure mode II) growth. For orthotropic materials the favored states are not pure modal, and have mode mixity that is a function of crack orientation. Drawing upon these principles, two crack turning prediction approaches are extended to include fracture resistance orthotropy---a second-order linear elastic method with a characteristic length parameter to incorporate T-stress/process-zone effects, and an elastic-plastic method that uses the Crack Tip Opening Displacement (CTOD) to determine the failure response. Together with a novel method for obtaining enhanced accuracy T-stress calculations, these methods are incorporated into an adaptive-mesh, finite-element fracture simulation code. A total of 43 fracture tests using symmetrically and asymmetrically loaded double cantilever beam specimens were run to develop crack turning parameters and compare predicted and observed crack paths.

The Development of Directional Decohesion Finite Elements for Multiscale Failure Analysis of Metallic Polycrystals

NASA Technical Reports Server (NTRS)

Saether, Erik; Glaessgen, Edward H.

2009-01-01

Atomistic simulations of intergranular fracture have indicated that grain-scale crack growth in polycrystalline metals can be direction dependent. At these material length scales, the atomic environment greatly influences the nature of intergranular crack propagation, through either brittle or ductile mechanisms, that are a function of adjacent grain orientation and direction of crack propagation. Methods have been developed to obtain cohesive zone models (CZM) directly from molecular dynamics simulations. These CZMs may be incorporated into decohesion finite element formulations to simulate fracture at larger length scales. A new directional decohesion element is presented that calculates the direction of Mode I opening and incorporates a material criterion for dislocation emission based on the local crystallographic environment to automatically select the CZM that best represents crack growth. The simulation of fracture in 2-D and 3-D aluminum polycrystals is used to illustrate the effect of parameterized CZMs and the effectiveness of directional decohesion finite elements.

A Comparison of Quasi-Static Indentation and Drop-Weight Impact Testing on Carbon-Epoxy Laminates

NASA Technical Reports Server (NTRS)

Prabhakaran, R.

2001-01-01

The project had two objectives: 1) The primary objective was to characterize damage tolerance of composite materials. To accomplish this, polymer matrix composites were to be subjected to static indentation as well as low-velocity impacts and the results analyzed. 2) A second objective was to investigate the effects of laser shock peening on the damage tolerance of aerospace materials, such as aluminum alloys, in terms of crack nucleation and crack propagation. The impact testing was proposed to be performed using a Dynatup drop tower. The specimens were to be placed over a square opening in a steel platen and impacted with a hemispherical tup. The damage was to be characterized in the laminate specimens. The damage tolerance of aerospace alloys was to be studied by conducting fatigue tests on aluminum alloy specimens with prior shock peening treatment. The crack length was to be monitored by a microscope and the crack propagation rate, da/dN, determined.

An efficient numerical method for the solution of the problem of elasticity for 3D-homogeneous elastic medium with cracks and inclusions

NASA Astrophysics Data System (ADS)

Kanaun, S.; Markov, A.

2017-06-01

An efficient numerical method for solution of static problems of elasticity for an infinite homogeneous medium containing inhomogeneities (cracks and inclusions) is developed. Finite number of heterogeneous inclusions and planar parallel cracks of arbitrary shapes is considered. The problem is reduced to a system of surface integral equations for crack opening vectors and volume integral equations for stress tensors inside the inclusions. For the numerical solution of these equations, a class of Gaussian approximating functions is used. The method based on these functions is mesh free. For such functions, the elements of the matrix of the discretized system are combinations of explicit analytical functions and five standard 1D-integrals that can be tabulated. Thus, the numerical integration is excluded from the construction of the matrix of the discretized problem. For regular node grids, the matrix of the discretized system has Toeplitz's properties, and Fast Fourier Transform technique can be used for calculation matrix-vector products of such matrices.

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.

Orientation effects on the measurement and analysis of critical CTOA in an aluminum alloy sheet

NASA Technical Reports Server (NTRS)

Sutton, M. A.; Dawicke, D. S.; Newman, J. C., Jr.

1994-01-01

Fracture tests were conducted on 76.2mm wide, 2.3mm thick middle crack tension (M(T)) specimens machined from 2024-T3 aluminum sheet. The specimens were tested on the T-L orientation and comparisons were made to similar tests conducted in the L-T orientation. Measurement of critical crack tip opening angle (CTOA), applied stress, and crack front shape were made as a function of crack extension. A two-dimensional, elastic-plastic finite element analysis was used to simulate the fracture behavior for both orientations. The results indicate that the T-L orientation had a 10 percent lower stress at fracture than similar tests conducted in the L-T orientation. Correspondingly, the critical CTOA in the T-L tests reached a constant value of 4.7 degrees after 2-3mm of crack extension and the L-T tests reached a value of 6 degrees. The fracture surfaces of the T-L specimens were observed to remain flat, while those of the L-T specimens transitioned to a 45 degree slant fracture after about 2-3mm of crack extension. The tunneling behavior of the two orientations also differed; the T-L specimens reached a deeply tunneled stabilized crack front shape while, the L-T specimens were observed to have only a small amount of tunneling once the crack began to grow on the 45 degree slant. The two-dimensional, elastic-plastic finite element analysis was able to simulate the fracture behavior for both the T-L and L-T orientations.

Urban cool

NASA Astrophysics Data System (ADS)

Ennos, Roland

2010-08-01

The picture-perfect summer for many involves dipping toes into the water's edge on a sandy beach, strolling through a park licking an ice cream or cracking open a bottle of cold beer as gorgeous smells waft from a barbecue nearby.

Differences in the Microstructure and Fatigue Properties of Dentin Between Residents of North and South America

PubMed Central

Ivancik, J.; Naranjo, M.; Correa, S.; Ossa, A.; Tay, F.R.; Pashley, D.H.; Arola, D.

2014-01-01

Spatial variations in the microstructure of dentin contribute to its mechanical behavior. Objective The objective of this investigation was to compare the microstructure and fatigue behavior of dentin from donors of two different countries. Methods Caries-free third molars were obtained from dental practices in Colombia, South America and the US to assemble two age-matched samples. The microstructure of the coronal dentin was evaluated at three characteristic depths (i.e. deep, middle and superficial dentin) using scanning electron microscopy and image processing techniques. The mechanical behavior of dentin in these three regions was evaluated by the fatigue crack growth resistance. Cyclic crack growth was achieved in-plane with the dentin tubules and the fatigue crack growth behavior was characterized in terms of the stress intensity threshold and the Paris Law parameters. Results There was no difference in the tubule density between the dentin of patients from the two countries. However, there were significant differences (p≤0.05) in the tubule lumen diameters between the two groups in the deep and peripheral regions. In regards to the fatigue resistance, there was a significant increase (p≤0.05) in threshold stress intensity range, and a significant decrease in fatigue crack growth coefficient with increasing distance from the pulp in teeth from the US donors. In contrast, these properties were independent of location for the dentin of teeth from the Colombian donors. Conclusions The microstructure of dentin and its mechanical behavior appear to be a function of patient background, which may include environmental factors and/or ethnicity. PMID:24960115

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.

Utilization of fractography in the evaluations of high temperature dynamic fatigue experiments

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

Breder, K.; Tennery, V.J.; Mroz, T.J.

1996-12-31

The slow crack growth properties of six structural ceramics were measured by dynamic fatigue in air and inert atmospheres over a range of elevated temperatures. The material response varied from no strength degradation as a function of stress and environment to significant strength degradation by slow crack growth (SCG) and by a combination of SCG and creep. The fractographic investigation showed that SCG was evidenced by growth of isolated cracks and often by an intergranular fracture mode, while creep was evidenced by accumulated damage such as void formation and opening of the microstructure at grain boundaries and triple junctions. Formore » the materials in which the strength was unaffected by the stress and environment, the fracture surfaces were essentially indistinguishable from the inert fracture surfaces.« less

Utilization of fractography in the evaluation of high temperature dynamic fatigue experiments

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

Breder, K.; Wereszczak, A.A.; Tennery, V.J.

1995-12-31

The slow crack growth properties of six structural ceramics were measured by dynamic fatigue in air and inert atmospheres over a range of elevated temperatures. The material response varied from no strength degradation as a function of stress and environment to significant strength degradation by slow crack growth (SCG) and by a combination of SCG and creep. The fractographic investigation showed that SCG was evidenced by growth of isolated cracks and often by an intergranular fracture mode, while creep was evidenced by accumulated damage such as void formation and opening of the microstructure at grain boundaries and triple junctions. Formore » the materials in which the strength was unaffected by the stress and environment, the fracture surfaces were essentially indistinguishable from the inert fracture surfaces.« less

Stress state of rock mass under open pit mining in the influence zone of tectonic disturbances (in terms of the Oktorkoi Fault, North Tien Shan)

NASA Astrophysics Data System (ADS)

Kozhogulov, KCh; Nikolskaya, OV; Rybin, AK; Kuzikov, SI

2018-03-01

The qualitative connection between the crack growth direction and the orientation of the main axes of horizontal deformations in rocks mass in the area of the Boordin gold ore province is revealed. The effect of the rock mass quality (RQD) and contact conditions of crack surfaces on the stability index of pit wall rock mass is evaluated, and the influence of the rock mass quality index on the pit wall stability is determined.

Addressing the stimulant treatment gap: A call to investigate the therapeutic benefits potential of cannabinoids for crack-cocaine use.

PubMed

Fischer, Benedikt; Kuganesan, Sharan; Gallassi, Andrea; Malcher-Lopes, Renato; van den Brink, Wim; Wood, Evan

2015-12-01

Crack-cocaine use is prevalent in numerous countries, yet concentrated primarily - largely within urban contexts - in the Northern and Southern regions of the Americas. It is associated with a variety of behavioral, physical and mental health and social problems which gravely affect users and their environments. Few evidence-based treatments for crack-cocaine use exist and are available to users in the reality of street drug use. Numerous pharmacological treatments have been investigated but with largely disappointing results. An important therapeutic potential for crack-cocaine use may rest in cannabinoids, which have recently seen a general resurgence for varied possible therapeutic usages for different neurological diseases. Distinct potential therapeutic benefits for crack-cocaine use and common related adverse symptoms may come specifically from cannabidiol (CBD) - one of the numerous cannabinoid components found in cannabis - with its demonstrated anxiolytic, anti-psychotic, anti-convulsant effects and potential benefits for sleep and appetite problems. The possible therapeutic prospects of cannabinoids are corroborated by observational studies from different contexts documenting crack-cocaine users' 'self-medication' efforts towards coping with crack-cocaine-related problems, including withdrawal and craving, impulsivity and paranoia. Cannabinoid therapeutics offer further benefits of being available in multiple formulations, are low in adverse risk potential, and may easily be offered in community-based settings which may add to their feasibility as interventions for - predominantly marginalized - crack-cocaine user populations. Supported by the dearth of current therapeutic options for crack-cocaine use, we are advocating for the implementation of a rigorous research program investigating the potential therapeutic benefits of cannabinoids for crack-cocaine use. Given the high prevalence of this grave substance use problem in the Americas, opportunities for such research should urgently be created and facilitated there. Copyright © 2015 Elsevier B.V. All rights reserved.

Asynchronous cracking with dissimilar paths in multilayer graphene.

PubMed

Jang, Bongkyun; Kim, Byungwoon; Kim, Jae-Hyun; Lee, Hak-Joo; Sumigawa, Takashi; Kitamura, Takayuki

2017-11-16

Multilayer graphene consists of a stack of single-atomic-thick monolayer graphene sheets bound with π-π interactions and is a fascinating model material opening up a new field of fracture mechanics. In this study, fracture behavior of single-crystalline multilayer graphene was investigated using an in situ mode I fracture test under a scanning electron microscope, and abnormal crack propagation in multilayer graphene was identified for the first time. The fracture toughness of graphene was determined from the measured load-displacement curves and the realistic finite element modelling of specimen geometries. Nonlinear fracture behavior of the multilayer graphene is discussed based on nonlinear elastic fracture mechanics. In situ scanning electron microscope images obtained during the fracture test showed asynchronous crack propagation along independent paths, causing interlayer shear stress and slippages. We also found that energy dissipation by interlayer slippages between the graphene layers is the reason for the enhanced fracture toughness of multilayer graphene. The asynchronous cracking with independent paths is a unique cracking and toughening mechanism for single-crystalline multilayer graphene, which is not observed for the monolayer graphene. This could provide a useful insight for the design and development of graphene-based composite materials for structural applications.

Microscopic Observation of the Side Surface of Dynamically-Tensile-Fractured 6061-T6 and 2219-T87 Aluminum Alloys with Pre-Fatigue

NASA Astrophysics Data System (ADS)

Itabashi, Masaaki; Nakajima, Shigeru; Fukuda, Hiroshi

After unexpected failure of metallic structure, microscopic investigation will be performed. Generally, such an investigation is limited to search striation pattern with a SEM (scanning electron microscope). But, when the cause of the failure was not severe repeated stress, this investigation is ineffective. In this paper, new microscopic observation technique is proposed to detect low cycle fatigue-impact tensile loading history. Al alloys, 6061-T6 and 2219-T87, were fractured in dynamic tension, after severe pre-fatigue. The side surface of the fractured specimens was observed with a SEM. Neighboring fractured surface, many opened cracks on the side surface have been generated. For each specimen, the number of the cracks was counted together with information of individual sizes and geometric features. For 6061-T6 alloy specimen with the pre-fatigue, the number of the cracks is greater than that for the specimen without the pre-fatigue. For 2219-T87 alloy, the same tendency can be found after a certain screening of the crack counting. Therefore, the crack counting technique may be useful to detect the existence of the pre-fatigue from the dynamically fractured specimen surface.

The Effect of Interface Cracks on the Electrical Performance of Solar Cells

NASA Astrophysics Data System (ADS)

Kim, Hansung; Tofail, Md. Towfiq; John, Ciby

2018-04-01

Among a variety of solar cell types, thin-film solar cells have been rigorously investigated as cost-effective and efficient solar cells. In many cases, flexible solar cells are also fabricated as thin films and undergo frequent stress due to the rolling and bending modes of applications. These frequent motions result in crack initiation and propagation (including delamination) in the thin-film solar cells, which cause degradation in efficiency. Reliability evaluation of solar cells is essential for developing a new type of solar cell. In this paper, we investigated the effect of layer delamination and grain boundary crack on 3D thin-film solar cells. We used finite element method simulation for modeling of both electrical performance and cracked structure of 3D solar cells. Through simulations, we quantitatively calculated the effect of delamination length on 3D copper indium gallium diselenide (CIGS) solar cell performance. Moreover, it was confirmed that the grain boundary of CIGS could improve the solar cell performance and that grain boundary cracks could decrease cell performance by altering the open circuit voltage. In this paper, the investigated material is a CIGS solar cell, but our method can be applied to general polycrystalline solar cells.

Non-double-couple mechanisms of microearthquakes induced by hydraulic fracturing

USGS Publications Warehouse

Sileny, J.; Hill, D.P.; Eisner, Leo; Cornet, F.H.

2009-01-01

We have inverted polarity and amplitude information of representative microearthquakes to investigate source mechanisms of seismicity induced by hydraulic fracturing in the Carthage Cotton Valley, east Texas, gas field. With vertical arrays of four and eight three-component geophones in two monitoring wells, respectively, we were able to reliably determine source mechanisms of the strongest events with the best signal-to-noise ratio. Our analysis indicates predominantly non-double-couple source mechanisms with positive volumetric component consistent with opening cracks oriented close to expected hydraulic fracture orientation. Our observations suggest the induced events are directly the result of opening cracks by fluid injection, in contrast to many previous studies where the seismicity is interpreted to be primarily shearing caused by pore pressure diffusion into the surrounding rock or associated with shear stresses created at the hydraulic fracture tip. Copyright 2009 by the American Geophysical Union.

The effect of fatigue cracks on fastener flexibility, load distribution, and fatigue crack growth

NASA Astrophysics Data System (ADS)

Whitman, Zachary Layne

Fatigue cracks typically occur at stress risers such as geometry changes and holes. This type of failure has serious safety and economic repercussions affecting structures such as aircraft. The need to prevent catastrophic failure due to fatigue cracks and other discontinuities has led to durability and damage tolerant methodologies influencing the design of aircraft structures. Holes in a plate or sheet filled with a fastener are common fatigue critical locations in aircraft structure requiring damage tolerance analysis (DTA). Often, the fastener is transferring load which leads to a loading condition involving both far-field stresses such as tension and bending, and localized bearing at the hole. The difference between the bearing stress and the tensile field at the hole is known as load transfer. The ratio of load transfer as well as the magnitude of the stresses plays a significant part in how quickly a crack will progress to failure. Unfortunately, the determination of load transfer in a complex joint is far from trivial. Many methods exist in the open literature regarding the analysis of splices, doublers and attachment joints to determine individual fastener loads. These methods work well for static analyses but greater refinement is needed for crack growth analysis. The first fastener in a splice or joint is typically the most critical but different fastener flexibility equations will all give different results. The constraint of the fastener head and shop end, along with the type of fastener, affects the stiffness or flexibility of the fastener. This in turn will determine the load that the fastener will transfer within a given fastener pattern. However, current methods do not account for the change in flexibility at a fastener as the crack develops. It is put forth that a crack does indeed reduce the stiffness of a fastener by changing its constraint, thus lessening the load transfer. A crack growth analysis utilizing reduced load transfer will result in a slower growing crack versus an analysis that ignores the effect.

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.

Elastic-Plastic J-Integral Solutions or Surface Cracks in Tension Using an Interpolation Methodology

NASA Technical Reports Server (NTRS)

Allen, P. A.; Wells, D. N.

2013-01-01

No closed form solutions exist for the elastic-plastic J-integral for surface cracks due to the nonlinear, three-dimensional nature of the problem. Traditionally, each surface crack must be analyzed with a unique and time-consuming nonlinear finite element analysis. To overcome this shortcoming, the authors have developed and analyzed an array of 600 3D nonlinear finite element models for surface cracks in flat plates under tension loading. The solution space covers a wide range of crack shapes and depths (shape: 0.2 less than or equal to a/c less than or equal to 1, depth: 0.2 less than or equal to a/B less than or equal to 0.8) and material flow properties (elastic modulus-to-yield ratio: 100 less than or equal to E/ys less than or equal to 1,000, and hardening: 3 less than or equal to n less than or equal to 20). The authors have developed a methodology for interpolating between the goemetric and material property variables that allows the user to reliably evaluate the full elastic-plastic J-integral and force versus crack mouth opening displacement solution; thus, a solution can be obtained very rapidly by users without elastic-plastic fracture mechanics modeling experience. Complete solutions for the 600 models and 25 additional benchmark models are provided in tabular format.

Inherent flexibility determines the transition mechanisms of the EF-hands of calmodulin.

PubMed

Tripathi, Swarnendu; Portman, John J

2009-02-17

We explore how inherent flexibility of a protein molecule influences the mechanism controlling allosteric transitions by using a variational model inspired from work in protein folding. The striking differences in the predicted transition mechanism for the opening of the two domains of calmodulin (CaM) emphasize that inherent flexibility is key to understanding the complex conformational changes that occur in proteins. In particular, the C-terminal domain of CaM (cCaM), which is inherently less flexible than its N-terminal domain (nCaM), reveals "cracking" or local partial unfolding during the open/closed transition. This result is in harmony with the picture that cracking relieves local stresses caused by conformational deformations of a sufficiently rigid protein. We also compare the conformational transition in a recently studied even-odd paired fragment of CaM. Our results rationalize the different relative binding affinities of the EF-hands in the engineered fragment compared with the intact odd-even paired EF-hands (nCaM and cCaM) in terms of changes in flexibility along the transition route. Aside from elucidating general theoretical ideas about the cracking mechanism, these studies also emphasize how the remarkable intrinsic plasticity of CaM underlies conformational dynamics essential for its diverse functions.

Environmental fatigue of an Al-Li-Cu alloy. Part 1: Intrinsic crack propagation kinetics in hydrogenous environments

NASA Technical Reports Server (NTRS)

Piascik, Robert S.; Gangloff, Richard P.

1991-01-01

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates (da/dN) in peak aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant delta K and K(sub max) loading. The da/dN are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 percent NaCl with anodic polarization, pure water vapor, moist air, and NaCl with cathodic polarization. While da/dN depends on delta K(sup 4.0) for the inert gases, water vapor and chloride induced multiple power-laws, and a transition growth rate 'plateau'. Environmental effects are strongest at low delta K. Crack tip damage is ascribed to hydrogen embrittlement because of the following: (1) accelerated da/dN due to part-per-million levels of H2O without condensation; (2) impeded molecular flow model predictions of the measured water vapor pressure dependence of da/dN as affected by mean crack opening; (3) the lack of an effect of film-forming O2; (4) the likelihood for crack tip hydrogen production in NaCl, and (5) the environmental and delta K-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li2CO3, and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075.

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

Kruska, Karen; Zhai, Ziqing; Bruemmer, Stephen M.

Due to its superior resistance to corrosion and stress corrosion cracking (SCC), high Cr, Ni-base Alloy 690 is now commonly used in pressurized water reactors (PWRs). Even though highly cold-worked (CW) Alloy 690 has been shown to be susceptible to SCC crack growth in PWR primary water environments, an open question remains whether SCC initiation was possible for these materials under constant load test conditions. Testing has been performed on a series of CW alloy 690 CRDM tubing specimens at constant load for up to 9,220 hours in 360°C simulated PWR primary water. A companion paper will discuss the overallmore » testing approach and describe results on different alloy 690 heats and cold work levels. The focus of the current paper is to illustrate the use of focused ion beam (FIB), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for the high-resolution investigation of precursor damage and intergranular (IG) crack nucleation in these specimens. Three-dimensional (3D) FIB/SEM imaging has been conducted on a series of grain boundary (GB) damage precursors, such as IG small cavities, local corrosion and even shallow cracks observed at the specimen surface. Contrast variations and EDS mapping were used to distinguish oxides, carbides and cavities from the matrix material. Nanometer-sized cavities were observed associated with GB carbides in the highly CW specimens. Shallow IG cracks were present in the 30%CW specimens and exhibited oxidized crack flanks and a higher density of cavities ahead of the oxide front in all cases. The shape and distribution of carbides and cavities in the plane of the cracked GBs was analyzed in 3D to gain a mechanistic understanding of the processes that may be leading to crack initiation in highly CW alloy 690.« less

Investigating glide snow avalanche release using seismic monitoring in combination with time-lapse photography

NASA Astrophysics Data System (ADS)

van Herwijnen, Alec; Failletaz, Jerome; Berhod, Nicole; Mitterer, Christoph

2013-04-01

Glide avalanches occur when the entire snowpack glides over the ground until an avalanche releases. These avalanches are difficult to forecast since the gliding process can take place over a few hours up to several weeks or months. The presence of liquid water at the interface between the snow cover and the ground surface is of primary importance as it reduces frictional support. Glide avalanches are often preceded by the opening of a tensile crack in the snow cover, called a glide crack. Past research has shown that glide crack opening accelerates prior to avalanche release. During the winter of 2012-2013, we monitored glide crack expansion using time-lapse photography in combination with a seismic sensor and two heat flux sensors on a slope with well documented glide avalanche activity in the Eastern Swiss Alps above Davos, Switzerland. To track changes in glide rates, the number of dark pixels in an area around the glide crack is counted in each image. Using this technique, we observed an increase in glide rates prior to avalanche release. Since the field site is located very close to the town of Davos, the seismic data was very noisy. Nevertheless, the accelerated snow gliding observed in the time-lapse images coincided with increased seismic activity. Overall, these results show that a combination of time-lapse photography with seismic monitoring could provide valuable insight into glide avalanche release. Recordings of the heat flux plates show that the energy input from the soil is fairly small and constant throughout the observed period. The results suggest that ground heat flux is a minor contributor to the water production at the snow-soil interface. Instead, the presence of water at the base of the snowpack is probably due to a strong hydraulic pressure gradient at the snow-soil interface.

Double Cantilever Beam and End Notched Flexure Fracture Toughness Testing of Two Composite Materials

NASA Technical Reports Server (NTRS)

Kessler, Jeff A.; Adams, Donald F.

1993-01-01

Two different unidirectional composite materials were provided by NASA Langley Research Center and tested by the Composite Materials Research Group within the Department of Mechanical Engineering at the University of Wyoming. Double cantilever beam and end notched flexure tests were performed to measure the mode I (crack opening) and mode II (sliding or shear) interlaminar fracture toughness of the two materials. The two composites consisted of IM7 carbon fiber combined with either RP46 resin toughened with special formulation of LaRC IA resin, known as JJS1356; or PES chain extended thermoplastic resin known as JJS1361. Double Cantilever Beam Specimen Configuration and Test Methods As received from NASA, the test specimens were nominally 0.5 inch wide, 6 inches long, and 0.2 inch thick. A 1 inch long Kapton insert at the midplane of one end of the specimen (placed during laminate fabrication) facilitated crack initiation and extension. It was noted that the specimens provided were smaller than the nominal 1.5 inch wide, 9.0 inch long configuration specified. Similarly, the Kapton inserts were of greater length than those in the present specimens. Hence, the data below should not be compared directly to those generated with the referenced methods. No preconditioning was performed on the specimens prior to testing. In general, the methodology was used for the present work. Crack opening loads were introduced to the specimens via piano hinges attached to the main specimen faces at a single end of each specimen. Hinges were bolted to the specimens using the technique presented. The cracks were extended a small distance from the end of the Kapton insert prior to testing. Just before precracking, the sides of the specimens were coated with water-soluble typewriter correction fluid to aid in crack visualization. Scribe marks were then made in the coating at half-inch intervals.

Experimental Observations of a Stitched Composite with a Notch Subjected to Combined Bending and Tension Loading

NASA Technical Reports Server (NTRS)

Palmer, Susan O.; Nettles, Alan T.; Poe, C. C.

1998-01-01

A series of tests was conducted to support development of an analytical model for predicting the failure strains of stitched warp-knit carbon/epoxy composite materials with through-thicknesss damage in the form of a crack-like notch. Measurements of strain near notch tips, crack opening displacement (COD), and applied load were monitored in all tests. The out-of-plane displacement at the center of the notch was also measured when the specimen was subjected to bending. Three types of loading were applied: pure bending, pure tension, and combined bending and tension.

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

Crack initiation and potential hot-spot formation around a cylindrical defect under dynamic compression

NASA Astrophysics Data System (ADS)

Ma, Xiao; Li, Xinguo; Zheng, Xianxu; Li, Kewu; Hu, Qiushi; Li, Jianling

2017-11-01

In recent decades, the hot-spot theory of condensed-phase explosives has been a compelling focus of scientific investigation attracting many researchers. The defect in the polymeric binder of the polymer-bonded explosive is called the intergranular defect. In this study, the real polymeric binder was substituted by poly(methyl methacrylate) (PMMA) as it is transparent and has similar thermodynamic properties to some binders. A set of modified split Hopkinson pressure bars equipped with a time-resolved shadowgraph was used to study the process of crack initiation and potential hot-spot formation around a cylindrical defect in PMMA. The new and significant phenomenon that the opening-mode crack emerged earlier than the shearing-mode crack from the cylindrical defect has been published for the first time in this paper. Furthermore, a two-dimensional numerical simulation was performed to show the evolution of both the stress field and the temperature field. The simulation results were in good agreement with the experiment. Finally, the law of potential hot-spot formation is discussed in detail.

Damage tolerance of woven graphite-epoxy buffer strip panels

NASA Technical Reports Server (NTRS)

Kennedy, John M.

1990-01-01

Graphite-epoxy panels with S glass buffer strips were tested in tension and shear to measure their residual strengths with crack-like damage. The buffer strips were regularly spaced narrow strips of continuous S glass. Panels were made with a uniweave graphite cloth where the S glass buffer material was woven directly into the cloth. Panels were made with different width and thickness buffer strips. The panels were loaded to failure while remote strain, strain at the end of the slit, and crack opening displacement were monitoring. The notched region and nearby buffer strips were radiographed periodically to reveal crack growth and damage. Except for panels with short slits, the buffer strips arrested the propagating crack. The strength (or failing strain) of the panels was significantly higher than the strength of all-graphite panels with the same length slit. Panels with wide, thick buffer strips were stronger than panels with thin, narrow buffer strips. A shear-lag model predicted the failing strength of tension panels with wide buffer strips accurately, but over-estimated the strength of the shear panels and the tension panels with narrow buffer strips.

Comparative Stress Corrosion Cracking and General Corrosion Resistance of Annealed and Hardened 440 C Stainless Steel - New Techniques in Stress Corrosion Testing

NASA Technical Reports Server (NTRS)

Mendreck, M. J.; Hurless, B. E.; Torres, P. D.; Danford, M. D.

1998-01-01

The corrosion and stress corrosion cracking (SCC) characteristics of annealed and hardened 440C stainless steel were evaluated in high humidity and 3.5-percent NaCl solution. Corrosion testing consisted of an evaluation of flat plates, with and without grease, in high humidity, as well as electrochemical testing in 3.5-percent NaCl. Stress corrosion testing consisted of conventional, constant strain, smooth bar testing in high humidity in addition to two relatively new techniques under evaluation at MSFC. These techniques involve either incremental or constant rate increases in the load applied to a precracked SE(B) specimen, monitoring the crack-opening-displacement response for indications of crack growth. The electrochemical corrosion testing demonstrated an order of magnitude greater general corrosion rate in the annealed 440C. All techniques for stress corrosion testing showed substantially better SCC resistance in the annealed material. The efficacy of the new techniques for stress corrosion testing was demonstrated both by the savings in time and the ability to better quantify SCC data.

Failure of disordered materials as a depinning transition

NASA Astrophysics Data System (ADS)

Ponson, Laurent

2010-03-01

Crack propagation is the fundamental process leading to material failure. However, its dynamics is far from being fully understood. In this work, we investigate both experimentally and theoretically the far-from-equilibrium propagation of a crack within a disordered brittle material. At first, we focus on the average dynamics of a crack, and study the variations of its growth velocity v with respect to the external driving force G [1]. Carefully measured on a brittle rock, these variations are shown to display two regimes: above a given threshold Gc, the velocity evolves as a power law v ˜(G- Gc)^0.8, while at low driving force, its variations are well described by a sub-critical creep law, characteristic of a thermally activated crack propagation. Extending the continuum theory of Fracture Mechanics to inhomogeneous media, we show that this behavior is reminiscent of a dynamical critical transition: critical failure occurs when the driving force is sufficiently large to depin the crack front from the material heterogeneities. Another way to reveal such a transition is to investigate the fluctuations of crack velocity [2]. Considering a crack at the heterogeneous interface between two elastic solids, we predict that its propagation occurs through sudden jumps, with power law distributed sizes and durations. These predictions compare quantitatively well with recent direct observations of interfacial crack propagation [3]. Such an interpretation of material failure opens new perspectives in the field of Engineering and Applied Science that will be finally discussed. [4pt] [1] L. Ponson, Depinning transition in failure of inhomogeneous brittle materials, Phys. Rev. Lett. 103, 055501 (2009). [0pt] [2] D. Bonamy, S. Santucci and L. Ponson, Crackling dynamics in material failure as a signature of a self-organized dynamic phase transition, Phys. Rev. Lett. 101, 045501 (2008). [0pt] [3] K.J. Måløy, S. Santucci, J. Schmittbuhl and R. Toussaint, Local waiting time fluctuations along a randomly pinned crack front, Phys. Rev. Lett. 96, 045501 (2006).

Analysis of the progressive failure of brittle matrix composites

NASA Technical Reports Server (NTRS)

Thomas, David J.

1995-01-01

This report investigates two of the most common modes of localized failures, namely, periodic fiber-bridged matrix cracks and transverse matrix cracks. A modification of Daniels' bundle theory is combined with Weibull's weakest link theory to model the statistical distribution of the periodic matrix cracking strength for an individual layer. Results of the model predictions are compared with experimental data from the open literature. Extensions to the model are made to account for possible imperfections within the layer (i.e., nonuniform fiber lengths, irregular crack spacing, and degraded in-situ fiber properties), and the results of these studies are presented. A generalized shear-lag analysis is derived which is capable of modeling the development of transverse matrix cracks in material systems having a general multilayer configuration and under states of full in-plane load. A method for computing the effective elastic properties for the damaged layer at the global level is detailed based upon the solution for the effects of the damage at the local level. This methodology is general in nature and is therefore also applicable to (0(sub m)/90(sub n))(sub s) systems. The characteristic stress-strain response for more general cases is shown to be qualitatively correct (experimental data is not available for a quantitative evaluation), and the damage evolution is recorded in terms of the matrix crack density as a function of the applied strain. Probabilistic effects are introduced to account for the statistical nature of the material strengths, thus allowing cumulative distribution curves for the probability of failure to be generated for each of the example laminates. Additionally, Oh and Finney's classic work on fracture location in brittle materials is extended and combined with the shear-lag analysis. The result is an analytical form for predicting the probability density function for the location of the next transverse crack occurrence within a crack bounded region. The results of this study verified qualitatively the validity of assuming a uniform crack spacing (as was done in the shear-lag model).

Strength and durability of near-surface mounted CFRP bars for shear strengthening reinforced concrete bridge girders : appendices.

DOT National Transportation Integrated Search

2012-01-01

During the interstate expansion of the 1950s, many conventionally reinforced concrete deck girder bridges were built throughout the country. These aging bridges commonly exhibit diagonal cracking and rate inadequately for shear, thus they are candida...

Strength and durability of near-surface mounted CFRP bars for shear strengthening reinforced concrete bridge girders.

DOT National Transportation Integrated Search

2012-03-01

During the interstate expansion of the 1950s, many conventionally reinforced concrete deck girder bridges were built throughout the country. These aging bridges commonly exhibit diagonal cracking and rate inadequately for shear, thus they are candida...

Seismic characteristics of tensile fracture growth induced by hydraulic fracturing

NASA Astrophysics Data System (ADS)

Eaton, D. W. S.; Van der Baan, M.; Boroumand, N.

2014-12-01

Hydraulic fracturing is a process of injecting high-pressure slurry into a rockmass to enhance its permeability. Variants of this process are used for unconventional oil and gas development, engineered geothermal systems and block-cave mining; similar processes occur within volcanic systems. Opening of hydraulic fractures is well documented by mineback trials and tiltmeter monitoring and is a physical requirement to accommodate the volume of injected fluid. Numerous microseismic monitoring investigations acquired in the audio-frequency band are interpreted to show a prevalence of shear-dominated failure mechanisms surrounding the tensile fracture. Moreover, the radiated seismic energy in the audio-frequency band appears to be a miniscule fraction (<< 1%) of the net injected energy, i.e., the integral of the product of fluid pressure and injection rate. We use a simple penny-shaped crack model as a predictive framework to describe seismic characteristics of tensile opening during hydraulic fracturing. This model provides a useful scaling relation that links seismic moment to effective fluid pressure within the crack. Based on downhole recordings corrected for attenuation, a significant fraction of observed microseismic events are characterized by S/P amplitude ratio < 5. Despite the relatively small aperture of the monitoring arrays, which precludes both full moment-tensor analysis and definitive identification of nodal planes or axes, this ratio provides a strong indication that observed microseismic source mechanisms have a component of tensile failure. In addition, we find some instances of periodic spectral notches that can be explained by an opening/closing failure mechanism, in which fracture propagation outpaces fluid velocity within the crack. Finally, aseismic growth of tensile fractures may be indicative of a scenario in which injected energy is consumed to create new fracture surfaces. Taken together, our observations and modeling provide evidence that failure mechanisms documented by passive monitoring of hydraulic fractures may contain a significant component of tensile failure, including fracture opening and closing, although creation of extensive new fracture surfaces may be a seismically inefficient process that radiates at sub-audio frequencies.

Three-dimensional fracture instability of a displacement-weakening planar interface under locally peaked nonuniform loading

NASA Astrophysics Data System (ADS)

Uenishi, Koji

2018-06-01

We consider stability of fracture on a three-dimensional planar interface subjected to a loading stress that is locally peaked spatially, the level of which increases quasi-statically in time. Similar to the earlier study on the two-dimensional case (Uenishi and Rice, 2003; Rice and Uenishi, 2010), as the loading stress increases, a crack, or a region of displacement discontinuity (opening gap in tension or slip for shear fracture), develops on the interface where the stress is presumed to decrease according to a displacement-weakening constitutive relation. Upon reaching the instability point at which no further quasi-static solution for the extension of the crack on the interface exists, dynamic fracture follows. For the investigation of this instability point, we employ a dimensional analysis as well as an energy approach that gives a Rayleigh-Ritz approximation for the dependence of crack size and maximum displacement discontinuity on the level and quadratic shape of the loading stress distribution. We show that, if the linear displacement-weakening law is applied and the crack may be assumed of an elliptical form, the critical crack size at instability is independent of the curvature of the loading stress distribution and it is of the same order for all two- and three-dimensional cases.

Creep Crack Initiation and Growth Behavior for Ni-Base Superalloys

NASA Astrophysics Data System (ADS)

Nagumo, Yoshiko; Yokobori, A. Toshimitsu, Jr.; Sugiura, Ryuji; Ozeki, Go; Matsuzaki, Takashi

The structural components which are used in high temperature gas turbines have various shapes which may cause the notch effect. Moreover, the site of stress concentration might have the heterogeneous microstructural distribution. Therefore, it is necessary to clarify the creep fracture mechanism for these materials in order to predict the life of creep fracture with high degree of accuracy. In this study, the creep crack growth tests were performed using in-situ observational testing machine with microscope to observe the creep damage formation and creep crack growth behavior. The materials used are polycrystalline Ni-base superalloy IN100 and directionally solidified Ni-base superalloy CM247LC which were developed for jet engine turbine blades and gas turbine blades in electric power plants, respectively. The microstructural observation of the test specimens was also conducted using FE-SEM/EBSD. Additionally, the analyses of two-dimensional elastic-plastic creep finite element using designed methods were conducted to understand the effect of microstructural distribution on creep damage formation. The experimental and analytical results showed that it is important to determine the creep crack initiation and early crack growth to predict the life of creep fracture and it is indicated that the highly accurate prediction of creep fracture life could be realized by measuring notch opening displacement proposed as the RNOD characteristic.

Effect of initial contact surface condition on the friction and wear properties of bearing steel in cyclic reciprocating sliding contact

NASA Astrophysics Data System (ADS)

Tanaka, Y.; Endo, M.; Moriyama, S.

2017-05-01

Delamination failure is one of the most important engineering problems. This failure can frequently be detrimental to rolling contact machine elements such as bearings, gear wheels, etc. This phenomenon, called rolling contact fatigue, has a close relationship not only with opening-mode but also with shear-mode fatigue crack growth. The crack face interference is known to significantly affect the shear-mode fatigue crack propagation and its threshold behavior. Quantitative investigation on friction and wear at fatigue crack faces in the material is essentially impossible. Previously, thus, a novel ring-on-ring test by making use of fatigue testing machine was proposed to simulate a cyclic reciprocating sliding contact of crack surfaces. However, this test procedure had some problems. For instance, in order to achieve the uniform contact at the start of test, the rubbing of specimens must be conducted in advance. By this treatment, the specimen surfaces were already damaged before the test. In this study, an improvement of experimental method was made to perform the test using the damage-free specimens. The friction and wear properties for heat-treated high carbon-chromium bearing steel were investigated with this new method and the results were compared to the results obtained by using the initially damaged specimens.

Analytical Round Robin for Elastic-Plastic Analysis of Surface Cracked Plates: Phase I Results

NASA Technical Reports Server (NTRS)

Wells, D. N.; Allen, P. A.

2012-01-01

An analytical round robin for the elastic-plastic analysis of surface cracks in flat plates was conducted with 15 participants. Experimental results from a surface crack tension test in 2219-T8 aluminum plate provided the basis for the inter-laboratory study (ILS). The study proceeded in a blind fashion given that the analysis methodology was not specified to the participants, and key experimental results were withheld. This approach allowed the ILS to serve as a current measure of the state of the art for elastic-plastic fracture mechanics analysis. The analytical results and the associated methodologies were collected for comparison, and sources of variability were studied and isolated. The results of the study revealed that the J-integral analysis methodology using the domain integral method is robust, providing reliable J-integral values without being overly sensitive to modeling details. General modeling choices such as analysis code, model size (mesh density), crack tip meshing, or boundary conditions, were not found to be sources of significant variability. For analyses controlled only by far-field boundary conditions, the greatest source of variability in the J-integral assessment is introduced through the constitutive model. This variability can be substantially reduced by using crack mouth opening displacements to anchor the assessment. Conclusions provide recommendations for analysis standardization.

Use of fiber reinforced concrete for concrete pavement slab replacement.

DOT National Transportation Integrated Search

2014-03-01

Unlike ordinary concrete pavement, replacement concrete slabs need to be open to traffic within 24 hours (sooner in : some cases). Thus, high early-strength concrete is used; however, it frequently cracks prematurely as a result of high : heat of hyd...

Fracture Growth Testing of Titanium 6AL-4V in AF-M315E

NASA Technical Reports Server (NTRS)

Sampson, Jeffrey W.; Martinez, Jonathan; McLean, Christopher

2015-01-01

The Green Propellant Infusion Mission (GPIM) will demonstrate the performance of AF-M315E monopropellant in orbit. Flight certification requires a safe-life analysis of the titanium alloy fuel tank to ensure inherent flaws will not cause failure during the design life. Material property inputs for this analysis require testing to determine the stress intensity factor for environmentally-assisted cracking (K (sub EAC)) of Ti 6Al-4V in combination with the AF-M315E monopropellant. Testing of single-edge notched specimens SE(B) representing the bulk tank membrane and weld material were performed in accordance with ASTM E1681. Specimens with fatigue pre-cracks were loaded into test fixtures so that the crack tips were exposed to the monopropellant at 50 degrees Centigrade for a duration of 1,000 hours. Specimens that did not fail during exposure were opened to inspect the crack surfaces for evidence of crack growth. The threshold stress intensity value, KEAC, is the highest applied stress intensity that produced neither a failure of the specimen during the exposure nor showed evidence of crack growth. The threshold stress intensity factor of the Ti 6Al-4V forged tank material when exposed to AF-M315E monopropellant was found to be at least 22.0 kilopounds per square inch. The stress intensity factor of the weld material was at least 31.3 kilopounds per square inch.

Elastic-Plastic J-Integral Solutions or Surface Cracks in Tension Using an Interpolation Methodology. Appendix C -- Finite Element Models Solution Database File, Appendix D -- Benchmark Finite Element Models Solution Database File

NASA Technical Reports Server (NTRS)

Allen, Phillip A.; Wells, Douglas N.

2013-01-01

No closed form solutions exist for the elastic-plastic J-integral for surface cracks due to the nonlinear, three-dimensional nature of the problem. Traditionally, each surface crack must be analyzed with a unique and time-consuming nonlinear finite element analysis. To overcome this shortcoming, the authors have developed and analyzed an array of 600 3D nonlinear finite element models for surface cracks in flat plates under tension loading. The solution space covers a wide range of crack shapes and depths (shape: 0.2 less than or equal to a/c less than or equal to 1, depth: 0.2 less than or equal to a/B less than or equal to 0.8) and material flow properties (elastic modulus-to-yield ratio: 100 less than or equal to E/ys less than or equal to 1,000, and hardening: 3 less than or equal to n less than or equal to 20). The authors have developed a methodology for interpolating between the goemetric and material property variables that allows the user to reliably evaluate the full elastic-plastic J-integral and force versus crack mouth opening displacement solution; thus, a solution can be obtained very rapidly by users without elastic-plastic fracture mechanics modeling experience. Complete solutions for the 600 models and 25 additional benchmark models are provided in tabular format.

A Newtonian interpretation of configurational forces on dislocations and cracks

NASA Astrophysics Data System (ADS)

Ballarini, Roberto; Royer-Carfagni, Gianni

2016-10-01

Configurational forces are fundamental concepts in the description of the motion of dislocations, cracks and other defects that introduce singularities within the solid state. They are defined by considering variations in energies associated with the movement of such defects, and are therefore different from the classical forces that enter the balance laws of classical Newtonian mechanics. Here, it is demonstrated how a configurational force can be viewed as the resultant of the (Newtonian) contact forces acting on the perturbed shape of an object of substance equivalent to the defect, and evaluated in the limit of the shape being restored to the primitive configuration. The expressions for the configurational forces on the paradigmatic examples of cracks and dislocations are in agreement with those determined using classical variational arguments. This finding opens a new prospective in the use of configurational forces by permitting their physical and intuitive visualization.

A Hybrid Multi-Scale Model of Crystal Plasticity for Handling Stress Concentrations

DOE PAGES

Sun, Shang; Ramazani, Ali; Sundararaghavan, Veera

2017-09-04

Microstructural effects become important at regions of stress concentrators such as notches, cracks and contact surfaces. A multiscale model is presented that efficiently captures microstructural details at such critical regions. The approach is based on a multiresolution mesh that includes an explicit microstructure representation at critical regions where stresses are localized. At regions farther away from the stress concentration, a reduced order model that statistically captures the effect of the microstructure is employed. The statistical model is based on a finite element representation of the orientation distribution function (ODF). As an illustrative example, we have applied the multiscaling method tomore » compute the stress intensity factor K I around the crack tip in a wedge-opening load specimen. The approach is verified with an analytical solution within linear elasticity approximation and is then extended to allow modeling of microstructural effects on crack tip plasticity.« less

A Hybrid Multi-Scale Model of Crystal Plasticity for Handling Stress Concentrations

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

Sun, Shang; Ramazani, Ali; Sundararaghavan, Veera

Microstructural effects become important at regions of stress concentrators such as notches, cracks and contact surfaces. A multiscale model is presented that efficiently captures microstructural details at such critical regions. The approach is based on a multiresolution mesh that includes an explicit microstructure representation at critical regions where stresses are localized. At regions farther away from the stress concentration, a reduced order model that statistically captures the effect of the microstructure is employed. The statistical model is based on a finite element representation of the orientation distribution function (ODF). As an illustrative example, we have applied the multiscaling method tomore » compute the stress intensity factor K I around the crack tip in a wedge-opening load specimen. The approach is verified with an analytical solution within linear elasticity approximation and is then extended to allow modeling of microstructural effects on crack tip plasticity.« less

Ductile fracture theories for pressurised pipes and containers

NASA Technical Reports Server (NTRS)

Erdogan, F.

1976-01-01

Two mechanisms of fracture are distinguished. Plane strain fractures occur in materials which do not undergo large-scale plastic deformations prior to and during a possible fracture deformation. Plane stress or high energy fractures are generally accompanied by large inelastic deformations. Theories for analyzing plane stress are based on the concepts of critical crack opening stretch, K(R) characterization, J-integral, and plastic instability. This last is considered in some detail. The ductile fracture process involves fracture initiation followed by a stable crack growth and the onset of unstable fracture propagation. The ductile fracture propagation process may be characterized by either a multiparameter (discrete) model, or some type of a resistance curve which may be considered as a continuous model expressed graphically. These models are studied and an alternative model is also proposed for ductile fractures which cannot be modeled as progressive crack growth phenomena.

Process for magnetic beneficiating petroleum cracking catalyst

DOEpatents

Doctor, R.D.

1993-10-05

A process is described for beneficiating a particulate zeolite petroleum cracking catalyst having metal values in excess of 1000 ppm nickel equivalents. The particulate catalyst is passed through a magnetic field in the range of from about 2 Tesla to about 5 Tesla generated by a superconducting quadrupole open-gradient magnetic system for a time sufficient to effect separation of said catalyst into a plurality of zones having different nickel equivalent concentrations. A first zone has nickel equivalents of about 6,000 ppm and greater, a second zone has nickel equivalents in the range of from about 2000 ppm to about 6000 ppm, and a third zone has nickel equivalents of about 2000 ppm and less. The zones of catalyst are separated and the second zone material is recycled to a fluidized bed of zeolite petroleum cracking catalyst. The low nickel equivalent zone is treated while the high nickel equivalent zone is discarded. 1 figures.

Process for magnetic beneficiating petroleum cracking catalyst

DOEpatents

Doctor, Richard D.

1993-01-01

A process for beneficiating a particulate zeolite petroleum cracking catalyst having metal values in excess of 1000 ppm nickel equivalents. The particulate catalyst is passed through a magnetic field in the range of from about 2 Tesla to about 5 Tesla generated by a superconducting quadrupole open-gradient magnetic system for a time sufficient to effect separation of said catalyst into a plurality of zones having different nickel equivalent concentrations. A first zone has nickel equivalents of about 6,000 ppm and greater, a second zone has nickel equivalents in the range of from about 2000 ppm to about 6000 ppm, and a third zone has nickel equivalents of about 2000 ppm and less. The zones of catalyst are separated and the second zone material is recycled to a fluidized bed of zeolite petroleum cracking catalyst. The low nickel equivalent zone is treated while the high nickel equivalent zone is discarded.

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.

Visualizing In Situ Microstructure Dependent Crack Tip Stress Distribution in IN-617 Using Nano-mechanical Raman Spectroscopy

NASA Astrophysics Data System (ADS)

Zhang, Yang; Mohanty, Debapriya P.; Tomar, Vikas

2016-11-01

Inconel 617 (IN-617) is a solid solution alloy, which is widely used in applications that require high-temperature component operation due to its high-temperature stability and strength as well as strong resistance to oxidation and carburization. The current work focuses on in situ measurements of stress distribution under 3-point bending at elevated temperature in IN-617. A nanomechanical Raman spectroscopy measurement platform was designed and built based on a combination of a customized open Raman spectroscopy (NMRS) system incorporating a motorized scanning and imaging system with a nanomechanical loading platform. Based on the scanning of the crack tip notch area using the NMRS notch tip, stress distribution under applied load with micron-scale resolution for analyzed microstructures is predicted. A finite element method-based formulation to predict crack tip stresses is presented and validated using the presented experimental data.

Strength and durability of near-surface mounted CFRP bars for shear strengthening reinforced concrete bridge girders : final report appendices.

DOT National Transportation Integrated Search

2012-03-01

During the interstate expansion of the 1950s, many conventionally reinforced concrete deck girder bridges were built throughout the country. These aging bridges commonly exhibit diagonal cracking and rate inadequately for shear, thus they are candida...

Strength and durability of near-surface mounted CFRP bars for shear strengthening reinforced concrete bridge girders : final report.

DOT National Transportation Integrated Search

2012-03-01

During the interstate expansion of the 1950s, many conventionally reinforced concrete deck girder bridges were built throughout the country. These aging bridges commonly exhibit diagonal cracking and rate inadequately for shear, thus they are candida...

Rupture Dynamics along Thrust Dipping Fault: Inertia Effects due to Free Surface Wave Interactions

NASA Astrophysics Data System (ADS)

Vilotte, J. P.; Scala, A.; Festa, G.

2017-12-01

We numerically investigate the dynamic interaction between free surface and up-dip, in-plane rupture propagation along thrust faults, under linear slip-weakening friction. With reference to shallow along-dip rupture propagation during large subduction earthquakes, we consider here low dip-angle fault configurations with fixed strength excess and depth-increasing initial stress. In this configuration, the rupture undergoes a break of symmetry with slip-induced normal stress perturbations triggered by the interaction with reflected waves from the free surface. We found that both body-waves - behind the crack front - and surface waves - at the crack front - can trigger inertial effects. When waves interact with the rupture before this latter reaches its asymptotic speed, the rupture can accelerate toward the asymptotic speed faster than in the unbounded symmetric case, as a result of these inertial effects. Moreover, wave interaction at the crack front also affects the slip rate generating large ground motion on the hanging wall. Imposing the same initial normal stress, frictional strength and stress drop while varying the static friction coefficient we found that the break of symmetry makes the rupture dynamics dependent on the absolute value of friction. The higher the friction the stronger the inertial effect both in terms of rupture acceleration and slip amount. When the contact condition allows the fault interface to open close to the free surface, the length of the opening zone is shown to depend on the propagation length, the initial normal stress and the static friction coefficient. These new results are shown to agree with analytical results of rupture propagation in bounded media, and open new perspectives for understanding the shallow rupture of large subduction earthquakes and tsunami sources.

New International Program to Asses the Reliability of Emerging Nondestructive Techniques (PARENT)

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

Prokofiev, Iouri; Cumblidge, Stephen E.; Csontos, Aladar A.

2013-01-25

The Nuclear Regulatory Commission (NRC) established the Program to Assess the Reliability of Emerging Nondestructive Techniques (PARENT) to follow on from the successful Program for the Inspection of Nickel alloy Components (PINC). The goal of the PARENT is to conduct a confirmatory assessment of the reliability of nondestructive evaluation (NDE) techniques for detecting and sizing primary water stress corrosion cracks (PWSCC) and applying the lessons learned from PINC to a series of round-robin tests. These open and blind round-robin tests will comprise a new set of typical pressure boundary components including dissimilar metal welds (DMWs) and bottom-mounted instrumentation penetrations. Openmore » round-robin tests will engage research and industry teams worldwide to investigate and demonstrate the reliability of emerging NDE techniques to detect and size flaws with a wide range of lengths, depths, orientations, and locations. Blind round-robin tests will utilize various testing organizations, whose inspectors and procedures are certified by the standards for the nuclear industry in their respective countries, to investigate the ability of established NDE techniques to detect and size flaws whose characteristics range from relatively easy to very difficult for detection and sizing. Blind and open round-robin testing started in late 2011 and early 2012, respectively. This paper will present the work scope with reports on progress, NDE methods evaluated, and project timeline for PARENT.« less

Residual strength of thin panels with cracks

NASA Technical Reports Server (NTRS)

Madenci, Erdogan

1994-01-01

The previous design philosophies involving safe life, fail-safe and damage tolerance concepts become inadequate for assuring the safety of aging aircraft structures. For example, the failure mechanism for the Aloha Airline accident involved the coalescence of undetected small cracks at the rivet holes causing a section of the fuselage to peel open during flight. Therefore, the fuselage structure should be designed to have sufficient residual strength under worst case crack configurations and in-flight load conditions. Residual strength is interpreted as the maximum load carrying capacity prior to unstable crack growth. Internal pressure and bending moment constitute the two major components of the external loads on the fuselage section during flight. Although the stiffeners in the form of stringers, frames and tear straps sustain part of the external loads, the significant portion of the load is taken up by the skin. In the presence of a large crack in the skin, the crack lips bulge out with considerable yielding; thus, the geometric and material nonlinearities must be included in the analysis for predicting residual strength. Also, these nonlinearities do not permit the decoupling of in-plane and out-of-plane bending deformations. The failure criterion combining the concepts of absorbed specific energy and strain energy density addresses the aforementioned concerns. The critical absorbed specific energy (local toughness) for the material is determined from the global specimen response and deformation geometry based on the uniaxial tensile test data and detailed finite element modeling of the specimen response. The use of the local toughness and stress-strain response at the continuum level eliminates the size effect. With this critical parameter and stress-strain response, the finite element analysis of the component by using STAGS along with the application of this failure criterion provides the stable crack growth calculations for residual strength predictions.

Wild capuchin monkeys (Cebus libidinosus) use anvils and stone pounding tools.

PubMed

Fragaszy, Dorothy; Izar, Patrícia; Visalberghi, Elisabetta; Ottoni, Eduardo B; de Oliveira, Marino Gomes

2004-12-01

We conducted an exploratory investigation in an area where nut-cracking by wild capuchin monkeys is common knowledge among local residents. In addition to observing male and female capuchin monkeys using stones to pound open nuts on stone "anvils," we surveyed the surrounding area and found physical evidence that monkeys cracked nuts on rock outcrops, boulders, and logs (collectively termed anvils). Anvils, which were identified by numerous shallow depressions on the upper surface, the presence of palm shells and debris, and the presence of loose stones of an appropriate size to pound nuts, were present even on the tops of mesas. The stones used to crack nuts can weigh >1 kg, and are remarkably heavy for monkeys that weigh <4 kg. The abundance of shell remains and depressions in the anvil surface at numerous anvil sites indicate that nut-cracking activity is common and long-enduring. Many of the stones found on anvils (presumably used to pound nuts) are river pebbles that are not present in the local area we surveyed (except on or near the anvils); therefore, we surmise that they were transported to the anvil sites. Ecologically and behaviorally, nut-cracking by capuchins appears to have strong parallels to nut-cracking by wild chimpanzees. The presence of abundant anvil sites, limited alternative food resources, abundance of palms, and the habit of the palms in this region to produce fruit at ground level all likely contribute to the monkeys' routine exploitation of palm nuts via cracking them with stones. This discovery provides a new reference point for discussions regarding the evolution of tool use and material culture in primates. Routine tool use to exploit keystone food resources is not restricted to living great apes and ancestral hominids. Copyright 2004 Wiley-Liss, Inc.

Hydroxyapatite-anatase-carbon nanotube nanocomposite coatings fabricated by electrophoretic codeposition for biomedical applications.

PubMed

Zhang, Bokai; Kwok, Chi Tat

2011-10-01

In order to eliminate micro-cracks in the monolithic hydroxyapatite (HA) and composite hydroxyapatite/carbon nanotube (HA/CNT) coatings, novel HA/TiO(2)/CNT nanocomposite coatings on Ti6Al4V were attempted to fabricate by a single-step electrophoretic codeposition process for biomedical applications. The electrophoretically deposited layers with difference contents of HA, TiO(2) (anatase) and CNT nanoparticles were sintered at 800°C for densification with thickness of about 7-10 μm. A dense and crack-free coating was achieved with constituents of 85 wt% HA, 10 wt% TiO(2) and 5 wt% CNT. Open-circuit potential measurements and cyclic potentiodynamic polarization tests were used to investigate the electrochemical corrosion behavior of the coatings in vitro conditions (Hanks' solution at 37°C). The HA/TiO(2)/CNT coatings possess higher corrosion resistance than that of the Ti6Al4V substrate as reflected by nobler open circuit potential and lower corrosion current density. In addition, the surface hardness and adhesion strength of the HA/TiO(2)/CNT coatings are higher than that of the monolithic HA and HA/CNT coatings without compromising their apatite forming ability. The enhanced properties were attributed to the nanostructure of the coatings with the appropriate TiO(2) and CNT contents for eliminating micro-cracks and micro-pores.

Numerical methods for coupled fracture problems

NASA Astrophysics Data System (ADS)

Viesca, Robert C.; Garagash, Dmitry I.

2018-04-01

We consider numerical solutions in which the linear elastic response to an opening- or sliding-mode fracture couples with one or more processes. Classic examples of such problems include traction-free cracks leading to stress singularities or cracks with cohesive-zone strength requirements leading to non-singular stress distributions. These classical problems have characteristic square-root asymptotic behavior for stress, relative displacement, or their derivatives. Prior work has shown that such asymptotics lead to a natural quadrature of the singular integrals at roots of Chebyhsev polynomials of the first, second, third, or fourth kind. We show that such quadratures lead to convenient techniques for interpolation, differentiation, and integration, with the potential for spectral accuracy. We further show that these techniques, with slight amendment, may continue to be used for non-classical problems which lack the classical asymptotic behavior. We consider solutions to example problems of both the classical and non-classical variety (e.g., fluid-driven opening-mode fracture and fault shear rupture driven by thermal weakening), with comparisons to analytical solutions or asymptotes, where available.

STS-121: Discovery Mission Management Team Briefing

NASA Technical Reports Server (NTRS)

2006-01-01

The briefing opened with Bruce Buckingham (NASA Public Affairs) introducing John Shannon (Chairman, Mission Management Team, JSC), John Chapman (External Tank Project Manager), Mike Leinbach (Shuttle Launch Director), and 1st Lt. Kaleb Nordgren (USAF 45th Weather Squadron). John Shannon reported that the team for hydrogen loading was proceeding well and the external tank detanking was completed. During detanking the inspection team cracked foam caused by condensation and ice formation as the tank expanded and contracted. Aerothermal analysis and analysis fro ice formation will be completed before launch. John Chapman explained the mechanics of the external tank design, the foam cracking, bracket design, etc. Mike Leinbach discussed the inspection teams and their inspection final inspection for ice formation before and after external tank filling. The inspection team of eight very experienced personnel also use telescopes with cameras to find any problems before launch. Kaleb Nordgren discussed weather and said there was a 40% chance of weather prohibiting launch. The floor was the opened for questions from the press.

Fracture resistance of a TiB2 particle/SiC matrix composite at elevated temperature

NASA Technical Reports Server (NTRS)

Jenkins, Michael G.; Salem, Jonathan A.; Seshadri, Srinivasa G.

1988-01-01

The fracture resistance of a comercial TiB2 particle/SiC matrix composite was evaluated at temperatures ranging from 20 to 1400 C. A laser interferometric strain gauge (LISG) was used to continuously monitor the crack mouth opening displacement (CMOD) of the chevron-notched and straight-notched, three-point bend specimens used. Crack growth resistance curves (R-curves) were determined from the load versus displacement curves and displacement calibrations. Fracture toughness, work-of-fracture, and R-curve levels were found to decrease with increasing temperature. Microstructure, fracture surface, and oxidation coat were examined to explain the fracture behavior.

Fracture resistance of a TiB2 particle/SiC matrix composite at elevated temperature

NASA Technical Reports Server (NTRS)

Jenkins, Michael G.; Salem, Jonathan A.; Seshadri, Srinivasa G.

1989-01-01

The fracture resistance of a commercial TiB2 particle/SiC matrix composite was evaluated at temperatures ranging from 20 to 1400 C. A laser interferometric strain gauge (LiSG) was used to continuously monitor the crack mouth opening displacement (CMOD) of the chevron-notched and straight-notched, three-point bend specimens used. Crack growth resistance curves (R-curves) were determined from the load versus displacement curves and displacement calibrations. Fracture toughness, work-of-fracture, and R-curve levels were found to decrease with increasing temperature. Microstructure, fracture surface, and oxidation coat were examined to explain the fracture behavior.

Model for the Effect of Fiber Bridging on the Fracture Resistance of Reinforced-Carbon-Carbon

NASA Technical Reports Server (NTRS)

Chan, Kwai S.; Lee, Yi-Der; Hudak, Stephen J., Jr.

2009-01-01

A micromechanical methodology has been developed for analyzing fiber bridging and resistance-curve behavior in reinforced-carbon-carbon (RCC) panels with a three-dimensional (3D) composite architecture and a silicon carbide (SiC) surface coating. The methodology involves treating fiber bridging traction on the crack surfaces in terms of a weight function approach and a bridging law that relates the bridging stress to the crack opening displacement. A procedure has been developed to deduce material constants in the bridging law from the linear portion of the K-resistance curve. This report contains information on the application of procedures and outcomes.

Stress and Pore Fluid Pressure Cycles Beneath the Seismogenic Layer Recorded by Veins

NASA Astrophysics Data System (ADS)

Nüchter, J. A.; Stöckhert, B.

2006-12-01

Metamorphic rocks approaching the crustal scale brittle-ductile transition (BDT) during exhumation are expected to become increasingly affected by short term stress fluctuations related to seismic activity in the overlying seismogenic layer, while still residing in a long-term viscous environment. The (micro-)structural record of monogenetic syntaxial quartz veins in metamorphic rocks from southern Evia, Greece, yields insight into the processes and conditions just beneath the long-term BDT at temperatures of about 300 to 350° C. The following features are characteristic: 1) The veins crosscut the foliation and all syn-metamorphic structures; 2) the veins have formed from tensile fractures, with a typical length on the order of 10-1 to 101 m; 3) some veins branch symmetrically with an aperture angle of 30°, which is interpreted to indicate high crack propagation rates similar to Raleigh wave speed; 4) the veins formed during a single sealing stage by mineral precipitation in open cavities; 5) the veins show a low aspect ratio of about 10 to 100 and a characteristic lenticular shape, controlled by distributed ductile deformation of the host rock, with vein-parallel shortening by typically less than 1 %; 6) the intensity of crystal plastic deformation in the vein quartz decreases from the vein walls towards the center; 7) fluid inclusions trapped in the vein quartz record a time series of pore fluid pressure (Pf) evolution during progressive sealing, with low Pf at the vein walls (early stage) to high Pf in the vein core (final stage). These features indicate: Opening of the fractures commenced immediately after crack arrest, controlled by ductile deformation of the host rock at temperatures between about 300 and 350° C. The crack opening rate exceeded the rate of sealing, so that the quartz crystals grew into an open cavity. For opening of cracks, the effective stress on the fracture walls must be tensile and the fluid pressure must be similar to that of the magnitude of the least principal stress Pf ≍ σ3. The evolution of Pf recorded by the fluid inclusions reflects the relaxation of stress during progressive opening and sealing of the vein. This is supported by the fact that the quartz crystals grown at the fracture walls, hence early in the opening and sealing history, record a more intense deformation compared to the crystals grown lately in the centre of the vein. The structural and microstructural record of these monogenetic veins reflects an isothermal switch from brittle failure to decelerating viscous creep. The total strain accumulated is low. The following scenario is inferred: Fracturing is proposed to be a consequence of co-seismic loading related to fault slip in the overlying upper crust. Within the underlying damage zone, fractures develop and dilation causes a drop in Pf. Subsequently, the fractures open controlled by viscous creep of the host rock with a concomitant rise in Pf during stress relaxation. Sealing of the fissures to become veins takes place continuously by precipitation of minerals from the pore fluid streaming through the evolving cavity. The lenticular monogenetic veins are therefore interpreted to record a short-term and episodic process. Such type of record of exhumed rocks provides insight into earthquake-related damage beneath the seismogenic layer and into the nature of transient crustal properties in the earthquake cycle.

Crack Growth Simulation and Residual Strength Prediction in Airplane Fuselages

NASA Technical Reports Server (NTRS)

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

1999-01-01

This is the final report for the NASA funded project entitled "Crack Growth Prediction Methodology for Multi-Site Damage." The primary objective of the project was to create a capability to simulate curvilinear fatigue crack growth and ductile tearing in aircraft fuselages subjected to widespread fatigue damage. The second objective was to validate the capability by way of comparisons to experimental results. Both objectives have been achieved and the results are detailed herein. In the first part of the report, the crack tip opening angle (CTOA) fracture criterion, obtained and correlated from coupon tests to predict fracture behavior and residual strength of built-up aircraft fuselages, is discussed. Geometrically nonlinear, elastic-plastic, thin shell finite element analyses are used to simulate stable crack growth and to predict residual strength. Both measured and predicted results of laboratory flat panel tests and full-scale fuselage panel tests show substantial reduction of residual strength due to the occurrence of multi-site damage (MSD). Detailed comparisons of n stable crack growth history, and residual strength between the predicted and experimental results are used to assess the validity of the analysis methodology. In the second part of the report, issues related to crack trajectory prediction in thin shells; an evolving methodology uses the crack turning phenomenon to improve the structural integrity of aircraft structures are discussed, A directional criterion is developed based on the maximum tangential stress theory, but taking into account the effect of T-stress and fracture toughness orthotropy. Possible extensions of the current crack growth directional criterion to handle geometrically and materially nonlinear problems are discussed. The path independent contour integral method for T-stress evaluation is derived and its accuracy is assessed using a p- and hp-version adaptive finite element method. Curvilinear crack growth is simulated in coupon tests and in full-scale fuselage panel tests. Both T-stress and fracture toughness orthotropy are found to be essential to predict the observed crack paths. The analysis methodology and software program (FRANC3D/STAGS) developed herein allows engineers to maintain aging aircraft economically while insuring continuous airworthiness. Consequently, it will improve the technology to support the safe operation of the current aircraft fleet as well as the design of more damage-tolerant aircraft for the next generation fleet.

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.

The fracture criticality of crustal rocks

NASA Astrophysics Data System (ADS)

Crampin, Stuart

1994-08-01

The shear-wave splitting observed along almost all shear-wave ray paths in the Earth's crust is interpreted as the effects of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore space. Once away from the free surface, where open joints and fractures may lead to strong anisotropy of 10 per cent or greater, intact ostensibly unfractured crustal rock exhibits a limited range of shear-wave splitting from about 1.5 to 4.5 per cent differential shear-wave velocity anisotropy. Interpreting this velocity anisotropy as normalized crack densities, a factor of less than two in crack radius covers the range from the minimum 1.5 per cent anisotropy observed in intact rock to the 10 per cent observed in heavily cracked almost disaggregated near-surface rocks. This narrow range of crack dimensions and the pronounced effect on rock cohesion suggests that there is a state of fracture criticality at some level of anisotropy between 4.5 and 10 per cent marking the boundary between essentially intact, and heavily fractured rock. When the level of fracture criticality is exceeded, cracking is so severe that there is a breakdown in shear strength, the likelihood of progressive fracturing and the dispersal of pore fluids through enhanced permeability. The range of normalized crack dimensions below fracture criticality is so small in intact rock, that any modification to the crack geometry by even minor changes of conditions or minor deformation (particularly in the presence of high pore-fluid pressures) may change rock from being essentially intact (below fracture criticality) to heavily fractured (above fracture criticality). This recognition of the essential compliance of most crustal rocks, and its effect on shear-wave splitting, has implications for monitoring changes in any conditions affecting the rock mass. These include monitoring changes in reservoir evolution during hydrocarbon production and enhanced oil recovery, and in monitoring changes before and after earthquakes, amongst others.

Health monitoring and rehabilitation of a concrete structure using intelligent materials

NASA Astrophysics Data System (ADS)

Song, G.; Mo, Y. L.; Otero, K.; Gu, H.

2006-04-01

This paper presents the concept of an intelligent reinforced concrete structure (IRCS) and its application in structural health monitoring and rehabilitation. The IRCS has multiple functions which include self-rehabilitation, self-vibration damping, and self-structural health monitoring. These functions are enabled by two types of intelligent (smart) materials: shape memory alloys (SMAs) and piezoceramics. In this research, Nitinol type SMA and PZT (lead zirconate titanate) type piezoceramics are used. The proposed concrete structure is reinforced by martensite Nitinol cables using the method of post-tensioning. The martensite SMA significantly increases the concrete's damping property and its ability to handle large impact. In the presence of cracks due to explosions or earthquakes, by electrically heating the SMA cables, the SMA cables contract and close up the cracks. In this research, PZT patches are embedded in the concrete structure to detect possible cracks inside the concrete structure. The wavelet packet analysis method is then applied as a signal-processing tool to analyze the sensor signals. A damage index is defined to describe the damage severity for health monitoring purposes. In addition, by monitoring the electric resistance change of the SMA cables, the crack width can be estimated. To demonstrate this concept, a concrete beam specimen with reinforced SMA cables and with embedded PZT patches is fabricated. Experiments demonstrate that the IRC has the ability of self-sensing and self-rehabilitation. Three-point bending tests were conducted. During the loading process, a crack opens up to 0.47 inches. Upon removal of the load and heating the SMA cables, the crack closes up. The damage index formed by wavelet packet analysis of the PZT sensor data predicts and confirms the onset and severity of the crack during the loading. Also during the loading, the electrical resistance value of the SMA cable changes by up to 27% and this phenomenon is used to monitor the crack width.

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.

Toughening mechanisms in laminated composites: A biomimetic study in mollusk shells

NASA Astrophysics Data System (ADS)

Kamat, Shekhar Shripad

2000-10-01

Mollusk shells can be described as structural biocomposite materials composed of a mineral (aragonite) and a continuous, albeit minor, organic (protein) component. The conch shell, Strombus Gigas, has intermediate strength and high fracture toughness. The high fracture toughness is a result of enhanced energy dissipation during crack propagation due to delamination, crack bridging, frictional sliding etc. A theoretical and experimental study was conducted on the crack bridging mechanisms operative in the shell. Four-point bend tests were conducted. Acoustic emission and post-mortem dye penetrants were used to characterize the crack propagation, together with conventional fractography. A two layer composite configuration is seen in the shells, with the tough and weak layers having a toughness ratio of ˜4 (Ktough = 2.2MPam1/2). This toughness ratio is a requisite for multiple cracking in the weak layer. A theoretical shear lag analysis of the crack bridging phenomena in the tough layer is shown to lead to a bridging law for the crack wake of the form of p = betau1/2 (p is the bridging traction for a crack opening u, with beta, being a constant of proportionality). Finite element analysis yielded a value of beta = 630 Nmm-5/2 and ucritical = 5 mum for the bridging law parameters. In a nonlinear fracture mechanics phenomenology, these values are relevant material parameters, rather than a critical stress intensity factor. The work of fracture for unnotched specimens is three orders of magnitude higher than mineral aragonite, and is demonstrated numerically incorporating the toughening mechanisms in the shell. Similar structural adaptations have been observed and studied in the red abalone shell, haliotis rufescens and the spines of the sea urchin, Heterocentrotus trigonarius. The toughening mechanisms seen in these shells give insight into structural design needs of brittle matrix composites (BMC) as well as conventional structural ceramics.

7 CFR 201.56-2 - Sunflower family, Asteraceae (Compositae).

Code of Federal Regulations, 2014 CFR

2014-01-01

... tissue remaining attached. (B) Less than half of the original cotyledon tissue free of necrosis or decay. (Remove attached seed coat for evaluation of cotyledons. Physiological necrosis is manifested by... degree of necrosis or decay. (iii) Hypocotyl: (A) Deep open cracks extending into the conducting tissue...

7 CFR 201.56-2 - Sunflower family, Asteraceae (Compositae).

Code of Federal Regulations, 2012 CFR

2012-01-01

... tissue remaining attached. (B) Less than half of the original cotyledon tissue free of necrosis or decay. (Remove attached seed coat for evaluation of cotyledons. Physiological necrosis is manifested by... degree of necrosis or decay. (iii) Hypocotyl: (A) Deep open cracks extending into the conducting tissue...

7 CFR 201.56-2 - Sunflower family, Asteraceae (Compositae).

Code of Federal Regulations, 2011 CFR

2011-01-01

... tissue remaining attached. (B) Less than half of the original cotyledon tissue free of necrosis or decay. (Remove attached seed coat for evaluation of cotyledons. Physiological necrosis is manifested by... degree of necrosis or decay. (iii) Hypocotyl: (A) Deep open cracks extending into the conducting tissue...

7 CFR 201.56-2 - Sunflower family, Asteraceae (Compositae).

Code of Federal Regulations, 2013 CFR

2013-01-01

... tissue remaining attached. (B) Less than half of the original cotyledon tissue free of necrosis or decay. (Remove attached seed coat for evaluation of cotyledons. Physiological necrosis is manifested by... degree of necrosis or decay. (iii) Hypocotyl: (A) Deep open cracks extending into the conducting tissue...

7 CFR 201.56-2 - Sunflower family, Asteraceae (Compositae).

Code of Federal Regulations, 2010 CFR

2010-01-01

... tissue remaining attached. (B) Less than half of the original cotyledon tissue free of necrosis or decay. (Remove attached seed coat for evaluation of cotyledons. Physiological necrosis is manifested by... degree of necrosis or decay. (iii) Hypocotyl: (A) Deep open cracks extending into the conducting tissue...

Cost-effectiveness of open graded base course with doweled and nondoweled transverse joints : research brief.

DOT National Transportation Integrated Search

2010-09-01

Most of the damage to concrete pavement results from poor drainage, which can lead to increased freeze-thaw damage, and when combined with heavy loading can contribute to cracking, spalling and surface damage that causes driver discomfort from increa...

29 CFR 1910.252 - General requirements.

Code of Federal Regulations, 2010 CFR

2010-07-01

...: (i) Combustible material. Wherever there are floor openings or cracks in the flooring that cannot be closed, precautions shall be taken so that no readily combustible materials on the floor below will be... nature and quantity of the combustible material exposed. (iii) Fire watch. (A) Fire watchers shall be...

Crack Coalescence in Molded Gypsum and Carrara Marble

NASA Astrophysics Data System (ADS)

Wong, N.; Einstein, H. H.

2007-12-01

This research investigates the fracturing and coalescence behavior in prismatic laboratory-molded gypsum and Carrara marble specimens, which consist of either one or two pre-existing open flaws, under uniaxial compression. The tests are monitored by a high speed video system with a frame rate up to 24,000 frames/second. It allows one to precisely observe the cracking mechanisms, in particular if shear or tensile fracturing takes place. Seven crack types and nine crack coalescence categories are identified. The flaw inclination angle, the ligament length and the bridging angle between two flaws have different extents of influence on the coalescence patterns. For coplanar flaws, as the flaw inclination angle increases, there is a general trend of variation from shear coalescence to tensile coalescence. For stepped flaws, as the bridging angle changes from negative to small positive, and further up to large positive values, the coalescence generally progresses from categories of no coalescence, indirect coalescence to direct coalescence. For direct coalescence, it generally progresses from shear, mixed shear-tensile to tensile as the bridging angle increases. Some differences in fracturing and coalescence processes are observed in gypsum and marble, particularly the crack initiation in marble is preceded by the development of macroscopic white patches, but not in gypsum. Scanning Electron Microprobe (SEM) study reveals that the white patches consist of zones of microcracks (process zones).

Space Shuttle Orbiter Atlantis Liquid Oxygen Prevalve Detent Roller Cracking Investigation

NASA Technical Reports Server (NTRS)

Holleman, Elizabeth; Eddleman, David; Richard, James; Jacobs, Rebecca

2008-01-01

During routine inspections of the Space Shuttle's Main Propulsion System (MPS) Liquid Oxygen (LO2) pre-valve, the mechanism provided to maintain the valve in the open position was found cracked. The mechanism is a Vespel roller held against the valve visor by a stack of Belleville springs. The roller has been found cracked 3 times. All three instances were in the same valve in the same location. There are 6 pre-valves on each orbiter, and only one has exhibited this problem. Every-flight inspections were instituted and the rollers were found to be cracked after only one flight. Engineers at Marshall Space Flight Center, Johnson Space Center and Kennedy Space Center worked together to determine a solution. There were several possible contributors to the failure: a mis-aligned visor, an out of specification edge with a sharp radius, an out of specification tolerance stack up of a Belleville spring stack that caused un-predicted loads on the Vespel SP-21 roller, and a dimple machined into the side of the roller to indicate LO2 compatibility that created a stress riser. The detent assembly was removed and replaced with parts that were on the low-side of the tolerance stack up to eliminate the potential for high loads on the detent roller. After one flight, the roller was inspected and showed fewer signs of wear and no cracks.

Space Shuttle Orbiter Atlantis Liquid Oxygen Pre-Valve Detent Roller Cracking Investigation

NASA Technical Reports Server (NTRS)

Holleman, Elizabeth; Eddleman, David; Jacobs, Rebecca; Richard, James

2008-01-01

During routine inspections of the Space Shuttle s Main Propulsion System Liquid Oxygen (LO2) pre-valve, the mechanism provided to maintain the valve in the open position was found cracked. The mechanism is a Vespel roller held against the valve visor by a stack of Belleville springs. The roller has been found cracked 3 times. All three instances were in the same valve in the same location. There are 6 pre-valves on each orbiter, and only one has exhibited this problem. Every-flight inspections were instituted and the rollers were found to be cracked after only one flight. Engineers at Marshall Space Flight Center, Johnson Space Center, and Kennedy Space Center worked together to determine a solution. There were several possible contributors to the failure: a misaligned visor, an out-of-specification edge with a sharp radius, an out-of-specification tolerance stack up of a Belleville spring stack that caused un-predicted loads on the Vespel SP-21 roller, and a dimple machined into the side of the roller to indicate LO2 compatibility that created a stress riser. The detent assembly was removed and replaced with parts that were on the low side of the tolerance stack up to eliminate the potential for high loads on the detent roller. After one flight, the roller was inspected and showed fewer signs of wear and no cracks.

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.

Characteristics of microearthquakes accompanying hydraulic fracturing as determined from studies of spectra of seismic waveforms

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

Fehler, M.; Bame, D.

1985-03-01

A study of the spectral properties of the waveforms recorded during hydraulic fracturing earthquakes has been carried out to obtain information about the physical dimensions of the earthquakes. We find two types of events. The first type has waveforms with clear P and S arrivals and spectra that are very similar to earthquakes occurring in tectonic regions. These events are interpreted as being due to shear slip along fault planes. The second type of event has waveforms that are similar in many ways to long period earthquakes observed at volcanoes and is called long period. Many waveforms of these eventsmore » are identical, which implies that these events represent repeated activation of a given source. We propose that the source of these long period events is the sudden opening of a channel that connects two cracks filled with fluid at different pressures. The sizes of the two cracks differ, which causes two or more peaks to appear in the spectra, each peak being associated with one physical dimension of the crack. From the frequencies at which spectral peaks occur, we estimate crack dimensions of between 3 and 22m. 13 refs., 8 figs.« less

Adhesion strength of norbornene-based self-healing agents to an amine-cured epoxy

NASA Astrophysics Data System (ADS)

Huang, Guang Chun; Lee, Jong Keun; Kessler, Michael R.; Yoon, Sungho

2009-07-01

Self-healing is triggered by crack propagation through embedded microcapsules in an epoxy matrix, which then release the liquid healing agent into the crack plane. Subsequent exposure of the healing agent to the chemical catalyst initiates ring-opening metathesis polymerization (ROMP) and bonding of the crack faces. In order to improve self-healing functionality, it is necessary to enhance adhesion of polymerized healing agent within the crack to the matrix resin. In this study, shear bond strength between different norbornene-based healing agents and an amine-cured epoxy resin was evaluated using the single lap shear test method (ASTM D3163, modified). The healing agents tested include endodicyclopentadiene (endo-DCPD), 5-ethylidene-2-norbornene (ENB) and DCPD/ENB blends. 5-Norbornene-2-methanol (NBM) was used as an adhesion promoter, containing hydroxyl groups to form hydrogen bonds with the amine-cured epoxy. A custom synthesized norbornene-based crosslinking agent was also added to improve adhesion for ENB by increasing the crosslinking density of the adhesive after ROMP. The healing agents were polymerized with varying loadings of the 1st generation Grubbs' catalyst at different reaction times and temperatures.

Research into topology optimization and the FDM method for a space cracked membrane

NASA Astrophysics Data System (ADS)

Hu, Qingxi; Li, Wanyuan; Zhang, Haiguang; Liu, Dali; Peng, Fujun; Duan, Yongchao

2017-07-01

The problem that the space membranes are easily torn open is the main focus in this paper, and a bionic strengthening-ribs structure is proposed for a space membrane based on interdisciplinary strengths, such as topology optimization, composite materials, and rapid prototyping. The optimization method and modeling method of membranes with bionic strengthening-ribs was studied. The PEEK and SCF/PEEK composite material which are applied to the space environment are chosen, and FDM technology is used. Through topology optimization, bionic strengthening-ribs with good tensile and tear capacities were obtained. Cracked membranes, cracked membranes with PEEK strengthening-ribs and SCF/PEEK strengthening-ribs were tested and test data were obtained. An extension situation and tension fracture were compared for three cases. The experimental results showed that membranes with the bionic strengthening-ribs structure have better mechanical properties, and the strength of the membranes with PEEK and SCF/PEEK strengthening-ribs were raised, respectively, up to 266.9% and 185.9%. The strengthening-ribs structure greatly improves the capacity to halt membrane crack-growth, which has an important significance to avoid membrane tear, and to ensure the spacecraft orbital lifetime.

Residual Strength Analysis Methodology: Laboratory Coupons to Structural Components

NASA Technical Reports Server (NTRS)

Dawicke, D. S.; Newman, J. C., Jr.; Starnes, J. H., Jr.; Rose, C. A.; Young, R. D.; Seshadri, B. R.

2000-01-01

The NASA Aircraft Structural Integrity (NASIP) and Airframe Airworthiness Assurance/Aging Aircraft (AAA/AA) Programs have developed a residual strength prediction methodology for aircraft fuselage structures. This methodology has been experimentally verified for structures ranging from laboratory coupons up to full-scale structural components. The methodology uses the critical crack tip opening angle (CTOA) fracture criterion to characterize the fracture behavior and a material and a geometric nonlinear finite element shell analysis code to perform the structural analyses. The present paper presents the results of a study to evaluate the fracture behavior of 2024-T3 aluminum alloys with thickness of 0.04 inches to 0.09 inches. The critical CTOA and the corresponding plane strain core height necessary to simulate through-the-thickness effects at the crack tip in an otherwise plane stress analysis, were determined from small laboratory specimens. Using these parameters, the CTOA fracture criterion was used to predict the behavior of middle crack tension specimens that were up to 40 inches wide, flat panels with riveted stiffeners and multiple-site damage cracks, 18-inch diameter pressurized cylinders, and full scale curved stiffened panels subjected to internal pressure and mechanical loads.

Ballistic Impact of Braided Composites With a Soft Projectile

NASA Technical Reports Server (NTRS)

Roberts, Gary D.; Pereira, J. Michael; Revilock, Duane M., Jr.; Binienda, Wieslaw; Xie, Ming; Braley, Mike

2004-01-01

Impact tests using a soft gelatin projectile were performed to identify failure modes that occur at high strain energy density during impact loading. Use of a soft projectile allows a large amount of kinetic energy to be transferred into strain energy in the target before penetration occurs. Failure modes were identified for flat aluminum plates and for flat composite plates made from a triaxial braid having a quasi-isotropic fiber architecture with fibers in the 0 and +/- 60 deg. directions. For the aluminum plates, a large hole formed as a result of crack propagation from the initiation site at the center of the plate to the fixed boundaries. For the composite plates, fiber tensile failure occurred in the back ply at the center of the plate. Cracks then propagated from this site along the +/- 60 deg. fiber directions until triangular flaps opened to allow the projectile to pass through the plate. The damage size was only slightly larger than the initial impact area. It was difficult to avoid slipping of the fixed edges of the plates during impact, and slipping was shown to have a large effect on the penetration threshold. Failure modes were also identified for composite half-rings fabricated with the 0 deg. fibers aligned circumferentially. Slipping of the edges was not a problem in the half-ring tests. For the composite half-rings, fiber tensile failure also occurred in the back ply. However, cracks initially propagated from this site in a direction transverse to the 0 deg. fibers. The cracks then turned to follow the +/- 60 deg. fibers for a short distance before turning again to follow 0 deg. fibers until two approximately rectangular flaps opened to allow the projectile to pass through the plate. The damage size in the composite half-rings was also only slightly larger than the initial impact area. Cracks did not propagate to the boundaries, and no delamination was observed. The damage tolerance demonstrated by the quasi-isotropic triaxial braid composites indicate that composites of this type can reasonably be considered as a lightweight alternative to metals for fan cases in commercial jet engines.

Residual Strength Analyses of Monolithic Structures

NASA Technical Reports Server (NTRS)

Forth, Scott (Technical Monitor); Ambur, Damodar R. (Technical Monitor); Seshadri, B. R.; Tiwari, S. N.

2003-01-01

Finite-element fracture simulation methodology predicts the residual strength of damaged aircraft structures. The methodology uses the critical crack-tip-opening-angle (CTOA) fracture criterion to characterize the fracture behavior of the material. The CTOA fracture criterion assumes that stable crack growth occurs when the crack-tip angle reaches a constant critical value. The use of the CTOA criterion requires an elastic- plastic, finite-element analysis. The critical CTOA value is determined by simulating fracture behavior in laboratory specimens, such as a compact specimen, to obtain the angle that best fits the observed test behavior. The critical CTOA value appears to be independent of loading, crack length, and in-plane dimensions. However, it is a function of material thickness and local crack-front constraint. Modeling the local constraint requires either a three-dimensional analysis or a two-dimensional analysis with an approximation to account for the constraint effects. In recent times as the aircraft industry is leaning towards monolithic structures with the intention of reducing part count and manufacturing cost, there has been a consistent effort at NASA Langley to extend critical CTOA based numerical methodology in the analysis of integrally-stiffened panels.In this regard, a series of fracture tests were conducted on both flat and curved aluminum alloy integrally-stiffened panels. These flat panels were subjected to uniaxial tension and during the test, applied load-crack extension, out-of-plane displacements and local deformations around the crack tip region were measured. Compact and middle-crack tension specimens were tested to determine the critical angle (wc) using three-dimensional code (ZIP3D) and the plane-strain core height (hJ using two-dimensional code (STAGS). These values were then used in the STAGS analysis to predict the fracture behavior of the integrally-stiffened panels. The analyses modeled stable tearing, buckling, and crack branching at the integral stiffener using different values of critical CTOA for different material thicknesses and orientation. Comparisons were made between measured and predicted load-crack extension, out-of-plane displacements and local deformations around the crack tip region. Simultaneously, three-dimensional capabilities to model crack branching and to monitor stable crack growth of multiple cracks in a large thick integrally-stiffened flat panels were implemented in three-dimensional finite element code (ZIP3D) and tested by analyzing the integrally-stiffened panels tested at Alcoa. The residual strength of the panels predicted from STAGS and ZP3D code compared very well with experimental data. In recent times, STAGS software has been updated with new features and now one can have combinations of solid and shell elements in the residual strength analysis of integrally-stiffened panels.

Sex and drug risk behavior pre- and post-emigration among Latino migrant men in post-Hurricane Katrina New Orleans.

PubMed

Mills, Jennifer; Burton, Nicole; Schmidt, Norine; Salinas, Oscar; Hembling, John; Aran, Alberto; Shedlin, Michele; Kissinger, Patricia

2013-06-01

High rates of sex and drug risk behaviors have been documented among Latino migrant men in the US. Whether these behaviors were established in the migrants' home countries or were adopted in the US has not been described and has implications for prevention strategies. Quarterly surveys were conducted to gather information on selected sex and drug risk practices of Latino migrant men who arrived in New Orleans after Hurricane Katrina seeking work. Both kappa scores and McNemar's tests were performed to determine if practice of these behaviors in home country was similar to practice post-emigration to the US. Female sex worker (FSW) patronage, same sex encounters (MSM), and crack cocaine use was more likely to occur post- rather than pre-emigration. Of those who ever engaged in these selected behaviors, most adopted the behavior in the US (i.e., 75.8% of FSW patrons, 72.7% of MSM participants, and 85.7% of crack cocaine users), with the exception of binge drinking (26.8%). Men who were living with a family member were less likely to adopt FSW patronage OR = 0.27, CI = 0.10-0.76, whereas men who earned >$465 per week were more likely to adopt crack cocaine use OR = 6.29 CI = 1.29, 30.57. Interventions that facilitate the maintenance of family cohesion and provide strategies for financial management may be useful for reducing sex and drug risk among newly arrived migrants.

Working session 2: Tubing inspection

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

Guerra, J.; Tapping, R.L.

1997-02-01

This session was attended by delegates from 10 countries, and four papers were presented. A wide range of issues was tabled for discussion. Realizing that there was limited time available for more detailed discussion, three topics were chosen for the more detailed discussion: circumferential cracking, performance demonstration (to focus on POD and sizing), and limits of methods. Two other subsessions were organized: one dealt with some challenges related to the robustness of current inspection methods, especially with respect to leaving cracked tubes in service, and the other with developing a chart of current NDE technology with recommendations for future development.more » These three areas are summarized in turn, along with conclusions and/or recommendations. During the discussions there were four presentations. There were two (Canada, Japan) on eddy current probe developments, both of which addressed multiarray probes that would detect a range of flaws, one (Spain) on circumferential crack detection, and one (JRC, Petten) on the recent PISC III results.« less

Changes in crack shape and saturation during water penetration into stressed rock

NASA Astrophysics Data System (ADS)

Masuda, K.; Nishizawa, O.

2012-12-01

Open cracks and cavities in rocks play important roles in fluid transport. Water penetration induced microcrack activities and caused the failure of rocks. Fluids in cracks affect earthquake generation mechanism through physical and physicochemical effects. Methods of characterizing crack shape and water saturation of rocks underground are needed for many scientific and industrial applications. It would be desirable to estimate the status of cracks using readily observable data such as elastic-wave velocities. We demonstrate a laboratory method for estimating crack status inside a cylindrical rock sample based on least-squares fitting of a cracked solid model to measured P- and S-wave velocities, and porosity derived from strain data. We used a cylinder (50 mm in diameter and 100 mm in length) of medium-grained granite. We applied a differential stress of 370 MPa, which corresponds to about 70% of fracture strength, to the rock sample under 30 MPa confining pressure and held it constant throughout the experiment. When the primary creep stage and acoustic emission (AE) caused by the initial loading had ceased, we injected distilled water into the bottom end of the sample at a constant pressure of 25 MPa until macroscopic fracture occurred. During water migration, we measured P waves and S waves (Sv and Sh), in five directions parallel to the top and bottom surfaces of the sample. We also measured strains of the sample surface and monitored AE. We created X-ray computer tomography (CT) images of the rock sample after the experiment in order to recognize the location and shape of fractured surfaces. We observed the different patterns of velocity changes in the upper and lower portions of the rock sample. Changes in P-wave velocities can be interpreted based on the crack density. S-waves showed the splitting with Vsv being faster than Vsh, corresponding to the second kind of anisotropy. We estimated two crack characteristics, crack shape and the degree of water saturation, and their changes during the loading and water migration into a granitic rock subjected to confining pressure and differential stress. We found that during injection of water to induce failure of a stressed rock sample, the aspect ratio of cracks increased and the degree of water saturation increased to about 70%. Laboratory derived method can be applicable for the well-planned observation in the field experiments. Monitoring in situ crack situations with seismic waves are useful for industrial and scientific applications such as sequestrations of carbon dioxide and waste, and measuring the regional stress field.

Guided-Wave Testing of Trunnion Rods at Greenup Lock and Dam, Kentucky

DTIC Science & Technology

2014-04-01

fatigue cracks which go through opening and closing periods during their progression, microcracks in post-tension trunnion anchor rods are believed to be...produce undesired interference effects. A new 1.5 in. diameter, medium-damped lead zirconate titanate ( PZT ), ceramic-based crystal (i.e., an Accuscan

40 CFR 61.344 - Standards: Surface impoundments.

Code of Federal Regulations, 2011 CFR

2011-07-01

... of less than 500 ppmv above background, initially and thereafter at least once per year by the... than 500 ppmv above background, as determined initially and thereafter at least once per year by the... no cracks or gaps occur and that access hatches and other openings are closed and gasketed properly...

Supporting Access to Open Online Courses for Learners of Developing Countries

ERIC Educational Resources Information Center

Nti, Kwame

2015-01-01

This paper examines how access to, and use of, open online courses may be enhanced for learners of developing countries from a learner perspective. Using analysis of the open education concept, factors that affect access to open educational resources content, and universal standards for delivering online learning, the author demonstrates that the…

Digital Classrooms Take Flight

ERIC Educational Resources Information Center

Schachter, Ron

2009-01-01

It used to be that anyone who made it through the semester without cracking a textbook was supposed to be very brilliant or very lazy. But schools across the country--driven by concerns ranging from the financial to the pedagogical--are changing that stereotype, as they switch from conventional textbooks to electronic versions of the same or…

Injection-Sensitive Mechanics of Hydraulic Fracture Interaction with Discontinuities

NASA Astrophysics Data System (ADS)

Chuprakov, D.; Melchaeva, O.; Prioul, R.

2014-09-01

We develop a new analytical model, called OpenT, that solves the elasticity problem of a hydraulic fracture (HF) contact with a pre-existing discontinuity natural fracture (NF) and the condition for HF re-initiation at the NF. The model also accounts for fluid penetration into the permeable NFs. For any angle of fracture intersection, the elastic problem of a blunted dislocation discontinuity is solved for the opening and sliding generated at the discontinuity. The sites and orientations of a new tensile crack nucleation are determined based on a mixed stress- and energy-criterion. In the case of tilted fracture intersection, the finite offset of the new crack initiation point along the discontinuity is computed. We show that aside from known controlling parameters such stress contrast, cohesional and frictional properties of the NFs and angle of intersection, the fluid injection parameters such as the injection rate and the fluid viscosity are of first-order in the crossing behavior. The model is compared to three independent laboratory experiments, analytical criteria of Blanton, extended Renshaw-Pollard, as well as fully coupled numerical simulations. The relative computational efficiency of OpenT model (compared to the numerical models) makes the model attractive for implementation in modern engineering tools simulating hydraulic fracture propagation in naturally fractured environments.

The feasibility study based on e-commerce instructions-focuses on detection and deletion of illegal content

NASA Astrophysics Data System (ADS)

Guo, Tianze; Bi, Siyu; Liu, Jiaming

2018-04-01

This essay legally restrains the illegal content based on the e-commerce directive and introduces that the European countries detect and notify illegal content through the instructions of competent authorities, notification of credible flaggers, user reports and technical tools. The illegal content should be deleted through the service terms and transparency report basing on prevent excessive deletions system. At the same time, use filters to detect and filter to against the recurrence of illegal content. By analyzing the advantages of China under the environment of cracking down on illegal content, this essay concludes that the success of China in cracking down on illegal content lies in all-round collaborative management model of countries, governments, enterprises and individuals. At the end of the essay, one is to build a training corpus that can automatically update the ability to identify the illegal content. And it proposes an optimization scheme that establish a complete set of address resolution procedures and classify IP address data according to big data analysis and DNS protection module to prevent hackers from spreading illegal content by tampering with DNS segments.

The problem of liquid fuels (for aircraft engines)

NASA Technical Reports Server (NTRS)

Gallo, Gino

1924-01-01

The crisis which troubles the world market for liquid fuel in general and for carburants in particular is doubtless one of the most serious ever experienced by modern industry. It is a national crisis of economic and political independence for countries like Italy and France. The solutions suggested for meeting the lack of liquid fuel may be summed up under two general headings: the economical use of the petroleum now available; creation of petroleum substitutes from natural sources within the country. The process of cracking is described at length.

Experimental Study on Fatigue Behaviour of Shot-Peened Open-Hole Steel Plates

PubMed Central

Wang, Zhi-Yu; Wang, Qing-Yuan; Cao, Mengqin

2017-01-01

This paper presents an experimental study on the fatigue behaviour of shot-peened open-hole plates with Q345 steel. The beneficial effects induced by shot peening on the fatigue life improvement are highlighted. The characteristic fatigue crack initiation and propagation modes of open-hole details under fatigue loading are revealed. The surface hardening effect brought by the shot peening is analyzed from the aspects of in-depth micro-hardness and compressive residual stress. The fatigue life results are evaluated and related design suggestions are made as a comparison with codified detail categories. In particular, a fracture mechanics theory-based method is proposed and demonstrated its validity in predicting the fatigue life of studied shot-peened open-hole details. PMID:28841160

Structural response of nuclear containment shield buildings with unanticipated construction openings

NASA Astrophysics Data System (ADS)

Mac Namara, Sinead Caitriona

As Nuclear Power Plants age many require steam generator replacement. There is a nickel alloy in the steam generator tubes that is susceptible to stress cracking and although these cracks can be sealed the generator becomes uneconomical without 10%-15% of the tubes. The steam generator in a typical nuclear power plant is housed in the containment structure next to the reactor. The equipment hatch is not big enough to facilitate steam generator replacement, thus construction openings in the dome of the containment structure are required. To date the structural consequences of construction openings in the dome have not been examined. This thesis examines the effects of such openings. The prototype concrete dome is made up of a 2 ft thick dome atop 3 ft thick and 170 ft high cylindrical walls (radius 65.5 ft) with a tension ring 15 ft high and 8 ft thick in between. The dome of the building is cast in two layers; a lower 9 inch layer that serves as the formwork for an upper 15 inch layer. The weight of the dome is carried in axial compression along the hoops and meridians of the dome. The first finite element model uses shell elements and considers two limiting load cases; where the two layers act as one, and where the lower layer carries the weight of both. The openings interrupt the hoops and meridians and the weight of the dome must be redistributed around the openings. Without openings, the stresses due to dead load in the structure are very low when compared to the material strength. The impact of the openings is increased compression stresses near the opening. The maximum stresses are approximately four times larger than in the original structure. These results are confirmed by the second model which is made from layers of solid elements. This model shows a significant difference between the compression on the top surface of the dome, in the affected areas, and that on the bottom surface, leading to shear stresses. These shear stresses are largest around the opening but are not large enough to cause delamination.

Influence of wetting effect at the outer surface of the pipe on increase in leak rate - experimental results and discussion

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

Isozaki, Toshikuni; Shibata, Katsuyuki

1997-04-01

Experimental and computed results applicable to Leak Before Break analysis are presented. The specific area of investigation is the effect of the temperature distribution changes due to wetting of the test pipe near the crack on the increase in the crack opening area and leak rate. Two 12-inch straight pipes subjected to both internal pressure and thermal load, but not to bending load, are modelled. The leak rate was found to be very susceptible to the metal temperature of the piping. In leak rate tests, therefore, it is recommended that temperature distribution be measured precisely for a wide area.

Materials characterization and fracture mechanics of a space grade dielectric silicone insulation

NASA Technical Reports Server (NTRS)

Abdel-Latif, A. I.; Tweedie, A. T.

1982-01-01

The present investigation is concerned with the DC 93-500 high voltage silicone insulation material employed to pot the gun and the collector end of a traveling wave tube (TWT) used on the Landsat D Satellite. The fracture mechanics behavior of the silicone resin was evaluated by measuring the slow crack velocity as a function of the opening mode of the stress intensity factor at +25 and -10 C, taking into account various uniaxial discrete strain values. It was found that the silicone resins slow crack growth is faster than that for a high voltage insulation polyurethane material at the same stress intensity factor value and room temperature.

Triboluminesence multifunctional cementitious composites with in situ damage sensing capability

NASA Astrophysics Data System (ADS)

Olawale, David O.; Dickens, Tarik; Uddin, Mohammed J.; Okoli, Okenwa O.

2012-04-01

Structural health monitoring of civil infrastructure systems like concrete bridges and dams has become critical because of the aging and overloading of these CIS. Most of the available SHM methods are not in-situ and can be very expensive. The triboluminescence multifunctional cementitious composites (TMCC) have in-built crack detection mechanism that can enable bridge engineers to monitor and detect abnormal crack formation in concrete structures so that timely corrective action can be taken to prevent costly or catastrophic failures. This article reports the fabrication process and test result of the flexural characterization of the TMCC. Accelerated durability test indicated that the 0.5 ZnS:Mn/Epoxy weight fraction ITOF sensor configuration to be more desirable in terms of durability. The alkaline environment at the highest temperature investigated (45 °C) resulted in significant reduction in the mean glass transition and storage moduli of the tested ITOF thin films. Further work is ongoing to correlate the TL response of the TMCC with damage, particularly crack opening.

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.

Fracture toughness determination using spiral-grooved cylindrical specimen and pure torsional loading

DOEpatents

Wang, Jy-An; Liu, Kenneth C.

2003-07-08

A method for determining fracture toughness K.sub.IC of materials ranging from metallic alloys, brittle ceramics and their composites, and weldments. A cylindrical specimen having a helical V-groove with a 45.degree. pitch is subjected to pure torsion. This loading configuration creates a uniform tensile-stress crack-opening mode, and a transverse plane-strain state along the helical groove. The full length of the spiral groove is equivalent to the thickness of a conventional compact-type specimen. K.sub.IC values are determined from the fracture torque and crack length measured from the test specimen using a 3-D finite element program (TOR3D-KIC) developed for the purpose. In addition, a mixed mode (combined tensile and shear stress mode) fracture toughness value can be determined by varying the pitch of the helical groove. Since the key information needed for determining the K.sub.IC value is condensed in the vicinity of the crack tip, the specimen can be significantly miniaturized without the loss of generality.

Fractography of composite delamination

NASA Technical Reports Server (NTRS)

1988-01-01

Studies were made of the fractography of mode 2 delamination and the impact damage of carbon fiber reinforced polymer composites. Laminates fractured under mode 2 loading were potted in a clear epoxy polymer, sectioned and polished and examined using transmission and reflection light microscopy. There were only occasional fibers bridging the mode 2 cracks. These cracks were not always visible probably because the crack opening displacement was too small as to be resolved using light microscopy. A study was made of the effect of repetitive impacts on laminates of AS4/3501-6 and IM6/3501-6. Plots of cumulative impact energy vs cumulative absorbed energy exhibited a sharp change in slope which corresponded to the damage area reaching the edges of the specimen. The initial slope was highly reproducible for both composite materials. On the other hand, the intersection point between the two slopes, where the damage area reaches the specimen edges, was highly variable between specimens. This variability is tentatively ascribed to differences in laminate quality.

Tension zones of deep-seated rockslides revealed by thermal anomalies and airborne laser scan data

NASA Astrophysics Data System (ADS)

Baroň, Ivo; Bečkovský, David; Gajdošík, Juraj; Opálka, Filip; Plan, Lukas; Winkler, Gerhard

2015-04-01

Open cracks, tension fractures and crevice caves are important diagnostic features of gravitationally deformed slopes. When the cracks on the upper part of the slope open to the ground surface, they transfer relatively warm and buoyant air from the underground in cold seasons and thus could be detected by the infrared thermography (IRT) as warmer anomalies. Here we present two IRT surveys of deep-seated rockslides in Austria and the Czech Republic. We used thermal imaging cameras Flir and Optris, manipulated manually from the ground surface and also from unmanned aerial vehicle and piloted ultralight-plane platforms. The surveys were conducted during cold days of winter 2014/2015 and early in the morning to avoid the negative effect of direct sunshine. The first study site is the Bad Fischau rockslide in the southern part of the Vienna Basin (Austria). It was firstly identified by the morphostructural analysis of 1-m digital terrain model from the airborne laser scan data. The rockslide is superimposed on, and closely related to the active marginal faults of the Vienna basin, which is a pull apart structure. There is the 80-m-deep Eisenstein Show Cave situated in the southern lateral margin of the rockslide. The cave was originally considered to be purely of hydrothermal (hypogene) karstification; however its specific morphology and position within the detachment zone of the rockslide suggests its relation to gravitational slope-failure. The IRT survey revealed the Eisenstein Cave at the ground surface and also several other open cracks and possible cleft caves along the margins, headscarp, and also within the body of the rockslide. The second surveyed site was the Kněhyně rockslide in the flysch belt of the Outer Western Carpathians in the eastern Czech Republic. This deep-seated translational rockslide formed about eight known pseudokarst crevice caves, which reach up to 57 m in depth. The IRT survey recognized several warm anomalies indicating very deep deformation of the slope. When compared to digital terain model, some of these thermal anomalies suggest large unexplored crack systems deep in the rock-slope failure. As a conclusion we notice that especially when compared to topographic structures visualized on high accuracy digital terrain models, detecting the thermal anomalies could significantly contribute to understanding the subsurface occurrence of the tension fractures and voids within deep-seated rockslide bodies.

Numerical Analysis of Crack Tip Plasticity and History Effects under Mixed Mode Conditions

NASA Astrophysics Data System (ADS)

Lopez-Crespo, Pablo; Pommier, Sylvie

The plastic behaviour in the crack tip region has a strong influence on the fatigue life of engineering components. In general, residual stresses developed as a consequence of the plasticity being constrained around the crack tip have a significant role on both the direction of crack propagation and the propagation rate. Finite element methods (FEM) are commonly employed in order to model plasticity. However, if millions of cycles need to be modelled to predict the fatigue behaviour of a component, the method becomes computationally too expensive. By employing a multiscale approach, very precise analyses computed by FEM can be brought to a global scale. The data generated using the FEM enables us to identify a global cyclic elastic-plastic model for the crack tip region. Once this model is identified, it can be employed directly, with no need of additional FEM computations, resulting in fast computations. This is done by partitioning local displacement fields computed by FEM into intensity factors (global data) and spatial fields. A Karhunen-Loeve algorithm developed for image processing was employed for this purpose. In addition, the partitioning is done such as to distinguish into elastic and plastic components. Each of them is further divided into opening mode and shear mode parts. The plastic flow direction was determined with the above approach on a centre cracked panel subjected to a wide range of mixed-mode loading conditions. It was found to agree well with the maximum tangential stress criterion developed by Erdogan and Sih, provided that the loading direction is corrected for residual stresses. In this approach, residual stresses are measured at the global scale through internal intensity factors.

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.

Oceanic crustal velocities from laboratory and logging measurements of Integrated Ocean Drilling Program Hole 1256D

NASA Astrophysics Data System (ADS)

Gilbert, Lisa A.; Salisbury, Matthew H.

2011-09-01

Drilling and logging of Integrated Ocean Drilling Program (IODP) Hole 1256D have provided a unique opportunity for systematically studying a fundamental problem in marine geophysics: What influences the seismic structure of oceanic crust, porosity or composition? Compressional wave velocities (Vp) logged in open hole or from regional refraction measurements integrate both the host rock and cracks in the crust. To determine the influence of cracks on Vp at several scales, we first need an accurate ground truth in the form of laboratory Vp on crack-free, or nearly crack-free samples. We measured Vp on 46 water-saturated samples at in situ pressures to determine the baseline velocities of the host rock. These new results match or exceed Vp logs throughout most of the hole, especially in the lower dikes and gabbros, where porosities are low. In contrast, samples measured at sea under ambient laboratory conditions, had consistently lower Vp than the Vp logs, even after correction to in situ pressures. Crack-free Vp calculated from simple models of logging and laboratory porosity data for different lithologies and facies suggest that crustal velocities in the lavas and upper dikes are controlled by porosity. In particular, the models demonstrate significant large-scale porosity in the lavas, especially in the sections identified as fractured flows and breccias. However, crustal velocities in the lower dikes and gabbros are increasingly controlled by petrology as the layer 2-3 boundary is approached.

Source Mechanism of Vulcanian Degassing at Popocatépetl Volcano, Mexico, Determined From Moment-Tensor Inversion of Very-long-period Seismic Waveforms

NASA Astrophysics Data System (ADS)

Chouet, B.; Dawson, P.; Arciniega, A.

2004-12-01

The source mechanism of very-long-period (VLP) signals accompanying degassing exhalations at Popocatépetl is analyzed in the 15-70~s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two events (04/23/00, 05/23/00) representative of mild Vulcanian eruptions are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500~m below the western perimeter of the summit crater, and the modeled source is composed of a shallow-dipping crack (sill with easterly dip of 10° ) intersecting a steeply-dipping crack (northeast striking dike with northwest dip of 83° ), whose surface trace bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation --- reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of 3-5~min. The largest moment release occurs in the sill, showing a maximum volume change of 500-1000\\:m3, pressure drop of 3-5~MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is 200-300\\:m3, with a corresponding pressure drop of 1-2~MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources is a single force with magnitude of 5 × 108~N, consistent with melt advection in response to the pressure transients. The source-time history of the three components of this force confirms that significant mass movement starts in the sill and triggers a mass movement response in the dike within ˜ 5~s. Such source behavior is consistent with the opening of an escape pathway for accumulated gases from slow pressurization of the sill driven by magma crystallization. The opening of a pathway for pent-up gases in the sill and rapid evacuation of this separated gas phase induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40~MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius, (100~m), crack aperture, (5~m), bubble number density, (1010 - 1012\\:m-3), initial bubble radius, (10-6\\:m), final bubble radius, ( ˜ 10-5\\:m), and net decrease of gas concentration in the melt, (0.01~wt%).

Source mechanism of Vulcanian degassing at Popocatépetl Volcano, Mexico, determined from waveform inversions of very long period signals

NASA Astrophysics Data System (ADS)

Chouet, Bernard; Dawson, Phillip; Arciniega-Ceballos, Alejandra

2005-07-01

The source mechanism of very long period (VLP) signals accompanying volcanic degassing bursts at Popocatépetl is analyzed in the 15-70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two eruptions (23 April and 23 May 2000) representative of mild Vulcanian activity are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500 m below the western perimeter of the summit crater, and the modeled source is composed of a shallow dipping crack (sill with easterly dip of 10°) intersecting a steeply dipping crack (northeast striking dike dipping 83° northwest), whose surface extension bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation, reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of 3-5 min. The largest moment release occurs in the sill, showing a maximum volume change of 500-1000 m3, pressure drop of 3-5 MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is less (200-300 m3), with a corresponding pressure drop of 1-2 MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources are single-force components with magnitudes of 108 N, consistent with melt advection in response to pressure transients. The source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds. Such source behavior is consistent with the opening of a pathway for escape of pent-up gases from slow pressurization of the sill driven by magma crystallization. The opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40 MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius (100 m), crack aperture (5 m), bubble number density (1010-1012 m-3), initial bubble radius (10-6 m), final bubble radius (˜10-5 m), and net decrease of gas concentration in the melt (0.01 wt %).

Source mechanism of Vulcanian degassing at Popocatépetl Volcano, Mexico, determined from waveform inversions of very long period signals

USGS Publications Warehouse

Chouet, Bernard A.; Dawson, Phillip B.; Arciniega-Ceballos, Alejandra

2005-01-01

The source mechanism of very long period (VLP) signals accompanying volcanic degassing bursts at Popocatépetl is analyzed in the 15–70 s band by minimizing the residual error between data and synthetics calculated for a point source embedded in a homogeneous medium. The waveforms of two eruptions (23 April and 23 May 2000) representative of mild Vulcanian activity are well reproduced by our inversion, which takes into account volcano topography. The source centroid is positioned 1500 m below the western perimeter of the summit crater, and the modeled source is composed of a shallow dipping crack (sill with easterly dip of 10°) intersecting a steeply dipping crack (northeast striking dike dipping 83° northwest), whose surface extension bisects the vent. Both cracks undergo a similar sequence of inflation, deflation, and reinflation, reflecting a cycle of pressurization, depressurization, and repressurization within a time interval of 3–5 min. The largest moment release occurs in the sill, showing a maximum volume change of 500–1000 m3, pressure drop of 3–5 MPa, and amplitude of recovered pressure equal to 1.2 times the amplitude of the pressure drop. In contrast, the maximum volume change in the dike is less (200–300 m3), with a corresponding pressure drop of 1–2 MPa and pressure recovery equal to the pressure drop. Accompanying these volumetric sources are single-force components with magnitudes of 108 N, consistent with melt advection in response to pressure transients. The source time histories of the volumetric components of the source indicate that significant mass movement starts within the sill and triggers a mass movement response in the dike within a few seconds. Such source behavior is consistent with the opening of a pathway for escape of pent-up gases from slow pressurization of the sill driven by magma crystallization. The opening of this pathway and associated rapid evacuation of volcanic gases induces the pressure drop. Pressure recovery in the magma filling the sill is driven by diffusion of gases from the resulting supersaturated melt into bubbles. Assuming a penny-shaped crack at ambient pressure of 40 MPa, the observed pressure and volume variations can be modeled with the following attributes: crack radius (100 m), crack aperture (5 m), bubble number density (1010–1012 m−3), initial bubble radius (10−6 m), final bubble radius (∼10−5 m), and net decrease of gas concentration in the melt (0.01 wt %).

The Effect of Microstructure on the Properties of High Strength Aluminum Alloys

DTIC Science & Technology

1980-02-01

precracked samples were loaded with opposing bolts to constant crack opening displacement (COD) values. The notch with the precrack was sealed with Scotch tape ...Respectfully submitted: Edgar K. Starke, Jr. Principal Investigator Si Il I REFERENCES 1. P. C. Paris and F. Erdogan , J. Basic Eng., Vol. 85, 1963, p. 528

Storms Spur Flood of Giving for Schools

ERIC Educational Resources Information Center

Robelen, Erik W.

2005-01-01

From elementary pupils cracking open their piggy banks to companies and foundations writing seven-figure checks, a flood of donations has reached schools and students affected by Katrina since the storm devastated parts of the Gulf Coast. Further help has gone to those affected by Hurricane Rita, which hit southwestern Louisiana and the coast of…

Giant desiccation fissures on the Black Rock and Smoke Creek Deserts, Nevada

USGS Publications Warehouse

Willden, R.; Mabey, D.R.

1961-01-01

Open fissures, from 100 to several hundred feet apart, that have produced polygonal patterns on the Black Rock Desert, Nevada, are believed to be giant desiccation cracks resulting from a secular trend toward aridity in the last few decades. Similar features on the Smoke Creek Desert probably have the same origin.

First report of Penicillium expansum isolates with reduced sensitivity to fludioxonil from a commercial packinghouse in Pennsylvania

USDA-ARS?s Scientific Manuscript database

Blue mold is caused by Penicillium expansum and is among the most economically significant disease of stored apples worldwide. The fungus gains ingress through cracks, natural openings, and wounds in the fruit and produces mycotoxins that contaminate processed apple products. All commercial apples a...

Bioactive compounds in cashew nut (Anacardium occidentale L.) kernels: effect of different shelling methods.

PubMed

Trox, Jennifer; Vadivel, Vellingiri; Vetter, Walter; Stuetz, Wolfgang; Scherbaum, Veronika; Gola, Ute; Nohr, Donatus; Biesalski, Hans Konrad

2010-05-12

In the present study, the effects of various conventional shelling methods (oil-bath roasting, direct steam roasting, drying, and open pan roasting) as well as a novel "Flores" hand-cracking method on the levels of bioactive compounds of cashew nut kernels were investigated. The raw cashew nut kernels were found to possess appreciable levels of certain bioactive compounds such as beta-carotene (9.57 microg/100 g of DM), lutein (30.29 microg/100 g of DM), zeaxanthin (0.56 microg/100 g of DM), alpha-tocopherol (0.29 mg/100 g of DM), gamma-tocopherol (1.10 mg/100 g of DM), thiamin (1.08 mg/100 g of DM), stearic acid (4.96 g/100 g of DM), oleic acid (21.87 g/100 g of DM), and linoleic acid (5.55 g/100 g of DM). All of the conventional shelling methods including oil-bath roasting, steam roasting, drying, and open pan roasting revealed a significant reduction, whereas the Flores hand-cracking method exhibited similar levels of carotenoids, thiamin, and unsaturated fatty acids in cashew nuts when compared to raw unprocessed samples.

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.

Effect of inclusions on heterogeneous crack nucleation in nanocomposites

NASA Astrophysics Data System (ADS)

Gutkin, M. Yu.; Ovid'Ko, I. A.; Skiba, N. V.

2007-02-01

A two-dimensional theoretical model is proposed for the heterogeneous nucleation of a grain-boundary nanocrack in a nanocomposite consisting of a nanocrystalline matrix and nanoinclusions whose elastic moduli are identical to those of the matrix. The inclusions have the form of rods with a rectangular cross section and undergo dilatation eigenstrain induced by the differences in the lattice parameters and thermal expansion coefficients of the matrix and inclusions. In terms of the model, a mode-I-II nanocrack nucleates at the negative disclination of a biaxial dipole consisting of wedge grain-boundary (or junction) disclinations; then, the nanocrack opens along a grain boundary and reaches an inclusion boundary. Depending on the relative positions and orientations of the initial segment of the nanocrack and the inclusion, the nanocrack can either penetrate into the inclusion or bypass it along the matrix-inclusion interface. The nanocrack nucleation probability increases near an inclusion with negative (compressive) dilatation eigenstrain. A decrease in the inclusion size decreases (increases) the probability of a crack opening along the interface if the dilatation eigenstrain is negative (positive).

The influence of loading on the corrosion of steel in cracked ordinary Portland cement and high performance concretes

NASA Astrophysics Data System (ADS)

Jaffer, Shahzma Jafferali

Most studies that have examined chloride-induced corrosion of steel in concrete have focused on sound concrete. However, reinforced concrete is seldom uncracked and very few studies have investigated the influence of cracked concrete on rebar corrosion. Furthermore, the studies that have examined the relationship between cracks and corrosion have focused on unloaded or statically loaded cracks. However, in practice, reinforced concrete structures (e.g. bridges) are often dynamically loaded. Hence, the cracks in such structures open and close which could influence the corrosion of the reinforcing steel. Consequently, the objectives of this project were (i) to examine the effect of different types of loading on the corrosion of reinforcing steel, (ii) the influence of concrete mixture design on the corrosion behaviour and (iii) to provide data that can be used in service-life modelling of cracked reinforced concretes. In this project, cracked reinforced concrete beams made with ordinary Portland cement concrete (OPCC) and high performance concrete (HPC) were subjected to no load, static loading and dynamic loading. They were immersed in salt solution to just above the crack level at their mid-point for two weeks out of every four (wet cycle) and, for the remaining two weeks, were left in ambient laboratory conditions to dry (dry cycle). The wet cycle led to three conditions of exposure for each beam: (i) the non-submerged region, (ii) the sound, submerged region and (iii) the cracked mid-section, which was also immersed in the solution. Linear polarization resistance and galvanostatic pulse techniques were used to monitor the corrosion in the three regions. Potentiodynamic polarization, electrochemical current noise and concrete electrical resistance measurements were also performed. These measurements illustrated that (i) rebar corroded faster at cracks than in sound concrete, (ii) HPC was more protective towards the rebar than OPCC even at cracks and (iii) there was a minor effect of the type of loading on rebar corrosion within the period of the project. These measurements also highlighted the problems associated with corrosion measurements, for example, identifying the actual corroding area and the influence of the length of rebar. The numbers of cracks and crack-widths in each beam were measured after the beam's initial exposure to salt solution and, again, after the final corrosion measurements. HPC beams had more cracks than the OPCC. Also, final measurements illustrated increased crack-widths in dynamically loaded beams, regardless of the concrete type. The cracks in both statically and dynamically loaded OPCC and HPC beams bifurcated at the rebar level and propagated parallel to the rebar. This project also examined the extent of corrosion on the rebars and the distribution of corrosion products in the concrete and on the concrete walls of the cracks. Corrosion occurred only at cracks in the concrete and was spread over a larger area on the rebars in HPC than those in OPCC. The damage due to corrosion was superficial in HPC and crater-like in OPCC. Regardless of the concrete type, there was a larger distribution of corrosion products on the crack walls of the dynamically loaded beams. Corrosion products diffused into the cement paste and the paste-aggregate interface in OPCC but remained in the crack in HPC. The most voluminous corrosion product identified was ferric hydroxide. Elemental analysis of mill-scale on rebar which was not embedded in concrete or exposed to chlorides was compared to that of the bars that had been embedded in uncontaminated concrete and in cracked concrete exposed to chlorides. In uncontaminated concrete, mill-scale absorbed calcium and silicon. At a crack, a layer, composed of a mixture of cement paste and corrosion products, developed between the mill-scale and the substrate steel. Based on the results, it was concluded that (i) corrosion occurred on the rebar only at cracks in the concrete, (ii) corrosion was initiated at the cracks immediately upon exposure to salt solution, (ii) the type of loading had a minor influence on the corrosion rates of reinforcing steel and (iv) the use of polarized area led to a significant underestimation of the current density at the crack. It is recommended that the effect of cover-depth on (i) the time to initiation of corrosion and (ii) the corrosion current density in cracked concrete be investigated.

Near-Field Acoustical Imaging using Lateral Bending Mode of Atomic Force Microscope Cantilevers

NASA Astrophysics Data System (ADS)

Caron, A.; Rabe, U.; Rödel, J.; Arnold, W.

Scanning probe microscopy techniques enable one to investigate surface properties such as contact stiffness and friction between the probe tip and a sample with nm resolution. So far the bending and the torsional eigenmodes of an atomic force microscope cantilever have been used to image variations of elasticity and shear elasticity, respectively. Such images are near-field images with the resolution given by the contact radius typically between 10 nm and 50 nm. We show that the flexural modes of a cantilever oscillating in the width direction and parallel to the sample surface can also be used for imaging. Additional to the dominant in-plane component of the oscillation, the lateral modes exhibit a vertical component as well, provided there is an asymmetry in the cross-section of the cantilever or in its suspension. The out-of-plane deflection renders the lateral modes detectable by the optical position sensors used in atomic force microscopes. We studied cracks which were generated by Vickers indents, in submicro- and nanocrystalline ZrO2. Images of the lateral contact stiffness were obtained by vibrating the cantilever close to a contact-resonance frequency. A change in contact stiffness causes a shift of the resonant frequency and hence a change of the cantilever vibration amplitude. The lateral contact-stiffness images close to the crack faces display a contrast that we attribute to altered elastic properties indicating a process zone. This could be caused by a stress-induced phase transformation during crack propagation. Using the contact mode of an atomic force microscope, we measured the crack-opening displacement as a function of distance from the crack tip, and we determined the crack-tip toughness Ktip. Furthermore, K1c was inferred from the length of radial cracks of Vickers indents that were measured using classical scanning acoustic microscopy

Cohesive zone model for intergranular slow crack growth in ceramics: influence of the process and the microstructure

NASA Astrophysics Data System (ADS)

Romero de la Osa, M.; Estevez, R.; Olagnon, C.; Chevalier, J.; Tallaron, C.

2011-10-01

Ceramic polycrystals are prone to slow crack growth (SCG) which is stress and environmentally assisted, similarly to observations reported for silica glasses. The kinetics of fracture are known to be dependent on the load level, the temperature and the relative humidity. In addition, evidence is available on the influence of the microstructure on the SCG rate with an increase in the crack velocity with decreasing the grain size. Crack propagation takes place beyond a load threshold, which is grain size dependent. We present a cohesive zone model for the intergranular failure process. The methodology accounts for an intrinsic opening that governs the length of the cohesive zone and allows the investigation of grain size effects. A rate and temperature-dependent cohesive model is proposed (Romero de la Osa M, Estevez R et al 2009 J. Mech. Adv. Mater. Struct. 16 623-31) to mimic the reaction-rupture mechanism. The formulation is inspired by Michalske and Freiman's picture (Michalske and Freiman 1983 J. Am. Ceram. Soc. 66 284-8) together with a recent study by Zhu et al (2005 J. Mech. Phys. Solids 53 1597-623) of the reaction-rupture mechanism. The present investigation extends a previous work (Romero de la Osa et al 2009 Int. J. Fracture 158 157-67) in which the problem is formulated. Here, we explore the influence of the microstructure in terms of grain size, their elastic properties and residual thermal stresses originating from the cooling from the sintering temperature down to ambient conditions. Their influence on SCG for static loadings is reported and the predictions compared with experimental trends. We show that the initial stress state is responsible for the grain size dependence reported experimentally for SCG. Furthermore, the account for the initial stresses enables the prediction of a load threshold below which no crack growth is observed: a crack arrest takes place when the crack path meets a region in compression.

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

Waveform inversion of oscillatory signatures in long-period events beneath volcanoes

USGS Publications Warehouse

Kumagai, H.; Chouet, B.A.; Nakano, M.

2002-01-01

The source mechanism of long-period (LP) events is examined using synthetic waveforms generated by the acoustic resonance of a fluid-filled crack. We perform a series of numerical tests in which the oscillatory signatures of synthetic LP waveforms are used to determine the source time functions of the six moment tensor components from waveform inversions assuming a point source. The results indicate that the moment tensor representation is valid for the odd modes of crack resonance with wavelengths 2L/n, 2W/n, n = 3, 5, 7, ..., where L and W are the crack length and width, respectively. For the even modes with wavelengths 2L/n, 2W/n, n = 2, 4, 6,..., a generalized source representation using higher-order tensors is required, although the efficiency of seismic waves radiated by the even modes is expected to be small. We apply the moment tensor inversion to the oscillatory signatures of an LP event observed at Kusatsu-Shirane Volcano, central Japan. Our results point to the resonance of a subhorizontal crack located a few hundred meters beneath the summit crater lakes. The present approach may be useful to quantify the source location, geometry, and force system of LP events, and opens the way for moment tensor inversions of tremor.

Hot Corrosion of Inconel 625 Overlay Weld Cladding in Smelting Off-Gas Environment

NASA Astrophysics Data System (ADS)

Mohammadi Zahrani, E.; Alfantazi, A. M.

2013-10-01

Degradation mechanisms and hot corrosion behavior of weld overlay alloy 625 were studied. Phase structure, morphology, thermal behavior, and chemical composition of deposited salt mixture on the weld overlay were characterized utilizing XRD, SEM/EDX, DTA, and ICP/OES, respectively. Dilution level of Fe in the weldment, dendritic structure, and degradation mechanisms of the weld were investigated. A molten phase formed on the weld layer at the operating temperature range of the boiler, which led to the hot corrosion attack in the water wall and the ultimate failure. Open circuit potential and weight-loss measurements and potentiodynamic polarization were carried out to study the hot corrosion behavior of the weld in the simulated molten salt medium at 873 K, 973 K, and 1073 K (600 °C, 700 °C, and 800 °C). Internal oxidation and sulfidation plus pitting corrosion were identified as the main hot corrosion mechanisms in the weld and boiler tubes. The presence of a significant amount of Fe made the dendritic structure of the weld susceptible to preferential corrosion. Preferentially corroded (Mo, Nb)-depleted dendrite cores acted as potential sites for crack initiation from the surface layer. The penetration of the molten phase into the cracks accelerated the cracks' propagation mainly through the dendrite cores and further crack branching/widening.

Sex and drug risk behavior pre- and post-emigration among Latino migrant men in post-Hurricane Katrina New Orleans

PubMed Central

Mills, Jennifer; Burton, Nicole; Schmidt, Norine; Salinas, Oscar; Hembling, John; Aran, Alberto; Shedlin, Michele; Kissinger, Patricia

2012-01-01

High rates of sex and drug risk behaviors have been documented among Latino migrant men in the U.S. Whether these behaviors were established in the migrants’ home countries or were adopted in the U.S. has not been described and has implications for prevention strategies. Quarterly surveys were conducted to gather information on selected sex and drug risk practices of Latino migrant men who arrived in New Orleans after Hurricane Katrina seeking work. Both kappa scores and McNemar’s tests were performed to determine if practice of these behaviors in home country was similar to practice post-emigration to the U.S. Female sex worker (FSW) patronage, same sex encounters (MSM), and crack cocaine use was more likely to occur post-rather than pre-emigration. Of those who ever engaged in these selected behaviors, most adopted the behavior in the U.S. (i.e. 75.8% of FSW patrons, 72.7% of MSM participants, and 85.7% of crack cocaine users), with the exception of binge drinking (26.8%). Men who were living with a family member were less likely to adopt FSW patronage OR=0.27, CI=0.10-0.76, whereas men who earned >$465 per week were more likely to adopt crack cocaine use OR=6.29 CI=1.29, 30.57. Interventions that facilitate the maintenance of family cohesion and provide strategies for financial management may be useful for reducing sex and drug risk among newly arrived migrants. PMID:22669638

Permeability measurements on rock samples from Unzen Scientific Drilling Project Drill Hole 4 (USDP-4)

NASA Astrophysics Data System (ADS)

Watanabe, Tohru; Shimizu, Yuhta; Noguchi, Satoshi; Nakada, Setsuya

2008-07-01

Permeability measurement was made on five rock samples from USDP-4 cores. Rock samples were collected from the conduit zone and its country rock. One sample (C14-1-1) is considered as a part of the feeder dyke for the 1991-1995 eruption. The transient pulse method was employed under confining pressure up to 50 MPa. Compressional wave velocity was measured along with permeability. The measured permeability ranges from 10 - 19 to 10 - 17 m 2 at the atmospheric pressure, and is as low as that reported for tight rocks such as granite. The permeability decreases with increasing confining pressure, while the compressional wave velocity increases. Assuming that pores are parallel elliptical tubes, the pressure dependence of permeability requires aspect ratio of 10 - 4 -10 - 2 at the atmospheric pressure. The pore aperture is estimated to be less than 1 μm. The estimated aspect ratio and pore aperture suggest that connectivity of pores is maintained by narrow cracks. The existence of cracks is supported by the pressure dependence of compressional wave velocity. Narrow cracks (< 1 μm) are observed in dyke samples, and they must have been created after solidification. Dyke samples do not provide us information of pore structures during degassing, since exsolved gas has mostly escaped and pores governing the gas permeable flow should have been lost. Both dyke and country rock samples provide us information of materials around ascending magma. Although the measured small-scale permeability cannot be directly applied to geological-scale processes, it gives constrains on studies of large-scale permeability.

Open and Flexible TVET in Commonwealth Pacific Countries

ERIC Educational Resources Information Center

Neal, Terry

2011-01-01

This report examines the current state of open and flexible technical-vocational education and training (TVET) in nine Pacific Commonwealth countries: Fiji, Kiribati, Nauru, Papua New Guinea, Samoa, Solomon Islands, Tonga, Tuvalu, and Vanuatu. The Commonwealth of Learning (COL) has commissioned the Open Polytechnic to complete this report to…

Results of crack-arrest tests on irradiated a 508 class 3 steel

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

Iskander, S.K.; Milella, P.P.; Pini, M.A.

1998-02-01

Ten crack-arrest toughness values for irradiated specimens of A 508 class 3 forging steel have been obtained. The tests were performed according to the American Society for Testing and Materials (ASTM) Standard Test Method for Determining Plane-Strain Crack-Arrest Fracture Toughness, K{sub la} of Ferritic Steels, E 1221-88. None of these values are strictly valid in all five ASTM E 1221-88 validity criteria. However, they are useful when compared to unirradiated crack-arrest specimen toughness values since they show the small (averaging approximately 10{degrees}C) shifts in the mean and lower-bound crack-arrest toughness curves. This confirms that a low copper content in ASTMmore » A 508 class 3 forging material can be expected to result in small shifts of the transition toughness curve. The shifts due to neutron irradiation of the lower bound and mean toughness curves are approximately the same as the Charpy V-notch (CVN) 41-J temperature shift. The nine crack-arrest specimens were irradiated at temperatures varying from 243 to 280{degrees}C, and to a fluence varying from 1.7 to 2.7 x 10{sup 19} neutrons/cm{sup 2} (> 1 MeV). The test results were normalized to reference values that correspond to those of CVN specimens irradiated at 284{degrees}C to a fluence of 3.2 x 10{sup 19} neutrons/cm{sup 2} (> 1 MeV) in the same capsule as the crack-arrest specimens. This adjustment resulted in a shift to lower temperatures of all the data, and in particular moved two data points that appeared to lie close to or lower than the American Society of Mechanical Engineers K{sub la} curve to positions that seemed more reasonable with respect to the remaining data. A special fixture was designed, fabricated, and successfully used in the testing. For reasons explained in the text, special blocks to receive the Oak Ridge National Laboratory clip gage were designed, and greater-than-standard crack-mouth opening displacements measured were accounted for. 24 refs., 13 figs., 12 tabs.« less

The Relationship Between Constraint and Ductile Fracture Initiation as Defined by Micromechanical Analyses

NASA Technical Reports Server (NTRS)

Panontin, Tina L.; Sheppard, Sheri D.

1994-01-01

The use of small laboratory specimens to predict the integrity of large, complex structures relies on the validity of single parameter fracture mechanics. Unfortunately, the constraint loss associated with large scale yielding, whether in a laboratory specimen because of its small size or in a structure because it contains shallow flaws loaded in tension, can cause the breakdown of classical fracture mechanics and the loss of transferability of critical, global fracture parameters. Although the issue of constraint loss can be eliminated by testing actual structural configurations, such an approach can be prohibitively costly. Hence, a methodology that can correct global fracture parameters for constraint effects is desirable. This research uses micromechanical analyses to define the relationship between global, ductile fracture initiation parameters and constraint in two specimen geometries (SECT and SECB with varying a/w ratios) and one structural geometry (circumferentially cracked pipe). Two local fracture criteria corresponding to ductile fracture micromechanisms are evaluated: a constraint-modified, critical strain criterion for void coalescence proposed by Hancock and Cowling and a critical void ratio criterion for void growth based on the Rice and Tracey model. Crack initiation is assumed to occur when the critical value in each case is reached over some critical length. The primary material of interest is A516-70, a high-hardening pressure vessel steel sensitive to constraint; however, a low-hardening structural steel that is less sensitive to constraint is also being studied. Critical values of local fracture parameters are obtained by numerical analysis and experimental testing of circumferentially notched tensile specimens of varying constraint (e.g., notch radius). These parameters are then used in conjunction with large strain, large deformation, two- and three-dimensional finite element analyses of the geometries listed above to predict crack initiation loads and to calculate the associated (critical) global fracture parameters. The loads are verified experimentally, and microscopy is used to measure pre-crack length, crack tip opening displacement (CTOD), and the amount of stable crack growth. Results for A516-70 steel indicate that the constraint-modified, critical strain criterion with a critical length approximately equal to the grain size (0.0025 inch) provides accurate predictions of crack initiation. The critical void growth criterion is shown to considerably underpredict crack initiation loads with the same critical length. The relationship between the critical value of the J-integral for ductile crack initiation and crack depth for SECT and SECB specimens has been determined using the constraint-modified, critical strain criterion, demonstrating that this micromechanical model can be used to correct in-plane constraint effects due to crack depth and bending vs. tension loading. Finally, the relationship developed for the SECT specimens is used to predict the behavior of circumferentially cracked pipe specimens.

Remelt Ingot Production Technology

NASA Astrophysics Data System (ADS)

Grandfield, J. F.

The technology related to the production of remelt ingots (small ingots, sows and T-Bar) is reviewed. Open mold conveyors, sow casting, wheel and belt casting and VDC and HDC casting are described and compared. Process economics, capacity, product quality and process problems are listed. Trends in casting machine technology such as longer open mold conveyor lines are highlighted. Safety issues related to the operation of these processes are discussed. The advantages and disadvantages of the various machine configurations and options e.g. such as dry filling with the mold out of water and wet filling with the mold in water for open mould conveyors are discussed. The effect of mold design on machine productivity, mold cracking and mold life is also examined.

Buffer strips in composites at elevated temperature

NASA Technical Reports Server (NTRS)

Bigelow, C. A.

1983-01-01

The composite material 'buffer strip' concept is presently investigated at elevated temperatures for the case of graphite/polyimide buffer strip panels using a (45/0/45/90)2S layup, where the buffer strip material was 0-deg S-glass/polyimide. Each panel was loaded in tension until it failed, and radiographs and crack opening displacements were recorded during the tests to determine fracture onset, fracture arrest, and the extent of damage in the buffer strip after crack arrest. At 177 + or - 3 C, the buffer strips increased the panel strength by at least 40 percent in comparison with panels without buffer strips. Compared to similar panels tested at room temperature, those tested at elevated temperature had lower residual strengths, but higher failure strains.

A comprehensive study on the damage tolerance of ultrafine-grained copper

PubMed Central

Hohenwarter, A.; Pippan, R.

2012-01-01

In this study the fracture behavior of ultrafine-grained copper was assessed by means of elasto-plastic fracture mechanics. For the synthesis of the material high pressure torsion was used. The fracture toughness was quantitatively measured by JIC as a global measure by recording the crack growth resistance curve. Additionally, the initiation toughness in terms of the crack opening displacement (CODi) was evaluated as a local fracture parameter. The results presented here exhibit a low fracture initiation toughness but simultaneously a remarkably high fracture toughness in terms of JIC. The origin of the large difference between these two parameters, peculiarities of the fracture surface and the fracture mechanical performance compared to coarse grained copper will be discussed. PMID:23471016

A review of shear wave splitting in the crack-critical crust

NASA Astrophysics Data System (ADS)

Crampin, Stuart; Chastin, Sebastien

2003-10-01

Over the last 15 years, it has become established that crack-induced stress-aligned shear wave splitting, with azimuthal anisotropy, is an inherent characteristic of almost all rocks in the crust. This means that most in situ rocks are pervaded by fluid-saturated microcracks and consequently are highly compliant. The evolution of such stress-aligned fluid-saturated grain-boundary cracks and pore throats in response to changing conditions can be calculated, in some cases with great accuracy, using anisotropic poro-elasticity (APE). APE is tightly constrained with no free parameters, yet dynamic modelling with APE currently matches a wide range of phenomena concerning anisotropy, stress, shear waves and cracks. In particular, APE has allowed the anisotropic response of a reservoir to injection to be calculated (predicted with hindsight), and the time and magnitude of an earthquake to be correctly stress-forecast. The reason for this calculability and predictability is that the microcracks in the crust are so closely spaced that they form critical systems. This crack-critical crust leads to a new style of geophysics that has profound implications for almost all aspects of pre-fracturing deformation of the crust and for solid-earth geophysics and geology. We review past, present and speculate about the future of shear wave splitting in the crack-critical crust. Shear wave splitting is seen to be a dynamic measure of the deformation of the rock mass. There is some good news and some bad news for conventional geophysics. Many accepted phenomena are no longer valid at high spatial and temporal resolution. A major effect is that the detailed crack geometry changes with time and varies from place to place in response to very small previously negligible changes. However, at least in some circumstances, the behaviour of the rock in the highly complex inhomogeneous Earth may be calculated and the response predicted, opening the way to possible control by feedback. The need is to devise ways to exploit these new opportunities in the crack-critical crust. Recent observations from the SMSITES Project at Húsavík in Northern Iceland, gathered while this review was being written, display the extraordinarily sensitivity of in situ rock to small changes at great distances. The effects are far too large to occur in a conventional elastic brittle crust, and their presence confirms the highly compliant nature of the crack-critical crust.

Hot cracking during welding and casting

NASA Astrophysics Data System (ADS)

Cao, Guoping

Aluminum welds are susceptible to liquation cracking in the partially melted zone (PMZ). Using the multicomponent Scheil model, curves of temperature vs. fraction solid (T-fS) during solidification were calculated for the PMZ and weld metals (WMs). These curves were used to predict the crack susceptibility by checking if the harmful condition of WM fS > PMZ fS exists during PMZ solidification and reduce the susceptibility by minimizing this condition. This approach was tested against full-penetration welds of alloys 7075 and 2024 and it can be used to guide the selection or development of filler metals. Liquation cracking in the PMZ in welds of Al-Si cast alloys was also investigated. The crack susceptibility was evaluated by circular-patch test, and full-penetration welds made with filler metals 1100, 4043, 4047 and 5356. Liquation cracking was significant with filler metals 1100 and 5356 but slight with filler metals 4043 and 4047. In all welds, liquation cracks were completely backfilled, instead of open as in full-penetration welds of wrought alloys 2219 and 6061. The T-fS curves showed that alloy A357 has a much higher fraction liquid for backfilling before PMZ solidification was essentially over. Hot tearing in Mg-xAl-yCa alloys was studied by constrained rod casting (CRC) in a steel mold. The hot tearing susceptibility decreased significantly with increasing Ca content (y) but did not change much with the Al content (x). An instrumented CRC with a steel mold was developed to detect the onset of hot tearing. The secondary phases, eutectic content, solidification path, and freezing range were examined. Hot tearing in Mg-Al-Sr alloys was also studied by CRC in a steel mold. With Mg-(4,6,8)Al-1.5Sr alloys, the hot tearing susceptibility decreased significantly with increasing Al content. With Mg-(4,6,8)Al-3Sr alloys, the trend was similar but not as significant. At the same Al content, the hot tearing susceptibility decreased significantly with increasing Sr content. Instrumented CRC with a steel mold was also used to test hot tearing of Mg-Al-Sr alloys. Cracking occurred at a higher temperature in alloys most susceptible to cracking than in alloys least susceptible.

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.

Influence of strain on the corrosion of magnesium alloys and zinc in physiological environments.

PubMed

Törne, Karin; Örnberg, Andreas; Weissenrieder, Jonas

2017-01-15

During implantation load-bearing devices experience stress that may influence its mechanical and corrosion profile and potentially lead to premature rupture. The susceptibility to stress corrosion cracking (SCC) of the Mg-Al alloy AZ61 and Zn was studied in simulated body fluid (m-SBF) and whole blood by slow strain rate (SSR) testing in combination with electrochemical impedance spectroscopy (EIS) and further ex situ analysis including scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy. AZ61 was found to be highly susceptible to SCC. EIS analysis show that although the majority of cracking occurred during the apparent plastic straining, cracking initiation occurs already in the elastic region at ∼50% of the ultimate tensile strength (UTS). Shifts in EIS phase angle and open circuit potential can be used to detect the onset of SCC. Zinc demonstrated a highly ductile behavior with limited susceptibility to SCC. No significant decrease in UTS was observed in m-SBF but a decrease in time to failure by ∼25% compared to reference samples indicates some effect on the mechanical properties during the ductile straining. The formation of micro cracks, ∼10μm deep, was indicated by the EIS analysis and later confirmed by ex situ SEM. The results of SSR analysis of zinc in whole blood showed a reduced effect compared to m-SBF and no cracks were detected. It appears that formation of an organic surface layer protects the corroding surface from cracking. These results highlight the importance of considering the effect of biological species on the degradation of implants in the clinical situation. Strain may deteriorate the corrosion properties of metallic implants drastically. We study the influence of load on the corrosion properties of a magnesium alloy and zinc by a combination of electrochemical impedance spectroscopy (EIS) and slow strain rate analysis. This combination of techniques has previously not been used for studying degradation in physiological relevant electrolytes. EIS provide valuable information on the initial formation of cracks, detecting crack nucleation before feasible in slow strain rate analysis. This sensitivity of EIS shows the potential for electrochemical methods to be used for in situ monitoring crack formation of implants in more applied studies. Copyright © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Bargaining for Open Skies

NASA Technical Reports Server (NTRS)

Wojahn, Oliver W.

2001-01-01

In this paper we analyze the bargaining problem between countries when negotiating bilateral air service agreements. To do so, we use the methods of bargaining and game theory. We give special attention to the case where a liberal minded country is trying to convince a less liberal country to agree to bilateral open skies, and the liberal country might also unilaterally open up its market. The following analysis is positive in the sense that the results help explain and predict the outcome of negotiations under different payoffs and structures of the bargaining process. They are normative in the sense that adequate manipulation of the bargaining conditions can ensure a desired outcome.

Diffusion of Appropriate Educational Technology in Open & Distance Learning in Developing Commonwealth Countries. Final Project Report.

ERIC Educational Resources Information Center

Williams, Roy

The Diffusion of Appropriate Educational Technology in Open and Distance Learning in Developing Countries project was designed to determine awareness and use of educational technologies and communications media in developing countries, to identify factors constraining wider use of educational technologies by developing nations, and to explore…

Lock-in thermography, penetrant inspection, and scanning electron microscopy for quantitative evaluation of open micro-cracks at the tooth-restoration interface

NASA Astrophysics Data System (ADS)

Streza, M.; Hodisan, I.; Prejmerean, C.; Boue, C.; Tessier, Gilles

2015-03-01

The evaluation of a dental restoration in a non-invasive way is of paramount importance in clinical practice. The aim of this study was to assess the minimum detectable open crack at the cavity-restorative material interface by the lock-in thermography technique, at laser intensities which are safe for living teeth. For the analysis of the interface, 18 box-type class V standardized cavities were prepared on the facial and oral surfaces of each tooth, with coronal margins in enamel and apical margins in dentine. The preparations were restored with the Giomer Beautifil (Shofu) in combination with three different adhesive systems. Three specimens were randomly selected from each experimental group and each slice has been analysed by visible, infrared (IR), and scanning electron microscopy (SEM). Lock-in thermography showed the most promising results in detecting both marginal and internal defects. The proposed procedure leads to a diagnosis of micro-leakages having openings of 1 µm, which is close to the diffraction limit of the IR camera. Clinical use of a thermographic camera in assessing the marginal integrity of a restoration becomes possible. The method overcomes some drawbacks of standard SEM or dye penetration testing. The results support the use of an IR camera in dentistry, for the diagnosis of micro-gaps at bio-interfaces.

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.

7 CFR 201.56-6 - Legume or pea family, Fabaceae (Leguminosae).

Code of Federal Regulations, 2013 CFR

2013-01-01

... necrosis or decay. (B) All other kinds: (1) Both missing and the seedling generally weak. (2) [Reserved... remaining attached. (B) Less than half of the original tissue free of necrosis or decay. (ii) Epicotyl: (A... necrosis or decay. (ii) Epicotyl: (A) Missing. (B) Less than one primary leaf. (C) Deep open cracks. (D...

7 CFR 201.56-6 - Legume or pea family, Fabaceae (Leguminosae).

Code of Federal Regulations, 2014 CFR

2014-01-01

... necrosis or decay. (B) All other kinds: (1) Both missing and the seedling generally weak. (2) [Reserved... remaining attached. (B) Less than half of the original tissue free of necrosis or decay. (ii) Epicotyl: (A... necrosis or decay. (ii) Epicotyl: (A) Missing. (B) Less than one primary leaf. (C) Deep open cracks. (D...

Assessment Matters: The Why and How of Cracking Open and Using Assessment Results

ERIC Educational Resources Information Center

Ahren, Chad; Ryan, Helen Grace; Massa-McKinley, Ryan

2008-01-01

The authors interviewed two dozen educators at several institutions who were responsible for administration of the survey on their campus to better understand how data from the National Survey of Student Engagement (NSSE) were (or were not) being used. The project managers they interviewed had successfully used strategies to make sense of NSSE…

Cracking Open the Curriculum: Lessons from a Community College

ERIC Educational Resources Information Center

Hanson, Chad; Amelotte, Patrick

2013-01-01

Community colleges comprise the largest single sector of the US higher education network. Forty percent of undergraduates attend one of our two-year schools. Some estimates suggest that, since the turnover is quicker than on four-year campuses, two-thirds of the students who attend a college at all attend a community college. For many of those…

Repairing Hard-to-Reach Cracks in Heat-Exchanger Tubes

NASA Technical Reports Server (NTRS)

Mills, R. C., Sr.; Duesberg, J.

1986-01-01

Inaccessible leaks repaired from accessible side of tube. Fish-Mouth insert placed in cut in leaky heat-exchanger tube. Insert welded or brazed to tube, and remaining open area of cut patched. Method developed for repairing leaks in nozzle coolant tubes of Space Shuttle main engine. Method also used on other types of tubular heat exchangers.

Detection of Microcracks in Trunnion Rods Using Ultrasonic Guided Waves

DTIC Science & Technology

2015-07-01

49 Figure 40. Larger second echo from PMN-PT vs. PZT transducer. ........................................................ 50...Figure. 41 Nonlinear crack simulations: two polished ends pulled together (top left), fatigued aluminum (bottom left), nut coupled and shims hammered... fatigued rods, can go through opening and closing variations during their deterioration. Microcracked rods need to be detected and quantified

HANDBOOK: SUB-SLAB DEPRESSURIZATION FOR LOW PERMEABILITY FILL MATERIAL DESIGN AND INSTALLATION OF A HOME RADON REDUCTION SYSTEM

EPA Science Inventory

Radon, a radioactive gas, comes from the natural decay of uranium. It moves to the earth's surface through tiny openings and cracks in soil and rocks. In outdoor air, radon is diluted to such low concentrations that it is usually nothing to worry about. However, radon can accumul...

Difficulties in Forecasting Flow Paths During the 2014-2015 Lava Flow Crisis at Kīlauea Volcano (Hawaíi)

NASA Astrophysics Data System (ADS)

Patrick, M. R.; Orr, T. R.; Trusdell, F.; Llewellin, E. W.; Kauahikaua, J. P.

2015-12-01

Kīlauea's East Rift Zone (ERZ) eruptive activity at Púu ´Ō´ō shifted to a new vent in June 2014, sparking a lava flow crisis that threatened critical infrastructure near the town of Pāhoa in east Hawaíi. The lava flow proved to be challenging to forecast because of the influence of ground cracks on flow direction, frequent fluctuations in lava supply, and the subtle interplay between ground slope and confining topography that prevented the flow from spreading laterally. After its onset, the "June 27th" flow, named informally for its start date, advanced northeast at up to several hundred m/day. The flow's path through heavy forest was forecast using steepest-descent paths derived from a digital elevation model (DEM). Flow path uncertainties were minimized using a multiple-run technique and built-in random DEM errors (modified from Favalli et al., 2005). In mid-August, the flow encountered and entered one of many deep, discontinuous ground cracks along Kīlauea's middle ERZ. The flow continued to advance out of sight in the crack, as inferred from a forward-progressing line of steam. A week later, lava spilled from the crack 1.3 km downslope, advancing along a different flow path than was forecast. By early September, the flow had entered and exited three more cracks sequentially, carrying the flow across slope, thus making flow path forecasts unreliable. Moreover, lava-occupied cracks dilated by up to 3 m. The lava accumulating in the ground cracks forced immense, but apparently mobile, blocks to shift. Thus, while an open crack was required to capture the lava, the lava was able to force its way beyond where the crack closed. In this way, the lava flow acted as an intruding dike. The flow eventually advanced beyond the area of cracks and onto a steepest-descent path that guided the flow toward the town of Pāhoa, where it destroyed one house, reached to within ~155 m of the main street in Pāhoa, and threatened the main highway and shopping center serving the east side of the Island of Hawaíi. The flow front stalled on March 13, 2015, owing to reservoir depressurization occurring at Kīlauea's summit. When the summit system recovered, activity withdrew to within ~9 km of the vent, ending the immediate threat to the Pāhoa area.

75 FR 5695 - Airworthiness Directives; PIAGGIO AERO INDUSTRIES S.p.A Model PIAGGIO P-180 Airplanes

Federal Register 2010, 2011, 2012, 2013, 2014

2010-02-04

... airworthiness information (MCAI) issued by an aviation authority of another country to identify and correct an... confirm the low stress level in that area, a reinforcement of the ``0'' pressure bulkhead is suggested to... aircraft from MSN 1106 up to 1189 could have the same cracks. Although calculations confirm the low stress...

Proceedings of the IAEA specialists` meeting on cracking in LWR RPV head penetrations

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

Pugh, C.E.; Raney, S.J.

1996-07-01

This report contains 17 papers that were presented in four sessions at the IAEA Specialists` meeting on Cracking in LWR RPV Head Penetrations held at ASTM Headquarters in Philadelphia on May 2-3, 1995. The papers are compiled here in the order that presentations were made in the sessions, and they relate to operational observations, inspection techniques, analytical modeling, and regulatory control. The goal of the meeting was to allow international experts to review experience in the field of ensuring adequate performance of reactor pressure vessel (RPV) heads and penetrations. The emphasis was to allow a better understanding of RPV materialmore » behavior, to provide guidance supporting reliability and adequate performance, and to assist in defining directions for further investigations. The international nature of the meeting is illustrated by the fact that papers were presented by researchers from 10 countries. There were technical experts present form other countries who participated in discussions of the results presented. This present document incorporates the final version of the papers as received from the authors. The final chapter includes conclusions and recommendations. Individual papers have been cataloged separately.« less

Crack-jump mechanism of microvein formation and its implications for stress cyclicity during extension fracturing

NASA Astrophysics Data System (ADS)

Caputo, Riccardo; Hancock, Paul L.

1998-11-01

It is well accepted and documented that faulting is produced by the cyclic behaviour of a stress field. Some extension fractures, such as veins characterised by the crack-seal mechanism, have also been presumed to result from repeated stress cycles. In the present note, some commonly observed field phenomena and relationships such as hackle marks and vein and joint spacing, are employed to argue that a stress field can also display cyclic behaviour during extensional fracturing. Indeed, the requirement of critical stress conditions for the occurrence of extensional failure events does not accord with the presence of contemporaneously open nearby parallel fractures. Therefore, because after each fracture event there is stress release within the surrounding volume of rock, high density sets of parallel extensional fractures also strongly support the idea that rocks undergo stress cyclicity during jointing and veining. A comparison with seismological data from earthquakes with dipole mechanical solutions, confirms that this process presently occurs at depth in the Earth crust. Furthermore, in order to explain dense sets of hair-like closely spaced microveins, a crack-jump mechanism is introduced here as an alternative to the crack-seal mechanism. We also propose that as a consequence of medium-scale stress cyclicity during brittle deformation, the re-fracturing of a rock mass occurs in either one or the other of these two possible ways depending on the ratio between the elastic parameters of the sealing material and those of the host rock. The crack-jump mechanism occurs when the former is stronger.

Effect of layerwise structural inhomogeneity on stress- corrosion cracking of steel tubes

NASA Astrophysics Data System (ADS)

Perlovich, Yu A.; Krymskaya, O. A.; Isaenkova, M. G.; Morozov, N. S.; Fesenko, V. A.; Ryakhovskikh, I. V.; Esiev, T. S.

2016-04-01

Based on X-ray texture and structure analysis data of the material of main gas pipelines it was shown that the layerwise inhomogeneity of tubes is formed during their manufacturing. The degree of this inhomogeneity affects on the tendency of tubes to stress- corrosion cracking under exploitation. Samples of tubes were cut out from gas pipelines located under various operating conditions. Herewith the study was conducted both for sections with detected stress-corrosion defects and without them. Distributions along tube wall thickness for lattice parameters and half-width of X-ray lines were constructed. Crystallographic texture analysis of external and internal tube layers was also carried out. Obtained data testifies about considerable layerwise inhomogeneity of all samples. Despite the different nature of the texture inhomogeneity of gas pipeline tubes, the more inhomogeneous distribution of texture or structure features causes the increasing of resistance to stress- corrosion. The observed effect can be explained by saturation with interstitial impurities of the surface layer of the hot-rolled sheet and obtained therefrom tube. This results in rising of lattice parameters in the external layer of tube as compared to those in underlying metal. Thus, internal layers have a compressive effect on external layers in the rolling plane that prevents cracks opening at the tube surface. Moreover, the high mutual misorientation of grains within external and internal layers of tube results in the necessity to change the moving crack plane, so that the crack growth can be inhibited when reaching the layer with a modified texture.

Hydrogen Environment Assisted Cracking of Modern Ultra-High Strength Martensitic Steels

NASA Astrophysics Data System (ADS)

Pioszak, Greger L.; Gangloff, Richard P.

2017-09-01

Martensitic steels (Aermet®100, Ferrium®M54™, Ferrium®S53®, and experimental CrNiMoWV at ultra-high yield strength of 1550 to 1725 MPa) similarly resist hydrogen environment assisted cracking (HEAC) in aqueous NaCl. Cracking is transgranular, ascribed to increased steel purity and rare earth addition compared to intergranular HEAC in highly susceptible 300M. Nano-scale precipitates ((Mo,Cr)2C and (W,V)C) reduce H diffusivity and the K-independent Stage II growth rate by 2 to 3 orders of magnitude compared to 300M. However, threshold K TH is similarly low (8 to 15 MPa√m) for each steel at highly cathodic and open circuit potentials. Transgranular HEAC likely occurs along martensite packet and {110}α'-block interfaces, speculatively governed by localized plasticity and H decohesion. Martensitic transformation produces coincident site lattice interfaces; however, a connected random boundary network persists in 3D to negate interface engineering. The modern steels are near-immune to HEAC when mildly cathodically polarized, attributed to minimal crack tip H production and uptake. Neither reduced Co and Ni in M54 and CrNiMoWV nor increased Cr in S53 broadly degrade HEAC resistance compared to baseline AM100. The latter suggests that crack passivity dominates acidification to widen the polarization window for HEAC resistance. Decohesion models predict the applied potential dependencies of K TH and d a/d t II with a single-adjustable parameter, affirming the importance of steel purity and trap sensitive H diffusivity.

Creep Strength Behavior of Boron Added P91 Steel and its Weld in the Temperature Range of 600-650°C

NASA Astrophysics Data System (ADS)

Swaminathan, J.; Das, C. R.; Baral, Jayashree; Phaniraj, C.; Ghosh, R. N.; Albert, S. K.; Bhaduri, A. K.

One of the promising ways for mitigation of Type IV cracking — a failure by cracking at the intercritical /fine grained heat affected zone, a life limiting problem in advanced 9-12 Cr ferritic steel weld like that of P91 is through modification of alloy composition by addition of boron. Addition of boron was observed to improve the microstructure at the weld zone and hence the creep strength. In the present work, boron (100 ppm with controlled nitrogen) added P91 steel after normalizing at 1050°C and 1150°C and tempered at 760°C were studied for the creep behavior in the base metal and welded condition in the temperature range of 600-650°C. Creep strength was characterized in terms of stress and temperature dependence of creep rate and rupture time. Weld creep life was reduced compared to the base metal with rupture occurring at the ICHAZ (Type IV crack). However at longer time (at lower stress levels) exposure creep crack moves from weld metal to HAZ (Type II crack). Rupture life was found to superior for the base and weld in the boron containing steel when higher normalizing temperature is used. Estimation of 105 h was attempted based on short term rupture data available and weld strength factors were calculated. Observed values are better for P91BH condition than the values for P91BLcondition as well as those available for P91 in open literature

Evaluation of damage in reinforced concrete bridge beams using acoustic emission technique

NASA Astrophysics Data System (ADS)

Vidya Sagar, R.; Raghu Prasad, B. K.; Sharma, Reema

2012-06-01

Acoustic emission (AE) testing is a well-known method for damage identification of various concrete structures including bridges. This article presents a method to assess damage in reinforced concrete (RC) bridge beams subjected to incremental cyclic loading. The specifications in the standard NDIS-2421 were used to classify the damage in RC bridge beams. Earlier researchers classified the damage occurring in bridge beams by using crack mouth opening displacement (CMOD) and AE released and proposed a standard (NDIS-2421: the Japanese Society for NonDestructive Inspection). In general, multiple cracks take place in RC beams under bending; therefore, utilisation of CMOD for crack detection may not be appropriate. In the present study, the damage in RC beams is classified by using the AE released, deflection, strains in steel and concrete, because the measurement of the strains in steel and concrete is easy and the codes of practice are specified for different limit states (IS-456:2000). The observations made in the present experimental study have some important practical applications in assessing the state of damage of concrete structural members.

Effects of programming and healing temperatures on the healing efficiency of a confined healable polymer composite

NASA Astrophysics Data System (ADS)

Yougoubare, Y. Quentin; Pang, Su-Seng

2014-02-01

In previous work, a biomimetic close-then-heal (CTH) healing mechanism was proposed and validated to repeatedly heal wide-open cracks in load carrying engineering structures by using constrained expansion of compression programmed thermoset shape memory polymers (SMPs). In this study, the effects on healing efficiencies of variation of temperature during both thermomechanical programming and shape recovery (healing) under three-dimensional (3D) confinement are evaluated. The polymer considered is a polystyrene shape memory polymer with 6% by volume of thermoplastic particle additives (copolyester) dispersed in the matrix. In addition to the programming and healing temperatures, some of the parameters investigated include the flexural strength, crack width and elemental composition at the crack interface. It is observed that while increase of the programming temperature is slightly beneficial to strength recovery, most of the strength recovered and damage repair are strongly dependent on the healing temperature. The best healing efficiency (63%) is achieved by a combination of a programming temperature above the glass transition temperature of the polymer and a healing temperature above the bonding point of the copolyester.

Accurate characterization of wafer bond toughness with the double cantilever specimen

NASA Astrophysics Data System (ADS)

Turner, Kevin T.; Spearing, S. Mark

2008-01-01

The displacement loaded double cantilever test, also referred to as the "Maszara test" and the "crack opening method" by the wafer bonding community, is a common technique used to evaluate the interface toughness or surface energy of direct wafer bonds. While the specimen is widely used, there has been a persistent question as to the accuracy of the method since the actual specimen geometry differs from the ideal beam geometry assumed in the expression used for data reduction. The effect of conducting the test on whole wafer pairs, in which the arms of cantilevers are wide plates rather than slender beams, is examined in this work using finite element analysis. A model is developed to predict the equilibrium shape of the crack front and to develop a corrected expression for calculating interface toughness from crack length measurements obtained in tests conducted on whole wafer pairs. The finite element model, which is validated through comparison to experiments, demonstrates that using the traditional beam theory-based expressions for data reduction can lead to errors of up to 25%.

Fragmentation of copper current collectors in Li-ion batteries during spherical indentation

NASA Astrophysics Data System (ADS)

Wang, Hsin; Watkins, Thomas R.; Simunovic, Srdjan; Bingham, Philip R.; Allu, Srikanth; Turner, John A.

2017-10-01

Large, areal, brittle fracture of copper current collector foils has been observed by 3D x-ray computed tomography (XCT) of a spherically indented Li-ion cell. This fracture is hidden and non-catastrophic to a degree because the graphite layers deform plastically, and hold the materials together so that the cracks in the foils cannot be seen under optical and electron microscopy. The cracking of copper foils could not be immediately confirmed when the cell is opened for post-mortem examination. However, 3D XCT on the indented cell reveals ;mud cracks; within the copper layer and an X-ray radiograph on a single foil of the Cu anode shows clearly that the copper foil has broken into multiple pieces. This failure mode of anodes in Li-ion cell has very important implications on the behavior of Li-ion cells under mechanical abuse conditions. The fragmentation of current collectors in the anode must be taken into consideration for the electrochemical responses which may lead to capacity loss and affect thermal runaway behavior of the cells.

Open access for operational research publications from low- and middle-income countries: who pays?

PubMed Central

Kumar, A. M. V.; Reid, A. J.; Van den Bergh, R.; Isaakidis, P.; Draguez, B.; Delaunois, P.; Nagaraja, S. B.; Ramsay, A.; Reeder, J. C.; Denisiuk, O.; Ali, E.; Khogali, M.; Hinderaker, S. G.; Kosgei, R. J.; van Griensven, J.; Quaglio, G. L.; Maher, D.; Billo, N. E.; Terry, R. F.; Harries, A. D.

2014-01-01

Open-access journal publications aim to ensure that new knowledge is widely disseminated and made freely accessible in a timely manner so that it can be used to improve people's health, particularly those in low- and middle-income countries. In this paper, we briefly explain the differences between closed- and open-access journals, including the evolving idea of the ‘open-access spectrum’. We highlight the potential benefits of supporting open access for operational research, and discuss the conundrum and ways forward as regards who pays for open access. PMID:26400799

Source Models of the June 17th, 2007 Kilauea Intrusion: Monte Carlo Optimization

NASA Astrophysics Data System (ADS)

Sinnett, D. K.; Montgomery-Brown, E. D.; Segall, P.; Miklius, A.; Poland, M.; Yun, S.; Zebker, H.

2007-12-01

Father's Day, 17 June 2007, marked the beginning of the 56th episode of the ongoing eruption of Kilauea volcano, Hawaii. The episode culminated in a short-lived eruption approximately 6 km west of Pu\\`{}u \\`{}O\\`{}o and 13 km southeast of Kilauea summit. The interruption of magma supply to, and withdrawal from, the reservoir beneath Pu\\`{}u \\`{}O\\`{}o caused cessation of activity and ~100 m of crater floor subsidence there. The continuous and campaign GPS, electronic tiltmeter, and seismic networks, as well as InSAR captured the episode in fine detail. Visual inspection of the data show subsidence at Kilauea summit and Pu\\`{}u \\`{}O\\`{}o, which fed the inflating dike. We began by modeling the intrusion with a Mogi source beneath Kilauea summit and a dislocation with uniform opening beneath the east rift zone embedded in an isotropic, homogenous, elastic, half space. We invert for the 12 source parameters (length, width, depth, dip, strike, horizontal position, and opening of the dike, and position, depth, and volume change of the Mogi source) using Monte Carlo optimization. The inversion used three component displacement data from 23 continuous and campaign GPS stations, diurnally and tidally filtered tilt from 6 stations, and an ENVISAT InSAR interferogram spanning 04/12/07 to 06/21/07 decimated using a quadtree algorithm. The optimum model included ~-4.1 * 106 m3 of volume loss from a reservoir 3 km beneath the summit, and a total dike volume of ~19*106 m3 (~4.84 km length x 2.45 km width x 1.6 m opening at 2.4 km depth). The discrepancy between summit volume loss and total dike volume suggests that other sources must have fed the dike. A crude estimate of volume loss from Pu\\`{}u \\`{}O\\`{}o is 8.5*106 m3 accounting for ~ 66% of the volume of the dike. The eruption site lies inside the eastern edge of the model, and ~0.5 km to the south of the best fit dike top. The best fit dike top parallels the northern margin of an area of ground cracking near Makaopuhui and terminates at its western margin near Mauna Ulu. The western termination is ~2.5 km east of the westernmost observed ground cracks. Within 95% bounds the dike top may intersect the eruption area and extend to all regions of ground cracking. It is also interesting to note that this dike is located in an area between the 1997 and 1999 intrusions. The best fit single dislocation model explains only 35% of the variance in the data. This is in part due to the inadequacies of a single planar dike with uniform opening to explain surface deformation and perhaps to inelastic deformation associated with ground cracking near the western edge of the dike. Models with distributed opening, in which the dike plane honors the optimization results as well as the region of decorrelation in the ENVISAT interferogram, explain 69% of the data (Montgomery-Brown et al., this session).