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1

The applications of 2D elasto-plastic stochastic finite element method in the field of fracture mechanics  

Microsoft Academic Search

In this paper the stochastic finite element method (SFEM) is applied to the field of fracture mechanics and cracked structures are analysed by using an elasto-plastic methodology. The main random variables considered are Young's modulus, Poisson's ratio and the crack size. The formulae of the mean and variance value of the J-integral for elasto-plastic deformation are discussed and an effective

Jian-Ping Zhao; Wen-Long Huang; Shu-Ho Dai

1998-01-01

2

A unified approach to the analysis and design of elasto-plastic structures with mechanical contact  

NASA Technical Reports Server (NTRS)

With structural design in mind, a new unified variational model has been developed which represents the mechanics of deformation elasto-plasticity with unilateral contact conditions. For a design problem formulated as maximization of the load carrying capacity of a structure under certain constraints, the unified model allows for a simultaneous analysis and design synthesis for a whole range of mechanical behavior.

Bendsoe, Martin P.; Olhoff, Niels; Taylor, John E.

1990-01-01

3

Elasto-plastic characteristics and mechanical properties of as-sprayed 8 mol% yttria-stabilized zirconia coating under nano-scales measured by nanoindentation  

NASA Astrophysics Data System (ADS)

The elasto-plastic characteristics and mechanical properties of as-received 8 mol% Y2O3-ZrO2 (8YSZ) coatings were studied by nanoindentation at ultra-low loads with a Berkovich indenter at room temperature. All experimental data including hardness H and elastic modulus E were analyzed by the Weibull statistical method due to the porous and heterogeneous nature of the tested samples. It was found that the hardness firstly exhibits interesting reverse indentation size effect, and then shows normal indentation size effect within different indentation scales. The average elastic modulus of 8YSZ was estimated as 214.8 ± 13.2 GPa. In order to reveal the elasto-plastic characteristics of 8YSZ at nano-scales, the distribution of resolved shear stresses underneath the indenter tip region was evaluated by Hertzian contact theory when the deformation behavior of 8YSZ changed from fully elastic to elasto-plastic stages. The results shed light on understanding possible foreign object damage mechanisms of thermal barrier coating systems.

Luo, J. M.; Dai, C. Y.; Shen, Y. G.; Mao, W. G.

2014-08-01

4

Mechanics of a contracting reservoir in an elastic half-space with an intervening visco-elasto-plastic layer  

Microsoft Academic Search

The extraction of fluids from porous rocks within the Earth's crust leads to localised volume strains. These in turn induce stress changes and displacements in the surrounding rock mass. The relationships between these processes are governed by the constitutive properties of the rocks. For the case of a poroelastic fluid reservoir in a linear-elastic matrix the mechanics are relatively well

A. Mossop; J. T. Fredrich

2004-01-01

5

Internal Stress in a Model Elasto-Plastic Fluid  

E-print Network

Plastic materials can carry memory of past mechanical treatment in the form of internal stress. We introduce a natural definition of the vorticity of internal stress in a simple two-dimensional model of elasto-plastic fluids, which generates the internal stress. We demonstrate how the internal stress is induced under external loading, and how the presence of the internal stress modifies the plastic behavior.

Takeshi Ooshida; Ken Sekimoto

2005-12-03

6

Pressure transients in damageable elasto-plastic pipes  

SciTech Connect

This paper is concerned with the analysis of pressure transients in damageable elasto-plastic piping systems. The fluid dynamics and pipewall deformation are modeled by the classical water hammer theory, whereas pipewall mechanical behavior is described by an internal variable constitutive theory. The resulting nonlinear hyperbolic problem is numerically approximated by means of a technique based on an additive decomposition of the operator together with the Glimm`s method and a special Euler-type time integration scheme. Examples concerning the structural integrity analysis of a reservoir-pipe-valve installation where hydraulic transients are generated by valve slam are presented to illustrate the applicability of both the theory and numerical method.

Freitas Rachid, F.B.; Costa Mattos, H.S. [Univ. Federal Fluminense, Niteroi, Rio de Janeiro (Brazil)

1995-11-01

7

Coupled THM processes in EDZ of crystalline rocks using an elasto-plastic cellular automaton  

NASA Astrophysics Data System (ADS)

This paper aims at a numerical study of coupled thermal, hydrological and mechanical processes in the excavation disturbed zones (EDZ) around nuclear waste emplacement drifts in fractured crystalline rocks. The study was conducted for two model domains close to an emplacement tunnel; (1) a near-field domain and (2) a smaller wall-block domain. Goodman element and weak element were used to represent the fractures in the rock mass and the rock matrix was represented as elasto-visco-plastic material. Mohr-Coulomb criterion and a non-associated plastic flow rule were adopted to consider the viscoplastic deformation in the EDZ. A relation between volumetric strain and permeability was established. Using a self-developed EPCA2D code, the elastic, elasto-plastic and creep analyses to study the evolution of stress and deformations, as well as failure and permeability evolution in the EDZ were conducted. Results indicate a strong impact of fractures, plastic deformation and time effects on the behavior of EDZ especially the evolution of permeability around the drift.

Pan, Peng-Zhi; Feng, Xia-Ting; Huang, Xiao-Hua; Cui, Qiang; Zhou, Hui

2009-05-01

8

Investigation of Optimal Seismic Design Methodology for Piping Systems Supported by Elasto-plastic Dampers  

NASA Astrophysics Data System (ADS)

In this study, the applicability of a previously developed optimal seismic design methodology, which can consider the structural integrity of not only piping systems but also elasto-plastic supporting devices, is studied for seismic waves with various frequency characteristics. This methodology employs a genetic algorithm and can search the optimal conditions such as the supporting location and the capacity and stiffness of the supporting devices. Here, a lead extrusion damper is treated as a typical elasto-plastic damper. Numerical simulations are performed using a simple piping system model. As a result, it is shown that the proposed optimal seismic design methodology is applicable to the seismic design of piping systems subjected to seismic waves with various frequency characteristics. The mechanism of optimization is also clarified.

Ito, Tomohiro; Michiue, Masashi; Fujita, Katsuhisa

9

Micropillar compression technique applied to micron-scale mudstone elasto-plastic deformation.  

SciTech Connect

Mudstone mechanical testing is often limited by poor core recovery and sample size, preservation and preparation issues, which can lead to sampling bias, damage, and time-dependent effects. A micropillar compression technique, originally developed by Uchic et al. 2004, here is applied to elasto-plastic deformation of small volumes of mudstone, in the range of cubic microns. This study examines behavior of the Gothic shale, the basal unit of the Ismay zone of the Pennsylvanian Paradox Formation and potential shale gas play in southeastern Utah, USA. Precision manufacture of micropillars 5 microns in diameter and 10 microns in length are prepared using an ion-milling method. Characterization of samples is carried out using: dual focused ion - scanning electron beam imaging of nano-scaled pores and distribution of matrix clay and quartz, as well as pore-filling organics; laser scanning confocal (LSCM) 3D imaging of natural fractures; and gas permeability, among other techniques. Compression testing of micropillars under load control is performed using two different nanoindenter techniques. Deformation of 0.5 cm in diameter by 1 cm in length cores is carried out and visualized by a microscope loading stage and laser scanning confocal microscopy. Axisymmetric multistage compression testing and multi-stress path testing is carried out using 2.54 cm plugs. Discussion of results addresses size of representative elementary volumes applicable to continuum-scale mudstone deformation, anisotropy, and size-scale plasticity effects. Other issues include fabrication-induced damage, alignment, and influence of substrate.

Michael, Joseph Richard; Chidsey, Thomas (Utah Geological Survey, Salt Lake City, UT); Heath, Jason E.; Dewers, Thomas A.; Boyce, Brad Lee; Buchheit, Thomas Edward

2010-12-01

10

A new thermo-elasto-plasticity constitutive equation for crystals  

NASA Astrophysics Data System (ADS)

Based on the crystal plasticity theory and interatomic potential, in this paper a new thermo-elasto-plasticity constitutive model is proposed to study the behavior of metal crystals at finite temperature. By applying the present constitutive model, the stress-strain curves under uniaxial tension at different temperatures are calculated for the typical crystal Al, and the calculated results are compared with the experimental results. From the comparisons, it can be seen that the present theory has the capability to describe the thermo-elasto-plastic behavior of metal crystals at finite temperature through a concise and explicit calculation process.

Chen, Cen; Tang, QiHeng; Wang, TzuChiang

2015-01-01

11

Small strain elasto-plastic multiphase-field model  

NASA Astrophysics Data System (ADS)

A small strain plasticity model, based on the principles of continuum mechanics, is incorporated into a phase-field model for heterogeneous microstructures in polycrystalline and multiphase material systems (Nestler et al., Phys Rev 71:1-6, 2005). Thereby, the displacement field is computed by solving the local momentum balance dynamically (Spatschek et al., Phys Rev 75:1-14, 2007) using the finite difference method on a staggered grid. The elastic contribution is expressed as the linear approximation according to the Cauchy stress tensor. In order to calculate the plastic strain, the Prandtl-Reuss model is implemented consisting of an associated flow rule in combination with the von Mises yield criterion and a linear isotropic hardening approximation. Simulations are performed illustrating the evolution of the stress and plastic strain using a radial return mapping algorithm for single phase system and two phase microstructures. As an example for interface evolution coupling with elasto-plastic effects, we present crack propagation simulations in ductile material.

Schneider, Daniel; Schmid, Stefan; Selzer, Michael; Böhlke, Thomas; Nestler, Britta

2015-01-01

12

Micropillar Compression Technique Applied to Micron-Scale Mudstone Elasto-Plastic Deformation  

NASA Astrophysics Data System (ADS)

Mudstone mechanical testing is often limited by poor core recovery and sample size, preservation and preparation issues, which can lead to sampling bias, damage, and time-dependent effects. A micropillar compression technique, originally developed by Uchic et al. 2004, here is applied to elasto-plastic deformation of small volumes of mudstone, in the range of cubic microns. This study examines behavior of the Gothic shale, the basal unit of the Ismay zone of the Pennsylvanian Paradox Formation and potential shale gas play in southeastern Utah, USA. Precision manufacture of micropillars 5 microns in diameter and 10 microns in length are prepared using an ion-milling method. Characterization of samples is carried out using: dual focused ion - scanning electron beam imaging of nano-scaled pores and distribution of matrix clay and quartz, as well as pore-filling organics; laser scanning confocal (LSCM) 3D imaging of natural fractures; and gas permeability, among other techniques. Compression testing of micropillars under load control is performed using two different nanoindenter techniques. Deformation of 0.5 cm in diameter by 1 cm in length cores is carried out and visualized by a microscope loading stage and laser scanning confocal microscopy. Axisymmetric multistage compression testing and multi-stress path testing is carried out using 2.54 cm plugs. Discussion of results addresses size of representative elementary volumes applicable to continuum-scale mudstone deformation, anisotropy, and size-scale plasticity effects. Other issues include fabrication-induced damage, alignment, and influence of substrate. This work is funded by the US Department of Energy, Office of Basic Energy Sciences. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Dewers, T. A.; Boyce, B.; Buchheit, T.; Heath, J. E.; Chidsey, T.; Michael, J.

2010-12-01

13

The elasto-plastic response of underground excavations in rock masses that satisfy the Hoek–Brown failure criterion  

Microsoft Academic Search

This paper is intended to illustrate the relationship between the Hoek–Brown parameters describing the strength of rock masses and the mechanical response of underground openings.A formulation of the elasto-plastic behavior of rock in terms of the Hoek–Brown criterion is presented. The analysis assumes that the joint system present in the rock mass has no preferred orientation so that the medium

C. Carranza-Torres; C. Fairhurst

1999-01-01

14

Elasto-plastic model with second order defect density tensor  

NASA Astrophysics Data System (ADS)

The paper deals with a second order finite elasto-plastic model, which involves the defect density tensor, as a measure of the extra material defects existing in the damaged microstructure. The material behaviour is described with respect to an anholonomic configuration, which is introduced through the second order plastic deformation, consisting in plastic distortion and plastic connection. The defect density tensor enters the expression of the plastic connection through its gradient and represents a measure of non-metricity. The constitutive and evolution equations are derived to be compatible with the free energy imbalance. The evolution equation for the defect density tensor is non-local and coupled with the plastic distortion.

Cleja-?igoiu, Sanda

2011-05-01

15

Visco-elasto-plastic rheology effects on free subduction dynamics  

NASA Astrophysics Data System (ADS)

Subduction dynamics on Earth is a typical three dimensional problem. Complex mantle flow can develop around and above the slabs. In addition, the 3D geometry of slabs (curvature, dip angle) evolves with time and can often be quite complicated. This last aspect strongly depends on their rheological properties and on the force balance in the bending region. Therefore understanding how bending processes, in a lithosphere with Earth-like rheology, affect the dynamics of subduction is key to compare numerical results with Earth subduction geometries. Scaling analysis based on 2D visco-elasto-plastic models has shown that one needs to take into account both the elastic and viscous strengths in order to correctly capture the bending effects on sinking velocities and dip angles. But most 3D numerical models developed so far use a visco-plastic rheology for slabs neglecting the role of elasticity in bending. We developed a new technique to model lithosphere-mantle interaction in 3D where lithosphere is a visco-elasto-plastic solid and mantle is a linear viscous fluid. At each iteration, a finite element solver is used to solve for creep in the oceanic plate and compute plate displacements. These displacements are then used as input in a boundary element software to calculate the full mantle circulation. New mantle tractions are then re-injected in the finite element software via a set of dashpots positioned around the oceanic plate. This technique has the advantage to calculate the full mantle drag only on the plate outer surface thus reducing considerably the computational time. We use this coupled approach to investigate the role of plate width and buoyancy on subduction behaviour.

Fourel, L.; Morra, G.; Goes, S.

2012-04-01

16

Investigation of Optimal Seismic Design Methodology for Piping Systems Supported by Elasto-Plastic Dampers  

NASA Astrophysics Data System (ADS)

In this study, the optimal seismic design methodology that can consider the structural integrity of not only the piping systems but also elasto-plastic supporting devices is developed. This methodology employs a genetic algorithm and can search the optimal conditions such as the supporting location, capacity and stiffness of the supporting devices. Here, a lead extrusion damper is treated as a typical elasto-plastic damper. Four types of evaluation functions are considered. It is found that the proposed optimal seismic design methodology is very effective and can be applied to the actual seismic design for piping systems supported by elasto-plastic dampers. The effectiveness of the evaluation functions is also clarified.

Ito, Tomohiro; Michiue, Masashi; Fujita, Katsuhisa

17

An Elasto-Plastic Analysis of Solids by the Local Meshless Method Based on Mls  

NASA Astrophysics Data System (ADS)

A pseudo-elastic local meshless formulation is developed in this paper for elasto-plastic analysis of solids. The moving least square (MLS) is used to construct the meshless shape functions, and the weighted local weak-form is employed to derive the system of equations. Hencky's total deformation theory is applied to define the effective Young's modulus and Poisson's ratio in the nonlinear analysis, which are obtained in an iterative manner using the strain controlled projection method. Numerical studies are presented for the elasto-plastic analysis of solids by the newly developed meshless formulation. It has demonstrated that the present pseudo-elastic local meshless approach is very effective for the elasto-plastic analysis of solids.

Gu, Y. T.

18

Influence of interphase layer on the overall elasto-plastic behaviors of HA/PEEK biocomposite.  

PubMed

A three-dimensional finite element unit cell model has been designed and constructed for studying mechanical properties of hydroxyapatite (HA) reinforced polyetheretherketone (PEEK) biocomposite. The model consists of an elastic-brittle HA spherical particle, an elasto-plastic matrix and an interphase layer between the particle and the matrix. The interphase layers with four different kinds of material behaviors have been taken into consideration to examine their effects on the overall properties of the composite. The damage evolution in the matrix and the interphase layer, and the interface failure, were also taken into account. Some other factors, such as mesh sensitivity, loading velocity and mass scale scheme, were also discussed in this investigation. A general-purpose finite element software package, ABAQUS, incorporated with a user-defined material subroutine, was used to perform the analysis. The predicted results were compared with the experimental data obtained from existing literatures. The results predicted by using the cell model with consideration of the matrix degradation and the effects of the damage and failure on the interphase layer are in good agreement with the experimental ones. Hence, the suitability of our proposed cell model incorporated with an appropriate type of the interphase layer for modeling the mechanical properties of the particulate biocomposite could be verified. PMID:15130721

Fan, J P; Tsui, C P; Tang, C Y; Chow, C L

2004-10-01

19

Analysis of the elasto-plastic response of a polygonal packing Fernando Alonso Marroqu n and Hans Herrmann  

E-print Network

is examined in the pre- failure regime. Besides the Young modulus and the Poisson ratio, an additionalAnalysis of the elasto-plastic response of a polygonal packing Fernando Alonso Marroqu#19;#16;n-dimensional packed samples of polygons using molecular dynamics simulation. The incremental elasto-plastic response

Luding, Stefan

20

Elasto-plasticity critical corrosive ratio model for RC structure corrosive expanding crack  

Microsoft Academic Search

The parameter of filling expanding ratio n, plasticity factor k\\u000a 1 and deformation parameter k\\u000a 2 is raised, and then the elasto-plasticity critical corrosive ratio model for RC structure corrosive expanding crack based\\u000a on elasto-plasticity theory is constructed in this paper. The influences of parameters such as filling expansion ratio n, plasticity factor k\\u000a 1, deformation parameter k\\u000a 2, Poisson

Yueshun Chen; Yiyan Lu; Li Liu

2007-01-01

21

Polycarbonate as an elasto-plastic material model for simulation of the microstructure hot imprint process.  

PubMed

The thermal imprint process of polymer micro-patterning is widely applied in areas such as manufacturing of optical parts, solar energy, bio-mechanical devices and chemical chips. Polycarbonate (PC), as an amorphous polymer, is often used in thermoforming processes because of its good replication characteristics. In order to obtain replicas of the best quality, the imprint parameters (e.g., pressure, temperature, time, etc.) must be determined. Therefore finite element model of the hot imprint process of lamellar periodical microstructure into PC has been created using COMSOL Multiphysics. The mathematical model of the hot imprint process includes three steps: heating, imprinting and demolding. The material properties of amorphous PC strongly depend on the imprint temperature and loading pressure. Polycarbonate was modelled as an elasto-plastic material, since it was analyzed below the glass transition temperature. The hot imprint model was solved using the heat transfer and the solid stress-strain application modes with thermal contact problem between the mold and polycarbonate. It was used for the evaluation of temperature and stress distributions in the polycarbonate during the hot imprint process. The quality of the replica, by means of lands filling ratio, was determined as well. PMID:23974153

Narijauskait?, Birut?; Palevi?ius, Arvydas; Gaidys, Rimvydas; Janušas, Giedrius; Sakalys, Rokas

2013-01-01

22

ElastoPlastic Fatigue Life Improvement of Bolted Joints and Introducing FBI Method  

Microsoft Academic Search

The failures of bolted joints, which are the basic and popular fasteners in industry, are mostly due to fatigue. This paper presents an effective method to improve the elasto-plastic fatigue life of a bolt. A complete bolted joint is considered and modeled with the finite element approach to calculate stress and strain. The code based on crack initiation theory is

Peyman Honarmandi; Jean W. Zu; Kamran Behdinan

2005-01-01

23

Elasto-plastic analysis of interface layers for fiber reinforced metal matrix composites  

NASA Technical Reports Server (NTRS)

The mismatch in coefficients of thermal expansion (CTE) of fiber and matrix in metal matrix composites reinforced with ceramic fibers induces high thermal stresses in the matrix. Elasto-plastic analyses - with different degrees of simplification and modelization - show that an interface layer with a sufficiently high CTE can reduce the tensile hoop stress in the matrix substantially.

Doghri, I.; Leckie, F. A.

1991-01-01

24

Theoretical and numerical aspects of fluid-saturated elasto-plastic soils  

SciTech Connect

The theoretical and numerical treatment of fluid-saturated porous solid materials generally falls into the category of porous media models, which are described within the framework of the classical theory of mixtures extended by the concept of volume fractions (porous media theories). In particular, this concept allows for the description of saturated, unsaturated and empty porous matrix materials, thus offering a well-founded theoretical background for a lot of engineering problems occurring, for instance, in the fields of geomechanics (soil and rock mechanics as well as glacier and rock ice mechanics), oil producing industries, sintering technologies, biomechanics, etc. In the present contribution, theoretical and numerical studies are outlined to describe a two-phase material composed of an incompressible elasto-plastic soil matrix saturated by an incompressible viscous pore fluid. In this context, the phenomenon of phase incompressibility is well known as a microscopic effect not implying bulk incompressibility in the macro regime. This is seen from the fact that even if the material density functions of the individual constituents are constant during deformation, the corresponding bulk densities can still change through changes in the volume fractions. Within the framework of a pure mechanical theory, constitutive equations are given for both the solid and the fluid partial stress tensors and for the interaction force acting between the two materials. Concerning the porous soil matrix, the elastic properties are described by an elasticity law of Hookean type, while the plastic range is governed by a {open_quote}single surface{close_quote} yield function exhibiting a smooth and closed shape in the principal stress space together with a non-associated flow rule. The viscosity effects of the pore fluid are included in the fluid stress tensor and in the drag force.

Ehlers, W. [Technische Hochschule Darmstadt (Germany)

1995-12-31

25

REVERSING CYCLIC ELASTO-PLASTIC DEMANDS ON STRUCTURES DURING STRONG MOTION EARTHQUAKE EXCITATION.  

USGS Publications Warehouse

Using the horizontal components from El Centro 1940, Taft 1952, and 4 accelerograms from the San Fernando earthquake of 2/9/71, the time history of the elasto-plastic displacement response was calculated for oscillators having periods within the range of 1 to 6 s and ductility factors within the range of 3 to 6. The Nth largest peak of the elasto-plastic response (N equals 2,4,8,16), when expressed as a percentage of maximum response (that is, N equals 1), is fairly independent of period within our period range. When considering only plastic peaks occurring, sometimes in a one-directional group of peaks, in the reverse direction from the preceding plastic peak, the amplitude of the Nth reversing plastic peak is similar to the Nth elastic peak, regardless of the ductility factor.

Perez, V.; Brady, A.G.; Safak, E.

1986-01-01

26

The parametric process design of tension levelling with an elasto-plastic finite element method  

Microsoft Academic Search

This paper is concerned with a simulation-based process design for the tension levelling of metallic strips based on elasto-plastic finite element analysis with reduced integration and hour-glass control. The tension levelling process is performed to elongate the strip plastically in combination with tensile and bending strain in a controlled manner so that all longitudinal fibers in the strip have an

Hoon Huh; Hyoung Wook Lee; Sang Rae Park; Geun Young Kim; Sung Hyun Nam

2001-01-01

27

Analysis of the elasto-plastic response of a polygonal packing  

Microsoft Academic Search

We investigate the constitutive response of two-dimensional packed samples of polygons using molecular dynamics simulation. The incremental elasto-plastic response is examined in the pre-failure regime. Besides the Young modulus and the Poisson ratio, an additional parameter must be included, which takes into account the anisotropy of the elastic response. The plastic deformations are described by the introduction of the yield

F. Alonso-Marroquin; H. J. Herrmann; S. Luding

2002-01-01

28

The p-version of the finite element method in incremental elasto-plastic analysis  

NASA Technical Reports Server (NTRS)

Whereas the higher-order versions of the finite elements method (the p- and hp-version) are fairly well established as highly efficient methods for monitoring and controlling the discretization error in linear problems, little has been done to exploit their benefits in elasto-plastic structural analysis. Aspects of incremental elasto-plastic finite element analysis which are particularly amenable to improvements by the p-version is discussed. These theoretical considerations are supported by several numerical experiments. First, an example for which an analytical solution is available is studied. It is demonstrated that the p-version performs very well even in cycles of elasto-plastic loading and unloading, not only as compared to the traditional h-version but also in respect to the exact solution. Finally, an example of considerable practical importance - the analysis of a cold-worked lug - is presented which demonstrates how the modeling tools offered by higher-order finite element techniques can contribute to an improved approximation of practical problems.

Holzer, Stefan M.; Yosibash, Zohar

1993-01-01

29

A numerical model for the thermo-elasto-plastic behaviour of a material  

NASA Technical Reports Server (NTRS)

This paper presents a numerical model for the thermo-elasto-plastic behavior of an isotropic material. The model is based on the assumption that the yielding of the material obeys von Mises distortion energy theory and the material exhibits isotropic strain hardening. This unique model can be used both for isothermal and non-isothermal cases. The original formulation for the non-isothermal three-dimensional case has been specialized for plane stress conditions and the equations for the computation of warping and thickness change are provided. The finite element implementation of this model is also outlined.

Ray, Sujit K.; Utki, Senol

1989-01-01

30

A review of developments in the theory of elasto-plastic flow  

NASA Technical Reports Server (NTRS)

The theory of elasto-plastic flow is developed so that it may accommodate features such as work-hardening, anisotropy, plastic compressibility, non-continuous loading including local or global unloading, and others. A complete theory is given in quasi-linear form; as a result, many useful attributes are accessible. Several integral theorems may be written, finite deformations may be incorporated, and efficient methods for solving problems may be developed; these and other aspects are described in some detail. The theory is reduced to special forms for 2-space, and extensive experience in solving such problems is cited.

Swedlow, J. L.

1973-01-01

31

Orthotropic elasto-plastic behavior of AS4/APC-2 thermoplastic composite in compression  

NASA Technical Reports Server (NTRS)

Uniaxial compression tests were performed on off-axis coupon specimens of unidirectional AS4/APC-2 thermoplastic composite at various temperatures. The elasto-plastic and strength properties of AS4/APC-2 composite were characterized with respect to temperature variation by using a one-parameter orthotropic plasticity model and a one-parameter failure criterion. Experimental results show that the orthotropic plastic behavior can be characterized quite well using the plasticity model, and the matrix-dominant compressive strengths can be predicted very accurately by the one-parameter failure criterion.

Sun, C. T.; Rui, Y.

1989-01-01

32

Elasto-Plastic Analysis of Tee Joints Using HOT-SMAC  

NASA Technical Reports Server (NTRS)

The Higher Order Theory - Structural/Micro Analysis Code (HOT-SMAC) software package is applied to analyze the linearly elastic and elasto-plastic response of adhesively bonded tee joints. Joints of this type are finding an increasing number of applications with the increased use of composite materials within advanced aerospace vehicles, and improved tools for the design and analysis of these joints are needed. The linearly elastic results of the code are validated vs. finite element analysis results from the literature under different loading and boundary conditions, and new results are generated to investigate the inelastic behavior of the tee joint. The comparison with the finite element results indicates that HOT-SMAC is an efficient and accurate alternative to the finite element method and has a great deal of potential as an analysis tool for a wide range of bonded joints.

Arnold, Steve M. (Technical Monitor); Bednarcyk, Brett A.; Yarrington, Phillip W.

2004-01-01

33

Constraints on bed scale fracture chronology with a FEM mechanical model of folding: The case of Split Mountain (Utah, USA)  

NASA Astrophysics Data System (ADS)

A technique is presented for improving the structural analysis of natural fractures development in large scale fold structures. A 3D restoration of a fold provides the external displacement loading conditions to solve, by the finite element method, the forward mechanical problem of an idealized rock material with a stress-strain relationship based on the activation of pervasive fracture sets. In this elasto-plasticity constitutive law, any activated fracture set contributes to the total plastic strain by either an opening or a sliding mode of rock failure. Inherited versus syn-folding fracture sets development can be studied using this mechanical model. The workflow of this methodology was applied to the Weber sandstone formation deformed by forced folding at Split Mountain Anticline, Utah for which the different fracture sets were created and developed successively during the Sevier and the syn-folding Laramide orogenic phases. The field observations at the top stratigraphic surface of the Weber sandstone lead to classify the fracture sets into a pre-fold WNW-ESE fracture set, and a NE-SW fracture set post-dating the former. The development and relative chronology of the fracture sets are discussed based on the geomechanical modeling results. Starting with a 3D restoration of the Split Mountain Anticline, three fold-fracture development models were generated, alternately assuming that the WNW-ESE fracture set is either present or absent prior to folding process. Depending on the initial fracture configuration, the calculated fracture patterns are markedly different, showing that assuming a WNW-ESE joint set to predate the fold best correlates with field observations. This study is a first step addressing the complex problem of identification of fold-related fracturing events using an elementary concept of rock mechanics. When tight to complementary field observations, including petrography, diagenesis and burial history, the approach can be used to better constrain fractured reservoir characterization.

Sassi, W.; Guiton, M. L. E.; Leroy, Y. M.; Daniel, J.-M.; Callot, J.-P.

2012-11-01

34

Elasto-Plastic Simulation Concepts For Profile Transfer And Flatness Prediction In Flat Hot Rolling  

NASA Astrophysics Data System (ADS)

For the prediction of the material flow behavior of wide strips in hot and cold rolling, highly sophisticated procedures are essential, which are able to couple the deformation of the strip and the elastic response of the rolls. Especially for thin, wide strips, where the aspect ratio width over thickness is extremely unfavorable for standard FEM-calculations, the determination of profile transfer and flatness obviously leads to extremely high calculation times with commercial FEM-programs. Therefore, a tailor-made FEM-code for the efficient simulation of the elasto-plastic material flow inside the roll gap has been developed. The underlying formalism for the strip-routines is based on pseudo-steady-state streamline-update techniques for the stress-field, coupled iteratively with the principle of virtual power for the determination of the velocity field and the contact stress distribution between strip and work roll. Coupling of the strip models with the routines for elastic roll stack deflection is a precondition to get reliable results concerning profile transfer and residual stresses inside the strip, which allows the prediction of flatness defects, such as buckling. Of particular interest is the dependence of the longitudinal stress distributions and of the corresponding specific rolling force-distribution across the strip width on the underlying constitutive elasto-viscoplastic laws including rate-dependence, work hardening, softening and creep effects.

Kainz, Alexander; Parteder, Erik; Widder, Markus; Zeman, Klaus

2007-05-01

35

ARMA 10-473 Comparison between elasto-plastic and rigid-plastic cohesive surface  

E-print Network

for presentation at the 44th US Rock Mechanics Symposium and 5th U.S.-Canada Rock Mechanics Symposium, held in Salt and embedded strong discontinuity finite element implementation of rock fracture Regueiro, R.A. and Yu, S, CO 80309, U.S.A. Copyright 2010 ARMA, American Rock Mechanics Association This paper was prepared

Regueiro, Richard A.

36

Elasto-Plastic Constitutive Behavior in Three Lithofacies of the Cambrian Mt. Simon Sandstone, Illinois Basin, USA  

NASA Astrophysics Data System (ADS)

The Mt. Simon Formation, a basal Cambrian sandstone underlying the Illinois Basin in the Central US, is a target for underground storage and waste injection which require an assessment of geomechanical behavior. The range of depositional environments, from braided streams and minor eolean features in the lower Mt Simon, to tidally-influenced near- and on-shore sands in the upper Mt. Simon, yield a heterogeneous formation with a range in porosity, permeability, and mechanical properties. We examine the experimental deformational behavior of three distinct Mt. Simon lithofacies via axisymmetric compressional testing. Initial yielding is confirmed with acoustic emissions in many of the tests and failure envelopes are determined for each lithofacies. The evolution of (assumed) isotropic elastic moduli are examined during testing by use of unload-reload cycles, which permit the separation of total measured strains into elastic and plastic (permanent) strains. The upper Mt Simon samples deform largely elastically at stresses encountered in the Illinois Basin, with very little modulus degradation. The lower Mt. Simon facies are weaker and deform plastically, with varying amounts of modulus degradation. Results are interpreted via petrographic observation of textural contrasts. This range in constitutive response is captured up to failure with a phenomenological elasto-plasticity model. Essential aspects to describe observed behavior used in the model include non-associative plasticity, stress-invariant dependent failure, an elliptical cap surface capturing shear effects on pore collapse, kinematic and isotropic hardening, nonlinear elasticity and elastic-plastic coupling, among other features. Static moduli derived from laboratory tests are compared to dynamic moduli from wellbore log response which can allow experimental results and model to be extrapolated to Mt. Simon occurrences across the basin. This work was funded in part by the Center for Frontiers of Subsurface Energy Security, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001114 and by the Department of Energy Office of Electricity. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Dewers, T.; Newell, P.; Broome, S. T.; Heath, J. E.; Bauer, S. J.

2012-12-01

37

A new uniformly valid asymptotic integration algorithm for elasto-plastic creep and unified viscoplastic theories including continuum damage  

NASA Technical Reports Server (NTRS)

A new scheme to integrate a system of stiff differential equations for both the elasto-plastic creep and the unified viscoplastic theories is presented. The method has high stability, allows large time increments, and is implicit and iterative. It is suitable for use with continuum damage theories. The scheme was incorporated into MARC, a commercial finite element code through a user subroutine called HYPELA. Results from numerical problems under complex loading histories are presented for both small and large scale analysis. To demonstrate the scheme's accuracy and efficiency, comparisons to a self-adaptive forward Euler method are made.

Chulya, Abhisak; Walker, Kevin P.

1991-01-01

38

Elasto-plastic flow in cracked bodies using a new finite element model. Ph.D. Thesis  

NASA Technical Reports Server (NTRS)

Cracked geometries were studied by finite element techniques with the aid of a new special element embedded at the crack tip. This model seeked to accurately represent the singular stresses and strains associated with the elasto-plastic flow process. The present model was not restricted to a material type and did not predetermine a singularity. Rather the singularity was treated as an unknown. For each step of the incremental process the nodal degrees of freedom and the unknown singularity were found through minimization of an energy-like functional. The singularity and nodal degrees of freedom were determined by means of an iterative process.

Karabin, M. E., Jr.

1977-01-01

39

A new uniformly valid asymptotic integration algorithm for elasto-plastic-creep and unified viscoplastic theories including continuum damage  

NASA Technical Reports Server (NTRS)

A new scheme to integrate a system of stiff differential equations for both the elasto-plastic creep and the unified viscoplastic theories is presented. The method has high stability, allows large time increments, and is implicit and iterative. It is suitable for use with continuum damage theories. The scheme was incorporated into MARC, a commercial finite element code through a user subroutine called HYPELA. Results from numerical problems under complex loading histories are presented for both small and large scale analysis. To demonstrate the scheme's accuracy and efficiency, comparisons to a self-adaptive forward Euler method are made.

Chulya, A.; Walker, K. P.

1989-01-01

40

Fluid focusing and breaching of low permeability layers in reacting and visco-elasto-plastically deforming reservoir rocks  

NASA Astrophysics Data System (ADS)

The on-going injection of one million tons per year of CO2 into the Utsira sand at Sleipner is used as an example for a highly successful CO2 storage operation. Even at Sleipner, however, we observe features that are not straightforward to explain and quantify with exiting models. One such feature is the so- called chimneys that show up in the time laps seismic images. They are zones of disturbed layering that cut nearly vertically through the interbedded thin shale layers in the reservoir sands, not unlink the frequently observed pipe structures due to fluid venting. These chimneys have been ascribed to artefacts in the data or pre-existing fractures or pipes, and these explanations are difficult to rule out. If we take the seismic interpretations at face value, however, then the data suggest that the intensity and extent of the chimneys changes through time. The extent and thickness of the observed plume supports that the injected CO2 is migrating through focused zones in the shales from the well at the bottom of the reservoir to the top layer immediately below the caprock much faster than predicted by Darcy flow through intact, low permeable shale layers. We developed a fully coupled numerical model for fluid flow through a reacting and deforming porous rock. Reactions may be upscaled to add a viscous component to the rheology, or be modelled explicitly. In laboratory experiments, viscous compaction has been shown to take place in typical reservoir rocks due to the high reactivity of CO2-rich brine. Other experimental studies show that unconsolidated sands, such as the Utsira sand, and clay-rich shales follow a visco-plastic flow law rather than behaving as purely poro-elastically. Hence, viisco-elasto-plastic deformation of the porous matrix is taken into account in our model and fluid focusing may occur due to non-linear couplings between porosity and permeability and viscosity. This phenomenon is known as a porosity wave. A non-linear viscous rheology (or viscous compaction in combination with plastic failure in decompaction) lead to the formation of high-porosity channels or jets. The formation of channels (or chimneys) in layered sediments depends on the rheological bulk behaviour of the rocks, the spacing and thickness of the low permeability layers, the permeability contrast between the shale and the sand and the effective pressure in the reservoir. We will present examples of models that predict the formation of high porosity channels at Sleipner using realistic input parameters. These channels also develop if the low-permeability layers are discontinuous. Another implication of our modelling results is that low permeability caprock may fail as a barrier to flow if significant fluid overpressure is build up by the injection and/or due to viscous compaction. These results may be applicable to observations at In Salah. However, the temporal and spatial scales for the onset of focused flow in porosity waves strongly depend on a set of poorly constrained parameters. Thus, more experimental and numerical work and comparison to field data is needed to correctly assess these coupled reaction-deformation-flow processes.

Simon, Nina S. C.

2013-04-01

41

The mechanism of fracture  

SciTech Connect

This book presents the papers given at a conference on the fracture mechanics of metals. Topics considered at the conference included microcrack mechanics, pressurized thermal shock behavior of LWR pressure vessels, stress intensity factors, submerged arc welding, weldments in power plants, pipeline weld quality, natural gas tanks, cast iron for spent nuclear fuel shipping casks, pipe ruptures, physical radiation effects, pressure tubes, hydrogen embrittlement, critical flaw size curves, and the fracture mechanics of steels in turbines of power stations.

Goel, V.S.

1985-01-01

42

Full-Field Strain Measurement On Titanium Welds And Local Elasto-Plastic Identification With The Virtual Fields Method  

SciTech Connect

One of the main problems in welding is the microstructural transformation within the area affected by the thermal history. The resulting heterogeneous microstructure within the weld nugget and the heat affected zones is often associated with changes in local material properties. The present work deals with the identification of material parameters governing the elasto--plastic behaviour of the fused and heat affected zones as well as the base material for titanium hybrid welded joints (Ti6Al4V alloy). The material parameters are identified from heterogeneous strain fields with the Virtual Fields Method. This method is based on a relevant use of the principle of virtual work and it has been shown to be useful and much less time consuming than classical finite element model updating approaches applied to similar problems. The paper will present results and discuss the problem of selection of the weld zones for the identification.

Tattoli, F.; Casavola, C. [Dipartimento di Ingegneria Meccanica e Gestionale, Politecnico di Bari, Viale Japigia 182, 70126 Bari (Italy); Pierron, F.; Rotinat, R.; Pappalettere, C. [Ecole Nationale Superieure d'Arts et Metiers, Rue Saint Dominique, BP508, 51006 Chalons en Champagne (France)

2011-01-17

43

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

NASA Technical Reports Server (NTRS)

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

Jones, D. P.

1972-01-01

44

A finite-element marker-in-cell simulation code for thermo-chemically coupled magma dynamics in a visco-elasto-plastic host rock  

NASA Astrophysics Data System (ADS)

Many prominent geodynamic scenarios such as subduction zones, plate collision and orogeny formation, mid-ocean ridges, continental rifting, etc. involve a significant amount of active magmatism. However, many numerical simulations used to address related questions focus on the deformation of the solid rock phase only, sometimes taking into account the magma dynamics in the form of some parameterized weakening mechanism. On the other hand, simulations specifically developed for magma dynamics problems have largely been restricted to the context of mantle dynamics and are not designed to deal with brittle tectonic deformation of the rock matrix as it occurs in the lithosphere and crust. Here, a 2-D finite-element marker-in-cell numerical method is presented capable of simulating thermally and compositionally coupled two-phase flow problems in a realistically deforming mantle, lithosphere and crust. The modeling approach is based on a set of well accepted equations for the conservation of mass, momentum, energy and composition, completed by constitutive laws for visco-elasto-plastic shear and compaction stresses and a much simplified yet thermodynamically consistent melting model depending on temperature, pressure and composition. The simulation code is written in Matlab and is capable of solving up to 500k degrees of freedom (requiring 2m marker particles) within few minutes per time step on a standard desktop computer. Long-term simulations of that size require run times of one to three weeks. The non-linear system of equations is solved using a Picard iterative scheme, where the linearized system of equations is solved directly during each iterative step. Sufficient convergence is usually obtained within less than 10 non-linear iterations. Generally, this numerical method is versatile, accessible and efficient enough for a wide range of 2-D problems.

Keller, Tobias

2014-05-01

45

Fracture mechanics validity limits  

NASA Technical Reports Server (NTRS)

Fracture behavior is characteristics of a dramatic loss of strength compared to elastic deformation behavior. Fracture parameters have been developed and exhibit a range within which each is valid for predicting growth. Each is limited by the assumptions made in its development: all are defined within a specific context. For example, the stress intensity parameters, K, and the crack driving force, G, are derived using an assumption of linear elasticity. To use K or G, the zone of plasticity must be small as compared to the physical dimensions of the object being loaded. This insures an elastic response, and in this context, K and G will work well. Rice's J-integral has been used beyond the limits imposed on K and G. J requires an assumption of nonlinear elasticity, which is not characteristic of real material behavior, but is thought to be a reasonable approximation if unloading is kept to a minimum. As well, the constraint cannot change dramatically (typically, the crack extension is limited to ten-percent of the initial remaining ligament length). Rice, et al investigated the properties required of J-type parameters, J(sub x), and showed that the time rate, dJ(sub x)/dt, must not be a function of the crack extension rate, da/dt. Ernst devised the modified-J parameter, J(sub M), that meets this criterion. J(sub M) correlates fracture data to much higher crack growth than does J. Ultimately, a limit of the validity of J(sub M) is anticipated, and this has been estimated to be at a crack extension of about 40-percent of the initial remaining ligament length. None of the various parameters can be expected to describe fracture in an environment of gross plasticity, in which case the process is better described by deformation parameters, e.g., stress and strain. In the current study, various schemes to identify the onset of the plasticity-dominated behavior, i.e., the end of fracture mechanics validity, are presented. Each validity limit parameter is developed in detail, and then data is presented and the various schemes for establishing a limit of the validity are compared. The selected limiting parameter is applied to a set of fracture data showing the improvement of correlation gained.

Lambert, Dennis M.; Ernst, Hugo A.

1994-01-01

46

A physically-based and fully coupled model of elasto-plasticity and damage for dynamic failure in ductile metals  

Microsoft Academic Search

It is well established that spall fracture and other rapid failures in ductile materials are often dominated by nucleation and growth of micro-voids. In the present work, a mechanistic model for failure by cumulative nucleation and growth of voids is fully coupled with the thermo-elastoplastic constitutive equations of the Mechanical Threshold Stress (MTS) which is used to model the evolution

O. Oussouaddi; L. Campagne; L. Daridon; S. Ahzi

2006-01-01

47

Statistical Physics of Elasto-Plastic Steady States in Amorphous Solids: Finite Temperatures and Strain Rates  

E-print Network

The effect of finite temperature $T$ and finite strain rate $\\dot\\gamma$ on the statistical physics of plastic deformations in amorphous solids made of $N$ particles is investigated. We recognize three regimes of temperature where the statistics are qualitatively different. In the first regime the temperature is very low, $Tstate exhibits highly correlated plastic events whose statistics are characterized by anomalous exponents. In the second regime $T_{\\rm cross}(N)physical mechanism of the cross-over is different for increasing temperature and increasing strain rate, since the plastic events are still dominated by the mechanical instabilities (seen as an eigenvalue of the Hessian matrix going to zero), and the effect of temperature is only to facilitate the transition. A third regime occurs above the second cross-over temperature $T_{\\rm max}(\\dot\\gamma)$ where stress fluctuations become dominated by thermal noise. Throughout the paper we demonstrate that scaling concepts are highly relevant for the problem at hand, and finally we present a scaling theory that is able to collapse the data for all the values of temperatures and strain rates, providing us with a high degree of predictability.

Smarajit Karmakar; Edan Lerner; Itamar Procaccia; Jacques Zylberg

2010-06-18

48

Fatigue crack propagation in a quasi one-dimensional elasto-plastic model  

E-print Network

Fatigue crack advance induced by the application of cyclic quasistatic loads is investigated both numerically and analytically using a lattice spring model. The system has a quasi-one-dimensional geometry, and consists in two symmetrical chains that are pulled apart, thus breaking springs which connect them, and producing the advance of a crack. Quasistatic crack advance occurs as a consequence of the plasticity included in the springs which form the chains, and that implies a history dependent stress-strain curve for each spring. The continuous limit of the model allows a detailed analytical treatment that gives physical insight of the propagation mechanism. This simple model captures key features that cause well known phenomenology in fatigue crack propagation, in particular a Paris-like law of crack advance under cyclic loading, and the overload retardation effect.

Tomás M. Guozden; Eduardo A. Jagla

2012-06-27

49

Electronics reliability fracture mechanics. Volume 2: Fracture mechanics  

NASA Astrophysics Data System (ADS)

This is the second of two volumes. The other volume (WL-TR-92-3015) is 'Causes of Failures of Shop Replaceable Units and Hybrid Microcircuits.' The objective of the Electronics Reliability Fracture Mechanics (ERFM) program was to develop and demonstrate a life prediction technique for electronic assemblies, when subjected to environmental stresses of vibration and thermal cycling, based upon the mechanical properties of the materials and packaging configurations which make up an electronic system. The application of fracture mechanics to microscale phenomena in electronic assemblies was a pioneering research effort. The small scale made the experiments very difficult; for example, the 1-mil-diameter bond wires in microelectronic devices are 1/3 the diameter of a human hair. A number of issues had to be resolved to determine whether a fracture mechanics modelling approach is correct for the selected failures; specifically, the following two issues had to be resolved: What fraction of the lifetime is spent in crack initiation? Are macro fracture mechanics techniques, used in large structures such as bridges, applicable to the tiny structures in electronic equipment? The following structural failure mechanisms were selected for modelling: bondwire fracture from mechanical cycling; bondwire fracture from thermal (power) cycling; plated through hole (PTH) fracture from thermal cycling. The bondwire fracture test specimens were A1-1 percent Si wires, representative of wires used in the parts in the modules selected for detailed investigation in this program (see Vol. 1 of this report); 1-mil-diameter wires were tested in this program. The PTH test specimens were sections of 14-layer printed wiring boards of the type used.

Kallis, J.; Duncan, L.; Buechler, D.; Backes, P.; Sandkulla, D.

1992-05-01

50

Mechanisms of basilar skull fracture.  

PubMed

Basilar skull fractures comprise a broad category of injuries that have been attributed to a variety of causal mechanisms. The objective of this work is to develop an understanding of the biomechanical mechanisms that result in basilar skull fractures, specifically focusing on mandibular impact and neck loading as potential mechanisms. In the characterization of the injury mechanisms, three experimental studies have been performed. The first study evaluated the response of the base of the skull to midsymphysis loading on the mental protuberance (chin) of the mandible. Five dynamic impacts using a vertical drop track and one quasi-static test in a servohydraulic test frame have been performed. In each test, clinically relevant mandibular fractures were produced but no basilar skull fractures were observed. The second study assessed the fracture tolerance of the base of the skull subject to direct loading on the temporomandibular joint in conjunction with tensile loading imposed locally around the foramen magnum to simulate the effect of the ligaments and musculature of the neck. Among four specimens that sustained either complete or incomplete basilar skull ring fractures remote from the sites of load application, the mean load at fracture was 4300 +/- 350 N. Energy to fracture was computed in three of those tests and averaged 13.0 +/- 1.7 J. Injuries produced were consistent with clinical observations that have attributed basilar skull ring fractures to mandibular impacts. In the third series of experimental tests, loading responses resulting from cranial vault impacts were investigated using unembalmed human cadaver heads and ligamentous cervical spines. Multiaxis load cells and accelerometers, coupled with high-speed digital video, were used to quantify impact dynamics. The results of these experiments suggest that while there is a greater probability of cervical spine injury, basilar skull ring fractures can result when the head is constrained on the impact surface and the inertia of the torso drives the vertebral column onto the occiput. PMID:8683618

McElhaney, J H; Hopper, R H; Nightingale, R W; Myers, B S

1995-08-01

51

Fracture mechanics of cellular glass  

NASA Technical Reports Server (NTRS)

The fracture mechanics of cellular glasses (for the structural substrate of mirrored glass for solar concentrator reflecting panels) are discussed. Commercial and developmental cellular glasses were tested and analyzed using standard testing techniques and models developed from linear fracture mechanics. Two models describing the fracture behavior of these materials were developed. Slow crack growth behavior in cellular glass was found to be more complex than that encountered in dense glasses or ceramics. The crack velocity was found to be strongly dependent upon water vapor transport to the tip of the moving crack. The existence of a static fatigue limit was not conclusively established, however, it is speculated that slow crack growth behavior in Region 1 may be slower, by orders of magnitude, than that found in dense glasses. Previously announced in STAR as N82-11209

Zwissler, J. G.; Adams, M. A.

1983-01-01

52

Fracture mechanics of cellular glass  

NASA Technical Reports Server (NTRS)

The fracture mechanics of cellular glasses (for the structural substrate of mirrored glass for solr concentrator reflecting panels) are discussed. Commercial and developmental cellular glasses were tested and analyzed using standard testing techniques and models developed from linear fracture mechanics. Two models describing the fracture behavior of these materials were developed. Slow crack growth behavior in cellular glass was found to be more complex than that encountered in dense glasses or ceramics. The crack velocity was found to be strongly dependent upon water vapor transport to the tip of the moving crack. The existence of a static fatigue limit was not conclusively established, however, it is speculated that slow crack growth behavior in Region 1 may be slower, by orders of magnitude, than that found in dense glasses.

Zwissler, J. G.; Adams, M. A.

1981-01-01

53

Fracture mechanics for piezoelectric ceramics  

NASA Astrophysics Data System (ADS)

We Study cracks either in piezoelectrics, or on interfaces between piezoelectrics and other materials such as metal electrodes or polymer matrices. The projected applications include ferroelectric actuators operating statically or cyclically, over the major portion of the samples, in the linear regime of the constitutive curve, but the elevated field around defects causes the materials to undergo hysteresis locally. The fracture mechanics viewpoint is adopted—that is, except for a region localized at the crack tip, the materials are taken to be linearly piezoelectric. The problem thus breaks into two subproblems: (i) determining the macroscopic field regarding the crack tip as a physically structureless point, and (ii) considering the hysteresis and other irreversible processes near the crack tip at a relevant microscopic level. The first Subproblem, which prompts a phenomenological fracture theory, receives a thorough investigation in this paper. Griffith's energy accounting is extended to include energy change due to both deformation and polarization. Four modes of square root singularities are identified at the tip of a crack in a homogeneous piezoelectric. A new type of singularity is discovered around interface crack tips. Specifically, the singularities in general form two pairs: r {1}/{2}±i? and r {1}/{2}±i? , where ?. and k are real numbers depending on the constitutive constants. Also solved is a class of boundary value problems involving many cracks on the interface between half-spaces. Fracture mechanics are established for ferroelectric ceramics under smallscale hysteresis conditions, which facilitates the experimental study of fracture resistance and fatigue crack growth under combined mechanical and electrical loading. Both poled and unpoled fcrroelectrie ceramics are discussed.

Suo, Z.; Kuo, C.-M.; Barnett, D. M.; Willis, J. R.

1992-05-01

54

(Fracture mechanics of inhomogeneous materials)  

SciTech Connect

Discussions were held with Japanese researchers concerning (1) the Elastic-Plastic Fracture Mechanics in Inhomogeneous Materials and Structures (EPI) Program, and (2) ongoing large-scale pressurized- thermal-shock (PTS) experiments in Japan. In the EPI Program, major activities in the current fiscal year include round-robin analyses of measured data from inhomogeneous base metal/weld metal compact- tension (CT) specimens fabricated from welded plates of A533 grade B class 1 steel. The round-robin task involves participants from nine research organizations in Japan and is scheduled for completion by the end of 1990. Additional experiments will be performed on crack growth in inhomogeneous CT specimens and three-point bend (3PB) specimens 10 mm thick. The data will be compared with that generated previously from 19-mm-thick-specimens. A new type of inhomogeneous surface-cracked specimen will be tested this year, with ratio of crack depth to surface length (a/c) satisfying 0.2 {le} (a/c) {le} 0. 8 and using a 3PB type of applied load. Plans are under way to fabricate a new welded plate of A533 grade B class 1 steel (from a different heat than that currently being tested) in order to provide an expanded fracture-toughness data base. Other topics concerning fracture-prevention issues in reactor pressure vessels were discussed with each of the host organizations, including an overview of ongoing work in the Heavy-Section Steel Technology (HSST) Program.

Bass, B.R.

1990-10-01

55

Section 3: Damage and fracture mechanics 1 Section 3: Damage and fracture mechanics  

E-print Network

Fracture Accounting for Brittle to Ductile Failure Mode Transition Abstract #12;Section 3: DamageSection 3: Damage and fracture mechanics 1 Section 3: Damage and fracture mechanics Organizers Fagerström (Chalmers University of Technology Göteborg): Ductile dynamic fracture modeling using embedded

Kohlenbach, Ulrich

56

Mechanical Coal-Face Fracturer  

NASA Technical Reports Server (NTRS)

Radial points on proposed drill bit take advantage of natural fracture planes of coal. Radial fracture points retracted during drilling and impacted by piston to fracture coal once drilling halts. Group of bits attached to array of pneumatic drivers to fracture large areas of coal face.

Collins, E. R., Jr.

1984-01-01

57

Compendium of fracture mechanics problems  

NASA Technical Reports Server (NTRS)

Fracture mechanics analysis results are presented from the following structures/components analyzed at Marshall Space Flight Center (MSFC) between 1982 and 1989: space shuttle main engine (SSME), Hubble Space Telescope (HST), external tank attach ring, B-1 stand LOX inner tank, and solid rocket booster (SRB). Results from the SSME high pressure fuel turbopump (HPFTP) second stage blade parametric analysis determine a critical flaw size for a wide variety of stress intensity values. The engine 0212 failure analysis was a time dependent fracture life assessment. Results indicated that the disk ruptured due to an overspeed condition. Results also indicated that very small flaws in the curvic coupling area could propagate and lead to failure under normal operating conditions. It was strongly recommended that a nondestructive evaluation inspection schedule be implemented. The main ring of the HST, scheduled to launch in 1990, was analyzed by safe-life and fail-safe analyses. First safe-life inspection criteria curves for the ring inner and outer skins and the fore and aft channels were derived. Afterwards the skins and channels were determined to be fail-safe by analysis. A conservative safe-life analysis was done on the 270 redesign external tank attach ring. Results from the analysis were used to determine the nondestructive evaluation technique required.

Stallworth, R.; Wilson, C.; Meyers, C.

1990-01-01

58

Entablature: fracture types and mechanisms  

NASA Astrophysics Data System (ADS)

Entablature is the term used to describe zones or tiers of irregular jointing in basaltic lava flows. It is thought to form when water from rivers dammed by the lava inundates the lava flow surface, and during lava-meltwater interaction in subglacial settings. A number of different fracture types are described in entablature outcrops from the Búrfell lava and older lava flows in Þjórsárdalur, southwest Iceland. These are: striae-bearing, column-bounding fractures and pseudopillow fracture systems that themselves consist of two different fracture types—master fractures with dimpled surface textures and subsidiary fractures with curved striae. The interaction of pseudopillow fracture systems and columnar jointing in the entablature produces the chevron fracture patterns that are commonly observed in entablature. Cube-jointing is a more densely fractured version of entablature, which likely forms when more coolant enters the hot lava. The entablature tiers display closely spaced striae and dendritic crystal shapes which indicate rapid cooling. Master fracture surfaces show a thin band with an evolved composition at the fracture surface; mineral textures in this band also show evidence of quenching of this material. This is interpreted as gas-driven filter pressing of late-stage residual melt that is drawn into an area of low pressure immediately preceding or during master fracture formation by ductile extensional fracture of hot, partially crystallised lava. This melt is then quenched by an influx of water and/or steam when the master fracture fully opens. Our findings suggest that master fractures are the main conduit for coolant entering the lava flow during entablature formation.

Forbes, A. E. S.; Blake, S.; Tuffen, H.

2014-05-01

59

Computer modeling of the mechanical behavior of composites -- Interfacial cracks in fiber-reinforced materials  

SciTech Connect

Computer modeling of materials and especially modeling the mechanical behavior of composites became increasingly popular in the past few years. Among them are examples of micromechanical modeling of real structures as well as idealized model structures of linear elastic and elasto-plastic material response. In this paper, Erdogan`s Integral Equation Method (IEM) is chosen as an example for a powerful method providing principle insight into elastic fracture mechanical situations. IEM or, alternatively, complex function techniques sometimes even allow for deriving analytical solutions such as in the case of a circumferential crack along a fiber/matrix interface. The analytical formulae of this interface crack will be analyzed numerically and typical results will be presented graphically.

Schmauder, S.; Haake, S. [Max-Planck-Inst. fuer Metallforschung, Stuttgart (Germany). Inst. fuer Werkstoffwissenschaft] [Max-Planck-Inst. fuer Metallforschung, Stuttgart (Germany). Inst. fuer Werkstoffwissenschaft; [Univ. Stuttgart (Germany). Staatliche Materialpruefungsanstalt; Mueller, W.H. [Max-Planck-Inst. fuer Metallforschung, Stuttgart (Germany). Inst. fuer Werkstoffwissenschaft] [Max-Planck-Inst. fuer Metallforschung, Stuttgart (Germany). Inst. fuer Werkstoffwissenschaft; [Univ. Paderborn (Germany). Lab. fuer Technische Mechanik

1996-06-15

60

The fracture mechanics of finite crack extension  

Microsoft Academic Search

This paper describes a modification to the traditional Griffith energy balance as used in linear elastic fracture mechanics (LEFM). The modification involves using a finite amount of crack extension (?a) instead of an infinitesimal extension (da) when calculating the energy release rate. We propose to call this method finite fracture mechanics (FFM). This leads to a change in the Griffith

David Taylor; Pietro Cornetti; Nicola Pugno

2005-01-01

61

MechanicalEngineering The Department of Mechanical Engineering presents  

E-print Network

for thrombogenic stimulus. Herein, we discuss our recent progress towards realization of next-generation titanium culture. Using elasto-plastic finite element analysis, we also explore the feasibility of planar stents, fabrication, and mechanical testing of a novel, titanium-based stent. Friday, January 24, 2014, 11:10AM-12

Mills, Allen P.

62

Equations For Selected Fracture-Mechanics Parameters  

NASA Technical Reports Server (NTRS)

Equations describing crack-mouth-opening displacements, stress-intensity factors, and related fracture-mechanics parameters of chevron-notched short bar and rod specimens presented in report. Equations in forms suitable for determining fracture toughnesses from maximum loads, for determining crack-extension-resistance curves, and for setting sensitivities of testing instruments. Useful in facilitating testing and interpretation of data from tests of brittle metals, ceramics, and glasses, formed into chevron-notched specimens for fracture testing according to concepts.

Bubsey, Raymond T.; Orange, Thomas W.; Pierce, William S.; Shannon, John L., Jr.

1994-01-01

63

Failure mechanism models for brittle fracture  

Microsoft Academic Search

This tutorial illustrates designs where brittle fracture can endanger system performance, thereby acting as an overstress failure mechanism. Analytic (physics-of-failure) methods, based on continuum fracture-mechanisms rather than on molecular micro-mechanics, are presented to design against such failures. The associated stress-analysis techniques and material characterizations have matured appreciably over the past 40 years and are routinely used in aerospace, automotive, and

Abhijit Dasgupta; Jun Ming Hu

1992-01-01

64

Fracture mechanics evaluation of GaAs  

NASA Technical Reports Server (NTRS)

A data base of mechanical and fracture properties for GaAs was generated. The data for single crystal GaAs will be used to design reusable GaAs solar modules. Database information includes; (1) physical property characterizations; (2) fracture behavior evaluations; and (3) strength of cells determined as a function of cell processing and material parameters.

Chen, C. P.

1984-01-01

65

Fracture mechanism maps for advanced structural ceramics  

Microsoft Academic Search

The static fatigue behaviour of a commercial sintered silicon nitride with yttria and alumina sintering aids was controlled by slow crack growth or creep fracture. A fracture map depicts the stress-temperature regimes where each mechanism is dominant. Devitrification of the grain boundary phase at intermediate temperatures (1000 and 1100 °C) radically altered the static fatigue resistance. The principal devitrification products

G. D. Quinn; W. R. Braue

1990-01-01

66

Brittle fracture in compression: Mechanisms, models and criteria  

Microsoft Academic Search

Although brittle fracture in compression has been a subject of study since the pioneering work of Griffith, some fundamental aspects of the fracture mechanism and phenomenon still remain unclear. Further, computer simulations of fracture processes are rather limited. Thus, a review and summary of existing typical models and fracture criteria for brittle fracture in compression are provided. A sound fracture

E. Z. Wang; N. G. Shrive

1995-01-01

67

Computational Solid Mechanics using a Vertex-based Finite Volume Method  

E-print Network

Computational Solid Mechanics using a Vertex-based Finite Volume Method G. A. Taylor, C. Bailey and using nite volume (FV) methods for computational solid mechanics (CSM). These methods are proving will be given. Key Words: Vertex-based, Finite Volume, Solid Mechanics, Elasto-plastic. 1. Introduction Over

Taylor, Gary

68

Fracture Mechanics of Rubber Epoxy Composites  

NASA Astrophysics Data System (ADS)

The effect of the addition of rubber micro-particles to epoxy matrix on the mechanical properties and the fracture toughness were investigated. Rubber epoxy composites were prepared with different weight percentages namely, 0, 5, 10, 15, 20, 25, 30 wt pct of rubber. Both quasi-static and dynamic ultrasonic measurements of the elastic modulus were found to decrease by 60 pct, and the critical value of the stress intensity factors was found to increase by approximately 45 pct for the rubber epoxy composites. This was also confirmed with the finite element analysis that had the same increasing trend. The fracture surface morphology reveals rough cleavage fracture in the epoxy matrix with brittle intergranular decohesion caused by the impurity segregation that exhibits relatively high micro-roughness of the fracture surfaces.

El-Hadek, Medhat Awad

2014-08-01

69

Reply to Davies: Hydraulic fracturing remains a possible mechanism for  

E-print Network

LETTER Reply to Davies: Hydraulic fracturing remains a possible mechanism for observed methane mechanisms were leaky gas well casings and the possibility that hydraulic fracturing might generate new- knowledged the possibility of hydraulic fracturing playing a role. Is it possible that hydraulic fracturing

Jackson, Robert B.

70

Coupled thermal-hydrological-mechanical analyses of the YuccaMountain Drift Scale Test - Comparison of field measurements topredictions of four different numerical models  

SciTech Connect

The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale underground heater test designed to study coupled thermal-hydrological-mechanical-chemical behavior in unsaturated fractured and welded tuff. As part of the international cooperative code-comparison project DECOVALEX, four research teams used four different numerical models to simulate and predict coupled thermal-hydrological-mechanical (THM) processes at the DST. The simulated processes included above-boiling temperature changes, liquid and vapor water movements, rock-mass stress and displacement, and THM-induced changes in fracture permeability. Model predictions were evaluated by comparison to measurements of temperature, water saturation,displacement, and air permeability. The generally good agreement between simulated and measured THM data shows that adopted continuum model approaches are adequate for simulating relevant coupled THM processes at the DST. Moreover, TM-induced rock-mass deformations were reasonably well predicted using elastic models, although some individual displacements appeared to be better captured using an elasto-plastic model. It is concluded that fracture closure/opening caused by change in normal stress across fractures is the dominant mechanism for TM-induced changes in intrinsic fracture permeability at the DST, whereas fracture shear dilation appears to be less significant. This indicates that TM-induced changes in intrinsic permeability at the DST, which are within one order of magnitude, tend to be reversible.

Rutqvist, J.; Barr, D.; Datta, R.; Gens, A.; Millard, A.; Olivella, S.; Tsang, C.-F.; Tsang, Y.

2004-08-30

71

Fracture Mechanics for Electroactive Materials  

Microsoft Academic Search

Electroactive materials include piezoelectrics, ferroelectrics and electroactive polymers. In the simplest form, this class\\u000a of materials also includes isotropic dielectrics where there is minimal coupling between the mechanics and electrostatics.\\u000a Indeed, electroactive polymers can be isotropic. Then they derive their useful properties of electrically stimulated actuation\\u000a from their low mechanical stiffness. In such materials, large actuation strains occur when electric

Robert M. McMeeking

72

A review of fracture mechanics life technology  

NASA Technical Reports Server (NTRS)

Lifetime prediction technology for structural components subjected to cyclic loads is examined. The central objectives of the project are: (1) to report the current state of the art, and (2) recommend future development of fracture mechanics-based analytical tools for modeling subcritical fatigue crack growth in structures. Of special interest is the ability to apply these tools to practical engineering problems and the developmental steps necessary to bring vital technologies to this stage. The authors conducted a survey of published literature and numerous discussions with experts in the field of fracture mechanics life technology. One of the key points made is that fracture mechanics analyses of crack growth often involve consideration of fatigue and fracture under extreme conditions. Therefore, inaccuracies in predicting component lifetime will be dominated by inaccuracies in environment and fatigue crack growth relations, stress intensity factor solutions, and methods used to model given loads and stresses. Suggestions made for reducing these inaccuracies include development of improved models of subcritical crack growth, research efforts aimed at better characterizing residual and assembly stresses that can be introduced during fabrication, and more widespread and uniform use of the best existing methods.

Besuner, P. M.; Harris, D. O.; Thomas, J. M.

1986-01-01

73

A finite element procedure for the analysis of thermo-mechanical solids in contact  

Microsoft Academic Search

We present a finite element procedure for the analysis of fully coupled thermo-elasto-plastic response of solids including contact conditions. The continuum mechanics formulation for the solid and contact conditions is summarized and effective finite element techniques for solution are given. The constraint function method is employed to impose the contact conditions at the Gauss points of the contact surface. Other

Daniel Pantuso; Klaus-Jürgen Bathe; Pavel A. Bouzinov

2000-01-01

74

Fracture Toughness of Metallic Glasses: Ductile-to-Brittle Transition?  

E-print Network

Quantitative understanding of the fracture toughness of metallic glasses, including the associated ductile-to-brittle transitions, is not yet available. Here we use a simple model of plastic deformation in glasses, coupled to an advanced Eulerian level set formulation for solving complex free boundary problems, to calculate the fracture toughness of metallic glasses as a function of the degree of structural relaxation corresponding to different annealing times near the glass temperature. Our main result indicates the existence of an elasto-plastic crack tip instability for sufficiently relaxed glasses, resulting in a marked drop in the toughness, which we interpret as a ductile-to-brittle transition similar to experimental observations.

Rycroft, Chris H

2012-01-01

75

RSRM nozzle actuator bracket\\/lug fracture mechanics qualification test  

Microsoft Academic Search

This is the final report for the actuator bracket\\/lug fracture mechanics qualification test. The test plan (CTP-0071) outlined a two-phase test program designed to answer questions about the fracture criticality of the redesigned solid rocket motor (RSRM) nozzle actuator bracket. An analysis conducted using the NASA\\/FLAGRO fracture mechanics computer program indicated that the actuator bracket might be a fracture critical

Peggy Kelley

1993-01-01

76

Some Fundamental Mechanisms of Hydraulic Fracturing.  

E-print Network

??This dissertation focuses mainly on three topics: (1) mixed-mode branching and segmentation of hydraulic fractures in brittle materials, (2) hydraulic fracture propagation in particulate materials,… (more)

Wu, Ruiting

2006-01-01

77

Fracture mechanics methodology: Evaluation of structural components integrity  

NASA Astrophysics Data System (ADS)

The application of fracture mechanics to structural-design problems is discussed in lectures presented in the AGARD Fracture Mechanics Methodology course held in Lisbon, Portugal, in June 1981. The emphasis is on aeronautical design, and chapters are included on fatigue-life prediction for metals and composites, the fracture mechanics of engineering structural components, failure mechanics and damage evaluation of structural components, flaw-acceptance methods, and reliability in probabilistic design. Graphs, diagrams, drawings, and photographs are provided.

Sih, G. C.; de Oliveira Faria, L.

1984-09-01

78

Fracture-toughening mechanisms responsible for differences in work to fracture of hydrated and dehydrated dentine  

Microsoft Academic Search

This study investigates the nature of deformation and differences in the mechanisms of fracture and properties of dentine where there has been a loss of moisture, as may occur with removal of the pulp in the endodontic treatment of teeth. Controlled fracture toughness testing was conducted on bovine teeth to determine the influence of hydration on the work of fracture

Bill Kahler; Michael V. Swain; Alex Moule

2003-01-01

79

Analogy between fluid cavitation and fracture mechanics  

NASA Technical Reports Server (NTRS)

When the stresses imposed on a fluid are sufficiently large, rupture or cavitation can occur. Such conditions can exist in many two-phase flow applications, such as the choked flows, which can occur in seals and bearings. Nonspherical bubbles with large aspect ratios have been observed in fluids under rapid acceleration and high shear fields. These bubbles are geometrically similar to fracture surface patterns (Griffith crack model) existing in solids. Analogies between crack growth in solid and fluid cavitation are proposed and supported by analysis and observation (photographs). Healing phenomena (void condensation), well accepted in fluid mechanics, have been observed in some polymers and hypothesized in solid mechanics. By drawing on the strengths of the theories of solid mechanics and cavitation, a more complete unified theory can be developed.

Hendricks, R. C.; Mullen, R. L.; Braun, M. J.

1983-01-01

80

References and conference proceedings towards the understanding of fracture mechanics  

NASA Technical Reports Server (NTRS)

A list of books, reports, periodicals, and conference proceedings, as well as individual papers, centered on specific aspects of fracture phenomenon has been compiled by the ASTM Committee E-24 on Fracture Testing. A list of basic references includes the articles on the development of fracture toughness, evaluation of stress intensity factors, fatigue crack growth, fracture testing, fracture of brittle materials, and fractography. Special attention is given to the references on application of fracture mechanics to new designs and on reevaluation of failed designs, many of them concerned with naval and aircraft structures.

Toor, P. M.; Hudson, C. M.

1986-01-01

81

Mechanical Behavior of Carbide-free Medium Carbon Bainitic Steels  

NASA Astrophysics Data System (ADS)

The effect of bainitic transformation time on the microstructure and mechanical properties was investigated in a steel containing 0.4 pct C-2.8 pct Mn-1.8 pct Si. The microstructure was characterized using optical and transmission electron microscopy; it consisted of bainitic ferrite, martensite, and retained austenite. The volume fraction of bainite increased from 0.4 for the shortest bainitic transformation time (30 minutes) to 0.9 at the longest time (120 minutes). The above microstructures exhibited an extended elasto-plastic transition leading to very high initial work-hardening rates. The work-hardening behavior was investigated in detail using strain-path reversals to measure the back stresses. These measurements point to a substantial kinematic hardening due to the mechanical contrast between the microstructural constituents. The onset of necking coincided with the saturation of kinematic hardening. Examination of the fracture surfaces indicated that the prior austenite grain boundaries play an important role in the fracture process.

Zhang, Xiaoxu; Xu, Guang; Wang, Xiang; Embury, David; Bouaziz, Olivier; Purdy, Gary R.; Zurob, Hatem S.

2013-11-01

82

A chemo-mechanical model of lithiation in silicon  

NASA Astrophysics Data System (ADS)

We present a chemo-mechanical model to investigate the lithiation-induced phase transformation, morphological evolution, stress generation and fracture in crystalline silicon nanowires (c-SiNWs). The model couples lithium (Li) diffusion with elasto-plastic deformation in a three-dimensional (3D) setting. Several key features observed from recent transmission electron microscopy (TEM) studies are incorporated into the model, including the sharp interface between the lithiated amorphous shell and unlithiated crystalline core, crystallographic orientation dependent Li-Si reaction rate, and large-strain plasticity. Our simulation results demonstrate that the model faithfully predicts the anisotropic swelling of lithiated SiNWs observed from previous experimental studies. Stress analysis from the finite-deformation model reveals that the SiNWs are prone to surface fracture at the angular sites where two adjacent {110} facets intersect, consistent with previous experimental observations. The mechanistic understanding of the morphological evolution and stress generation sheds light on the design of failure-resistant nanostructured electrodes. Our model offers a framework for the study of the chemo-mechanical degradation in high-capacity electrode materials.

Yang, Hui; Fan, Feifei; Liang, Wentao; Guo, Xu; Zhu, Ting; Zhang, Sulin

2014-10-01

83

Mechanical mechanism analysis of tension type anchor based on shear displacement method  

Microsoft Academic Search

Based on the fact that the shear stress along anchorage segment is neither linearly nor uniformly distributed, the load transfer\\u000a mechanism of the tension type anchor was studied and the mechanical characteristic of anchorage segment was analyzed. Shear\\u000a stress-strain relationship of soil surrounding anchorage body was simplified into three-folding-lines model consisting of\\u000a elastic phase, elasto-plastic phase and residual phase considering

Shu-jun Xiao; Chang-fu Chen

2008-01-01

84

Solution-adaptive finite element method in computational fracture mechanics  

NASA Technical Reports Server (NTRS)

Some recent results obtained using solution-adaptive finite element method in linear elastic two-dimensional fracture mechanics problems are presented. The focus is on the basic issue of adaptive finite element method for validating the applications of new methodology to fracture mechanics problems by computing demonstration problems and comparing the stress intensity factors to analytical results.

Min, J. B.; Bass, J. M.; Spradley, L. W.

1993-01-01

85

Finite element analysis of grain-matrix micro-cracking in shale within the context of a multiscale modeling approach for fracture (Invited)  

NASA Astrophysics Data System (ADS)

The paper models grain-scale micro-cracking in shale at grain-matrix interfaces, assuming constituents are composed of quart silt grains and compacted clay matrix for a typical shale. The influence of grain-matrix-grain interaction on micro-crack patterns is investigated. Elasto-plastic pressure-sensitive cohesive-surface models are inserted at grain-matrix interfaces and intra-clay-matrix finite element facets, while a bulk elasto-plasticity model with bifurcation is employed for the clay matrix to compare to the intra-clay-matrix cohesive-surface model. Numerical examples are presented under two-dimensional plane strain condition at small strains. A procedure is proposed to upscale grain-scale micro-cracking to predict macro-fracture nucleation and propagation in shale and other bound particulate materials. It is shown that using cohesive surface elements (CSEs) at all finite element facets in the clay matrix mesh to simulate micro-cracking in the clay matrix leads to mesh-dependent results. Using CSEs at grain-clay-matrix interfaces is physical and not mesh dependent. We also considered using bulk pressure-sensitive elasto-plasticity with bifurcation condition within the clay matrix to attempt to predict onset of localization around grains in the simulations. It was encouraging to see that for both the single grain and multiple grain simulations, the finite element region in the clay matrix meshes where bifurcation was first detected around the grains was nearly the same. This gives us confidence that once a proper post-bifurcation constitutive model is implemented within an embedded discontinuity formulation, micro-cracking nucleation and propagation at the grain-scale in shale can be properly simulated, which will provide the basis for up-scaling to macro-cracks within a multiscale method for fracture in shale. Other items to address in future research are: (i) include transverse isotropy (elastic and plastic) for the bulk clay matrix elasto-plasticity model, (ii) include influence of moisture content on elasto-plasticity and fracture properties, (iii) extend to three-dimensions, and (iii) coordinate with an experimental effort to calibrate parameters and validate the multiscale modeling approach.

Regueiro, R. A.; Yu, S.

2010-12-01

86

Fracture mechanics approach to penetration of surface crevasses on glaciers  

Microsoft Academic Search

In linear elastic fracture mechanics, the stress intensity factor is used to describe elastic stresses near the tip of a crack. Crack growth occurs when the stress intensity factor is larger than a critical value, the fracture toughness, which is a material parameter that applies to cracks of any size. For surface crevasses on glaciers, the net stress intensity factor

C. J. van der Veen

1998-01-01

87

Damage mechanics for the fracture prediction of metal forming tools  

Microsoft Academic Search

In this paper, a damage mechanics approach is shown to be useful for the prediction of fatigue fracture in metal forming tools. The approach can be applied in the general situation of under complex loading at elevated temperature. A damage criterion used considers initiation and propagation of cracks and fracture to be the accumulation of damage up to certain critical

R. Lapovok; S. Smirnov; V. Shveykin

2000-01-01

88

Some recent theoretical and experimental developments in fracture mechanics  

NASA Technical Reports Server (NTRS)

Recent theoretical and experimental developments in four distinct areas of fracture mechanics research are described. These are as follows: experimental comparisons of different nonlinear fracture toughness measures, including the nonlinear energy, R curve, COD and J integral methods; the singular elastic crack-tip stress and displacement equations and the validity of the proposition of their general adequacy as indicated, for example, by the biaxially loaded infinite sheet with a flat crack; the thermodynamic nature of surface energy induced by propagating cracks in relation to a general continuum thermodynamic description of brittle fracture; and analytical and experimental aspects of Mode II fracture, with experimental data for certain aluminum, steel and titanium alloys.

Liebowitz, H.; Eftis, J.; Hones, D. L.

1978-01-01

89

A nonlinear high temperature fracture mechanics basis for strainrange partitioning  

NASA Technical Reports Server (NTRS)

A direct link was established between Strainrange Partitioning (SRP) and high temperature fracture mechanics by deriving the general SRP inelastic strain range versus cyclic life relationships from high temperature, nonlinear, fracture mechanics considerations. The derived SRP life relationships are in reasonable agreement based on the experience of the SRP behavior of many high temperature alloys. In addition, fracture mechanics has served as a basis for derivation of the Ductility-Normalized SRP life equations, as well as for examination of SRP relations that are applicable to thermal fatigue life prediction. Areas of additional links between nonlinear fracture mechanics and SRP were identified for future exploration. These include effects of multiaxiality as well as low strain, nominally elastic, long life creep fatigue interaction.

Kitamura, Takayuki; Halford, Gary R.

1989-01-01

90

“Electrochemical Shock” of Intercalation Electrodes: A Fracture Mechanics Analysis  

E-print Network

Fracture of electrode particles due to diffusion-induced stress has been implicated as a possible mechanism for capacity fade and impedance growth in lithium-ion batteries. In brittle materials, including many lithium ...

Chiang, Yet-Ming

91

Adaptive Finite-Element Computation In Fracture Mechanics  

NASA Technical Reports Server (NTRS)

Report discusses recent progress in use of solution-adaptive finite-element computational methods to solve two-dimensional problems in linear elastic fracture mechanics. Method also shown extensible to three-dimensional problems.

Min, J. B.; Bass, J. M.; Spradley, L. W.

1995-01-01

92

Work of fracture of a composite resin: fracture-toughening mechanisms.  

PubMed

The aim of this work was to investigate those mechanical parameters able to describe the fracture behavior of dental composite resins. A commercially available fine-particle micro-hybrid resin composite was used. Classical parameters as Young's modulus, strength distribution, and critical stress intensity factor were considered. Strength values were determined using the diametrical compression of discs test and for the critical stress intensity factor both unstable and controlled fracture tests were used. Controlled fracture tests allowed determining the work of fracture. Microstructure was studied by optical and field emission scanning electron microscopy. The obtained properties have been Young's modulus, 17.7 +/- 0.6 GPa; Weibull modulus, m = 14 (upper and lower limits for 90% confidence: 17 and 10); characteristic strength 51 MPa (upper and lower limits for 90% confidence: 53 and 49 MPa); critical stress intensity factor in mode I, K(IC) = 1.3 +/- 0.1 and work of fracture, gamma(wof) = 8-9 J/m(2). Pores and bubbles formed during the packing of the composite were identified as critical defects in the tested specimens. Crack deflection and branching have been identified as toughening mechanisms. Classical mechanical parameters (Young's modulus, hardness...) are not able to efficiently predict the major clinical failure mode of composite resins by fatigue. Work of fracture analysis, which is dependant on microstructural parameters such as particle size and shape, have to be included when testing mechanical properties of dental composite resins in future research studies. PMID:18465813

Baudin, Carmen; Osorio, Raquel; Toledano, Manuel; de Aza, Salvador

2009-06-01

93

In Vitro Fracture of Human Cortical Bone: Local Fracture Criteria and Toughening Mechanisms  

SciTech Connect

A micro-mechanistic understanding of bone fracture that encompasses how cracks interact with the underlying microstructure and defines their local failure mode is lacking, despite extensive research on the response of bone to a variety of factors like aging, loading, and/or disease. Micro-mechanical models for fracture incorporating such local failure criteria have been widely developed for metallic and ceramic materials systems; however, few such deliberations have been undertaken for the fracture of bone. In fact, although the fracture event in mineralized tissues such as bone is commonly believed to be locally strain controlled, until recently there has been little experimental evidence to support this widely held belief. In the present study, a series of in vitro experiments involving a double-notch bend test geometry are performed in order to shed further light on the nature of the local cracking events that precede catastrophic fracture in bone and to define their relationship to the microstructure. Specifically, crack-microstructure interactions are examined to determine the salient toughening mechanisms in human cortical bone and to characterize how these may affect the anisotropy in fracture properties. Based on preliminary micro-mechanical models of these processes, in particular crack deflection and uncracked ligament bridging, the relative importance of these toughening mechanisms is established.

Nalla, R; Stolken, J; Kinney, J; Ritchie, R

2004-08-18

94

Material properties and fracture mechanics in relation to ceramic machining  

SciTech Connect

Material removal rate, surface finish, and subsurface damage are largely governed by fracture mechanics and plastic deformation, when ceramics are machined using abrasive methods. A great deal of work was published on the fracture mechanics of ceramics in the late 1970s and early 1980s, although this work has never resulted in a comprehensive model of the fixed abrasive grinding process. However, a recently published model describes many of the most important features of the loose abrasive machining process, for example depth of damage, surface roughness, and material removal rate. Many of the relations in the loose abrasive machining model can be readily discerned from fracture mechanics models, in terms of material properties. By understanding the mechanisms of material removal, from a material properties perspective, we can better estimate how one material will machine in relation to another. Although the fracture mechanics models may have been developed for loose abrasive machining, the principles of crack initiation and propagation are equally valuable for fixed abrasive machining. This report provides a brief review of fracture in brittle materials, the stress distribution induced by abrasives, critical indenter loads, the extension of cracks, and the relation of the fracture process to material removal.

Griffith, L.V.

1993-12-02

95

Comparison of Fracture Initiation and Propagation in Untreated and Thermally Damaged Westerly granites Using a Geophysical Imaging Cell  

NASA Astrophysics Data System (ADS)

By performing controlled thermal cracking experiments, we compare quantitatively the influence of microcracks on strength, deformation, elastic wave velocities (compressional and shear waves) and acoustic emission activity of a pre-heated (850C) specimen with that of an unheated (room temperature, RT) specimen of Westerly granite. Comparison shows a distinctive difference as they undergo a similar stress path during the experiment. The RT specimen shows a failure strength of 420 MPa and axial strain of 0.013, characterized by an elasto-plastic stress-strain curve exhibiting class-2 unstable post-failure behaviour. The 850C specimen fails at 360 MPa with an axial strain of 0.022, characterized by a plasto-elasto-plastic stress-strain curve showing class-1 stable post-failure fracture propagation behaviour. VP measured in the axial direction for the 850C specimen increases steadily from ~4.6 to 5.7 km/s up to the failure point and then decrease to 5.4 km/s at post-failure. VP measured in both horizontal directions increases from 4.6 km/s to a maximum of 5.25 km/s, then decreases to 4.9 km/s during the 33 minutes period prior to failure and settles at 4.3 km/s after the full development of fracture planes. Propagation of axial fractures parallel to sigma1 cause the reduction of VP measured normal to the axial planes which is concomitant to the change in the pre-failure nature of the stress-strain curve shown by the 850C specimen when it experiences increased plasticity at the beginning of dilation. Fracture clustering, coalescence and propagation happened at three distinct stages, as demonstrated from analysis of the AE hit count, AE source location and stress-time curve for a period of 3 to 4 minutes during the pre to post-failure regime of the 850C specimen. AE sensors show a peak number of AE hits/s roughly 25 seconds prior to each stress drop. Induced AE source location analysis demonstrates the evolution of fracture propagation corresponding to each stress drop. Source mechanisms were computed for events that occurred in a 15 minute window covering both the pre and post failure regimes of the 850C specimen. Prior to failure the observed events are close to a +Dipole type event appearing outside the significantly deviatoric region on the source type plot and showing a 20-30% volumetric component. At each of the subsequent three stress drops the source mechanisms migrate into the significantly deviatoric region on the source type plot and cluster around a Double Couple shear type event. Between the second and third stress drops a cluster of compressive events resembling -Dipole and +Crack type events appear. Evolution of the stress-strain curve for the RT specimen shows only one stress drop during the pre-failure to post-failure regime, which is accompanied by only one peak for AE hits/s prior to sudden brittle failure. Evolution of AE for RT specimen shows fracture initiation taking place at the middle of the specimen, propagating rapidly towards the outer edges of the specimen along an inclined fault plane. This study aims at comparing the evolution of strength, deformation, seismic wave velocities and AE response for two different media representing intact and highly damaged rocks under a similar stress regime.

Nasseri, M. B.; Goodfellow, S.; Young, R.

2011-12-01

96

Adhesive fracture mechanics. [stress analysis for bond line interface  

NASA Technical Reports Server (NTRS)

In studies of fracture mechanics the adhesive fracture energy is regarded as a fundamental property of the adhesive system. It is pointed out that the value of the adhesive fracture energy depends on surface preparation, curing conditions, and absorbed monolayers. A test method reported makes use of a disk whose peripheral part is bonded to a substrate material. Pressure is injected into the unbonded central part of the disk. At a certain critical pressure value adhesive failure can be observed. A numerical stress analysis involving arbitrary geometries is conducted.

Bennett, S. J.; Devries, K. L.; Williams, M. L.

1974-01-01

97

Trabecular Bone Mechanical Properties in Patients with Fragility Fractures  

Microsoft Academic Search

Fragility fractures are generally associated with substantial loss in trabecular bone mass and alterations in structural anisotropy.\\u000a Despite the high correlations between measures of trabecular mass and mechanical properties, significant overlap in density\\u000a measures exists between individuals with osteoporosis and those who do not fracture. The purpose of this paper is to provide\\u000a an analysis of trabecular properties associated with

Jaclynn M. Kreider; Steven A. Goldstein

2009-01-01

98

RSRM nozzle actuator bracket/lug fracture mechanics qualification test  

NASA Technical Reports Server (NTRS)

This is the final report for the actuator bracket/lug fracture mechanics qualification test. The test plan (CTP-0071) outlined a two-phase test program designed to answer questions about the fracture criticality of the redesigned solid rocket motor (RSRM) nozzle actuator bracket. An analysis conducted using the NASA/FLAGRO fracture mechanics computer program indicated that the actuator bracket might be a fracture critical component. In the NASA/FLAGRO analysis, a simple lug model was used to represent the actuator bracket. It was calculated that the bracket would fracture if subjected to an actuator stall load in the presence of a 0.10 in. corner crack at the actuator attachment hole. The 0.10 in. crack size corresponds to the nondestructive inspection detectability limit for the actuator bracket. The inspection method used is the dye penetrant method. The actuator stall load (103,424 lb) is the maximum load which the actuator bracket is required to withstand during motor operation. This testing was designed to establish the accuracy of the analytical model and to directly determine whether the actuator bracket is capable of meeting fracture mechanics safe-life requirements.

Kelley, Peggy

1993-01-01

99

Finite deformation of elasto-plastic solids  

NASA Technical Reports Server (NTRS)

A theoretical basis is established for analysis of finite deformation of metals. The observation that finite deformation of such elastoplastic materials may be viewed as a process rather than an event leads to derivation of a complete initial and boundary value problem distinguished by its quasilinear nature. This feature of the formulation motivates adoption of an incremental approach to numerical problem solving. Numerical solution capability is established for problems of plane stress and plane strain. The validity of the theory and numerical analysis is demonstrated by consideration of a number of problems of homogeneous finite deformation for which analytic solutions are available. Subsequently the analysis is employed for the investigation of necking in flat metal tensile bars. The results of this investigation provide the first full numerical solutions for tensile necking in plane stress and plane strain. In addition a basis is provided for assessment of the validity of stress-strain relations inferred from tensile test data.

Osias, J. R.

1973-01-01

100

Single State ElastoPlastic Friction Models  

Microsoft Academic Search

For control applications involving small displace- ments and velocities, friction modeling and compensation can be very important. In particular, the modeling of presliding displacement (motion prior to fully developed slip) can play a pivotal role. In this paper, it is shown that existing single-state friction models exhibit a nonphysical drift phenomenon which results from modeling presliding as a combination of

Pierre Dupont; Vincent Hayward; Brian Armstrong; Friedhelm Altpeter

2002-01-01

101

Fracture mechanism of amorphous polymers at strain fields.  

PubMed

Owing to the wide application of polymeric materials, understanding the fracture mechanism of amorphous polymers at strain fields is a fundamentally important challenge. In this work, we use molecular dynamics simulations to investigate the uniaxial deformation of amorphous polyethylene and further monitor the polyethylene fracture process induced by stretching. Results indicate that the polyethylene systems with chain lengths of 600-800 united atoms exhibit the fracture behavior at a temperature T < 200 K and the strain of 1.0. Further study shows that in the stretching process, the disentanglement and orientation of chains lead to the formation of small cavities in the middle region of the system, and the small cavities subsequently form a large hole, causing the fracture of the whole system. Definitely, the fracture is determined by the two factors of mobility and entanglement of chains. The polyethylene systems with a high chain mobility or a high chain entanglement do not fracture. Finally, a schematic diagram is put forward to illustrate the fracture behavior. PMID:25322468

Huang, Lan; Yang, Xiaoping; Jia, Xiaolong; Cao, Dapeng

2014-12-01

102

Probabilistic fracture mechanics analysis of APT blanket tubes  

SciTech Connect

A probabilistic fracture mechanics (PFM) model that is specific to the Accelerator Production of Tritium (APT) helium tubes was developed. The model performs Monte Carlo analyses of potential failure modes caused by cyclic stresses generated by beam trips and depressurizations 60m normal operation, coupled with material aging due to irradiation. Dominant failure probabilities are due to crack through-growth while brittle fracture and ductile tearing have lower probability. Failure mechanisms of global plastic collapse and buckling or crack initiation mechanisms of fatigue or local fracture (upon loss of ductility) have negligible probability. For the population of (7,311) tubes in the APT blanket, the worst-case, annual probability of one tube failing is 3 percent. The probability of 2 or more failures is substantially lower; therefore, unavailability impacts are driven by single failure. The average annual loss of production (unavailability) is below about 0.2 percent. Helium outflow and water inflow rates were characterized for the failures.

Barsell, A. W. (Arthur W.); Kern, K. T. (Kristen T.)

2001-01-01

103

Solid particle erosion mechanisms of protective coatings for aerospace applications  

NASA Astrophysics Data System (ADS)

The main objective of this PhD project is to investigate the material loss mechanisms during Solid Particle Erosion (SPE) of hard protective coatings, including nanocomposite and nanostructured systems. In addition, because of the complex nature of SPE mechanisms, rigorous testing methodologies need to be employed and the effects of all testing parameters need to be fully understood. In this PhD project, the importance of testing methodology is addressed throughout in order to effectively study the SPE mechanisms of brittle materials and coatings. In the initial stage of this thesis, we studied the effect of the addition of silicon (Si) on the microstructure, mechanical properties and, more specifically, on the SPE resistance of thick CrN-based coatings. It was found that the addition of Si significantly improved the erosion resistance and that SPE correlated with the microhardness values, i.e. the coating with the highest microhardness also had the lowest erosion rate (ER). In fact, the ERs showed a much higher dependence on the surface hardness than what has been proposed for brittle erosion mechanisms. In the first article, we study the effects of the particle properties on the SPE behavior of six brittle bulk materials using glass and alumina powders. First, we apply a robust methodology to accurately characterize the elasto-plastic and fracture properties of the studied materials. We then correlate the measured ER to materials' parameters with the help of a morphological study and an analysis of the quasi-static elasto-plastic erosion models. Finally, in order to understand the effects of impact on the particles themselves and to support the energy dissipation-based model proposed here, we study the particle size distributions of the powders before and after erosion testing. It is shown that tests using both powders lead to a material loss mechanism related to lateral fracture, that the higher than predicted velocity exponents point towards a velocity-dependent damage accumulation mechanism correlated to target yield pressure, and that damage accumulation effects are more pronounced for the softer glass powder because of kinetic energy dissipation through different means. In the second article, we study the erosion mechanisms for several hard coatings deposited by pulsed DC magnetron sputtering. We first validate a new methodology for the accurate measurement of volume loss, and we show the importance of optimizing the testing parameters in order to obtain results free from experimental artefacts. We then correlate the measured ERs to the material parameters measured by depth-sensing indentation. In order to understand the material loss mechanisms, we study three of the coating systems in greater detail with the help of fracture characterization and a morphological study of the eroded surfaces. Finally, we study the particle size distributions of the powders before and after erosion testing in an effort to understand the role of particle fracture. We demonstrate that the measured ERs of the coatings are strongly dependent on the target hardness and do not correlate with coating toughness. In fact, the material removal mechanism is found to occur through repeated ductile indentation and cutting of the surface by the impacting particles and that particle breakup is not sufficiently large to influence the results significantly. Studying SPE mechanisms of hard protective coating systems in detail has proven to be quite challenging in the past, given that conventional SPE testing is notoriously inaccurate due to its aggressive nature and its many methodological uncertainties. In the third article, we present a novel in situ real-time erosion testing methodology using a quartz crystal microbalance, developed in order to study the SPE process of hard protective coating systems. Using conventional mass loss SPE testing, we validate and discuss the advantages and challenges related to such a method. In addition, this time-resolved technique enables us to discuss some transient events present during SPE testing of hard coatin

Bousser, Etienne

104

Fracture mechanics life analytical methods verification testing  

NASA Technical Reports Server (NTRS)

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.

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

1994-01-01

105

Fracture mechanics approaches for characterizing creep-fatigue crack growth  

NASA Astrophysics Data System (ADS)

Recent developments and advances in linear and nonlinear fracture mechanics are reviewed. Current approaches for predicting crack growth under creep-fatigue loading are described and evaluated with particular emphasis on concepts of nonlinear fracture mechanics. The importance of loading frequency and waveform on the creep-fatigue crack growth behavior of high temperature alloys is demonstrated. Several examples of creep-fatigue crack growth data are given. Methods for determining crack tip parameters and the cyclic J integral are described. Experimental methods for obtaining creep-fatigue crack growth data and models for the prediction of crack growth in composites are discussed. Recommendations for areas of future research are given.

Saxena, Ashok

1993-01-01

106

Fracture mechanics evaluation for at typical PWR primary coolant pipe  

SciTech Connect

For the primary coolant piping of PWRs in Japan, cast duplex stainless steel which is excellent in terms of strength, corrosion resistance, and weldability has conventionally been used. The cast duplex stainless steel contains the ferrite phase in the austenite matrix and thermal aging after long term service is known to change its material characteristics. It is considered appropriate to apply the methodology of elastic plastic fracture mechanics for an evaluation of the integrity of the primary coolant piping after thermal aging. Therefore we evaluated the integrity of the primary coolant piping for an initial PWR plant in Japan by means of elastic plastic fracture mechanics. The evaluation results show that the crack will not grow into an unstable fracture and the integrity of the piping will be secured, even when such through wall crack length is assumed to equal the fatigue crack growth length for a service period of up to 60 years.

Tanaka, T. [Kansai Electric Power Company, Osaka (Japan); Shimizu, S.; Ogata, Y. [Mitsubishi Heavy Industries, Ltd., Kobe (Japan)

1997-04-01

107

Measurements of residual stress in fracture mechanics coupons  

SciTech Connect

This paper describes measurements of residual stress in coupons used for fracture mechanics testing. The primary objective of the measurements is to quantify the distribution of residual stress acting to open (and/or close) the crack across the crack plane. The slitting method and the contour method are two destructive residual stress measurement methods particularly capable of addressing that objective, and these were applied to measure residual stress in a set of identically prepared compact tension (C(T)) coupons. Comparison of the results of the two measurement methods provides some useful observations. Results from fracture mechanics tests of residual stress bearing coupons and fracture analysis, based on linear superposition of applied and residual stresses, show consistent behavior of coupons having various levels of residual stress.

Prime, Michael B [Los Alamos National Laboratory; Hill, Michael R [U.C. DAVIS; Nav Dalen, John E [HILL ENGINEERING

2010-01-01

108

Experimental study and analysis of mechanical properties of frozen rabbit aorta by fracture mechanics approach.  

PubMed

In order to investigate fracture problems associated with cryopreservation of aorta, experiments were carried out to study the effects of temperature, freezing rate and cryoprotective agent on the fracture mechanics properties of frozen rabbit aorta and a test method for crack criterion of frozen rabbit aorta was developed. Results showed that as temperature decreases, the fracture modes of frozen rabbit aorta changed from typical ductile fracture to typical brittle fracture, the crack was more difficult to grow as temperature decreased, and its resist-fracture ability weakened remarkably from -20 to -80 degrees C. However, freezing rates had no effects on the fracture modes when sample was cooled to -50 degrees C, while the resist-fracture ability was stronger when the sample was cooled at a higher freezing rate. Due to the hydration action of dimethyl sulphoxide (DMSO), the rabbit aorta permeated by 10% (v/v) DMSO presented typical ductile fracture when it was cooled to -50 degrees C, so its resist-fracture ability was enhanced obviously. Compared to the axial sample, the circumferential sample's resist-fracture ability is better because of its larger deformation energy available. PMID:17980371

Xu, Yi; Hua, Tse-Chao; Sun, Da-Wen; Zhou, Guo-Yan

2008-01-01

109

Investigation of the fracture mechanics of boride composites  

NASA Technical Reports Server (NTRS)

Significant results were obtained in fabrication studies of the role of metallic additives of Zr, Ti, Ni, Fe and Cr on the densification of ZrB2. All elemental additions lower the processing temperatures required to effect full densification of ZrB2. Each addition effects enhanced densification by a clearly distinguishable and different mechanism and the resulting fabricated materials are different. A significant improvement in strength and fracture toughness was obtained for the ZrB2/Ti composition. Mechanical characterization studies for the ZrB2/SiC/C composites and the new ZrB2/Metal materials produced data relevant to the effect of impacting load on measured impact energies, a specimen configuration for which controlled fracture could occur in a suitably hard testing apparatus, and fracture strength data. Controlled fracture--indicative of measurable fracture toughness--was obtained for the ZrB2-SiC-C composite, and a ZrB2/Ti composite fabricated from ZrB2 with an addition of 30 weight per cent Ti. The increased strength and toughness of the ZrB2/Ti composite is consistent with the presence of a significantly large amount of a fine grained acicular phase formed by reaction of Ti with ZrB2 during processing.

Clougherty, E. V.; Pober, R. L.; Kaufman, L.

1972-01-01

110

Ris-R-1411(EN) Fracture mechanics characterisation of  

E-print Network

, aircrafts and wind turbine blades are made of composite structures that are joined by adhesive bonds. It is therefore of interest to establish approaches for safe design of adhesively bonded compos- ite structuresRisø-R-1411(EN) Fracture mechanics characterisation of medium-size adhesive joint specimens Bent F

111

Integral equations with hypersingular kernelstheory and applications to fracture mechanics  

E-print Network

Integral equations with hypersingular kernels­­theory and applications to fracture mechanics Youn in revised form 3 July 2002; accepted 3 July 2002 Abstract Hypersingular integrals of the type IaðTn; m; r; Fredholm integral equation; Hypersingular integrals; Asymptotic analysis; Chebyshev polynomials

Fannjiang, Albert

112

FINITE FRACTURE MECHANICS OF MATRIX MICROCRACKING IN COMPOSITES  

E-print Network

,2]). Tensile loading, fatigue loading, environment, and thermal cycling can all lead to microcrack formation damage following complex loading conditions. This chapter describes a fracture mechanics approach the extent or number of microcracks that form under various loading histories. Some microcracking models have

Nairn, John A.

113

Fracture mechanics applied to the machining of brittle materials  

SciTech Connect

Research has begun on incorporating fracture mechanics into a model of the orthogonal cutting of brittle materials. Residual stresses are calculated for the machined material by a combination of Eulerian and Lagrangian finite element models and then used in the calculation of stress intensity factors by the Green`s Function Method.

Hiatt, G.D.; Strenkowski, J.S.

1988-12-01

114

Fracture mechanics and strength of materials: Advances and prospects  

Microsoft Academic Search

We analyze the principal stages of the development of fracture mechanics and strength of materials in the second half of the last century. Our attention is mainly focused on the analysis of the computational models of limiting equilibrium of deformable solid bodies with sharp stress concentrators (cracks), on the development of the methods aimed at evaluation of the stress intensity

V. V. Panasyuk

2004-01-01

115

Fracture mechanics and strength of materials: advances and prospects  

Microsoft Academic Search

We analyze the principal stages of the development of fracture mechanics and strength of materials in the second half of the last century. Our attention is mainly focused on the analysis of the computational models of limiting equilibrium of deformable solid bodies with sharp stress concentrators (cracks), on the development of the methods aimed at evaluation of the stress intensity

V. V. Panasyuk

2004-01-01

116

Strength of Silicon Wafers: Fracture Mechanics Approach  

SciTech Connect

This paper describes a model to predict mechanical strength distribution of silicon wafers. A generalized expression, based on a multimodal Weibull distribution, is proposed to describe the strength of a brittle material with surface, edge, and bulk flaws. The specific case of a cast, unpolished photovoltaic (PV) wafer is further analyzed. Assuming that surface microcracks constitute the dominant mechanism of wafer breakage, this model predicts the strength distribution of PV silicon that matches well the experimental results available in the literature.

Rupnowski, P.; Sopori, B.

2009-01-01

117

Unique Mechanism of Chance Fracture in a Young Adult Male  

PubMed Central

Since the first description of the Chance fracture in 1948, there have been few case reports of unique mechanisms causing this classical flexion-extension injury to the spine in motor vehicle accidents, sports injury, and falls. To our knowledge, this injury has not been reported from a fall with the mechanistic forces acting laterally on the spine and with spinal support in place. We present a 21-year-old male who slid down a flight of stairs onto his side wearing a heavy mountaineering style backpack, subsequently sustaining a Chance fracture of his first lumbar vertebrae. PMID:23599852

Birch, Aaron; Walsh, Ryan; Devita, Diane

2013-01-01

118

Fracture initiation mechanisms in ?-alumina under hypervelocity impact  

NASA Astrophysics Data System (ADS)

Atomistic mechanisms of crack initiation and fracture in ?-alumina under hypervelocity impact are investigated using large-scale molecular-dynamics simulations involving 540×106 atoms. The simulations reveal crack initiation due to amorphization, structural phase changes, and interaction between high strain rate deformation modes, which can advance at sustained speeds of 8km/s. Fracture toughness in the impact-damaged substrate is estimated to be around 2MPa?m . Spallation at the interface of the amorphous region and twins causes eventual failure. The favorable cleavage planes normal to basal planes are found to be {21¯1¯0}.

Zhang, Cheng; Kalia, Rajiv K.; Nakano, Aiichiro; Vashishta, Priya

2007-09-01

119

Mechanical Properties for Fracture Analysis of Mild Steel Storage Tasks  

Microsoft Academic Search

Mechanical properties of 1950's vintage, A285 Grade B carbon steels have been compiled for elastic-plastic fracture mechanics analysis of storage tanks (Lam and Sindelar, 1999). The properties are from standard Charpy V-notch (CVN), 0.4T planform Compact Tension (C(T)), and Tensile (T) specimens machined from archival steel from large water piping. The piping and storage tanks were constructed in the 1950s

R. L. Sindelar; P. S. Lam; G. R. Caskey

1999-01-01

120

Structure, mechanical properties, and fracture of 20GL cast steel  

NASA Astrophysics Data System (ADS)

The structure and mechanical properties of 20GL steel are studied. It is shown that a significant decrease in the ductility and impact toughness of the steel is caused by intercrystalline fracture, which is induced by a weakening of the intercrystallite bonds due to the existence of coarse lamellar pearlite and nonmetallic inclusions, namely, film inclusions and eutectic-type oxysulfides, at the boundaries of primary crystals. Annealing from a temperature in the intercritical range is found to improve the mechanical properties.

Schastlivtsev, V. M.; Tabatchikova, T. I.; Yakovleva, I. L.; Klyueva, S. Yu.

2014-04-01

121

Hydraulic fracture extending into network in shale: reviewing influence factors and their mechanism.  

PubMed

Hydraulic fracture in shale reservoir presents complex network propagation, which has essential difference with traditional plane biwing fracture at forming mechanism. Based on the research results of experiments, field fracturing practice, theory analysis, and numerical simulation, the influence factors and their mechanism of hydraulic fracture extending into network in shale have been systematically analyzed and discussed. Research results show that the fracture propagation in shale reservoir is influenced by the geological and the engineering factors, which includes rock mineral composition, rock mechanical properties, horizontal stress field, natural fractures, treating net pressure, fracturing fluid viscosity, and fracturing scale. This study has important theoretical value and practical significance to understand fracture network propagation mechanism in shale reservoir and contributes to improving the science and efficiency of shale reservoir fracturing design. PMID:25032240

Ren, Lan; Zhao, Jinzhou; Hu, Yongquan

2014-01-01

122

Hydraulic Fracture Extending into Network in Shale: Reviewing Influence Factors and Their Mechanism  

PubMed Central

Hydraulic fracture in shale reservoir presents complex network propagation, which has essential difference with traditional plane biwing fracture at forming mechanism. Based on the research results of experiments, field fracturing practice, theory analysis, and numerical simulation, the influence factors and their mechanism of hydraulic fracture extending into network in shale have been systematically analyzed and discussed. Research results show that the fracture propagation in shale reservoir is influenced by the geological and the engineering factors, which includes rock mineral composition, rock mechanical properties, horizontal stress field, natural fractures, treating net pressure, fracturing fluid viscosity, and fracturing scale. This study has important theoretical value and practical significance to understand fracture network propagation mechanism in shale reservoir and contributes to improving the science and efficiency of shale reservoir fracturing design. PMID:25032240

Ren, Lan; Zhao, Jinzhou; Hu, Yongquan

2014-01-01

123

Critical review of the state-of-the-art of fracture mechanics with emphasis on layered rocks  

SciTech Connect

Results are presented of a literature survey of over 70 pertinent publications and critical reviews of fracture mechanics emphasizing the fracture behavior of layered rocks. Historical perspective, fracture mechanisms, linear and nonlinear fracture mechanics, energy theories, ductile and brittle fractures, process regions, specific work of fracture, J-integrals, failure theories, static and dynamic fractures, rock fracture mechanics, fracture toughness of layered rocks (e.g., oil shale), experimental and numerical methods are reviewed and discussed. Innovative and promising methods tailored for the fracture mechanics of layered rocks are recommended.

Kuruppu, M.D.; Cheng, K.P.; Edl, J.N. Jr.

1983-07-01

124

Nonlinear fracture mechanics formability criterion for plastic deformation processes  

SciTech Connect

This work presents a model and an experimental procedure for building up the formability maps for metal forming based on nonlinear fracture mechanics. Formability is the limiting condition from which superficial or internal fractures propagate inside the plastic region in metal forming by compressive manufacturing process. The formability criterion developed in this paper through the Noether`s generalized invariant integral theorem is the best way to deal with near ductile metal in the, manufacturing processes. Model analysis will lead to the construction of the formability maps. It will show the limitation of plasticity theory for the analysis of the compressive type metal forming operations near a fracture region. It may put an upper boundary on the construction of an easy applicable formability criterion on practical problems from a day to day basis. This work establishes an experimental method based on the Noether`s invariant theorem, the HRR model for stress distribution near the crack tip in the deformed region, and Hill`s plasticity theorem. It results in an experimental method of low cost, and easy implementation for any single industrial laboratory. The mentioned procedure is applied to the analysis of a flat open die cold forging of a precipitation hardened UNS A92024 Al-Cu alloy. This will allow the construction of a formability model based on experimental results. The model to be discussed would lead to the generalization of a formability criterion in metal forming studies that would relate to on many practical problems. The authors carry out a review of the literature on nonlinear fracture mechanics, the conservation laws in finite elasticity and plasticity, and energy-release rates due to fracture propagation. Here the show the applied mechanics models that will collapse in a variational model for formability map construction.

Gomes, E. [Universidade de Sao Paulo (Brazil); Filho, E.B. [Universidade Estadual de Campinas (Brazil)

1995-12-31

125

Quantitative Integration of Ndt with Probabilistic Fracture Mechanics for the Assessment of Fracture Risk in Pipelines  

NASA Astrophysics Data System (ADS)

In the context of probabilistic paradigm of fracture risk assessment in structural components a computer simulation rationale is presented which has at the base the integration of Quantitative Non-destructive Inspection and Probabilistic Fracture Mechanics. In this study the static failure under static loading is assessed in the format known as Failure Assessment Diagram (FAD). The fracture risk is evaluated in probabilistic terms. The superposed probabilistic pattern over the deterministic one is implemented via Monte-Carlo sampling. The probabilistic fracture simulation yields a more informative analysis in terms of probability of failure. The ability to simulate the influence of the quality and reliability of non-destructive inspection (NDI) is an important feature of this approach. It is achieved by integrating, algorithmically, probabilistic FAD analysis and the Probability of Detection (POD). The POD information can only be applied in a probabilistic analysis and leads to a refinement of the assessment. By this means, it can be ascertained the decrease of probability of failure when POD-characterized NDI is applied. Therefore, this procedure can be used as a tool for inspection based life time conceptions. In this paper results of sensitivity analyses are presented with the aim to outline, in terms of non-failure probabilities, the benefits of applying NDI, in various qualities, in comparison with the situation when NDI is lacking. A better substantiation is enabled of both the component reliability management and the costs-effectiveness of NDI timing.

Kurz, J. H.; Cioclov, D.; Dobmann, G.; Boller, C.

2010-02-01

126

Extrinsic fracture mechanisms in two laminated metal composites  

SciTech Connect

The crack growth behavior and fracture toughness of two laminated metal composites (6090/SiC/25p laminated with 5182 and ultrahigh-carbon steel laminated with brass) have been studied in both ``crack arrester`` and ``crack divider`` orientations. The mechanisms of crack growth were analyzed and extrinsic toughening mechanisms were found to contribute significantly to the toughness. The influence of laminate architecture (layer thickness and component volume function), component material properties and residual stress on these mechanisms and the resulting crack growth resistance are discussed.

Lesuer, D.; Syn, C.; Riddle, R.; Sherby, O.

1994-11-29

127

A field theory of distortion incompatibility for coupled fracture and plasticity  

NASA Astrophysics Data System (ADS)

The displacement discontinuity arising between the crack surfaces is assigned to smooth areal/tensorial densities of crystal defects referred to as disconnections, through the incompatibility of the continuous distortion tensor. In a dual way, the disconnections are defined as line defects terminating surfaces where the displacement encounters a discontinuity. A conservation argument for their strength (the crack opening displacement) provides a natural framework for their dynamics in the form of a transport law for the disconnection densities. Similar methodology is applied to the discontinuity of the plastic displacement arising from the presence of dislocations in the body, which results in the concurrent involvement of the dislocation density tensor in the analysis. The present model can therefore be viewed as an extension of the mechanics of dislocation fields to the case where continuity of the body is disrupted by cracks. From the continuity of the elastic distortion tensor, it is expected that the stress field remains bounded everywhere in the body, including at the crack tip. Thermodynamic arguments provide the driving forces for disconnection and dislocation motion, and guidance for the formulation of constitutive relationships insuring non-negative dissipation. The conventional Peach-Koehler force on dislocations is retrieved in the analysis, and a Peach-Koehler-type force on disconnections is defined. A threshold in the disconnection driving force vs. disconnection velocity constitutive relationship provides for a Griffith-type fracture criterion. Application of the theory to the slit-crack (Griffith-Inglis crack) in elastic and elasto-plastic solids through finite element modeling shows that it allows recovering earlier results on the stress field around cracks, and that crack propagation can be consistently described by the transport scheme. Shielding/anti-shielding of cracks by dislocations is considered to illustrate the static/dynamic interactions between dislocations and disconnections resulting from the theory. Sample size effects on crack growth are evidenced in solids encountering plastic yielding.

Fressengeas, Claude; Taupin, Vincent

2014-08-01

128

Mechanical properties and fracture dynamics of silicene membranes.  

PubMed

As graphene has become one of the most important materials, there is renewed interest in other similar structures. One example is silicene, the silicon analogue of graphene. It shares some of the remarkable graphene properties, such as the Dirac cone, but presents some distinct ones, such as a pronounced structural buckling. We have investigated, through density functional based tight-binding (DFTB), as well as reactive molecular dynamics (using ReaxFF), the mechanical properties of suspended single-layer silicene. We calculated the elastic constants, analyzed the fracture patterns and edge reconstructions. We also addressed the stress distributions, unbuckling mechanisms and the fracture dependence on the temperature. We analysed the differences due to distinct edge morphologies, namely zigzag and armchair. PMID:25102369

Botari, T; Perim, E; Autreto, P A S; van Duin, A C T; Paupitz, R; Galvao, D S

2014-09-28

129

Mechanical Properties and Fracture Dynamics of Silicene Membranes  

E-print Network

As graphene became one of the most important materials today, there is a renewed interest on others similar structures. One example is silicene, the silicon analogue of graphene. It share some the remarkable graphene properties, such as the Dirac cone, but presents some distinct ones, such as a pronounced structural buckling. We have investigated, through density functional based tight-binding (DFTB), as well as reactive molecular dynamics (using ReaxFF), the mechanical properties of suspended single-layer silicene. We calculated the elastic constants, analyzed the fracture patterns and edge reconstructions. We also addressed the stress distributions, unbuckling mechanisms and the fracture dependence on the temperature. We analysed the differences due to distinct edge morphologies, namely zigzag and armchair.

Botari, T; Autreto, P A S; van Duin, A C T; Paupitz, R; Galvao, D S

2014-01-01

130

Combining damage and fracture mechanics to model calving  

NASA Astrophysics Data System (ADS)

Calving of icebergs is a major negative component of polar ice-sheet mass balance. We present a new calving modeling framework relying on both continuum damage mechanics and linear elastic fracture mechanics. This combination accounts for both the slow sub-critical surface crevassing and fast propagation of crevasses when calving occurs. First, damage of the ice occurs over long timescales and enhances the viscous flow of ice. Then brittle fracture propagation happens downward, over very short timescales, in ice considered as an elastic medium. The model is validated on Helheim Glacier, South-West Greenland, one of the most monitored fast-flowing outlet glacier. This allows to identify sets of model parameters giving a consistent response of the model and producing a dynamic equilibrium in agreement with observed stable position of the Helheim ice front between 1930 and today.

Krug, J.; Weiss, J.; Gagliardini, O.; Durand, G.

2014-03-01

131

Combining damage and fracture mechanics to model calving  

NASA Astrophysics Data System (ADS)

Calving of icebergs is a major negative component of polar ice-sheet mass balance. Here we present a new calving model relying on both continuum damage mechanics and linear elastic fracture mechanics. This combination accounts for both the slow sub-critical surface crevassing and the rapid propagation of crevasses when calving occurs. First, damage to the ice occurs over long timescales and enhances the viscous flow of ice. Then brittle fractures propagate downward, at very short timescales, when the ice body is considered as an elastic medium. The model was calibrated on Helheim Glacier, Southeast Greenland, a well-monitored glacier with fast-flowing outlet. This made it possible to identify sets of model parameters to enable a consistent response of the model and to produce a dynamic equilibrium in agreement with the observed stable position of the Helheim ice front between 1930 and today.

Krug, J.; Weiss, J.; Gagliardini, O.; Durand, G.

2014-11-01

132

Discrete fracture patterns of virus shells reveal mechanical building blocks  

NASA Astrophysics Data System (ADS)

Viral shells are self-assembled protein nanocontainers with remarkable material properties. They combine simplicity of construction with toughness and complex functionality. To date we know little about how virus structure determines assembly pathways and shell mechanics. We have used atomic force microscopy to study structural failure of the shells of the bacteriophage ?29. We observed rigidity patterns following the symmetry of the capsid proteins and under prolonged force exertion, we see fractures along well-defined lines of the 2D crystal lattice. We found the mechanically most stable building block of the shells was a trimer. Our approach of ``reverse engineering'' the virus shells thus made it possible to identify stable structural intermediates. Such stable intermediates point to a hierarchy of interactions among equal building blocks correlated with distinct next-neighbor interactions. The results also demonstrate that concepts from macroscopic materials science, such as fracture, can be usefully employed in molecular engineering.

Ivanovska, Irena L.; Miranda, Roberto; Carrascosa, José. L.; Wuite, Gijs J. L.; Schmidt, Christoph F.

2012-02-01

133

Fracture mechanics database for nuclear piping materials (PIFRAC)  

SciTech Connect

This paper describes a material database called PIFRAC for nuclear piping materials and its application and use in the nuclear piping industry. The PIFRAC database currently has 914 Tensile specimens, 820 J-R specimens and 920 Charpy specimens. The first section of the paper describes the details of the database tables and structure. The second section of the paper describes two applications where the database was used in performing fracture mechanics calculations.

Ghadiali, N.; Wilkowski, G. [Battelle Columbus Labs., OH (United States). Engineering Mechanics Group

1996-12-01

134

Results of fracture mechanics tests on PNC SUS 304 plate  

SciTech Connect

PNC provided SUS 304 plate to be irradiated in FFTF at about 400/sup 0/C to a target fluence of 5 x 10/sup 21/ n/cm/sup 2/ (E > 0.1 MeV). The actual irradiation included two basically different exposure levels to assure that information would be available for the exposure of interest. After irradiation, tensile properties, fatigue-crack growth rates and J-integral fracture toughness response were determined. These same properties were also measured for the unirradiated material so radiation damage effects could be characterized. This report presents the results of this program. It is expected that these results would be applicable for detailed fracture analysis of reactor components. Recent advances in elastic-plastic fracture mechanics enable reasonably accurate predictions of failure conditions for flawed stainless steel components. Extensive research has focused on the development of J-integral-based engineering approach for assessing the load carrying capacity of low-strength, high-toughness structural materials. Furthermore, Kanninen, et al., have demonstrated that J-integral concepts can accurately predict the fracture response for full-scale cracked structures manufactured from Type 304 stainless steel.

Mills, W.J.; James, L.A.; Blackburn, L.D.

1985-08-01

135

Mechanical and fracture behavior of calcium phosphate cements  

NASA Astrophysics Data System (ADS)

Apatite-based calcium phosphate cements are currently employed to a limited extent in the biomedical and dental fields. They present significant potential for a much broader range of applications, particularly as a bone mineral substitute for fracture fixation. Specifically, hydroxyapatite (HA) is known for its biocompatibility and non-immunogenicity, attributed to its similarity to the mineral phase of natural bone. The advantages of a cement-based HA include injectability, greater resorbability and osteoconductivity compared to sintered HA, and an isothermal cement-forming reaction that avoids necrosis during cement setting. Although apatite cements demonstrate good compressive strength, tensile properties are very weak compared to natural bone. Applications involving normal weight-bearing require better structural integrity than apatite cements currently provide. A more thorough understanding of fracture behavior can elucidate failure mechanisms and is essential for the design of targeted strengthening methods. This study investigated a hydroxyapatite cement using a fracture mechanics approach, focusing on subcritical crack growth properties. Subcritical crack growth can lead to much lower load-bearing ability than critical strength values predict. Experiments show that HA cement is susceptible to crack growth under both cyclic fatigue-crack growth and stress corrosion cracking conditions, but only environmental, not mechanical, mechanisms contribute to crack extension. This appears to be the first evidence ever presented of stress corrosion crack growth behavior in calcium phosphate cements. Stress corrosion cracking was examined for a range of environmental conditions. Variations in pH have surprisingly little effect. Behavior in water at elevated temperature (50°C) is altered compared to water at ambient temperature (22°C), but only for crack-growth velocities below 10-7 m/s. However, fracture resistance of dried HA cement in air increases significantly compared to in water. Based on observed trends, mechanisms of stress corrosion cracking are considered. Strengthening methods using proteins as second phase additions to HA cement were also investigated. Critical flexure strength of these composites increases to a limited extent, primarily due to bridging of the fracture surfaces by organic phases. Despite the increase for critical values, stress corrosion crack growth of cement-albumin composites remains similar to unreinforced cement. This discrepancy between critical and subcritical behavior is discussed.

Jew, Victoria Chou

136

LOW VELOCITY SURFACE FRACTURE PATTERNS IN BRITTLE MATERIAL: A NEWLY EVIDENCED MECHANICAL INSTABILITY  

E-print Network

LOW VELOCITY SURFACE FRACTURE PATTERNS IN BRITTLE MATERIAL: A NEWLY EVIDENCED MECHANICAL is well known to occur in brittle fracture and significant advances have recently been obtained in nominally brittle materials are well described by Linear Elastic Fracture Mechanisms, without any

Paris-Sud XI, Université de

137

Predicting Aperture and Length Population Statistics Using a Fracture Mechanics Model  

NASA Astrophysics Data System (ADS)

Diagenesis and fracturing are often linked processes in deformed rock, particularly with regard to veins. The nature of preserved aperture distributions in veins, crack-seal textures, cement bridges across fractures, and fracture porosity preservation under effective compressive stress suggest that mineral precipitation and fracture growth often occur simultaneously. A fracture mechanics model is used to examine the systematics of aperture and length distributions generated by subcritical crack growth under uniform-rate, extensional-strain boundary conditions for stratabound fractures, with and without simulated diagenesis. Numerical results show considerable variability in fracture aperture distributions, but in many cases, aperture populations follow negative exponential curve shapes, consistent with published field observations for stratabound fractures in sedimentary rock. Fracture aperture closure under effective compressive stress is examined with respect to diagenetic stiffening of the rock matrix during the fracturing event. The numerical model also predicts episodic crack propagation that could be analogous to crack-seal vein growth. Crack growth reconstructions through time of numerically-generated orthogonal patterns show that the longest fractures are those that grow first, and these early fractures reach their fullest extent before shorter fractures begin growing. The generated length distributions are predominantly power-law in shape, with the slope established early in the fracture growth history by the longer fractures. Shorter fractures grow to the extent required to extend the power-law shape to smaller sizes. Uniform orientation (single set) patterns of straight fractures show negative exponential to power-law length distributions with different growth histories.

Olson, J. E.

2005-12-01

138

Measurement of residual stresses using fracture mechanics weight functions  

SciTech Connect

A residual stress measurement method has been developed to quantify through-the-thickness residual stresses. Accurate measurement of residual stresses is crucial for many engineering structures. Fabrication processes such as welding and machining generate residual stresses that are difficult to predict. Residual stresses affect the integrity of structures through promoting failures due to brittle fracture, fatigue, stress corrosion cracking, and wear. In this work, the weight function theory of fracture mechanics is used to measure residual stresses. The weight function theory is an important development in computational fracture mechanics. Stress intensity factors for arbitrary stress distribution on the crack faces can be accurately and efficiently computed for predicting crack growth. This paper demonstrates that the weight functions are equally useful in measuring residual stresses. In this method, an artificial crack is created by a thin cut in a structure containing residual stresses. The cut relieves the residual stresses normal to the crack-face and allows the relieved residual stresses to deform the structure. Strain gages placed adjacent to the cut measure the relieved strains corresponding to incrementally increasing depths of the cut. The weight functions of the cracked body relate the measured strains to the residual stresses normal to the cut within the structure. The procedure details, such as numerical integration of the singular functions in applying the weight function method, will be discussed.

Fan, Y.

2000-10-01

139

Damage mechanisms in the dynamic fracture of nominally brittle polymers  

E-print Network

Linear Elastic Fracture Mechanics (LEFM) provides a consistent framework to evaluate quantitatively the energy flux released to the tip of a growing crack. Still, the way in which the crack selects its velocity in response to this energy flux remains far from completely understood. To uncover the underlying mechanisms, we experimentally studied damage and dissipation processes that develop during the dynamic failure of polymethylmethacrylate (PMMA), classically considered as the archetype of brittle amorphous materials. We evidenced a well-defined critical velocity along which failure switches from nominally-brittle to quasi-brittle, where crack propagation goes hand in hand with the nucleation and growth of microcracks. Via post-mortem analysis of the fracture surfaces, we were able to reconstruct the complete spatiotemporal microcracking dynamics with micrometer/nanosecond resolution. We demonstrated that the true local propagation speed of individual crack fronts is limited to a fairly low value, which can be much smaller than the apparent speed measured at the continuum-level scale. By coalescing with the main front, microcracks boost the macroscale velocity through an acceleration factor of geometrical origin. We discuss the key role of damage-related internal variables in the selection of macroscale fracture dynamics.

Davy Dalmas; Claudia Guerra; Julien Scheibert; Daniel Bonamy

2013-04-23

140

Biomechanical mechanisms of orbital wall fractures - a transient finite element analysis.  

PubMed

As the biomechanical mechanisms of orbital wall fractures are still under research, three different fracture mechanisms were tested in a finite element based investigation. In contrast to earlier studies, a finer skeletal model and a transient dynamic simulation were used to test pure hydraulic, pure buckling and a mixed force transmission. Results showed that each set-up led to different orbital fracture patterns, which correlate well with clinical findings. Therefore the conclusion is that different mechanisms may act together explaining the variety of clinical fracture situations. Biomechanical testing has proven to be appropriate in answering questions regarding fracture mechanisms. PMID:22417768

Schaller, Andreas; Huempfner-Hierl, Heike; Hemprich, Alexander; Hierl, Thomas

2013-12-01

141

A fracture-mechanical cellular automaton model of seismicity  

NASA Astrophysics Data System (ADS)

We present a cellular automaton model which simulates the process of seismogenesis using rules for evolution which are derived from the field of fracture mechanics, and include an interplay of positive and negative feedbacks. We describe the implementation of this model, and its analysis, in a massively parallel environment using the Connection Machine. Starting from a lattice with a fractal distribution of fracture toughnesses, the b value evolves in a way which closely mimics both the evolutions of b value observed in the laboratory and derived from earthquake catalogues, reaching a broad and irregular maximum in the period preceding a major event, and declining rapidly during catastrophic failure. We conclude that the processes modelled are a reasonable representation of those occurring in Nature, and that the cellular automaton paradigm is a valuable way of simulating these processes on a large scale in an economical manner.

Henderson, Jeremy R.; Main, Ian G.; Maclean, Calum; Norman, Michael G.

1994-09-01

142

Effects of hydrogen on mechanical properties and fracture mechanism of 8090 Al-Li alloy  

NASA Astrophysics Data System (ADS)

The effects of hydrogen and strain rate on the mechanical properties and fracture mechanism of 8090 Al-Li alloy under electrochemical charging conditions have been studied. Experimental results demonstrate that the tensile strength [ultimate tensile strength (UTS) and yield strength (YS)] and plasticity [reduction of area (RA) and elongation (EL)] drop linearly with the decrease of strain rate. The charged hydrogen increases the tensile strength but markedly impair the plasticity. The susceptibility of hydrogen embrittlement increases with the decreases of strain rate, and the susceptibility of the charged specimens was larger than that of the uncharged ones over the strain-rate range. Observation by scanning electron microscope (SEM) reveals that the charged hydrogen enhances intergranular delamination cracking on the fracture surface. The fracture model of charged specimens at low strain rates(dot \\varepsilon {text{< 3}}{text{.4 X 10}}^{{text{ - 4}}} /s) is grain boundary brittle fracture (GBBF), while that of other conditions is grain boundary ductile fracture (GBDF). Secondary ion mass spectroscopy (SIMS) study shows that the atomic binding energy of Al and Li in the alloy decreased after hydrogen charging, and the atomic binding energy drop of the former is more than the latter. In this article, the hydrogen transport through the mobile dislocation mechanism of hydrogen-induced fracture and the hydrogen effect on atomic binding energy were also discussed in detail.

Chen, Lian; Chen, Wenxiu; Liu, Zhonghao; Shao, Yuxia; Hu, Zhuangqi

1993-06-01

143

Mechanics and fracture of hybrid material interface bond  

NASA Astrophysics Data System (ADS)

Considering current and future applications of hybrid materials and structures in civil engineering, the strength and durability of interface bond between the conventional materials and composites are critical to development of such products. Conventional methods mostly used for analysis of isotropic materials may not be well suitable or accurate enough for a system made of anisotropic materials with relatively low shear stiffness. A need exists for developing more accurate and explicit analytical solutions for hybrid material interface analysis and related novel experimental characterization techniques. In this study, a combined analytical and experimental approach to characterize hybrid material interface bond is developed. Using a shear deformable plate theory and an elastic interface model, a mechanics approach for interface analysis of hybrid material bond under general loading is first proposed. The resulting closed-form solution of interface stress distribution is used to compute strain energy release rate (SERB) and stress intensity factor (SIF) of the interface with or without adhesive bond. This approach is then extended to delamination of composite structures under generic loading conditions. Second, novel experimental approaches for characterization of hybrid material bonded interfaces are presented. To account for the crack tip deformations, a tapered beam on elastic foundation (TBEF) is developed. Based on the TBEF model, analysis and design of two novel fracture specimens, Tapered Double Cantilever Beam (TDCB) and Tapered End Notched Flexure (TENF), are proposed, and they are effectively used in fracture toughness tests of bonded interface under Mode-I and Mode-II loadings, respectively. A constant compliance rate change over certain crack length range is achieved for the TDCB and TENF specimens, and it alleviates the necessity of experimental compliance calibration tests. The fracture toughness data obtained from the experiments are useful to predict potential crack growth of adhesively bonded joints. In summary, a comprehensive study of mechanics and fracture of hybrid material interface bond is presented. The shear deformable-based mechanics and fracture model developed can be effectively and accurately used to predict the SERR and SIF of hybrid material interface bond, and the analysis and design of novel TDCB and TENF specimens provide useful and efficient techniques to experimentally characterize the hybrid material bonded interfaces.

Wang, Jialai

144

Mechanical degradation of fuel cell membranes under fatigue fracture tests  

NASA Astrophysics Data System (ADS)

The effects of cyclic stresses on the fatigue and mechanical stability of perfluorosulfonic acid (PFSA) membranes are experimentally investigated under standard fuel cell conditions. The experiments are conducted ex-situ by subjecting membrane specimens to cyclic uniaxial tension at controlled temperature and relative humidity. The fatigue lifetime is measured in terms of the number of cycles until ultimate fracture. The results indicate that the membrane fatigue lifetime is a strong function of the applied stress, temperature, and relative humidity. The fatigue life increases exponentially with reduced stresses in all cases. The effect of temperature is found to be more significant than that of humidity, with reduced fatigue life at high temperatures. The maximum membrane strain at fracture is determined to decrease exponentially with increasing membrane lifetime. At a given fatigue life, a membrane exposed to fuel cell conditions is shown to accommodate more plastic strain before fracture than one exposed to room conditions. Overall, the proposed ex-situ membrane fatigue experiment can be utilized to benchmark the fatigue lifetime of new materials in a fraction of the time and cost associated with conventional in-situ accelerated stress testing methods.

M. H. Khorasany, Ramin; Sadeghi Alavijeh, Alireza; Kjeang, Erik; Wang, G. G.; Rajapakse, R. K. N. D.

2015-01-01

145

Failure mechanisms and surface roughness statistics of fractured Fontainebleau sandstone.  

PubMed

In an effort to investigate the link between failure mechanisms and the geometry of fractures of compacted grains materials, a detailed statistical analysis of the surfaces of fractured Fontainebleau sandstones has been achieved. The roughness of samples of different widths W is shown to be self-affine with an exponent zeta=0.46+/-0.05 over a range of length scales ranging from the grain size d up to an upper cutoff length xi approximately =0.15 W. This low zeta value is in agreement with measurements on other sandstones and on sintered materials. The probability distributions pi delta z(delta h) of the variations of height over different distances delta z>d can be collapsed onto a single Gaussian distribution with a suitable normalization and do not display multiscaling features. The roughness amplitude, as characterized by the height-height correlation over fixed distances delta z, does not depend on the sample width, implying that no anomalous scaling of the type reported for other materials is present. It is suggested, in agreement with recent theoretical work, to explain these results by the occurrence of brittle fracture (instead of damage failure in materials displaying a higher value of zeta approximately =0.8 ). PMID:17930307

Ponson, L; Auradou, H; Pessel, M; Lazarus, V; Hulin, J P

2007-09-01

146

(Environmental and geophysical modeling, fracture mechanics, and boundary element methods)  

SciTech Connect

Technical discussions at the various sites visited centered on application of boundary integral methods for environmental modeling, seismic analysis, and computational fracture mechanics in composite and smart'' materials. The traveler also attended the International Association for Boundary Element Methods Conference at Rome, Italy. While many aspects of boundary element theory and applications were discussed in the papers, the dominant topic was the analysis and application of hypersingular equations. This has been the focus of recent work by the author, and thus the conference was highly relevant to research at ORNL.

Gray, L.J.

1990-11-09

147

Fracture mechanics analyses for skin-stiffener debonding  

NASA Technical Reports Server (NTRS)

The debond configurations presently subjected to 3D FEM fracture mechanics analyses are respectively of the flange-skin strip and skin-stiffener configuration type. Two methods employing the virtual crack closure technique were used to evaluate the strain energy release rate, or 'G-value' distributions across the debond front. Both methods yielded nearly identical G-value distributions for the debond configurations studied; they were compared with plane strain and shell analyses results from the literature for the flange skin strip configuration, and found to be in good agreement. Mode II is dominant for the skin-stiffener debond configuration.

Raju, I. S.; Sistla, R.; Krishnamurthy, T.; Lotts, C. G.

1993-01-01

148

Probabilistic/Fracture-Mechanics Model For Service Life  

NASA Technical Reports Server (NTRS)

Computer program makes probabilistic estimates of lifetime of engine and components thereof. Developed to fill need for more accurate life-assessment technique that avoids errors in estimated lives and provides for statistical assessment of levels of risk created by engineering decisions in designing system. Implements mathematical model combining techniques of statistics, fatigue, fracture mechanics, nondestructive analysis, life-cycle cost analysis, and management of engine parts. Used to investigate effects of such engine-component life-controlling parameters as return-to-service intervals, stresses, capabilities for nondestructive evaluation, and qualities of materials.

Watkins, T., Jr.; Annis, C. G., Jr.

1991-01-01

149

Application of elastic-plastic fracture mechanics to marine structures  

E-print Network

, but the specific toughness tests for material purchase or quality control are in terms of auxiliary test specimens such as the CVN impact test specimen. Therefore to Journal Model is ASTM Journal of Testing aad Esaluation be able to use fracture mechanics ef... length are both greater than a certain minimum value. For example, the American Society for Testing and Material (ASTM) standard for Itic testing includes the following requirement. B, b = 2. 5 (1. 22) where B is is the specimen thickness and b...

Pathi, Amarkumar

2012-06-07

150

Variational fracture mechanics The fracture pattern in stressed bodies is defined through the minimization of a two-field pseudo-spatial-  

E-print Network

., Francfort, G. A. and J. J. Marigo, Numerical experiments in revisited brittle fracture. J. Mech. PhysVariational fracture mechanics The fracture pattern in stressed bodies is defined through-deviatoric and masonry-like fractures. Remarkably, this latter formulation rigorously avoid material overlapping

Segatti, Antonio

151

Molecular dynamics simulations of atomic-level brittle fracture mechanisms in amorphous silica  

E-print Network

Molecular dynamics simulations of atomic-level brittle fracture mechanisms in amorphous silica Abstract We have examined the atomic dynamics of the brittle fracture process in amorphous silica using the change in local coordina- tion of atoms. Introduction The brittle fracture process has been a subject

Deymier, Pierre

152

[Current concepts of the mechanisms of formation of gunshot fractures].  

PubMed

The mechanisms of formation of gunshot fractures in flat bones inflicted by a semispherical bullet were investigated using expert and experimental materials. The process of crack formation was considered in terms of the Hertzian contact problem and the Hill-Johnson model. It was shown that the fracture develops as a result of combination of stresses and strains in the bone tissue leading to the formation of a hydrostatic nucleus prior to tissue fragmentation. Dynamic fluctuations (waves) generated in the zone of hydrostatic compression resulting from the gunshot injury propagate with the velocity of sound from the nucleus in the direction of the bullet movement. According to the Hill-Johnson model, the waves propagate in the direction of the impact within a parabolically expanding space; this accounts for the mechanism of formation of parabolic cracks and the specific shape of the defect that the bullet produces in the flat bones. The dynamic load applied by means of an indentor forms at a higher rate than the velocity of sound in the affected material. It gives reason to consider the effect of a bullet moving with the speed of 250 m/s as quasi-static loading. The results obtained in this study make a contribution to the theory of impact effect of a bullet and provide a deeper insight into the physical nature of the direct and sideway action of a gunshot projectile. Moreover, they explain the cause behind the widening of the outlet part of the perforating fracture in the flat bones. PMID:25474911

2013-01-01

153

[Current concepts of the mechanisms of formation of gunshot fractures].  

PubMed

The mechanisms of formation of gunshot fractures in flat bones inflicted by a semispherical bullet were investigated using expert and experimental materials. The process of crack formation was considered in terms of the Hertzian contact problem and the Hill-Johnson model. It was shown that the fracture develops as a result of combination of stresses and strains in the bone tissue leading to the formation of a hydrostatic nucleus prior to tissue fragmentation. Dynamic fluctuations (waves) generated in the zone of hydrostatic compression resulting from the gunshot injury propagate with the velocity of sound from the nucleus in the direction of the bullet movement. According to the Hill-Johnson model, the waves propagate in the direction of the impact within a parabolically expanding space; this accounts for the mechanism of formation of parabolic cracks and the specific shape of the defect that the bullet produces in the flat bones. The dynamic load applied by means of an indentor forms at a higher rate than the velocity of sound in the affected material. It gives reason to consider the effect of a bullet moving with the speed of 250 m/s as quasi-static loading. The results obtained in this study make a contribution to the theory of impact effect of a bullet and provide a deeper insight into the physical nature of the direct and sideway action of a gunshot projectile. Moreover, they explain the cause behind the widening of the outlet part of the perforating fracture in the flat bones. PMID:25508690

2013-01-01

154

Mechanical Properties and Fracture Behavior of Nanoporous Au  

SciTech Connect

Nanoporous metals have recently attracted considerable interest fueled by potential sensor and actuator applications. From a material science point of view, one of the key issues in this context is the synthesis of nanoporous metals with both high tensile and compressive strength. Nanoporous gold (np-Au) has been suggested as a candidate material for this application due to its monolithic character. The material can be synthesized by electrochemically-driven dealloying of Ag-Au alloys, and exhibits an open sponge-like structure of interconnecting ligaments with a typical pore size distribution on the nanometer length scale. However, besides the observation of a ductile-brittle transition very little is known about the mechanical behavior of this material. Here, we present our results regarding the mechanical properties and the fracture behavior of np-Au. Depth-sensing nanoindentation reveals that the yield strength of np-Au is almost one order of magnitude higher than the value predicted by scaling laws developed for macroscopic open-cell foams. The unexpectedly high value of the yield strength indicates the presence of a distinct size effect of the mechanical properties due to the sub-micron dimensions of the ligaments, thus potentially opening a door to a new class of high yield strength--low density materials. The failure mechanism of np-Au under tensile stress was evaluated by microscopic examination of fracture surfaces using scanning electron microscopy. On a macroscopic level, np-Au is a very brittle material. However, microscopically np-Au is very ductile as ligaments strained by as much as 200% can be observed in the vicinity of crack tips. Cell-size effects on the microscopic failure mechanism were studied by annealing experiments whereby increasing the typical pore size/ligament diameter from {approx}100 nm to {approx}1{micro}m.

Biener, J; Hodge, A M; Wang, Y M; Hayes, J R; Hamza, A V

2005-06-16

155

Fundamental mechanisms of tensile fracture in aluminum sheet undirectionally reinforced with boron filament  

NASA Technical Reports Server (NTRS)

Results are presented from an experimental study of the tensile-fracture process in aluminum sheet unidirectionally reinforced with boron filament. The tensile strength of the material is severely limited by a noncumulative fracture mechanism which involves the initiation and sustenance of a chain reaction of filament fractures at a relatively low stress level. Matrix fracture follows in a completely ductile manner. The minimum filament stress for initiation of the fracture mechanism is shown to be approximately 1.17 GN/sq m (170 ksi), and appears to be independent of filament diameter, number of filament layers, and the strength of the filament-matrix bond. All the commonly observed features of tensile fracture surfaces are explained in terms of the observed noncumulative fracture mechanism.

Herring, H. W.

1972-01-01

156

Chemical and Mechanical Alteration of Fractures: Micro-Scale Simulations and Comparison to Experimental Results  

NASA Astrophysics Data System (ADS)

Fractures are often the main pathways for subsurface fluid flow especially in rocks with low matrix porosity. Therefore, the hydro-mechanical properties of fractures are of fundamental concern for subsurface CO2 sequestration, enhanced geothermal energy production, enhanced oil recovery, and nuclear waste disposal. Chemical and mechanical stresses induced during these applications may lead to significant alteration of the hydro-mechanical properties of fractures. Laboratory experiments aimed at understanding the chemo-hydro-mechanical response of fractures have shown a range of results that contradict simple conceptual models. For example, under conditions favoring mineral dissolution, where one would expect an overall increase in permeability and fracture aperture, permeability increases under some conditions and decreases under others. Recent experiments have attempted to link these core-scale observations to the relevant small-scale processes occurring within fractures. Results suggest that the loss of mechanical strength in asperities due to chemical alteration may cause non-uniform deformation and alteration of fracture apertures. However, it remains difficult to directly measure the coupled chemical and mechanical processes that lead to alteration of contacting fracture surfaces, which challenges our ability to predict the long-term evolution of the hydro-mechanical properties of fractures. Here, we present a computational model that uses micro-scale surface roughness and explicitly couples dissolution and elastic deformation to calculate local alterations in fracture aperture under chemical and mechanical stresses. Chemical alteration of the fracture surfaces is modeled using a depth-averaged algorithm of fracture flow and reactive transport. Then, we deform the resulting altered fracture-surfaces using an algorithm that calculates the elastic deformation. Nonuniform dissolution may cause the location of the resultant force between the two contacting surfaces to shift away from the equilibrium location. We apply a relative rotation of the fracture surfaces to preserve force equilibrium during each iteration. The results of the model are compared with flow-through experiments conducted on fractured limestone cores and on analogue rough-surfaced KDP-glass fractures. The fracture apertures are mapped before, during (for some) and after the experiments. These detailed aperture measurements are used as input to our new coupled model. The experiments cover a wide range of transport and reaction conditions; some exhibit permeability increase due to channel formation and others exhibit fracture closure due to deformation of contacting asperities. Simulation results predict these general trends as well as the small-scale details in regions of contacting asperities.n example of an aperture field under chemical and mechanical alterations. The color scale is in microns.

Ameli, P.; Detwiler, R. L.; Elkhoury, J. E.; Morris, J. P.

2012-12-01

157

Effects of hydrogen on mechanical properties and fracture mechanism of 8090 Al-Li alloy  

Microsoft Academic Search

The effects of hydrogen and strain rate on the mechanical properties and fracture mechanism of 8090 Al-Li alloy under electrochemical\\u000a charging conditions have been studied. Experimental results demonstrate that the tensile strength [ultimate tensile strength\\u000a (UTS) and yield strength (YS)] and plasticity [reduction of area (RA) and elongation (EL)] drop linearly with the decrease\\u000a of strain rate. The charged hydrogen

Lian Chen; Wenxiu Chen; Zhonghao Liu; Yuxia Shao; Zhuangqi Hu

1993-01-01

158

On study of nonclassical problems of fracture and failure mechanics and related mechanisms  

Microsoft Academic Search

Nonclassical problems of fracture and failure mechanics that have been analyzed by the author and his collaborators at the\\u000a S. P. Timoshenko Institute of Mechanics (Kiev, National Academy of Sciences of Ukraine) during the past forty years are considered\\u000a in brief. The results of the analysis are presented in a form that would be quite informative for the majority of

Alexander N. Guz

2009-01-01

159

Mechanical rock properties, fracture propagation and permeability development in deep geothermal reservoirs  

NASA Astrophysics Data System (ADS)

Deep geothermal reservoirs are rock units at depths greater than 400 m from which the internal heat can be extracted using water as a transport means in an economically efficient manner. In many geothermal reservoirs, fluid flow is largely, and may be almost entirely, controlled by the permeability of the fracture network. No flow, however, takes place along a particular fracture network unless the fractures are interconnected. For fluid flow to occur from one site to another there must be at least one interconnected cluster of fractures that links these sites, that is, the percolation threshold must be reached. In "hydrothermal systems", only the natural fracture system (extension and shear fractures) creates the rock or reservoir permeability that commonly exceeds the matrix permeability by far; in "petrothermal systems", by contrast, interconnected fracture systems are formed by creating hydraulic fractures and massive hydraulic stimulation of the existing fracture system in the host rock. Propagation (or termination, that is, arrest) of both natural extension and shear fractures as well as man-made hydraulic fractures is mainly controlled by the mechanical rock properties, particularly rock toughness, stiffness and strengths, of the host rock. Most reservoir rocks are heterogeneous and anisotropic, in particular they are layered. For many layered rocks, the mechanical properties, particularly their Young's moduli (stiffnesses), change between layers, that is, the rocks are mechanically layered. Mechanical layering may coincide with changes in grain size, mineral content, fracture frequencies, or facies. For example, in sedimentary rocks, stiff limestone or sandstone layers commonly alternate with soft shale layers. In geothermal reservoirs fracture termination is important because non-stratabound fractures, that is, fractures not affected by layering, are more likely to form an interconnected fracture network than stratabound fractures, confined to single rock layers. Thus, to minimise exploration risks and for effective stimulation, the geometry of the fracture system and the mechanical properties of the host rock must be known. Here we present first results of structural geological field studies of fracture systems in outcrop analogues studies of rocks that could be used to host man-made geothermal reservoirs in sedimentary rocks in the North German Basin. As examples, we show data from different lithologies, including Buntsandstein (Lower Triassic), a sandstone-shale succession and Muschelkalk (Middle Triassic), a limestone-marl succession. We analyse natural fracture systems and the effects of rock heterogeneities, particularly stiffness variations between layers (mechanical layering) on the propagation of natural fractures. Important fracture parameters include attitude, aperture and interconnectivity to fracture systems. The field studies are supplemented by laboratory measurements of the above mentioned rock mechanical properties. Our field studies indicate that many fractures become arrested at layer contacts, particularly at contacts between layers with contrasting mechanical properties. Measurements of thousands of fractures indicate that even very thin layers (mm to cm-scale thicknesses) of shale or marl may be responsible for the arrest of many fractures. Our results suggest that the propagation and aperture variation of fractures are important parameters in the permeability development of deep geothermal reservoirs. These studies provide a basis for models of fracture networks and fluid transport in future man-made reservoirs. We conclude that the likely permeability of a man-made geothermal reservoir can be inferred from field data from outcrop analogues, laboratory measurements, and numerical models.

Leonie Philipp, Sonja; Reyer, Dorothea

2010-05-01

160

Thermal-Hydrologic-Mechanical Behavior of Single Fractures in EGS Reservoirs  

NASA Astrophysics Data System (ADS)

Enhanced Geothermal Systems (EGS) rely on the creation a connected fracture system or the enhancement of existing (natural) fractures by hydraulic and chemical treatments. EGS studies at Fenton Hill (New Mexico, USA) and Hijiori (Japan) have revealed that only a limited number of fractures contribute to the effective heat transfer surface area. Thus, the economic viability of EGS depends strongly on the creation and spacing of single fractures in order to efficiently mine heat from given volume of rock. Though there are many similarities between EGS and natural geothermal reservoirs, a major difference between the reservoir types is the (typically) high pumping pressures and induced thermal stresses at the injection wells of an EGS reservoir. These factors can be responsible for fracture dilation/extension and thermal short circuiting and depend strongly on the surrounding state of stress in the reservoir and mechanical properties. We will present results from our study of the thermal-hydrologic-mechanical (THM) behavior of a single fracture in a realistic subsurface stress field. We will show that fracture orientation, the stress environment, fracture permeability structure, and the relationship between permeability changes in a fracture resulting from mechanical displacement are all important when designing and managing an EGS reservoir. Lastly, we present a sensitivity analysis of the important parameters that govern fracture behavior with respect to field measurements. Temperature in high permeability fracture in an EGS reservoir

Zyvoloski, G.; Kelkar, S.; Yoshioka, K.; Rapaka, S.

2010-12-01

161

PVrisk: an approach integrating non-destructive testing an d probabilistic fracture mechanics  

Microsoft Academic Search

The integration of non-destructive testing and probabil istic fracture mechanics offers the possibility to gain realistic fracture mechanical assessment results considering nearly all uncertainties of the input values. Therefore, a probabilistic analysis has to be performed using distributions instead of discrete values as input parameters. This concept was realized wi th the developed software PVrisk which allows deterministic, parametric and

Jochen H. Kurz; Dragos D. Cioclov; Gerd Dobmann

2007-01-01

162

Structural, mechanical properties and fracture mechanism of RuB(1.1).  

PubMed

Polycrystalline RuB(1.1) has been prepared by using an arc-melting method and its structure and mechanical properties including elastic modulus, hardness and fracture behavior have been characterized. Also, the electronic structure and bond characteristics for this compound have been investigated by first-principles calculations. The lattice parameters of RuB(1.1) have been precisely determined by a Rietveld refinement. First-principles calculations show that this compound has a high bulk modulus and a big Poisson's ratio compared to RuB2. The measured hardness of ~10.6 GPa for RuB(1.1) is three times lower than the theoretical value. This low hardness can be attributed to bond characteristics such as the bonding state and orientation, and fracture mechanism, in which the features of the Ru-B bonds plays an important role in the hardness. We found that there is an isosceles triangle bonding state including the B-B and Ru-B bonds, and the two-dimensionally inclined Ru-B bonds along the a-b plane weaken the hardness and C33. The scanning electron microscopy images show that this RuB(1.1) compound exhibits a twinning fracture, and this fracture model is also confirmed by first-principle calculations. PMID:24496462

Pan, Yong; Guan, Weiming; Zheng, Weitao

2014-04-01

163

Linear elastic fracture mechanics in anisotropic solids : application to fluid-driven crack propagation  

E-print Network

Fracture mechanics is a field of continuum mechanics with the objective to predict how cracks initiate and propagate in solids. It has a wide domain of application. While aerospace engineers want to make sure a defect in ...

Laubie, Hadrien Hyacinthe

2013-01-01

164

Investigation of translaminar fracture in fibrereinforced composite laminates---applicability of linear elastic fracture mechanics and cohesive-zone model  

NASA Astrophysics Data System (ADS)

With the extensive application of fiber-reinforced composite laminates in industry, research on the fracture mechanisms of this type of materials have drawn more and more attentions. A variety of fracture theories and models have been developed. Among them, the linear elastic fracture mechanics (LEFM) and cohesive-zone model (CZM) are two widely-accepted fracture models, which have already shown applicability in the fracture analysis of fiber-reinforced composite laminates. However, there remain challenges which prevent further applications of the two fracture models, such as the experimental measurement of fracture resistance. This dissertation primarily focused on the study of the applicability of LEFM and CZM for the fracture analysis of translaminar fracture in fibre-reinforced composite laminates. The research for each fracture model consisted of two sections: the analytical characterization of crack-tip fields and the experimental measurement of fracture resistance parameters. In the study of LEFM, an experimental investigation based on full-field crack-tip displacement measurements was carried out as a way to characterize the subcritical and steady-state crack advances in translaminar fracture of fiber-reinforced composite laminates. Here, the fiber-reinforced composite laminates were approximated as anisotropic solids. The experimental investigation relied on the LEFM theory with a modification with respect to the material anisotropy. Firstly, the full-field crack-tip displacement fields were measured by Digital Image Correlation (DIC). Then two methods, separately based on the stress intensity approach and the energy approach, were developed to measure the crack-tip field parameters from crack-tip displacement fields. The studied crack-tip field parameters included the stress intensity factor, energy release rate and effective crack length. Moreover, the crack-growth resistance curves (R-curves) were constructed with the measured crack-tip field parameters. In addition, an error analysis was carried out with an emphasis on the influence of out-of-plane rotation of specimen. In the study of CZM, two analytical inverse methods, namely the field projection method (FPM) and the separable nonlinear least-squares method, were developed for the extraction of cohesive fracture properties from crack-tip full-field displacements. Firstly, analytical characterizations of the elastic fields around a crack-tip cohesive zone and the cohesive variables within the cohesive zone were derived in terms of an eigenfunction expansion. Then both of the inverse methods were developed based on the analytical characterization. With the analytical inverse methods, the cohesive-zone law (CZL), cohesive-zone size and position can be inversely computed from the cohesive-crack-tip displacement fields. In the study, comprehensive numerical tests were carried out to investigate the applicability and robustness of two inverse methods. From the numerical tests, it was found that the field projection method was very sensitive to noise and thus had limited applicability in practice. On the other hand, the separable nonlinear least-squares method was found to be more noise-resistant and less ill-conditioned. Subsequently, the applicability of separable nonlinear least-squares method was validated with the same translaminar fracture experiment for the study of LEFM. Eventually, it was found that the experimental measurements of R-curves and CZL showed a great agreement, in both of the fracture energy and the predicted load carrying capability. It thus demonstrated the validity of present research for the translaminar fracture of fiber-reinforced composite laminates.

Hou, Fang

165

Fracture mechanics analysis for various fiber/matrix interface loadings  

NASA Technical Reports Server (NTRS)

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.

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

1992-01-01

166

A numerical study on intended and unintended failure mechanisms in blanking of sandwich plates  

NASA Astrophysics Data System (ADS)

Metal-polymer-metal sandwich plates are widely used in the automotive and aerospace industry. As for different applications the sandwich plates can be divided into two types. They are sound-damping laminates with a polymer core much thinner than the metallic faces and low-density laminates with a core thickness of approximately 40-60% of the total thickness. One frequent process step in production of parts made of these plates is the blanking process whose hereditary effects draw the limits of further forming stages or service performance and life; e.g. the failure of the adhesive in the thermoplastic polymer interface affects the sound-damping efficiency intensively. With this motivation, we present FE simulation of an axi-symmetric blanking process of steel/polyethylene/steel sound-damping laminates. The mechanical behavior of the metallic layers was characterized by finite strain rate independent elasto-plasticity where progressive material deterioration and fracture are given account for using continuum damage mechanics (CDM). This material model is made accessible via implementations as VUMAT subroutines for ABAQUS/Explicit. Possible failure of the thermoplastic polymer which may lead to delamination of the metallic layers is modeled using ABAQUS built-in cohesive zone elements. The results show that existing intended and unintended failure modes, e.g. blanking of the metallic and thermoplastic polymer constituents as well as failure of polymer layer under shear and compression, can be effectively studied with the proposed framework for process enhancement. As a future work, a damage coupled nonlinear visco-elastic constitutive model will be devised for the simulation of the thermoplastic layer in low-density laminates.

Chen, L.; Soyarslan, C.; Tekkaya, A. E.

2013-05-01

167

Fracture mechanisms of retrieved titanium screw thread in dental implant  

Microsoft Academic Search

Titanium and its alloy are increasingly attracting attention for use as biomaterials. However, delayed fracture of titanium dental implants has been reported, and factors affecting the acceleration of corrosion and fatigue have to be determined. The fractured surface of a retrieved titanium screw and metallurgical structures of a dental implant system were analyzed. The outer surface of the retrieved screw

Ken’ichi Yokoyama; Tetsuo Ichikawa; Hiroki Murakami; Youji Miyamoto; Kenzo Asaoka

2002-01-01

168

Mechanical properties of bioactive glass (13-93) scaffolds fabricated by robotic deposition for structural bone repair.  

PubMed

There is a need to develop synthetic scaffolds to repair large defects in load-bearing bones. Bioactive glasses have attractive properties as a scaffold material for bone repair, but data on their mechanical properties are limited. The objective of the present study was to comprehensively evaluate the mechanical properties of strong porous scaffolds of silicate 13-93 bioactive glass fabricated by robocasting. As-fabricated scaffolds with a grid-like microstructure (porosity 47%, filament diameter 330?m, pore width 300?m) were tested in compressive and flexural loading to determine their strength, elastic modulus, Weibull modulus, fatigue resistance, and fracture toughness. Scaffolds were also tested in compression after they were immersed in simulated body fluid (SBF) in vitro or implanted in a rat subcutaneous model in vivo. As fabricated, the scaffolds had a strength of 86±9MPa, elastic modulus of 13±2GPa, and a Weibull modulus of 12 when tested in compression. In flexural loading the strength, elastic modulus, and Weibull modulus were 11±3MPa, 13±2GPa, and 6, respectively. In compression, the as-fabricated scaffolds had a mean fatigue life of ?10(6) cycles when tested in air at room temperature or in phosphate-buffered saline at 37°C under cyclic stresses of 1-10 or 2-20MPa. The compressive strength of the scaffolds decreased markedly during the first 2weeks of immersion in SBF or implantation in vivo, but more slowly thereafter. The brittle mechanical response of the scaffolds in vitro changed to an elasto-plastic response after implantation for longer than 2-4weeks in vivo. In addition to providing critically needed data for designing bioactive glass scaffolds, the results are promising for the application of these strong porous scaffolds in loaded bone repair. PMID:23438862

Liu, Xin; Rahaman, Mohamed N; Hilmas, Gregory E; Bal, B Sonny

2013-06-01

169

Chemically- and mechanically-mediated influences on the transport and mechanical characteristics of rock fractures  

SciTech Connect

A model is presented to represent changes in the mechanical and transport characteristics of fractured rock that result from coupled mechanical and chemical effects. The specific influence is the elevation of dissolution rates on contacting asperities, which results in a stress- and temperature-dependent permanent closure. A model representing this pressure-dissolution-like behavior is adapted to define the threshold and resulting response in terms of fundamental thermodynamic properties of a contacting fracture. These relations are incorporated in a stress-stiffening model of fracture closure to define the stress- and temperature-dependency of aperture loss and behavior during stress and temperature cycling. These models compare well with laboratory and field experiments, representing both decoupled isobaric and isothermal responses. The model was applied to explore the impact of these responses on heated structures in rock. The result showed a reduction in ultimate induced stresses over the case where chemical effects were not incorporated, with permanent reduction in final stresses after cooling to ambient conditions. Similarly, permeabilities may be lower than they were in the case where chemical effects were not considered, with a net reduction apparent even after cooling to ambient temperature. These heretofore-neglected effects may have a correspondingly significant impact on the performance of heated structures in rock, such as repositories for the containment of radioactive wastes.

Min, K.-B.; Rutqvist, J.; Elsworth, D.

2009-02-01

170

Title: Experimental Fracture Mechanics for the Bond between Composite Overlays and Concrete Substrate  

E-print Network

environmental attacks, and as a quality control tool. THEORETICAL BACKGROUND The durability of the adhesion the fracture toughness of adhesive bonds between symmetric metallic plates using the double cantilever beam1 Title: Experimental Fracture Mechanics for the Bond between Composite Overlays and Concrete

Giurgiutiu, Victor

171

Cracked finite elements proposed for NASTRAN. [based on application of finite element method to fracture mechanics  

NASA Technical Reports Server (NTRS)

The recent introduction of special crack-tip singularity elements, usually referred to as cracked elements, has brought the power and flexibility of the finite-element method to bear much more effectively on fracture mechanics problems. This paper recalls the development of two cracked elements and presents the results of some applications proving their accuracy and economy. Judging from the available literature on numerical methods in fracture mechanics, it seems clear that the elements described have been used more extensively than any others in practical fracture mechanics applications.

Aberson, J. A.; Anderson, J. M.

1973-01-01

172

An extension of fracture mechanics/technology to larger and smaller cracks/defects.  

PubMed

Fracture mechanics/technology is a key science and technology for the design and integrity assessment of the engineering structures. However, the conventional fracture mechanics has mostly targeted a limited size of cracks/defects, say of from several hundred microns to several tens of centimeters. The author and his group has tried to extend that limited size and establish a new version of fracture technology for very large cracks used in geothermal energy extraction and for very small cracks/defects or damage often appearing in the combination of mechanical and electronic components of engineering structures. Those new versions are reviewed in this paper. PMID:19907123

Abé, Hiroyuki

2009-01-01

173

A numerical model of hydro-thermo-mechanical coupling in a fractured rock mass  

SciTech Connect

Coupled hydro-thermo-mechanical codes with the ability to model fractured materials are used for predicting groundwater flow behavior in fractured aquifers containing thermal sources. The potential applications of such a code include the analysis of groundwater behavior within a geothermal reservoir. The capability of modeling hydro-thermo systems with a dual porosity, fracture flow model has been previously developed in the finite element code, FEHM. FEHM has been modified to include stress coupling with the dual porosity feature. FEHM has been further developed to implicitly couple the dependence of fracture hydraulic conductivity on effective stress within two dimensional, saturated aquifers containing fracture systems. The cubic law for flow between parallel plates was used to model fracture permeability. The Bartin-Bandis relationship was used to determine the fracture aperture within the cubic law. The code used a Newton Raphson iteration to implicitly solve for six unknowns at each node. Results from a model of heat flow from a reservoir to the moving fluid in a single fracture compared well with analytic results. Results of a model showing the increase in fracture flow due to a single fracture opening under fluid pressure compared well with analytic results. A hot dry rock, geothermal reservoir was modeled with realistic time steps indicating that the modified FEHM code does successfully model coupled flow problems with no convergence problems.

Bower, K.M.

1996-06-01

174

6. Fracture mechanics lead author: J, R. Rice  

E-print Network

, crack tip processes under fatigue loading, and the need for new methodologies for effective fracture in artificial heart valves, reactor piping, welding affected zones of bridges and offshore structures, aircraft

175

Atypical subtrochanteric femoral shaft fractures: role for mechanics and bone quality  

PubMed Central

Bisphosphonates are highly effective agents for reducing osteoporotic fractures in women and men, decreasing fracture incidence at the hip and spine up to 50%. In a small subset of patients, however, these agents have recently been associated with 'atypical femoral fractures' (AFFs) in the subtrochanteric region or the diaphysis. These fractures have several atypical characteristics, including occurrence with minimal trauma; younger age than typical osteoporotic fractures; occurrence at cortical, rather than cancellous sites; early radiographic appearance similar to that of a stress fracture; transverse fracture pattern rather than the familiar spiral or transverse-oblique morphologies; initiation on the lateral cortex; and high risk of fracture on the contralateral side, at the same location as the initial fracture. Fracture is a mechanical phenomenon that occurs when the loads applied to a structure such as a long bone exceed its load-bearing capacity, either due to a single catastrophic overload (traumatic failure) or as a result of accumulated damage and crack propagation at sub-failure loads (fatigue failure). The association of AFFs with no or minimal trauma suggests a fatigue-based mechanism that depends on cortical cross-sectional geometry and tissue material properties. In the case of AFFs, bisphosphonate treatment may alter cortical tissue properties, as these agents are known to alter bone remodeling. This review discusses the use of bisphosphonates, their effects on bone remodeling, mechanics and tissue composition, their significance as an effective therapy for osteoporosis, and why these agents may increase fracture risk in a small population of patients. PMID:22958475

2012-01-01

176

Porosity Dependence and Mechanism of Brittle Fracture in Sandstones  

Microsoft Academic Search

Brittle fracture tests of 105 fine-grained quartz arenites were conducted at 25øC, 1.0-kb confining pressure, a constant strain rate of 6.5. X 10-5\\/set, and pore pressure ranging from 0 to 750 bars. Orientation of planar anisotropy (bedding or cross-bedding) with respect to principal stresses has little influence on the fracture strength. The Donath orientation effect depends on rock type. Strong

David E. Dunn; Lester J. Lafountain; Robert E. Jackson

1973-01-01

177

The peel test in experimental adhesive fracture mechanics  

NASA Technical Reports Server (NTRS)

Several testing methods have been proposed for obtaining critical energy release rate or adhesive fracture energy in bond systems. These tests include blister, cone, lap shear, and peel tests. Peel tests have been used for many years to compare relative strengths of different adhesives, different surface preparation techniques, etc. The present work demonstrates the potential use of the peel test for obtaining adhesive fracture energy values.

Anderson, G. P.; Devries, K. L.; Williams, M. L.

1974-01-01

178

Injection-Sensitive Mechanics of Hydraulic Fracture Interaction with Discontinuities  

NASA Astrophysics Data System (ADS)

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.

Chuprakov, D.; Melchaeva, O.; Prioul, R.

2014-09-01

179

Fracture mechanisms and fracture pattern in men and women aged 50 years and older: a study of a 12-year population-based injury register, Umeå, Sweden  

Microsoft Academic Search

Summary  In a study of a 12-year population-based injury register, Umeå, Sweden, we analyzed the fracture mechanisms and fracture pattern\\u000a in men and women 50 years and older. Low-energy trauma was responsible for the major and costliest part of the fracture panorama,\\u000a but the pattern differs between age groups.\\u000a \\u000a \\u000a \\u000a Introduction  Osteoporosis-related fracture is a major health problem: the number of hip fractures is

U. Bergström; U. Björnstig; H. Stenlund; H. Jonsson; O. Svensson

2008-01-01

180

3D Modeling of Coupled Rock Deformation and Thermo-Poro-Mechanical Processes in Fractures  

E-print Network

on combining Displacement Discontinuity (DD) Boundary Element Method (BEM) and Finite Element Method (FEM) to solve the governing equations of thermo-poro-mechanical processes involving fracture/reservoir matrix. The integration of BEM and FEM is accomplished...

Rawal, Chakra

2012-07-16

181

Fracture mechanics analysis on the resistance of welded details under variable amplitude long life loading  

E-print Network

Fracture mechanics approach has been used to analyze the behavior of fatigue resistance of welded details existing in highway steel bridges under variable amplitude long life loading which means most of the stress ranges will be below constant...

Zhou, Minjian

1993-01-01

182

Three-dimensional mixed mode linear elastic fracture mechanics analysis using domain interaction integrals  

E-print Network

Three-dimensional mixed-mode linear elastic fracture mechanics analysis is presented using domain interaction integrals. An out-of-plane sinusoidal crack was analyzed using a commercially available finite element package ...

Esmen, Ekrem Alp, 1977-

2004-01-01

183

[Bone fracture and the healing mechanisms. Application of the extracorporeal shock wave on treatment of fracture].  

PubMed

We developed a new and unique extracorporeal shock waves (ESW) generator for bone application in 1992. ESW was applied to the canine's femurs. Gross fracture was not seen in any of the dogs. Detachment of the periosteum and subperiosteal bleeding were seen. Callus formation was seen beneath the periosteum at the focusing surface of the cortex and inner surface of the cortex. We applied ESW to eight patients with delayed union of the fracture. The sites of the ESW application were 4 tibiae, 2 humerus, one radius, one femur. ESW was applied to the delayed union site avoiding a metal plate, major vessels, and peripheral nerves. Five out of 8 cases achieved bone union. Average period until fusion achieved after ESW was 4.3 months. There was no complication except mild subcutaneous hemorrhage that disappeared within a week. We conclude that ESW is a safe and effective procedure for the treatment of delayed union of the fracture. PMID:19398841

Ikeda, Kazuo

2009-05-01

184

Fracture initiation mechanisms in -alumina under hypervelocity impact Cheng Zhang, Rajiv K. Kalia, Aiichiro Nakano, and Priya Vashishtaa  

E-print Network

and fracture in -alumina under hypervelocity impact are investigated using large-scale molecular-dynamicsFracture initiation mechanisms in -alumina under hypervelocity impact Cheng Zhang, Rajiv K. Kalia about atomistic mechanisms of fracture process under hypervelocity impact strain rate 104 s-1 , which

Southern California, University of

185

Mechanisms of hydrogen-assisted fracture in austenitic stainless steel welds  

Microsoft Academic Search

The objective of this study was to quantify the hydrogen-assisted fracture susceptibility of gas-tungsten arc (GTA) welds in the nitrogen-strengthened, austenitic stainless steels 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-5Mn (22-13-5). In addition, mechanisms of hydrogen-assisted fracture in the welds were identified using electron microscopy and finite-element modeling. Elastic-plastic fracture mechanics experiments were conducted on hydrogen-charged GTA welds at 25 C. Results showed

Dorian K. Balch; Petros Sofronis; Brian P. Somerday; Paul Novak

2005-01-01

186

Integrity of the osteocyte bone cell network in osteoporotic fracture: Implications for mechanical load adaptation  

NASA Astrophysics Data System (ADS)

The human skeleton has the ability to modify its material composition and structure to accommodate loads through adaptive modelling and remodelling. The osteocyte cell network is now considered to be central to the regulation of skeletal homeostasis; however, very little is known of the integrity of the osteocyte cell network in osteoporotic fragility fracture. This study was designed to characterise osteocyte morphology, the extent of osteocyte cell apoptosis and expression of sclerostin protein (a negative regulator of bone formation) in trabecular bone from the intertrochanteric region of the proximal femur, for postmenopausal women with fragility hip fracture compared to age-matched women who had not sustained fragility fracture. Osteocyte morphology (osteocyte, empty lacunar, and total lacunar densities) and the degree of osteocyte apoptosis (percent caspase-3 positive osteocyte lacunae) were similar between the fracture patients and non-fracture women. The fragility hip fracture patients had a lower proportion of sclerostin-positive osteocyte lacunae in comparison to sclerostin-negative osteocyte lacunae, in contrast to similar percent sclerostin-positive/sclerostin-negative lacunae for non-fracture women. The unexpected finding of decreased sclerostin expression in trabecular bone osteocytes from fracture cases may be indicative of elevated bone turnover and under-mineralisation, characteristic of postmenopausal osteoporosis. Further, altered osteocytic expression of sclerostin may be involved in the mechano-responsiveness of bone. Optimal function of the osteocyte cell network is likely to be a critical determinant of bone strength, acting via mechanical load adaptation, and thus contributing to osteoporotic fracture risk.

Kuliwaba, J. S.; Truong, L.; Codrington, J. D.; Fazzalari, N. L.

2010-06-01

187

Hydro-mechanical modelling of geological CO2 storage and the study of possible caprock fracture mechanisms  

E-print Network

lead to damage for high pressure build-ups. Mechanically induced degradation is split into two cases-mechanical properties of the materials modelled are chosen to be representative of a potential injection site. For high injection rates, local effective stress modifications may lead to various fracture mechanisms induced

188

A Fracture-Mechanical Model of Crack Growth and Interaction: Application to Pre-eruptive Seismicity  

NASA Astrophysics Data System (ADS)

A greater understanding of the physical processes occurring within a volcano is a key aspect in the success of eruption forecasting. By considering the role of fracture growth, interaction and coalescence in the formation of dykes and conduits as well as the source mechanism for observed seismicity we can create a more general, more applicable model for precursory seismicity. The frequency of volcano-tectonic earthquakes, created by fracturing of volcanic rock, often shows a short-term increase prior to eruption. Using fracture mechanics, the model presented here aims to determine the conditions necessary for the acceleration in fracture events which produces the observed pre-eruptive seismicity. By focusing on the cause of seismic events rather than simply the acceleration patterns observed, the model also highlights the distinction between an accelerating seismic sequence ending with an eruption and a short-term increase which returns to background levels with no activity occurring, an event also observed in the field and an important capability if false alarms are to be avoided. This 1-D model explores the effects of a surrounding stress field and the distribution of multi-scale cracks on the interaction and coalescence of these cracks to form an open pathway for magma ascent. Similarly to seismic observations in the field, and acoustic emissions data from the laboratory, exponential and hyperbolic accelerations in fracturing events are recorded. Crack distribution and inter-crack distance appears to be a significant controlling factor on the evolution of the fracture network, dominating over the effects of a remote stress field. The generality of the model and its basis on fundamental fracture mechanics results makes it applicable to studies of fracture networks in numerous situations. For example looking at the differences between high temperature fracture processes and purely brittle failure the model can be similarly applied to fracture dynamics in the edifice of a long repose volcano and a lava dome.

Matthews, C.; Sammonds, P.; Kilburn, C.

2007-12-01

189

The application of fracture mechanics to failure analysis of photovoltaic solar modules  

NASA Technical Reports Server (NTRS)

Cracking of silicon solar cells and solar module transparent cover panels such as glass or polymethylmethacrylate (PMMA) is a major cause of photovoltaic solar module failure in field service. Silicon and cover materials are brittle, and cracking of these materials is expected to result from the extension of preexisting flaws under stress. Study of the cracking mechanisms is therefore an appropriate area for the application of fracture mechanics principles. In this study, fracture mechanics techniques were employed to identify the mode of crack propagation, to examine the fracture-initiating flaw, to estimate the nature and magnitude of fracture stress in the field, and to predict analytically the service lifetime. Recommendations for corrective actions are also made.

Chen, C. P.; Leipold, M. H.

1981-01-01

190

Mechanical and hydraulic behavior of a rock fracture under shear deformation  

NASA Astrophysics Data System (ADS)

With regard to crystalline rock that constitutes deep geology, attempts have been made to explore its hydraulic characteristics by focusing on the network of numerous fractures within. As the hydraulic characteristics of a rock are the accumulation of hydraulic characteristics of each fracture, it is necessary to develop the hydraulic model of a single fracture to predict the large-scale hydraulic behavior. To this end, a simultaneous permeability and shear test device is developed, and shear-flow coupling tests are conducted on specimens having fractures with varied levels of surface roughness in the constant normal stiffness conditions. The results show that the permeability characteristics in the relation between shear displacement and transmissivity change greatly at the point where the stress path reaches the Mohr-Coulomb failure curve. It is also found that there exists a range in which transmissivity is not proportional to the cube of mechanical aperture width, which seems to be because of the occurrence of channeling phenomenon at small mechanical aperture widths. This channeling flow disappears with increasing shear and is transformed into a uniform flow. We develop a simulation technique to evaluate the macroscopic permeability characteristics by the lattice gas cellular automaton method, considering the microstructure of fracture, namely the fracture surface roughness. With this technique, it is shown that the formation of the Hagen-Poiseuille flow is affected by the fracture microstructure under shear, which as a result determines the relationship between the mechanical aperture width and transmissivity.

Nishiyama, Satoshi; Ohnishi, Yuzo; Ito, Hisao; Yano, Takao

2014-12-01

191

Measurement of residual stresses using fracture mechanics weight functions  

Microsoft Academic Search

A residual stress measurement method has been developed to quantify through-the-thickness residual stresses. Accurate measurement of residual stresses is crucial for many engineering structures. Fabrication processes such as welding and machining generate residual stresses that are difficult to predict. Residual stresses affect the integrity of structures through promoting failures due to brittle fracture, fatigue, stress corrosion cracking, and wear. In

Fan

2000-01-01

192

Mechanisms of orbital floor fractures: a clinical, experimental, and theoretical study.  

PubMed Central

PURPOSE: The purpose of this study was to investigate the two accepted mechanisms of the orbital blow-out fracture (the hydraulic and the buckling theories) from a clinical, experimental, and theoretical standpoint. METHODS: Clinical cases in which blow-out fractures resulted from both a pure hydraulic mechanism and a pure buckling mechanism are presented. Twenty-one intact orbital floors were obtained from human cadavers. A metal rod was dropped, experimentally, onto each specimen until a fracture was produced, and the energy required in each instance was calculated. A biomathematical model of the human bony orbit, depicted as a thin-walled truncated conical shell, was devised. Two previously published (by the National Aeronautics Space Administration) theoretical structural engineering formulas for the fracture of thin-walled truncated conical shells were used to predict the energy required to fracture the bone of the orbital floor via the hydraulic and buckling mechanisms. RESULTS: Experimentally, the mean energy required to fracture the bone of the human cadaver orbital floor directly was 78 millijoules (mj) (range, 29-127 mj). Using the engineering formula for the hydraulic theory, the predicted theoretical energy is 71 mj (range, 38-120 mj); for the buckling theory, the predicted theoretical energy is 68 mj (range, 40-106 mj). CONCLUSION: Through this study, we have experimentally determined the amount of energy required to fracture the bone of the human orbital floor directly and have provided support for each mechanism of the orbital blow-out fracture from a clinical and theoretical basis. Images FIGURE 2 FIGURE 3 FIGURE 4 FIGURE 5A FIGURE 5B FIGURE 5E FIGURE 5F PMID:10703119

Bullock, J D; Warwar, R E; Ballal, D R; Ballal, R D

1999-01-01

193

Evolution of Stiffness and Permeability in Fractures Subject to - and Mechanically-Activated Dissolution  

NASA Astrophysics Data System (ADS)

Strong feedbacks link thermal gradients (T), hydrologic flow (H), chemical alteration (C) and mechanical deformation (M) in fractured rock. These processes are strongly interconnected since one process effects the initiation and progress of another. Dissolution and precipitation of minerals are affected by temperature and stress, and can result in significant changes in permeability and solute transport characteristics. Understanding these couplings is important for oil, gas, and geothermal reservoir engineering and for waste disposal in underground repositories and reservoirs. In order to experimentally investigate the interactions between THCM processes in a natural stressed fracture, we report on heated ( up to 150C) flow-through experiments on fractured core samples of Westerly granite. These experiments are performed to examine the influence of thermally and mechanically activated dissolution on the mechanical (stress/strain) and transport (permeability) characteristics of fractures. The evolutions of both the permeability and stiffness of the sample are recorded as the experimental thermal conditions change and chemical alteration progresses. Furthermore efflux of dissolved mineral mass is measured periodically to provide a record of the net mass removal, to correlate this with observed changes in fracture aperture, defined by the flow test. During the experiments the fracture shows high hydraulic sensitivity to the changing conditions of stress and temperature. Significant variation of the effluent fluid chemistry is observed. We argue that the formation of clay (Kaolinite) is the main mechanism responsible for the permanent change in permeability recorded at higher confining stresses (40 MPa).

Faoro, I.; Elsworth, D.; Candela, T.

2013-12-01

194

Fracture Permeability Alteration due to Chemical and Mechanical Processes: A Coupled High-Resolution Model  

NASA Astrophysics Data System (ADS)

Reactive fluid-flow experiments in fractures subjected to normal stress suggest the potential for either increased or decreased permeability resulting from fracture-surface dissolution. We present a computational model that couples mechanical deformation and chemical alteration of fractures subjected to constant normal stress and reactive fluid flow. The model explicitly represents micro-scale roughness of the fracture surfaces and calculates elastic deformation of the rough surfaces using a semi-analytical approach that ensures the surfaces remain in static equilibrium. A depth-averaged reactive transport model calculates chemical alteration of the surfaces, which leads to alteration of the contacting fracture surfaces. The mechanical deformation and chemical alteration calculations are explicitly coupled, which is justified by the disparate timescales required for equilibration of mechanical stresses and reactive transport processes. An idealized analytical representation of dissolution from a single contacting asperity shows that under reaction-limited conditions, contacting asperities can dissolve faster than the open regions of the fracture. Computational simulations in fractures with hundreds of contacting asperities show that the transition from transport-limited conditions (low flow rates) to reaction-rate-limited conditions (high flow rates) causes a shift from monotonically increasing permeability to a more complicated process in which permeability initially decreases and then increases as contacting asperities begin to dissolve. These results are qualitatively consistent with a number of experimental observations reported in the literature and suggest the potential importance of the relative magnitude of mass transport and reaction kinetics on the evolution of fracture permeability in fractures subjected to combined normal stress and reactive fluid flow.

Ameli, Pasha; Elkhoury, Jean E.; Morris, Joseph P.; Detwiler, Russell L.

2014-09-01

195

Tensile fracture mechanisms of ferritic/martensitic structural materials  

NASA Astrophysics Data System (ADS)

Radiation-induced defect structures are known to elevate material yield strength and reduce material ductility so that small strains induce plastic instability. This process is commonly known as flow localization. Recent research indicates that the flow localization in face-centered cubic (FCC) materials is controlled by critical stress, the true stress of onset of necking. Critical stress dose not change with irradiation dose but has strong temperature dependence. Because critical stress acts as a material intrinsic property similar to yield strength, it is important to understand the controlling mechanism and the temperature dependence of critical stress. It is believed that critical stress is associated with critical interfacial strength between particle and matrix, which determines the void nucleation. Void nucleation has received extensive attention motivated by the need to enhance damage tolerance and avoid fracture in structural materials. Several initiation models have been built to predict critical interfacial strength of void nucleation. However, all of these models are limited because they were derived from post-deformation (static) micro-structural characterization, which contains no information about the dynamic processes involved in the evolution of failure. This dissertation project aims to examine the controlling mechanism of critical stress, and the effects of temperature and particle characters by investigating the void nucleation through in-situ tensile tests with high energy X-ray diffraction and small angle X-ray scattering. By combining stress and void evolution from X-ray experiments, the critical interfacial strength right at the beginning of void nucleation is directly measured for four types of Fe-Cr-C model alloys: Fe-9%Cr-0.1%C, Fe-9%Cr-0.5%C, Fe-12%Cr-0.2%C, and Fe-12%Cr-0.5%C, which are promising candidates for high-temperature advanced nuclear system applications. The effects of temperature, particle size, particle volume fraction, particle crystal orientation, and chemical composition on critical interfacial strength have been inspected thoroughly. Results from this study demonstrate that critical stress is strongly linearly correlated with critical interfacial strength. The effects of temperature and particle size on critical stress are correspondingly determined by how temperature and particle size influence critical interfacial strength. In addition, a new Micro-Structural Model (MSM) is developed to predict critical interfacial strength using macro tensile testing parameters. This MSM model is consistent with experimental measurements under all testing conditions. Pre-existing dislocation models are also evaluated by the X-ray measurements. Results show that the MSM model can provide better prediction than dislocation models. X-ray measurements and analysis not only support the hypothesis of the association between critical stress and critical interfacial strength, but also offer a new method to determine the true strain/stress behavior during deformation. This method is called the Universal Micro-structural Geometry-corrected Model (UMGM) in this dissertation. Due to the development of tri-axial stress and severe distortion of grain structure in the necking region, it is a challenge to determine the true strain/stress curve, a critical input in finite element modeling (FEM) analysis. Previous scholars have developed several analytical methods to characterize true strain/stress behavior after necking merely based on geometry correction or direct linear extrapolation. Grounded with experimental data, the UMGM provides empirical validation for these existing methods.

Pan, Xiao

196

Incidence, mechanism of injury, and treatment of fractures of the patella in children.  

PubMed

Fractures of the patella in skeletally immature patients are rare. The charts of 185 patients treated for patella fractures at the University of Kentucky Medical Center between 1976 and 1988 were retrospectively reviewed. The 12 patients of these 185 aged 8 to 16 years were included in this study. The incidence was calculated to be 6.5% of all patella fractures. All patients studied were male with an average age of 12.7 years. Sleeve fractures were the most common type of patella fracture observed (five), followed by transverse fractures (four). Ten of the 12 cases required operative management ranging from irrigation and debridement to open reduction and internal fixation. Partial patellectomy was performed when indicated. Indications for operative management in this age group were similar to those for adults. As in adults, the mechanism of injury was predominantly motor vehicle and motorcycle crashes. Laws requiring seatbelt restraints for children should have a positive effect on the incidence of such fractures resulting from dashboard injuries. One mechanism of injury not reported previously was that of a flexed knee striking the gym wall after performing a basketball lay-up because the basket was placed flush with the wall. PMID:1569620

Ray, J M; Hendrix, J

1992-04-01

197

Mechanical and fracture behavior of veneer–framework composites for all-ceramic dental bridges  

Microsoft Academic Search

ObjectivesHigh-strength ceramics are required in dental posterior restorations in order to withstand the excessive tensile stresses that occur during mastication. The aim of this study was to investigate the fracture behavior and the fast-fracture mechanical strength of three veneer–framework composites (Empress 2\\/IPS Eris, TZP\\/Cercon S and Inceram–Zirconia\\/Vita VM7) for all-ceramic dental bridges.

André R. Studart; Frank Filser; Peter Kocher; Heinz Lüthy; Ludwig J. Gauckler

2007-01-01

198

Investigation of the thermal, mechanical, and fracture properties of alumina–epoxy composites  

Microsoft Academic Search

A combination of dynamic shear rheology, thermomechanical analysis (TMA), scanning electron microscopy (SEM), Near-Edge X-ray Absorption Fine Structure (NEXAFS), and fracture toughness testing was utilized to characterize the thermal, mechanical, chemical, and fracture properties of alumina (?-Al2O3)-filled epoxy resins as a function of average filler size, size distribution, particle shape, loading, and epoxy crosslink density. In general the cured properties

Laura M. McGrath; Richard S. Parnas; Saskia H. King; John L. Schroeder; Daniel A. Fischer; Joseph L. Lenhart

2008-01-01

199

Experimental and theoretical fracture mechanics applied to volcanic conduits and domes  

NASA Astrophysics Data System (ADS)

We present an integrated modelling and experimental approach to magma deformation and fracture, which we attempt to validate against field observations of seismicity. The importance of fracture processes in magma ascent dynamics and lava dome growth and collapse are apparent from the associated seismicity. Our laboratory experiments have shown that brittle fracture of magma can occur at high temperature and stress conditions prevalent in the shallow volcanic system. Here, we use a fracture mechanics approach to model seismicity preceding volcanic eruptions. Starting with the fracture mechanics concept of a crack in an elastic body, we model crack growth around the volcanic conduit through the processes of crack interactions, leading either to the propagation and linkage of cracks, or crack avoidance and the inhibition of crack propagation. The nature of that interaction is governed by the temperature and plasticity of the magma. We find that fracture mechanics rules can account for the style of seismicity preceding eruptions. We have derived the changes in seismic b-value predicted by the model and interpret these in terms of the style of fracturing, fluid flow and heat transport. We compare our model with results from our laboratory experiments where we have deformed lava at high temperatures under triaxial stresses. These experiments were conducted in dry and water saturated conditions at effective pressures up to 10 MPa, temperatures up to 1000°C and strain rates from 10-4 s-1 to 10-6 s-1. The behaviour of these magmas was largely brittle under these conditions. We monitored the acoustic emission emitted and calculate the change in micro-seismic b-value with deformation. These we find are in accord with volcano seismicity and our fracture mechanics model.

Sammonds, P.; Matthews, C.; Kilburn, C.; Smith, R.; Tuffen, H.; Meredith, P.

2008-12-01

200

Investigation of the fracture mechanics of boride composites  

NASA Technical Reports Server (NTRS)

Fracture energies of WC-6Co, Boride 5 (ZrB2+SiC), Boride 8(ZrB2+SiC+C) and Boride 8-M2(ZrB2+SiC+C) were measured by slow bend and impact tests of notched charpy bars. Cobalt bonded tungsten carbide exhibited impact energies of 0.76 ft-lb or 73.9 in-lb/square inch. Boride 5 and the Boride 8 exhibit impact energies one third and one quarter of that observed for WC-6Co comparing favorably with measurements for SiC and Si3N4. Slow bend-notched bar-fracture energies for WC-6Co were near 2.6 in-lb/square inch or 1/20 the impact energies. Slow bend energies for Boride 8-M2, Boride 8 and Boride 5 were 58%, 42% and 25% of the value observed for WC-6Co. Fractograph showed differences for WC-6Co where slow bend testing resulted in smooth transgranular cleavage while samples broken by impact exhibited intergranular failures. By contrast the boride fractures showed no distinction based on testing method. Fabrication studies were conducted to effect alteration of the boride composites by alloying and introduction of graphite cloth.

Kaufman, L.; Clougherty, E. V.; Nesor, H.

1971-01-01

201

Fracture mechanics analyses of ceramic/veneer interface under mixed-mode loading.  

PubMed

Few studies have focused on the interface fracture performance of zirconia/veneer bilayered structure, which plays an important role in dental all-ceramic restorations. The purpose of this study was to evaluate the fracture mechanics performance of zirconia/veneer interface in a wide range of mode-mixities (at phase angles ranging from 0° to 90°), and to examine the effect of mechanical properties of the materials and the interface on the fracture initiation and crack path of an interfacial crack. A modified sandwich test configuration with an oblique interfacial crack was proposed and calibrated to choose the appropriate geometry dimensions by means of finite element analysis. The specimens with different interface inclination angles were tested to failure under three-point bending configuration. Interface fracture parameters were obtained with finite element analyses. Based on the interfacial fracture mechanics, three fracture criteria for crack kinking were used to predict crack initiation and propagation. In addition, the effects of residual stresses due to coefficient of thermal expansion mismatch between zirconia and veneer on the crack behavior were evaluated. The crack initiation and propagation were well predicted by the three fracture criteria. For specimens at phase angle of 0, the cracks propagated in the interface; whereas for all the other specimens the cracks kinked into the veneer. Compressive residual stresses in the veneer can improve the toughness of the interface structure. The results suggest that, in zirconia/veneer bilayered structure the veneer is weaker than the interface, which can be used to explain the clinical phenomenon that veneer chipping rate is larger than interface delamination rate. Consequently, a veneer material with larger fracture toughness is needed to decrease the failure rate of all-ceramic restorations. And the coefficient of thermal expansion mismatch of the substrates can be larger to produce larger compressive stresses in the veneer. PMID:25123435

Wang, Gaoqi; Zhang, Song; Bian, Cuirong; Kong, Hui

2014-11-01

202

Brittleness of twig bases in the genus Salix: fracture mechanics and ecological relevance.  

PubMed

The twig bases within the genus Salix were investigated. Brittleness of twig bases as defined in the literature neither correlates with Young's modulus nor with growth strains, which were measured for S. alba, S. fragilis and S. x rubens. For the species S. alba, S. appendiculata, S. eleagnos, S. fragilis, S. purpurea, S. triandra, S. viminalis, and S. x rubens, fracture surfaces of broken twigs were investigated and semiquantitatively described in terms of 'relative roughness' (ratio of rough area of fracture surface over whole area of fracture surface). The relative roughness clearly corresponds with the classification into brittle and nonbrittle species given in the literature. An attempt was made to quantify brittleness with mechanical tests. The absolute values of stress and strain do not correlate with the brittleness of the twig bases as defined by the relative roughness. However, the 'index stress' (ratio of stress at yield over stress at fracture) or the 'index strain' (ratio of strain at yield over strain at fracture), correlate well with the relative roughness. The graphic analysis of index stress against index strain reveals a straight line on which the eight species are ordered according to their brittleness. Depending on growth form and habitat, brittle twig bases of willows may function ecologically as mechanical safety mechanisms and, additionally, as a propagation mechanism. PMID:10938818

Beismann, H; Wilhelmi, H; Baillères, H; Spatz, H C; Bogenrieder, A; Speck, T

2000-03-01

203

Updated Fatigue-Crack-Growth And Fracture-Mechanics Software  

NASA Technical Reports Server (NTRS)

NASA/FLAGRO 2.0 developed as analytical aid in predicting growth and stability of preexisting flaws and cracks in structural components of aerospace systems. Used for fracture-control analysis of space hardware. Organized into three modules to maximize efficiency in operation. Useful in: (1) crack-instability/crack-growth analysis, (2) processing raw crack-growth data from laboratory tests, and (3) boundary-element analysis to determine stresses and stress-intensity factors. Written in FORTRAN 77 and ANSI C.

Forman, Royce G.; Shivakumar, Venkataraman; Newman, James C., Jr.

1995-01-01

204

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

USGS Publications Warehouse

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.

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

2009-01-01

205

Fracture processes and mechanisms of crack growth resistance in human enamel  

NASA Astrophysics Data System (ADS)

Human enamel has a complex micro-structure that varies with distance from the tooth’s outer surface. But contributions from the microstructure to the fracture toughness and the mechanisms of crack growth resistance have not been explored in detail. In this investigation the apparent fracture toughness of human enamel and the mechanisms of crack growth resistance were evaluated using the indentation fracture approach and an incremental crack growth technique. Indentation cracks were introduced on polished surfaces of enamel at selected distances from the occlusal surface. In addition, an incremental crack growth approach using compact tension specimens was used to quantify the crack growth resistance as a Junction of distance from the occlusal surface. There were significant differences in the apparent toughness estimated using the two approaches, which was attributed to the active crack length and corresponding scale of the toughening mechanisms.

Bajaj, Devendra; Park, Saejin; Quinn, George D.; Arola, Dwayne

2010-07-01

206

A new failure mechanism in thin film by collaborative fracture and delamination: interacting duos of cracks  

E-print Network

When a thin film moderately adherent to a substrate is subjected to residual stress, the cooperation between fracture and delamination leads to unusual fracture patterns such as spirals, alleys of crescents and various types of strips, all characterized by a robust characteristic length scale. We focus on the propagation of a duo of cracks: two fractures in the film connected by a delamination front and progressively detaching a strip. We show experimentally that the system selects an equilibrium width on the order of 25 times the thickness of the coating and independent of both fracture and adhesion energies. We investigate numerically the selection of the width and the condition for propagation by considering Griffith's criterion and the principle of local symmetry. In addition, we propose a simplified model based on maximum of energy release rate, which provides insights of the physical mechanisms leading to these regular patterns, and predicts the effect of material properties on the selected with of the detaching strip.

Joel Marthelot; Jose Bico; Francisco Melo; Benoit Roman

2014-12-02

207

Hydraulic and mechanical properties of natural fractures in low-permeability rock  

SciTech Connect

The results of a comprehensive laboratory study of the mechanical displacement, permeability, and void geometry of single rock fractures in a quartz monzonite are summarized and analyzed. A metal-injection technique was developed that provided quantitative data on the precise geometry of the void spaces between the fracture surfaces and the areas of contact at different stresses. At effective stresses of less than 20 MPa fluid flow was proportional to the mean fracture aperture raised to a power greater than 3. As stress was increased, contact area was increased and void spaces become interconnected by small tortuous channels that constitute the principal impediment to fluid flow. At effective stresses higher than 20 MPa, the mean fracture aperture continued to diminish with increasing stress, but this had little effect on flow because the small tortuous flow channels deformed little with increasing stress.

Pyrack-Nolte, L.J.; Myer, L.R.; Cook, N.G.W.; Witherspoon, P.A.

1987-01-01

208

Fracture mechanics in fiber reinforced composite materials, taking as examples B/A1 and CRFP  

NASA Technical Reports Server (NTRS)

The validity of linear elastic fracture mechanics and other fracture criteria was investigated with laminates of boron fiber reinforced aluminum (R/A1) and of carbon fiber reinforced epoxide (CFRP). Cracks are assessed by fracture strength Kc or Kmax (critical or maximum value of the stress intensity factor). The Whitney and Nuismer point stress criterion and average stress criterion often show that Kmax of fiber composite materials increases with increasing crack length; however, for R/A1 and CFRP the curve showing fracture strength as a function of crack length is only applicable in a small domain. For R/A1, the reason is clearly the extension of the plastic zone (or the damage zone n the case of CFRP) which cannot be described with a stress intensity factor.

Peters, P. W. M.

1982-01-01

209

Elasto-plasticity in wrinkled polymerized lipid membranes  

NASA Astrophysics Data System (ADS)

Biomembranes shown to behave like elastic sheets, can also suffer plastic deformations. Neutron scattering experiments on partially polymerised wrinkled membranes revealed that when a critical degree of polymerisation is crossed, the wrinkled membranes do not resume their spherical shapes. Instead they remain wrinkled and rigid while their non-polymerised counterparts resume their spherical floppy shapes. The yield stress of these membranes, measured for the first time via the fractal dimension, is intimately related to the degree of polymerisation probably through a 2D disorder that quenches the lateral diffusion of the lipid molecules. This work might shed light on the physical reason behind the irreversible deformation of echinocytes, acanthocytes and malaria infected red blood cells.

Chaieb, Sahraoui

2014-01-01

210

Elasto-plasticity in wrinkled polymerized lipid membranes.  

PubMed

Biomembranes shown to behave like elastic sheets, can also suffer plastic deformations. Neutron scattering experiments on partially polymerised wrinkled membranes revealed that when a critical degree of polymerisation is crossed, the wrinkled membranes do not resume their spherical shapes. Instead they remain wrinkled and rigid while their non-polymerised counterparts resume their spherical floppy shapes. The yield stress of these membranes, measured for the first time via the fractal dimension, is intimately related to the degree of polymerisation probably through a 2D disorder that quenches the lateral diffusion of the lipid molecules. This work might shed light on the physical reason behind the irreversible deformation of echinocytes, acanthocytes and malaria infected red blood cells. PMID:24424373

Chaieb, Sahraoui

2014-01-01

211

Accurate in vitro identification of fracture onset in bones: failure mechanism of the proximal human femur.  

PubMed

Bone fractures have extensively been investigated, especially for the proximal femur. While failure load can easily be recorded, and the fracture surface is readily accessible, identification of the point of fracture initiation is difficult. Accurate location of fracture initiation is extremely important to understand the multi-scale determinants of bone fracture. In this study, a recently developed technique based on electro-conductive lines was applied to the proximal femoral metaphysis to elucidate the fracture mechanism. Eight cadaveric femurs were prepared with 15-20 electro-conductive lines (crack-grid) covering the proximal region. The crack-grid was connected to a dedicated data-logger that monitored electrical continuity of each line at 700 kHz. High-speed videos (12,000 frames/s, 0.1-0.2 mm pixel size) of the destructive tests were acquired. Most crack-grid-lines failed in a time-span of 0.08-0.50 ms, which was comparable to that identified in the high-speed videos, and consistent with previous video recordings. However, on all specimens 1-3 crack-grid-lines failed significantly earlier (2-200 ms) than the majority of the crack-grid-lines. The first crack-grid-line to fail was always the closest one to the point of fracture initiation identified in the high-speed videos (superior-lateral neck region). Then the crack propagated simultaneously, at comparable velocity on the anterior and posterior sides of the neck. Such a failure pattern has never been observed before, as spatial resolution of the high-speed videos prevented from observing the initial opening of a crack. This mechanism (fracture onset, time-lag, followed by catastrophic failure) can be explained with a transfer of load to the internal trabecular structure caused by the initial fracture of the thin cortical shell. This study proves the suitability of the crack-grid method to investigate bone fractures associated to tensile stress. The crack-grid method enables significantly faster sampling than high-speed cameras. The present findings elucidate some aspects of the failure mechanism of the proximal human femoral metaphysis. PMID:23218142

Juszczyk, Mateusz Maria; Cristofolini, Luca; Salvà, Marco; Zani, Lorenzo; Schileo, Enrico; Viceconti, Marco

2013-01-01

212

Effect of root canal preparation, type of endodontic post and mechanical cycling on root fracture strength  

PubMed Central

Objective To evaluate the impact of the type of root canal preparation, intraradicular post and mechanical cycling on the fracture strength of roots. Material and Methods eighty human single rooted teeth were divided into 8 groups according to the instruments used for root canal preparation (manual or rotary instruments), the type of intraradicular post (fiber posts- FRC and cast post and core- CPC) and the use of mechanical cycling (MC) as follows: Manual and FRC; Manual, FRC and MC; Manual and CPC; Manual, CPC and MC; Rotary and FRC; Rotary, FRC and MC; Rotary and CPC; Rotary, CPC and MC. The filling was performed by lateral compactation. All root canals were prepared for a post with a 10 mm length, using the custom #2 bur of the glass fiber post system. For mechanical cycling, the protocol was applied as follows: an angle of incidence of 45°, 37°C, 88 N, 4 Hz, 2 million pulses. All groups were submitted to fracture strength test in a 45° device with 1 mm/ min cross-head speed until failure occurred. Results The 3-way ANOVA showed that the root canal preparation strategy (p<0.03) and post type (p<0.0001) affected the fracture strength results, while mechanical cycling (p=0.29) did not. Conclusion The root canal preparation strategy only influenced the root fracture strength when restoring with a fiber post and mechanical cycling, so it does not seem to be an important factor in this scenario. PMID:25025556

RIPPE, Marília Pivetta; SANTINI, Manuela Favarin; BIER, Carlos Alexandre Souza; BALDISSARA, Paolo; VALANDRO, Luiz Felipe

2014-01-01

213

Hierarchy of non-extensive mechanical processes in fracturing sea ice  

NASA Astrophysics Data System (ADS)

The fracture-induced oscillations in sea ice were detected by seismographs and seismic tiltmeters established on the Arctic ice pack. Field observations were supplemented with a laboratory experiment. The energy distributions in elastic waves generated during: (i) large-scale ice pack fragmentation over area of about 105 km2, (ii) local crack propagation in ice floe, and (iii) laboratory ice crashing were constructed and analyzed using principles of the Tsallis statistics. The energy release regimes at different stages of fracturing were characterized by the parameter of nonextensivity q. In terms of the non-extensive statistical mechanics, q > 1 evidences the correlated (non-extensive) dynamics of the process in nonequilibrium system, q = 1 responds to the additivity of events occurring in equilibrium system, and q < 1 takes place when the energy release is additive and limited by an upper cut-off. All these scenarios were revealed in fracture processes occurring at three hierarchic levels. The variation of the q-value demonstrates high thermodynamic changeability of the fracture process driven by irregular external source. The role of energy conservation in fracturing sea ice is discussed in connection with the observed reversible transitions between extensive and non-extensive modes of fracture.

Chmel, Alexandre; Smirnov, Victor; Shcherbakov, Igor

2012-06-01

214

Mechanical properties of highly defective graphene: from brittle rupture to ductile fracture  

NASA Astrophysics Data System (ADS)

Defects are generally believed to deteriorate the superlative performance of graphene-based devices but may also be useful when carefully engineered to tailor the local properties and achieve new functionalities. Central to most defect-associated applications is the defect coverage and arrangement. In this work, we investigate, by molecular dynamics simulations, the mechanical properties and fracture dynamics of graphene sheets with randomly distributed vacancies or Stone-Wales defects under tensile deformations over a wide defect coverage range. With defects presented, an sp-sp2 bonding network and an sp-sp2-sp3 bonding network are observed in vacancy-defected and Stone-Wales-defected graphene, respectively. The ultimate strength degrades gradually with increasing defect coverage and saturates in the high-ratio regime, whereas the fracture strain presents an unusual descending-saturating-improving trend. In the dense vacancy defect situation, the fracture becomes more plastic and super-ductility is observed. Further fracture dynamics analysis reveals that the crack trapping by sp-sp2 and sp-sp2-sp3 rings and the crack-tip blunting account for the ductile fracture, whereas geometric rearrangement on the entire sheet for vacancy defects and geometric rearrangement on the specific defect sites for Stone-Wales defects account for their distinctive rules of the evolution of the fracture strain.

Xu, Lanqing; Wei, Ning; Zheng, Yongping

2013-12-01

215

Mechanical properties of highly defective graphene: from brittle rupture to ductile fracture.  

PubMed

Defects are generally believed to deteriorate the superlative performance of graphene-based devices but may also be useful when carefully engineered to tailor the local properties and achieve new functionalities. Central to most defect-associated applications is the defect coverage and arrangement. In this work, we investigate, by molecular dynamics simulations, the mechanical properties and fracture dynamics of graphene sheets with randomly distributed vacancies or Stone-Wales defects under tensile deformations over a wide defect coverage range. With defects presented, an sp-sp(2) bonding network and an sp-sp(2)-sp(3) bonding network are observed in vacancy-defected and Stone-Wales-defected graphene, respectively. The ultimate strength degrades gradually with increasing defect coverage and saturates in the high-ratio regime, whereas the fracture strain presents an unusual descending-saturating-improving trend. In the dense vacancy defect situation, the fracture becomes more plastic and super-ductility is observed. Further fracture dynamics analysis reveals that the crack trapping by sp-sp(2) and sp-sp(2)-sp(3) rings and the crack-tip blunting account for the ductile fracture, whereas geometric rearrangement on the entire sheet for vacancy defects and geometric rearrangement on the specific defect sites for Stone-Wales defects account for their distinctive rules of the evolution of the fracture strain. PMID:24270887

Xu, Lanqing; Wei, Ning; Zheng, Yongping

2013-12-20

216

Evaluation of fracture strength of metal/epoxy joint by interface mechanics  

SciTech Connect

Tension tests of metal/epoxy joints with or without interface cracks were conducted and fracture criteria of the joints were discussed based on interface mechanics. The variation of the fracture strength of each specimen was large, and the strength showed Gaussian distribution. The fracture strength of smooth specimens was lower for wider specimens, but the cumulative probability of fracture of smooth specimens was not controlled by the stress singularity parameter. In interface cracked specimens, the cracks were propagated either along the interface or in epoxy resin, depending on crack length. When cracks propagated along the interface, the cumulative probability of the fracture of the specimen was controlled by the real part of the complex stress intensity factor along the interface, K{sub 1}. When cracks kinked to epoxy resin, the angle was almost identical to that of the maximum tangential stress, {sigma}{sub {theta}max}. In this case, the cumulative probability of fracture was controlled by the value of K{sub {theta}max}.

Nakai, Yoshikazu [Kobe Univ., Hyogo (Japan). Dept. of Mechanical Engineering

1995-11-01

217

Probabilistic fracture mechanics analysis for the life extension estimate of the high flux isotope reactor vessel  

SciTech Connect

The state of the vessel steel embrittlement as a result of neutron irradiation can be measured by its increase in the nil ductility temperature (NDT). This temperature is sometimes referred to as the brittle-ductile transition temperature (DBT) for fracture. The life extension of the High Flux Isotope Reactor (HFIR) vessel is calculated by using the method of fracture mechanics. A new method of fracture probability calculation is presented in this paper. The fracture probability as a result of the hydrostatic pressure test (hydrotest) is used to determine the life of the vessel. The hydrotest is performed in order to determine a safe vessel static pressure. It is then followed by using fracture mechanics to project the safe reactor operation time from the time of the satisfactory hydrostatic test. The life extension calculation provides the following information on the remaining life of the reactor as a function of the NDT increase: (1) the life of the vessel is determined by the probability of vessel fracture as a result of hydrotest at several hydrotest pressures and vessel embrittlement conditions, (2) the hydrotest time interval vs the NDT increase rate, and (3) the hydrotest pressure vs the NDT increase rate. It is understood that the use of a complete range of uncertainties of the NDT increase is equivalent to the entire range of radiation damage that can be experienced by the vessel steel. From the numerical values for the probabilities of the vessel fracture as a result of hydrotest, it is estimated that the reactor vessel life can be extended up to 50 EFPY (100 MW) with the minimum vessel operating temperature equal to 85{degrees}F.

Chang, S.J.

1997-05-01

218

Weakly singular, singular and hypersingular integrals in 3?D elasticity and fracture mechanics  

Microsoft Academic Search

This article considers weakly singular, singular and hypersingular integrals which arise when the boundary integral equations (BIE) are used to solve problems in the theory of elasticity and fracture mechanics. For their regularization, an approach based on the application of the Gauss?Ostrogradskii and the Green theorems has been used. The expressions, which allow an easy calculation of the weakly singular,

V. V. Zozulya

1999-01-01

219

Fracture mechanisms of the Strombus gigas conch shell: II-micromechanics analyses of multiple cracking  

E-print Network

Fracture mechanisms of the Strombus gigas conch shell: II-micromechanics analyses of multiple shell of the giant Queen conch, Strombus gigas, are presented. The crossed lamellar microstructure pink Queen conch, Strombus gigas, is characterized by high toughness and moderate strength, a direct

Ballarini, Roberto

220

JOURNAL OF MATERIALS SCIENCE 31 (1996) 6583-6594 Fracture mechanisms of the Strombus gigas  

E-print Network

JOURNAL OF MATERIALS SCIENCE 31 (1996) 6583-6594 Fracture mechanisms of the Strombus gigas conch release rates were measured for wet and dry specimens of the Strombus gigas conch shell. This shell has laminates to the Strombus gigas conch shell. Conchs are active burrowers, and require a hard, abrasion

Ballarini, Roberto

221

Structural Reliability of Ceramics at High Temperature: Mechanisms of Fracture and Fatigue Crack Growth  

SciTech Connect

Final report of our DOE funded research program. Aim of the research program was to provide a fundamental basis from which the mechanical reliability of layered structures may be understood, and to provide guidelines for the development of technologically relevant layered material structures with optimum resistance to fracture and subcritical debonding. Progress in the program to achieve these goals is described.

Reinhold H. Dauskardt

2005-08-01

222

Life assessment by fracture mechanics analysis and damage monitoring technique on combustion liners  

Microsoft Academic Search

A methodology has been developed and tested including a multi-disciplinary framework towards integrated analysis of gas turbine combustors. The sub-elements consist of combustion dynamics, stress and modal analysis, fracture mechanics and structural health monitoring have been interlinked indicating the damage evaluation to life assessment. The interaction between the interrelated combustion driven flame dynamics, acoustic pressure fluctuations and liner wall vibration

A. Can Altunlu; Hoogt van der Peter; Boer de André

2011-01-01

223

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

E-print Network

Enhanced fatigue endurance of metallic glasses through a staircase-like fracture mechanism Bernd, their fatigue resistance can be poor and inconsistent, severely limiting their potential as reliable structural materials. As fatigue limits are invariably governed by the local arrest of microscopically small cracks

Ritchie, Robert

224

RIS-M-2586 ELASTIC-PLASTIC FRACTURE MECHANICS ANALYSIS OF A CT-SPECIMEN  

E-print Network

RIS�-M-2586 ELASTIC-PLASTIC FRACTURE MECHANICS ANALYSIS OF A CT-SPECIMEN - A TWO-DIMENSIONAL - Risø National Laboratory, Denmark. The problem to be dealt with is a two-dimensional finite the finite- element code ADINA and the postprocessor code JINT. June 1986 Risø National Laboratory, DK-4000

225

Failure modes and fracture mechanisms in flexure of Kevlar-epoxy composites  

Microsoft Academic Search

The results of testing in three-point bending of aramid fibre-reinforced epoxy composites are described. This loading mode has been chosen in order to increase the variety of failure modes and of fracture mechanisms. The main failure modes observed are tensile and delamination, with a transition at a fibre volume fraction of about 46%. This mode transition is detectable by monitoring

M. Davidovitz; A. Mittelman; I. Roman; G. Marom

1984-01-01

226

Adaptive finite element methods for two-dimensional problems in computational fracture mechanics  

NASA Technical Reports Server (NTRS)

Some recent results obtained using solution-adaptive finite element methods in two-dimensional problems in linear elastic fracture mechanics are presented. The focus is on the basic issue of adaptive finite element methods for validating the new methodology by computing demonstration problems and comparing the stress intensity factors to analytical results.

Min, J. B.; Bass, J. M.; Spradley, L. W.

1994-01-01

227

Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states  

Microsoft Academic Search

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states, three kinds of representative triaxial stress states have been adopted, namely smooth tensile, notch tensile, and pure shear. The results of the study indicate the following. During the notch tensile test, a relatively higher stress triaxiality appears in the root of the notch. With the

Hao ZHU; Liang ZHU; Jianhong CHEN

2008-01-01

228

Advances in Fatigue and Fracture Mechanics Analyses for Aircraft Structures  

NASA Technical Reports Server (NTRS)

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.

Newman, J. C., Jr.

1999-01-01

229

Fracture mechanics research at NASA related to the aging commercial transport fleet  

NASA Astrophysics Data System (ADS)

NASA is conducting the Airframe Structural Integrity Program in support of the aging commercial transport fleet. This interdisciplinary program is being worked in cooperation with the U.S. airframe manufacturers, airline operators, and the FAA. Advanced analysis methods are under development and an extensive testing program is under way to study fatigue crack growth and fracture in complex built-up shell structures. Innovative nondestructive examination technologies are also being developed to provide large area inspection capability to detect corrosion, disbonds, and cracks. Recent fracture mechanics results applicable to predicting the growth of cracks under monotonic and cyclic loading at rivets in fuselage lap-splice joints are reviewed.

Newman, James C., Jr.; Harris, Charles E.

1992-07-01

230

Identification of Fracture Toughness for Discrete Damage Mechanics Analysis of Glass-Epoxy Laminates  

NASA Astrophysics Data System (ADS)

A methodology for determination of the intralaminar fracture toughness is presented, based on fitting discrete damage mechanics (DDM) model predictions to available experimental data. DDM is constitutive model that, when incorporated into commercial finite element software via user material subroutines, is able to predict intralaminar transverse and shear damage initiation and evolution in terms of the fracture toughness of the composite. The applicability of the DDM model is studied by comparison to available experimental data for Glass-Epoxy laminates. Sensitivity of the DDM model to h- and p-refinement is studied. Also, the effect of in-situ correction of strength is highlighted.

Barbero, E. J.; Cosso, F. A.; Martinez, X.

2014-08-01

231

Fracture Mechanics Approach to Stress Corrosion Cracking of Pipeline Steels: When Hydrogen Is the Circumstance  

Microsoft Academic Search

\\u000a Stress corrosion cracking is a problem of major concern in pipeline steels. This paper deals with the fracture mechanics approach\\u000a to the phenomenon. Paraphrasing the famous sentence by Ortega y Gasset “yo soy yo y mi circunstancia”, one could say that\\u000a the material is itself and its circumstance, the latter being the physico-chemical and mechanical environment. The paper analyzes\\u000a situations

J. Toribio

232

Phenomenological and mechanics aspects of nondestructive evaluation and characterization by sound and ultrasound of material and fracture properties  

NASA Technical Reports Server (NTRS)

Developments in fracture mechanics and elastic wave theory enhance the understanding of many physical phenomena in a mathematical context. Available literature in the material, and fracture characterization by NDT, and the related mathematical methods in mechanics that provide fundamental underlying principles for its interpretation and evaluation are reviewed. Information on the energy release mechanism of defects and the interaction of microstructures within the material is basic in the formulation of the mechanics problems that supply guidance for nondestructive evaluation (NDE).

Fu, L. S. W.

1982-01-01

233

Mechanical testing and fracture studies of a nickel-alumina functionally graded material system  

NASA Astrophysics Data System (ADS)

Elastic modulus and fracture toughness was determined for a layered nickel-alumina functionally graded material (FGM) system. FGM samples with a linear compositional gradient profile were formed by sequentially stacking different Ni-Al 2O3 cermet compositions and bonding together by solid-state sintering. From these bulk samples tensile and compact-tension (C-T) specimen geometries were used to determine the elastic moduli and fracture response within each specific layer of the FGM structure, respectively. Moire interferometry techniques were used extensively to determine the physical displacement across the entire tensile specimen surface and the surface directly adjacent to the crack tip during testing. Interferometry results were used to verify the accuracy of a series of computer simulations calculating the fracture response through a material interface. While the macroscopic effects of residual stress resulting from the layered gradient structure were anticipated for this study, defects within the microstructure were shown to dominate the mechanical and fracture response in this material system. The level of microdamage was illustrated by much lower elastic modulus values measured for cermet layers possessing an interpenetrating network microstructure. These microstructures had considerable contact between the two constituents, nickel and alumina, providing maximum opportunity for microdamage within these cermet interlayers. Using the measured moduli values the elastic fracture response for a crack propagating from an 80%-Al2O3 layer toward a 60%-Al 2O3 layer was both measured experimentally and calculated using finite element modeling techniques. As expected, the fracture toughness of the FGM increased as a crack propagated within the more ductile layer. Relatively straight, brittle fracture was seen in the 80%-Al2O 3 microstructure while crack bridging, deflection, and bifurcation were all observed within the 60%-Al2O3 layer. A gradual rise in fracture toughness as the crack approached the sharp material interface was calculated from computer simulations illustrating the effect on fracture toughness as the material (and thus elastic modulus) is changed in front of the crack tip. Again, microstructural damage dominated the fracture response in the form of significantly reduced elastic moduli while macroscopic residual stresses had only a minimal effect, due in part to the crack propagation direction.

Windes, William E.

234

Hydro-mechanical model for wetting/drying and fracture development in geomaterials  

NASA Astrophysics Data System (ADS)

This paper presents a modeling approach for studying hydro-mechanical coupled processes, including fracture development, within geological formations. This is accomplished through the novel linking of two codes: TOUGH2, which is a widely used simulator of subsurface multiphase flow based on the finite volume method; and an implementation of the Rigid-Body-Spring Network (RBSN) method, which provides a discrete (lattice) representation of material elasticity and fracture development. The modeling approach is facilitated by a Voronoi-based discretization technique, capable of representing discrete fracture networks. The TOUGH-RBSN simulator is intended to predict fracture evolution, as well as mass transport through permeable media, under dynamically changing hydrologic and mechanical conditions. Numerical results are compared with those of two independent studies involving hydro-mechanical coupling: (1) numerical modeling of swelling stress development in bentonite; and (2) experimental study of desiccation cracking in a mining waste. The comparisons show good agreement with respect to moisture content, stress development with changes in pore pressure, and time to crack initiation. The observed relationship between material thickness and crack patterns (e.g., mean spacing of cracks) is captured by the proposed modeling approach.

Asahina, D.; Houseworth, J. E.; Birkholzer, J. T.; Rutqvist, J.; Bolander, J. E.

2014-04-01

235

Critical Chemical-Mechanical Couplings that Define Permeability Modifications in Pressure-Sensitive Rock Fractures  

SciTech Connect

This work examined and quantified processes controlling changes in the transport characteristics of natural fractures, subjected to coupled thermal-mechanical-chemical (TMC) effects. Specifically, it examined the effects of mineral dissolution and precipitation mediated by mechanical effects, using laboratory through-flow experiments concurrently imaged by X-ray CT. These were conducted on natural and artificial fractures in cores using water as the permeant. Fluid and mineral mass balances are recorded and are correlated with in-sample saturation, porosity and fracture aperture maps, acquired in real-time by X-ray CT-imaging at a maximum spatial resolution of 15-50 microns per pixel. Post-test, the samples were resin-impregnated, thin-sectioned, and examined by microscopy to define the characteristics of dissolution and precipitation. The test-concurrent X-ray imaging, mass balances, and measurements of permeability, together with the post-test microscopy, were used to define dissolution/precipitation processes, and to constrain process-based models. These models define and quantify key processes of pressure solution, free-face dissolution, and shear-dilation, and the influence of temperature, stress level, and chemistry on the rate of dissolution, its distribution in space and time, and its influence on the mechanical and transport properties of the fracture.

Derek Elsworth; Abraham Grader; Susan Brantley

2007-04-25

236

Coupled chemical alteration and mechanical deformation in fractures: Insights from laboratory-scale imaging  

NASA Astrophysics Data System (ADS)

Perturbations from mechanical and chemical equilibrium can lead to significant alterations in fracture permeability and corresponding changes in fracture mechanical properties. Under conditions favoring mineral dissolution, alterations caused by chemical disequilibrium depend on the dimensionless Damkohler number (Da=kL/Vb, where k is an effective reaction rate, L is the length scale of the fracture, V is a mean flow velocity and b is the mean fracture aperture). At small values of Da, dissolution is relatively uniform throughout the fracture, whereas at high values of Da, instabilities in the reaction front lead to the formation of dissolution channels, and increased permeability at all values of Da. However, the addition of mechanical stresses can lead to contrasting behavior (i.e., either permeability increase or permeability decrease) due to the alteration of contact regions between the fracture surfaces. Quantifying the rates and relative impacts of different mechanisms in such experiments is necessary, particularly if we wish to use results to support predictions of field-scale behavior under different conditions. However, laboratory-scale experiments aimed at understanding coupled chemical and mechanical disequilibrium typically rely upon core-scale observations that provide insufficient details about the evolution of contacting asperities and the resulting stress induced deformation. We present results from a set of experiments in fractured cores through which we flowed reactive fluids. The cores were reservoir- and cap-rock (limestone and anhydrite, respectively) from the Weyburn CO2 sequestration project in Saskatchewan. In addition to differential pore pressures and effluent chemistry we measured the surface topography (+/- 1 ?m) of each sample before and after each experiment at high spatial resolution (20 x 20 ?m). We fabricated a jig for accurately aligning the halves of the core on the profilometer stage and developed and tested numerical routines for reconstructing fracture apertures from the measured surfaces. All cores were subjected to an effective confining stress of 14 MPa and reactive fluids (CO2-equilibrated brine or DI water) were flowed through the cores at different values of Da. The limestone cores showed increased fracture apertures at all values of Da. At the lowest value of Da, the core diameter decreased during the experiment, which was consistent with the measured erosion of contacting asperities contact. Thus, in these experiments, the rate of deformation was slower than the rate of chemical dissolution of the fracture surfaces. In the anhydrite cores, at the lowest value of Da, the permeability decreased by approximately two orders of magnitude to near the value of the intact core. This was consistent with the observed formation of a zone of gypsum adjacent to the fracture plane that deformed due to the confining stress. Results from these experiments are extended via numerical simulations of dissolution that consider the influence of stress-induced deformation of contacts and provide a means for studying the influence of the competing processes of mechanical deformation and mineral dissolution at much larger scales.

Detwiler, R. L.; Elkhoury, J. E.; Ameli, P.

2012-12-01

237

Nontraumatic fracture of the femoral condylar prosthesis in a total knee arthroplasty leading to mechanical failure.  

PubMed

This paper reports a case of fatigue fracture of the femoral component in a cruciate-retaining cemented total knee arthroplasty (TKA). A 64-year-old man had undergone a primary TKA for osteoarthritis 10 years previously at another institution using the PFC-Sigma prosthesis. The patient recovered fully and was back to his regular activities. He presented with a history of sudden onset pain and locking of the left knee since the preceding three months. There was no history of trauma, and the patient was mobilizing with difficulty using crutches. Radiographs revealed fracture of the posterior condyle of the femoral prosthesis. Revision surgery was performed as an elective procedure revealing the broken prosthesis. The TC3RP-PFC revision prosthesis was used with a medial parapatellar approach. The patient recovered fully without any squeal. Mechanical failure of the knee arthroplasty prosthesis is rare, and nontraumatic fracture of the femoral metallic component has not been reported before. PMID:24587928

Swamy, Girish N; Quah, Conal; Bagouri, Elmunzar; Badhe, Nitin P

2014-01-01

238

Nontraumatic Fracture of the Femoral Condylar Prosthesis in a Total Knee Arthroplasty Leading to Mechanical Failure  

PubMed Central

This paper reports a case of fatigue fracture of the femoral component in a cruciate-retaining cemented total knee arthroplasty (TKA). A 64-year-old man had undergone a primary TKA for osteoarthritis 10 years previously at another institution using the PFC-Sigma prosthesis. The patient recovered fully and was back to his regular activities. He presented with a history of sudden onset pain and locking of the left knee since the preceding three months. There was no history of trauma, and the patient was mobilizing with difficulty using crutches. Radiographs revealed fracture of the posterior condyle of the femoral prosthesis. Revision surgery was performed as an elective procedure revealing the broken prosthesis. The TC3RP-PFC revision prosthesis was used with a medial parapatellar approach. The patient recovered fully without any squeal. Mechanical failure of the knee arthroplasty prosthesis is rare, and nontraumatic fracture of the femoral metallic component has not been reported before. PMID:24587928

Swamy, Girish N.; Quah, Conal; Bagouri, Elmunzar; Badhe, Nitin P.

2014-01-01

239

Mechanical Properties and Fracture Behavior of Directionally Solidified NiAl-V Eutectic Composites  

NASA Astrophysics Data System (ADS)

Directional solidification of eutectic alloys has been recognized as promising technique for producing in situ composite materials exhibiting balance of properties. Therefore, an in situ NiAl-V eutectic composite has been successfully directionally solidified using Bridgman technique. The mechanical behavior of the composite including fracture resistance, microhardness, and compressive properties at room and elevated temperatures was investigated. Damage evolution and fracture characteristics were also discussed. The obtained results indicate that the NiAl-V eutectic retains high yield strength up to 1073 K (800 °C), above which there is a rapid decrease in strength. Its yield strength is higher than that of binary NiAl and most of the NiAl-based eutectics. The exhibited fracture toughness of 28.5 MPa?m is the highest of all other NiAl-based systems investigated so far. The material exhibited brittle fracture behavior of transgranular type and all observations pointed out that the main fracture micromechanism was cleavage.

Milenkovic, Srdjan; Caram, Rubens

2015-02-01

240

The fifth metatarsal base: anatomic evaluation regarding fracture mechanism and treatment algorithms.  

PubMed

Fractures occurring within the 1.5-cm proximal portion of the fifth metatarsal are commonly considered avulsion fractures. The exact mechanisms of such fractures are controversial. The present study focused on determining the likely mechanism of fracture according to the exact anatomy to allow for more successful treatment. The research sample included 10 frozen cadaveric specimens. The lateral band of the plantar fascia, peroneus brevis, and articular surface were identified and separated from their attachments, thereby splitting the fifth metatarsal base into zones A, B, and C. In zone A, the attachment of the plantar fascia was 6.6 ± 2.2 mm from the inferior aspect, 9.5 ± 2.9 mm from the proximal aspect, and 11.5 ± 0.9 mm from the lateral aspect. In zone B, the attachment of the peroneus brevis was 12.0 ± 2.2 mm from the inferior aspect, 10.2 ± 2.2 mm from the proximal aspect, and 11.5 ± 0.9 mm from the lateral aspect. Zone C was measured from the border of zone B and encompassed the articulation of the fifth metatarsal to the cuboid. We propose that fractures occurring in the most proximal end of the fifth metatarsal, zone A, are caused by a lateral band of plantar fascia and might be able to be treated conservatively by immobilization with weightbearing. We also propose that fractures occurring in zones B and C result from traumatic tension on peroneus brevis and might need to be treated with strict immobilization and non-weightbearing or open reduction internal fixation. PMID:25441854

DeVries, J George; Taefi, Erfan; Bussewitz, Bradly W; Hyer, Christopher F; Lee, Thomas H

2015-01-01

241

Can deterministic mechanical size effects contribute to fracture and microdamage accumulation in trabecular bone?  

PubMed

Failure of bone under monotonic and cyclic loading is related to the bone mineral density, the quality of the bone matrix, and the evolution of microcracks. The theory of linear elastic fracture mechanics has commonly been applied to describe fracture in bone. Evidence is presented that bone failure can be described through a non-linear theory of fracture. Thereby, deterministic size effects are introduced. Concepts of a non-linear theory are applied to discern how the interaction among bone matrix constituents (collagen and mineral), microcrack characteristics, and trabecular architecture can create distinctively differences in the fracture resistance at the bone tissue level. The non-linear model is applied to interpret pre-clinical data concerning the effects of anti-osteoporotic agents on bone properties. The results show that bisphosphonate (BP) treatments that suppress bone remodeling will change trabecular bone in ways such that the size of the failure process zone relative to the trabecular thickness is reduced. Selective estrogen receptor modulators (SERMs) that suppress bone remodeling will change trabecular bone in ways such that the size of the failure process zone relative to the trabecular thickness is increased. The consequences of these changes are reflected in bone mechanical response and predictions are consistent with experimental observations in the animal model which show that BP treatment is associated with more brittle fracture and microcracks without altering the average length of the cracks, whereas SERM treatments lead to a more ductile fracture and mainly increase crack length with a smaller increase in microcrack density. The model suggests that BPs may be more effective in cases in which bone mass is very low, whereas SERMS may be more effective when milder osteoporotic symptoms are present. PMID:20398678

Siegmund, Thomas; Allen, Matthew R; Burr, David B

2010-07-21

242

Accelerated fatigue fracture mechanism of medium density polyethylene pipe material  

Microsoft Academic Search

Fatigue crack propagation studies were performed in medium density polyethylene pipe to elucidate the damage mechanism associated with pipe failure. Past pipe testing methods required up to several years to produce failures which mimicked those observed in the field. However, by fatiguing a specially designed test specimen, brittle failure, resembling that observed under service conditions, was produced in only three

J. J. Strebel; A. Moet

1991-01-01

243

Electronics reliability fracture mechanics. Volume 1: Causes of failures of shop replaceable units and hybrid microcircuits  

NASA Astrophysics Data System (ADS)

This is the first of two volumes. The other volume (WL-TR-91-3119) is 'Fracture Mechanics'. The objective of the Electronics Reliability Fracture Mechanics (ERFM) program was to develop and demonstrate a life prediction technique for electronic assemblies, when subjected to environmental stress of vibration and thermal cycling, based upon the mechanical properties of the materials and packaging configurations which make up an electronic system. A detailed investigation was performed of the following two shop replaceable units (SRUs): Timing and Control Module (P/N 3562102) and Linear Regulator Module (P/N 3569800). The SRUs are in the Programmable Signal Processor (3137042) Line Replaceable Unit (LRU) of the Hughes AN/APG-63 Radar for the F-15 Aircraft.

Kallis, J.; Buechler, D.; Erickson, J.; Westerhuyzen, D. V.; Strokes, R.

1992-05-01

244

Exploring particulate retention mechanisms through visualization of E. coli transport through a single, saturated fracture  

NASA Astrophysics Data System (ADS)

Groundwater is an extremely valuable resource; a large body of work has been conducted towards remediating, tracking and reducing its contamination. Even so, there are large gaps within the current understanding of groundwater flow and contaminant transport, particularly within fractured media. Fractured media has the ability transport contaminants over longer distances in less time relative to porous media. Furthermore, colloids display unique transport characteristics in comparison to dissolved constituents, including the fact that they typically exhibit earlier initial arrival times. Of particular concern to human health are pathogenic microorganisms, which often originate from fecal contamination. Escherichia coli is a common indicator for fecal contamination; some strains are pathogenic, causing acute illness and sometimes death, in humans. A comprehensive understanding of the transport and retention of E. coli in fractured media will improve our ability to accurately assess whether a site is at risk of becoming contaminated by pathogenic microorganisms. Therefore, the goal of this work is to expand our mechanistic understanding particulate retention, specifically E. coli, in fractures, and the influence of flow rate on these mechanisms. In order to achieve this goal, clear epoxy casts were fabricated of two dolomitic limestone fractures retrieved from a quarry in Guelph, Ontario. Each aperture field was characterized through hydraulic and tracer tests, and measured directly using the light transmission technique. E. coli RS2-GFP, which is a non-pathogenic strain of E. coli that has been tagged with a green fluorescent protein, was injected into the cast under three separate specific discharges ranging from 5 - 30 m/d. These experiments were conducted on an ultraviolet light source, and a high resolution charged-couple device (CCD) camera was employed to take photos at regular intervals in order to capture the dominant flow paths and the areas of retention within the epoxy fracture. Samples were drawn downstream to obtain the E. coli breakthrough curve and determine the percent retained within the fracture. This paper will present the dominant retention mechanisms of E. coli at various effective flow rates as determined from an analysis of the images showing trapped E. coli, together with the aperture field information from the direct measurement. This information will help to improve the robustness and of contaminant transport models in fractures, and will therefore improve the ability to assess the risk posed by using bedrock aquifers as drinking water sources.

Burke, M. G.; Dickson, S. E.; Schutten, M.

2011-12-01

245

Development of hyperplasticity models for soil mechanics  

NASA Astrophysics Data System (ADS)

Hyperplasticity theory was developed by Collins and Houlsby (Proc. Roy. Soc. Lon. A 1997; 453:1975-2001) and Houlsby and Puzrin (Int. J. Plasticity 2000; 16(9):1017-1047). Further research has extended the method to continuous hyperplasticity, in which smooth transitions between elastic and plastic behaviour can be modelled. This paper illustrates a development of a new constitutive model for soils using hyperplasticity theory. The research begins with a simple one-dimensional elasticity model. This is extended in stages to an elasto-plastic model with a continuous internal function. The research aims to develop a soil model, which addresses some of the shortcomings of the modified cam-clay model, specifically the fact that it cannot model small strain stiffness, or the effects of immediate stress history. All expressions used are consistent with critical state soil mechanics terminology. Finally, a numerical implementation of the model using a rate-dependent algorithm is described.

Likitlersuang, S.; Houlsby, G. T.

2006-03-01

246

Nonlinear fracture mechanics-based analysis of thin wall cylinders  

NASA Technical Reports Server (NTRS)

This paper presents a simple analysis technique to predict the crack initiation, growth, and rupture of large-radius, R, to thickness, t, ratio (thin wall) cylinders. The method is formulated to deal both with stable tearing as well as fatigue mechanisms in applications to both surface and through-wall axial cracks, including interacting surface cracks. The method can also account for time-dependent effects. Validation of the model is provided by comparisons of predictions to more than forty full scale experiments of thin wall cylinders pressurized to failure.

Brust, Frederick W.; Leis, Brian N.; Forte, Thomas P.

1994-01-01

247

Fracture Mechanics Analyses of Reinforced Carbon-Carbon Wing-Leading-Edge Panels  

NASA Technical Reports Server (NTRS)

Fracture mechanics analyses of subsurface defects within the joggle regions of the Space Shuttle wing-leading-edge RCC panels are performed. A 2D plane strain idealized joggle finite element model is developed to study the fracture behavior of the panels for three distinct loading conditions - lift-off and ascent, on-orbit, and entry. For lift-off and ascent, an estimated bounding aerodynamic pressure load is used for the analyses, while for on-orbit and entry, thermo-mechanical analyses are performed using the extreme cold and hot temperatures experienced by the panels. In addition, a best estimate for the material stress-free temperature is used in the thermo-mechanical analyses. In the finite element models, the substrate and coating are modeled separately as two distinct materials. Subsurface defects are introduced at the coating-substrate interface and within the substrate. The objective of the fracture mechanics analyses is to evaluate the defect driving forces, which are characterized by the strain energy release rates, and determine if defects can become unstable for each of the loading conditions.

Raju, Ivatury S.; Phillips, Dawn R.; Knight, Norman F., Jr.; Song, Kyongchan

2010-01-01

248

International Journal of Rock Mechanics & Mining Sciences 45 (2008) 11951210 Consolidation settlements above deep tunnels in fractured crystalline  

E-print Network

International Journal of Rock Mechanics & Mining Sciences 45 (2008) 1195­1210 Consolidation settlements above deep tunnels in fractured crystalline rock: Part 1--Investigations above the Gotthard associated with tunnelling in fractured crystalline rock masses is rarely considered to be large enough

249

International Journal of Rock Mechanics & Mining Sciences 45 (2008) 12111225 Consolidation settlements above deep tunnels in fractured  

E-print Network

International Journal of Rock Mechanics & Mining Sciences 45 (2008) 1211­1225 Consolidation settlements above deep tunnels in fractured crystalline rock: Part 2--Numerical analysis of the Gotthard of subsidence. Subsidence of this magnitude in relation to a deep tunnel excavated in fractured crystalline rock

250

Simulation of fluid flow mechanisms in high permeability zones (Super-K) in a giant naturally fractured carbonate reservoir  

E-print Network

the Super-K Zone was investigated. It is known that these zones are connected to naturally occurring fractures. Fluid flow in naturally fractured reservoirs is a very difficult mechanism to understand. To accomplish this mission, the Super-K Zone...

Abu-Hassoun, Amer H.

2009-05-15

251

Strontium Is Incorporated into the Fracture Callus but Does Not Influence the Mechanical Strength of Healing Rat Fractures  

Microsoft Academic Search

Strontium ranelate (SrR) is a new agent used in the treatment of osteoporosis and is suggested to reduce bone resorption and\\u000a increase bone formation. We investigated whether SrR influences the macro- and nanomechnical properties of healing fractures\\u000a in rats. A closed tibia fracture model was used to study fracture healing in rats after 3 and 8 weeks of healing. Two groups

Annemarie Brüel; Jakob Olsen; Henrik Birkedal; Malene Risager; Troels Torp Andreassen; Anders Christer Raffalt; Jens Enevold Thaulov Andersen; Jesper Skovhus Thomsen

2011-01-01

252

Mechanisms of hydrogen-assisted fracture in austenitic stainless steel welds.  

SciTech Connect

The objective of this study was to quantify the hydrogen-assisted fracture susceptibility of gas-tungsten arc (GTA) welds in the nitrogen-strengthened, austenitic stainless steels 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-5Mn (22-13-5). In addition, mechanisms of hydrogen-assisted fracture in the welds were identified using electron microscopy and finite-element modeling. Elastic-plastic fracture mechanics experiments were conducted on hydrogen-charged GTA welds at 25 C. Results showed that hydrogen dramatically lowered the fracture toughness from 412 kJ/m{sup 2} to 57 kJ/m{sup 2} in 21-6-9 welds and from 91 kJ/m{sup 2} to 26 kJ/m{sup 2} in 22-13-5 welds. Microscopy results suggested that hydrogen served two roles in the fracture of welds: it promoted the nucleation of microcracks along the dendritic structure and accelerated the link-up of microcracks by facilitating localized deformation. A continuum finite-element model was formulated to test the notion that hydrogen could facilitate localized deformation in the ligament between microcracks. On the assumption that hydrogen decreased local flow stress in accordance with the hydrogen-enhanced dislocation mobility argument, the finite-element results showed that deformation was localized in a narrow band between two parallel, overlapping microcracks. In contrast, in the absence of hydrogen, the finite-element results showed that deformation between microcracks was more uniformly distributed.

Balch, Dorian K.; Sofronis, Petros (University of Illinois, Urbana, IL); Somerday, Brian P.; Novak, Paul (University of Illinois, Urbana, IL)

2005-03-01

253

ASPECTS OF THERMO-HYDRO-MECHANICAL SIMULATION OF A PROTOTYPE REPOSITORY IN FRACTURED ROCK  

Microsoft Academic Search

A numerical investigation into the thermo-hydro-mechanical behaviour of a single deposition-hole in a nuclear waste repository is presented. In particular, two simulations are presented modelling first a deposition-hole in largely unfractured rock, and a second including a discrete fracture of 2m in radius intersecting the deposition-hole. The impact upon the hydraulic phase is discussed in detail and compared with experimental

254

Applications of FEM and BEM in two-dimensional fracture mechanics problems  

NASA Technical Reports Server (NTRS)

A comparison of the finite element method (FEM) and boundary element method (BEM) for the solution of two-dimensional plane strain problems in fracture mechanics is presented in this paper. Stress intensity factors (SIF's) were calculated using both methods for elastic plates with either a single-edge crack or an inclined-edge crack. In particular, two currently available programs, ANSYS for finite element analysis and BEASY for boundary element analysis, were used.

Min, J. B.; Steeve, B. E.; Swanson, G. R.

1992-01-01

255

Computational Investigation of Fundamental Mechanisms Contributing to Fracture Dissolution and the Evolution of Hypogene Karst Systems  

NASA Astrophysics Data System (ADS)

Hypogene karst systems evolve by dissolution resulting from the cooling of water flowing upward against the geothermal gradient in limestone formations. We present a comprehensive coupled-process model of fluid flow, heat transfer, reactive transport and buoyancy effects to investigate the origin of hypogene karst systems by fracture dissolution. Our model incorporates the temperature and pressure dependence of the solubility and dissolution kinetics of calcite. Our formulation inherently incorporates mechanisms such as “mixing corrosion” that have been implicated in the formation of hypogene cave systems. It also allows for rigorous representation of temperature-dependent fluid density and its consequences at various stages of karstification. The model is applied to investigate karstification over geological time scales in a network of faults/fractures that serves as a vertical conduit for upward flow. We considered two different conceptual hydrogeologic models. In the first model, the upward flow is controlled by a constant pressure gradient. In the second model, the flow is induced by topographic effects in a mountainous hydrologic system. During the very early stages of fracture growth, there is a positive feedback between fluid flow rate, heat transfer and dissolution. In this stage the dissolution rate is largely controlled by the retrograde solubility of calcite and aperture growth occurs throughout the fracture. For the first model, there is a period of slow continuous increase in the mass flow rate through the fracture, which is followed by an abrupt rapid increase. We refer to the time when this rapid increase occurs as the maturation time. For the second model of a mountainous hydrologic system, the fluid flux through the fracture remains nearly constant even though the fracture permeability and aperture increase. This is largely because the permeability of the country rock does not increase significantly. While this limits the fluid flux through the system, it does not impede karstification. At later stages, forced convection and buoyant convection effects arise in both models due to the increased permeability of the evolving fracture system. Our results suggest that there is s strong tendency for buoyant convection cells to form under a wide range of conditions. A modified Rayleigh number provides a unified quantitative criterion for the onset of buoyant convection across all cases considered. Once buoyant convection cells are set up, dissolution is sustained in the upward flow portions of the cells, while precipitation occurs in the regions of downward flow. We discuss the implications of this type of flow pattern for the formation of hot springs and mazework caves, both of which are characteristic of hypogene karst environments. We also investigate the sensitivity of karst evolution to various physical and geochemical factors.

Chaudhuri, A.; Rajaram, H.; Viswanathan, H. S.; Zyvoloski, G.; Stauffer, P. H.

2009-12-01

256

Effects of mechanical fracturing and experimental trampling on Hawaiian corals.  

PubMed

In situ trampling occurred under experimental conditions to quantify the differences in the responses to anthropogenic trampling in four dominant species of Hawaiian corals, Porites compressa, Porites lobata, Montipora capitata, and Pocillopora meandrina. Trampling was simulated daily for a period of nine days at which time further breakage was minimal. Forty treatment colonies produced 559 fragments. Trampling was followed by an 11-month recovery period. Coral colony and fragment mortality was low. All four species were highly tolerant of inflicted damage, suggesting that some species of corals can withstand limited pulse events that allow time for recovery. Growth rates following trampling were significantly lower in the treatment groups for three of the four species. This study demonstrated that very few trampling events can produce significant changes in growth even after a long recovery period. Survivorship of fragments is clearly size- and species-dependent in M. capitata and P. compressa. Smaller fragments (<5 cm) had higher mortality than larger fragments (>5 cm). High breakage rates for M. capitata and P. compressa are consistent with the near shore, low-energy regions they inhabit-the same environment frequented by skin divers and waders. Mechanical tests were conducted to determine tensile and compressive strengths. Pocillopora meandrina exhibited the strongest skeletal strength, followed in decreasing order by Porites lobata, Porites compressa, and Montipora capitata. The skeletal strength obtained from the experiments correlate with the wave energy present in the environments in the regions they inhabit, suggesting that structural strength of corals is an adaptive response to hydraulic stress. PMID:12592453

Rodgers, Ku'ulei; Cox, Evelyn; Newtson, Craig

2003-03-01

257

A study of fracture mechanisms in ATD roller bearing  

NASA Technical Reports Server (NTRS)

The purpose was to investigate how microstructures, especially anisotropy, affects internal stresses and the overall mechanical response of bearings. Samples with the stress axis along the aligned carbide direction possessed high modulus values compared to those with their axis perpendicular to the carbide stringers. The difference in the modulus was found to be more than a factor of two. A series of experiments was conducted on rolled samples to further investigate this effect; the two sets of results were consistent with each other. The degree of anisotropy of the microstructure in terms of the carbide and matrix orientations was determined using x-ray diffraction. The stress state determination was conducted using neutron diffraction. It was found that there was little variation in the distribution of the internal stresses amongst different samples, indicating that small changes in the processing and geometrical conditions did not result in significant variations in the internal stress. A nominal tensile hoop stress of 39 ksi was obtained for the inner raceway. Furthermore, during the course of the investigation, it was apparent that there was a need to determine the stress state induced by the shrink fitting process. Therefore, a series of experiments was conducted using strain gages to identify the stress distribution in a shrink fitting process in three different types of geometries. Correlations were obtained to estimate the highest stress values in the outer and inner groove geometry. A finite element program based on the ANSYS system was developed to compute the stress distribution in the inner raceway geometry. This analysis indicates that the highest tensile stress in the system occurs at the ID of the ring with a stress value of over 5 times that of the applied radial stress. Results from all these facets were correlated with one another. It appears that the material does not fail as a result of any one single factor, but results from a combination of the various factors investigated.

Zee, Ralph H.

1990-01-01

258

Incorporation of Interfacial Intermetallic Morphology in Fracture Mechanism Map for Sn-Ag-Cu Solder Joints  

NASA Astrophysics Data System (ADS)

A fracture mechanism map (FMM) is a powerful tool which correlates the fracture behavior of a material to its microstructural characteristics in an explicit and convenient way. In the FMM for solder joints, an effective thickness of the interfacial intermetallic compound (IMC) layer ( t eff) and the solder yield strength ( ? ys,eff) are used as abscissa and ordinate axes, respectively, as these two predominantly affect the fracture behavior of solder joints. Earlier, a definition of t eff, based on the uniform thickness of IMC ( t u) and the average height of the IMC scallops ( t s), was proposed and shown to aptly explain the fracture behavior of solder joints on Cu. This paper presents a more general definition of t eff that is more widely applicable to a range of metallizations, including Cu and electroless nickel immersion gold (ENIG). Using this new definition of t eff, mode I FMM for SAC387/Cu joints has been updated and its validity was confirmed. A preliminary FMM for SAC387/Cu joints with ENIG metallization is also presented.

Huang, Z.; Kumar, P.; Dutta, I.; Sidhu, R.; Renavikar, M.; Mahajan, R.

2014-01-01

259

Investigation of the Thermal, Mechanical, and Fracture Properties of Alumina-Epoxy Composites  

SciTech Connect

A combination of dynamic shear rheology, thermomechanical analysis (TMA), scanning electron microscopy (SEM), Near-Edge X-ray Absorption Fine Structure (NEXAFS), and fracture toughness testing was utilized to characterize the thermal, mechanical, chemical, and fracture properties of alumina (a-Al2O3)-filled epoxy resins as a function of average filler size, size distribution, particle shape, loading, and epoxy crosslink density. In general the cured properties of the filled composites were robust. Small changes in particle size, shape, and size distribution had little impact on the final properties. Resin crosslink density and filler loading were the most critical variables, causing changes in all properties. However, most applications could likely tolerate small changes in these variables also. SEM and NEXAFS characterization of the fracture surfaces revealed that the fracture occurs at the filler interface and the interfacial epoxy composition is similar to the bulk resin, indicating a weak epoxy-alumina interaction. These results are critical for implementation of particulate-filled polymer composites in practical applications because relaxed material specifications and handling procedures can be incorporated in production environments to improve efficiency.

McGrath,L.; Parnas, R.; King, S.; Schroeder, J.; Fischer, D.; Lenhart, J.

2008-01-01

260

Study of fracture mechanisms of short fiber reinforced AS composite by acoustic emission technique  

SciTech Connect

The fracture mechanisms of short fiber reinforced AS composites are studied by acoustic emission technique for examining the effects of fiber contents. The loads P{sub b} and P{sub c} which the damage mechanisms change are obtained at the inflection points of the total AE energy curve the energy gradient method. The damages are generated by fiber breaking at the load point of P{sub b} and P{sub c} in B material, and by the fiber breaking and the debonding between resin and fiber at the load points of P{sub b} and P{sub c} in C material.

Kida, Sotoaki; Suzuki, Megumu [Kanazawa Inst. of Tech., Ishikawa (Japan). Dept. of Mechanical Engineering

1995-11-01

261

Fatigue analysis of post-weld fatigue improvement treatments using a strain-based fracture mechanics model  

Microsoft Academic Search

Untreated and post-weld treated (peened) fatigue details common to welded steel structures are analyzed herein using a strain-based fracture mechanics model. The model is first described and then evaluated by comparison with data from several test-based studies as well as analytical results obtained using two linear elastic fracture mechanics (LEFM) models. The strain-based model is then used to perform several

Scott Walbridge

2008-01-01

262

Linear elastic fracture mechanics predicts the propagation distance of frictional slip  

E-print Network

When a frictional interface is subject to a localized shear load, it is often (experimentally) observed that local slip events initiate at the stress concentration and propagate over parts of the interface by arresting naturally before reaching the edge. We develop a theoretical model based on linear elastic fracture mechanics to describe the propagation of such precursory slip. The model's prediction of precursor lengths as a function of external load is in good quantitative agreement with laboratory experiments as well as with dynamic simulations, and provides thereby evidence to recognize frictional slip as a fracture phenomenon. We show that predicted precursor lengths depend, within given uncertainty ranges, mainly on the kinetic friction coefficient, and only weakly on other interface and material parameters. By simplifying the fracture mechanics model we also reveal sources for the observed non-linearity in the growth of precursor lengths as a function of the applied force. The discrete nature of precursors as well as the shear tractions caused by frustrated Poisson's expansion are found to be the dominant factors. Finally, we apply our model to a different, symmetric set-up and provide a prediction of the propagation distance of frictional slip for future experiments.

David S. Kammer; Mathilde Radiguet; Jean-Paul Ampuero; Jean-François Molinari

2014-08-18

263

Interfacial fracture mechanisms in solid solution directionally solidified eutectic oxide composites  

SciTech Connect

The interfacial fracture behavior of the solid solution directionally solidified eutectic oxide, Co{sub 1-x}Ni{sub x}O/ZrO{sub 2}(CaO), is investigated via indentation testing. An abrupt transition from interfacial delamination to interfacial penetration is observed as a function of NiO fraction (x>0.33). The use of a focused ion beam technique is introduced as a means for exploring sub-surface cracking in brittle materials. The sub-surface observations revealed a compositional transition from energy dissipative mechanisms (e.g., delamination, secondary cracking) for cracking to brittle cracking behavior, in the Co{sub 1-x}Ni{sub x}O phase. It is proposed that the transition in interfacial fracture behavior for the directionally solidified eutectic material is the result of competing dislocation-based crack nucleation mechanisms. The observations and analyses have significant implications for fracture behavior in composites with one phase exhibiting (pseudo-) plastic behavior.

Brewer, L.N.; Guruz, M.U.; Dravid, V.P

2004-08-02

264

ADDITIONAL STRESS AND FRACTURE MECHANICS ANALYSES OF PRESSURIZED WATER REACTOR PRESSURE VESSEL NOZZLES  

SciTech Connect

In past years, the authors have undertaken various studies of nozzles in both boiling water reactors (BWRs) and pressurized water reactors (PWRs) located in the reactor pressure vessel (RPV) adjacent to the core beltline region. Those studies described stress and fracture mechanics analyses performed to assess various RPV nozzle geometries, which were selected based on their proximity to the core beltline region, i.e., those nozzle configurations that are located close enough to the core region such that they may receive sufficient fluence prior to end-of-life (EOL) to require evaluation of embrittlement as part of the RPV analyses associated with pressure-temperature (P-T) limits. In this paper, additional stress and fracture analyses are summarized that were performed for additional PWR nozzles with the following objectives: To expand the population of PWR nozzle configurations evaluated, which was limited in the previous work to just two nozzles (one inlet and one outlet nozzle). To model and understand differences in stress results obtained for an internal pressure load case using a two-dimensional (2-D) axi-symmetric finite element model (FEM) vs. a three-dimensional (3-D) FEM for these PWR nozzles. In particular, the ovalization (stress concentration) effect of two intersecting cylinders, which is typical of RPV nozzle configurations, was investigated. To investigate the applicability of previously recommended linear elastic fracture mechanics (LEFM) hand solutions for calculating the Mode I stress intensity factor for a postulated nozzle corner crack for pressure loading for these PWR nozzles. These analyses were performed to further expand earlier work completed to support potential revision and refinement of Title 10 to the U.S. Code of Federal Regulations (CFR), Part 50, Appendix G, Fracture Toughness Requirements, and are intended to supplement similar evaluation of nozzles presented at the 2008, 2009, and 2011 Pressure Vessels and Piping (PVP) Conferences. This work is also relevant to the ongoing efforts of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel (B&PV) Code, Section XI, Working Group on Operating Plant Criteria (WGOPC) efforts to incorporate nozzle fracture mechanics solutions into a revision to ASME B&PV Code, Section XI, Nonmandatory Appendix G.

Walter, Matthew [Structural Integrity Associates, Inc.; Yin, Shengjun [ORNL; Stevens, Gary [U.S. Nuclear Regulatory Commission; Sommerville, Daniel [Structural Integrity Associates, Inc.; Palm, Nathan [Westinghouse Electric Company, Cranberry Township, PA; Heinecke, Carol [Westinghouse Electric Company, Cranberry Township, PA

2012-01-01

265

Investigation of the fracture mechanism in Ti-5Al-2.5Sn at cryogenic temperatures  

NASA Technical Reports Server (NTRS)

The influence of microstructure on the fracture mechanism and plane-strain fracture toughness of Ti-5Al-2.5Sn was studied through the use of fractography and metallographic sectioning techniques. One-inch thick plates of extra low interstitial (ELI) and normal interstitial Ti-5Al-2.5Sn were mill annealed at 815 C followed by either air or furnace cooling. These variations in composition and cooling rate resulted in differences in the volume fraction and internal structure of the iron-stabilized phase, and in the crystallographic texture and ordering of the alpha matrix. The tensile properties of these plates were determined at 20 K, 77 K, and 295 K. The air-cooled ELI plate was the toughest material evaluated.

Vanstone, R. H.; Low, J. R., Jr.; Shannon, J. L., Jr.

1977-01-01

266

Grain boundary effects on plastic deformation and fracture mechanisms in Cu nanowires: Molecular dynamics simulations  

NASA Astrophysics Data System (ADS)

Metallic nanowires have many attractive properties such as ultra-high yield strength and large tensile elongation. However, recent experiments show that metallic nanowires often contain grain boundaries, which are expected to significantly affect mechanical properties. By using molecular dynamics simulations, here, we demonstrate that polycrystalline Cu nanowires exhibit tensile deformation behavior distinctly different from their single-crystal counterparts. A significantly lowered yield strength was observed as a result of dislocation emission from grain boundaries rather than from free surfaces, despite of the very high surface to volume ratio. Necking starts from the grain boundary followed by fracture, resulting in reduced tensile ductility. The high stresses found in the grain boundary region clearly play a dominant role in controlling both inelastic deformation and fracture processes in nanoscale objects. These findings have implications for designing stronger and more ductile structures and devices on nanoscale.

Cao, Ajing; Wei, Yueguang; Ma, En

2008-05-01

267

On a 2D hydro-mechanical lattice approach for modelling hydraulic fracture  

NASA Astrophysics Data System (ADS)

A 2D lattice approach to describe hydraulic fracturing is presented. The interaction of fluid pressure and mechanical response is described by Biot's theory. The lattice model is applied to the analysis of a thick-walled cylinder, for which an analytical solution for the elastic response is derived. The numerical results obtained with the lattice model agree well with the analytical solution. Furthermore, the coupled lattice approach is applied to the fracture analysis of the thick-walled cylinder. It is shown that the proposed lattice approach provides results that are independent of the mesh size. Moreover, a strong geometrical size effect on nominal strength is observed which lies between analytically derived lower and upper bounds. This size effect decreases with increasing Biot's coefficient.

Grassl, P.; Fahy, C.; Gallipoli, D.; Wheeler, S. J.

2015-02-01

268

Dyke propagation and tensile fracturing at high temperature and pressure, insights from experimental rock mechanics.  

NASA Astrophysics Data System (ADS)

It is well known that magma ascends trough the crust by the process of dyking. To enable dyke emplacement, basement rocks typically fail in a mode 1 fracture, which acts as conduits for magma transport. An overpressure of the ascending magma will further open/widen the fracture and permit the fracture to propagate. In order to further understand the emplacement and arrest of dykes in the subsurface, analogue and numerical studies have been conducted. However, a number of assumptions regarding rock mechanical behaviour frequently has to be made as such data are very hard to directly measure at the pressure/temperature conditions of interest: high temperatures at relatively shallow depths. Such data are key to simulating the magma intrusion dynamics through the lithologies that underlie the volcanic edifice. Here we present a new laboratory setup, which allows us to investigate the tensile fracturing properties under both temperature and confining pressure, and the emplacement of molten material within the newly formed fracture. We have modified a traditional tri-axial test assembly setup to be able to use a Paterson type High Pressure, High Temperature deformation apparatus. Sample setup consists of cylindrical rock samples with a 22 mm diameter and a 8 mm bore at their centre, filled with a material chosen as such that it's in a liquid state at the experimental temperature and solid at room temperature to enable post-experiment analysis. The top and lower parts of the rock sample are fitted with plugs, sealing in the melt. The assembly is then placed between ceramic pistons to ensure there are no thermal gradients across the sample. The assembly is jacketed to ensure the confining medium (Ar) cannot enter the assembly. A piston is driven into the sample such that the inner conduit materials pressure is slowly increased. At some point a sufficient pressure difference between the inner and outer surfaces causes the sample to deform and fail in the tensile regime. Tensile fractures can occur when the hoop stress exerted on the outer shell exceeds its tensile strength. The molten conduit material is then likely to flow into the newly formed fracture, depending on its viscosity and the fracture dimensions, allowing comparisons to be made between the temperature and intrusions dynamics of the simulated dyke process. As a starting point, we are now testing an analogue material to replace the magma to avoid complex multi-phase rheology (bubbles, crystals) and the need for high experimental temperatures, relying of maintaining similar temperature/viscosity ratios between magma/country rock in the laboratory and the field. We chose PMMA (commonly known as plexiglass) for this task as it displays a large range in viscosities (log(visc)range = 10 -1) with temperatures between 100 and 300 °C, making it an excellent analogue material. In the future experiments at higher temperatures will be conducted with NIST-glasses and field collected glasses. 2D and 3D imaging of post-deformation samples show no preferential location of the fractures. Fractures are formed both around and trough crystals. Some evidence suggests the formation of microcracks and linking up as predicted by Griffith's theory. Correcting for apparatus distortion and friction of o-rings and the filler material we can calculate the conduit stress and arrive at a tensile strength values for different basement rock types in the order of 1-15 MPa.

Bakker, Richard; Benson, Philip; Vinciguerra, Sergio

2014-05-01

269

Fracture mechanics of human cortical bone: The relationship of geometry, microstructure and composition with the fracture of the tibia, femoral shaft and the femoral neck  

NASA Astrophysics Data System (ADS)

Bone fracture is a major health problem in old population with its complications leading to mortality and morbidity. Therapies mostly involve preventing bone mass loss. Individuals with high bone mass, however, may still suffer fractures suggesting that additional components such as bone microstructure and composition may be responsible for increased fracture risk in the elderly. The relationship of bone constituents with bone fragility, however, is not well-understood. A better understanding of these relationships will help improving therapies by controlling the relevant biological processes. Bone is a composite material with many constituents such as osteons embedded with vascular channels, collagen fibers, mineral crystals, etc. The nature of interfacing between these constituents makes bone a more complex material. Bone also has a structure that adapts itself, both internally and externally, to better fit its needs. This suggested that, unlike man-made materials, a relationship between material properties and structural properties may exist. Because bone has some similarities with engineering composite materials and also experiences microcracks, a fracture mechanics approach would be more appropriate for investigating its fragility. Choosing mode I and mode II fracture toughness (Gsb{Ic} and Gnsb{IIc}, respectively) as indicators of bone fragility, their relationship with bone microstructure (porosity, osteon morphology, mineral crystal imperfection and microdamage), composition (density, mineral, organic, water and collagen content) and macrostructure (thickness, diameter and moment of inertia of the shaft and angle between the femoral neck and femoral shaft from different views) was investigated. Use of x-ray radiogrammetry for detecting the latter was tested. Differences among the femoral shaft, femoral neck and the tibia were investigated for an age range of 22-94 years. In general, fracture toughness increased with increasing bone quantity. However, the influence of bone quality, i.e., mineralization, water content, osteon size, area and number, microdamage and crystallinity differed between different locations, age groups and fracture mode. Fracture toughness was also significantly correlated with clinical parameters such as cortical index and Singh index, significance level being dependent upon bone location, fracture mode and age. Several mechanistic models to predict how bone microstructure influences bone fracture toughness were proposed based on experimental results and available literature.

Yeni, Yener Nail

270

Breaking the limits of structural and mechanical imaging of the heterogeneous structure of coal macerals  

NASA Astrophysics Data System (ADS)

The correlation between local mechanical (elasto-plastic) and structural (composition) properties of coal presents significant fundamental and practical interest for coal processing and for the development of rheological models of coal to coke transformations. Here, we explore the relationship between the local structural, chemical composition, and mechanical properties of coal using a combination of confocal micro-Raman imaging and band excitation atomic force acoustic microscopy for a bituminous coal. This allows high resolution imaging (10s of nm) of mechanical properties of the heterogeneous (banded) architecture of coal and correlating them to the optical gap, average crystallite size, the bond-bending disorder of sp2 aromatic double bonds, and the defect density. This methodology allows the structural and mechanical properties of coal components (lithotypes, microlithotypes, and macerals) to be understood, and related to local chemical structure, potentially allowing for knowledge-based modeling and optimization of coal utilization processes.

Collins, L.; Tselev, A.; Jesse, S.; Okatan, M. B.; Proksch, R.; Mathews, J. P.; Mitchell, G. D.; Rodriguez, B. J.; Kalinin, S. V.; Ivanov, I. N.

2014-10-01

271

Breaking the limits of structural and mechanical imaging of the heterogeneous structure of coal macerals.  

PubMed

The correlation between local mechanical (elasto-plastic) and structural (composition) properties of coal presents significant fundamental and practical interest for coal processing and for the development of rheological models of coal to coke transformations. Here, we explore the relationship between the local structural, chemical composition, and mechanical properties of coal using a combination of confocal micro-Raman imaging and band excitation atomic force acoustic microscopy for a bituminous coal. This allows high resolution imaging (10s of nm) of mechanical properties of the heterogeneous (banded) architecture of coal and correlating them to the optical gap, average crystallite size, the bond-bending disorder of sp(2) aromatic double bonds, and the defect density. This methodology allows the structural and mechanical properties of coal components (lithotypes, microlithotypes, and macerals) to be understood, and related to local chemical structure, potentially allowing for knowledge-based modeling and optimization of coal utilization processes. PMID:25299223

Collins, L; Tselev, A; Jesse, S; Okatan, M B; Proksch, R; Mathews, J P; Mitchell, G D; Rodriguez, B J; Kalinin, S V; Ivanov, I N

2014-10-31

272

Characterization of an injectable, degradable polymer for mechanical stabilization of mandibular fractures.  

PubMed

This study investigated the use of injectable poly(propylene fumarate) (PPF) formulations for mandibular fracture stabilization applications. A full factorial design with main effects analysis was employed to evaluate the effects of the PPF:N-vinyl pyrrolidone (NVP, crosslinking agent) ratio and dimethyl toluidine (DMT, accelerator) concentration on key physicochemical properties including setting time, maximum temperature, mechanical properties, sol fraction, and swelling ratio. Additionally, the effects of formulation crosslinking time on the mechanical and swelling properties were investigated. The results showed that increasing the PPF:NVP ratio from 3:1 to 4:1 or decreasing the DMT concentration from 0.05 to 0.01 v/w % significantly decreased all mechanical properties as well as significantly increased the sol fraction and swelling ratio. Also, increasing the crosslinking time at 37°C from 1 to 7 days significantly increased all mechanical properties and decreased both the sol fraction and swelling ratio. This study further showed that the flexural stiffness of ex vivo stabilized rabbit mandibles increased from 1.7?±?0.3 N/mm with a traditional mini-plate fixator to 14.5?±?4.1 N/mm for the 4:1 (0.05 v/w % DMT) PPF formulation at day 1. Overall, the formulations tested in this study were found to have properties suitable for potential further consideration in mandibular fracture fixation applications. © 2014 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 00B, 2014. PMID:24934595

Henslee, Allan M; Yoon, Diana M; Lu, Benjamin Y; Yu, Joseph; Arango, Andrew A; Marruffo, Liann P; Seng, Luke; Anver, Tamir D; Ather, Hunaiza; Nair, Manitha B; Piper, Sean O; Demian, Nagi; Wong, Mark E K; Kasper, F Kurtis; Mikos, Antonios G

2014-06-17

273

A nonlinear fracture mechanics approach to the growth of small cracks  

NASA Technical Reports Server (NTRS)

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

Newman, J. C., Jr.

1983-01-01

274

CONDENSED MATTER: STRUCTURE, MECHANICAL AND THERMAL PROPERTIES: Dynamic Response and Fracture Mechanism of a Novel Structural Radar Absorbing Material  

NASA Astrophysics Data System (ADS)

A novel structural radar absorbing material (SRAM), which gives the normal resin-base composites new function, is prepared. The dynamic compressive tests of SRAM are carried out in both in-plane and normal directions of composites by means of the split Hopkinson pressure bar (SHPB). In the compressive test along in-plane direction, failure occurs at the interface between a fibre and the matrix. A fracture mode and mechanism was proposed to explain these results. The addition of absorbing particles results in the deterioration of the compressive properties. However, there is no obvious decrease on compressive strength of SRAM with the radar absorbing properties.

Cao, Mao-Sheng; Zhou, Wei; Qu, Gui-Min; Rong, Ji-Li

2008-08-01

275

Mechanisms Predisposing Penile Fracture and Long-Term Outcomes on Erectile and Voiding Functions  

PubMed Central

Purpose. To determine the mechanisms predisposing penile fracture as well as the rate of long-term penile deformity and erectile and voiding functions. Methods. All fractures were repaired on an emergency basis via subcoronal incision and absorbable suture with simultaneous repair of eventual urethral lesion. Patients' status before fracture and voiding and erectile functions at long term were assessed by periodic follow-up and phone call. Detailed history included cause, symptoms, and single-question self-report of erectile and voiding functions. Results. Among the 44 suspicious cases, 42 (95.4%) were confirmed, mean age was 34.5 years (range: 18–60), mean follow-up 59.3 months (range 9–155). Half presented the classical triad of audible crack, detumescence, and pain. Heterosexual intercourse was the most common cause (28 patients, 66.7%), followed by penile manipulation (6 patients, 14.3%), and homosexual intercourse (4 patients, 9.5%). “Woman on top” was the most common heterosexual position (n = 14, 50%), followed by “doggy style” (n = 8, 28.6%). Four patients (9.5%) maintained the cause unclear. Six (14.3%) patients had urethral injury and two (4.8%) had erectile dysfunction, treated by penile prosthesis and PDE-5i. No patient showed urethral fistula, voiding deterioration, penile nodule/curve or pain. Conclusions. “Woman on top” was the potentially riskiest sexual position (50%). Immediate surgical treatment warrants long-term very low morbidity. PMID:24822062

Reis, Leonardo O.; Cartapatti, Marcelo; Marmiroli, Rafael; de Oliveira Júnior, Eduardo Jeronimo; Saade, Ricardo Destro; Fregonesi, Adriano

2014-01-01

276

Anomalous surface states modify the size-dependent mechanical properties and fracture of silica nanowires  

NASA Astrophysics Data System (ADS)

Molecular dynamics simulations of amorphous silica nanowires under tension were analyzed for size and surface stress effects on mechanical properties and for structural modifications via bond angle distributions. Their fracture behavior was also investigated beyond the elastic limit. The Young’s moduli of silica nanowires were predicted to be about 75–100 GPa, depending on the nanowire size. The ultimate strength was calculated to be ?10 GPa, depending on the diameter, which is in excellent agreement with the experiments. The dependence of the Young’s modulus on nanowire diameter is explained in terms of surface compressive stress effects. The fracture behavior of nanowires was also found to be influenced by surface compressive stresses. Bond angle distribution analysis of various nanowires reveals significant compressive surface states, as evidenced by the appearance of a secondary peak in the Si-O-Si bond angle distribution at ?97°, which is absent in bulk silica. The strain rate was found to have a negligible effect on the Young’s modulus of the silica nanowires, but it has a critical role in determining their fracture mode.

Tang, Chun; Dávila, Lilian P.

2014-10-01

277

Mechanisms for the formation of a perched water zone in fractured tuff: A natural analogue study  

SciTech Connect

Perched water zones have been identified in the fractured, welded tuff in the semi-arid to arid environments of Yucca Mountain, Nevada and near Superior, Arizona. An understanding of the formation of such zones is necessary in order to predict where future perched water might form at Yucca Mountain, the proposed site of a high-level nuclear waste repository. The formation or growth of a perched zone near a repository is one of the factors to be considered in the risk assessment of the Yucca Mountain site. The Apache Leap Research Site near Superior, Arizona is a natural analog to the Yucca Mountain site in terms of geology, hydrology, and climate. Information used to study possible mechanisms for the formation of the perched zone included data regarding isotopic and geochemical properties of the waters in and above the perched water zone; measured hydrologic parameters of the perched zone; geophysical and measured parameters of the tuff; megascopic and microscopic observations of the tuff, including mineralogical, alteration, and structural features; and the lateral and vertical extent of perched water in the region. Aquifer test, geophysical, geochemical, and radioisotopic data show that fractures are the means by which water is recharging the perched zone. The reduced hydraulic conductivity of the formation in the perched zone appears to result from both a severe reduction in matrix porosity and permeability caused by welding, devitrification, and vapor phase crystallization; and by an increase in fracture filling which restricts the pathways for flow.

Woodhouse, E.G.; Bassett, R.L. [Univ. of Arizona, Tucson, AZ (United States). Dept. of Hydrology and Water Resources

1997-12-31

278

Mechanical properties of hydroxyapatite single crystals from nanoindentation data  

PubMed Central

In this paper we compute elasto-plastic properties of hydroxyapatite single crystals from nanindentation data using a two-step algorithm. In the first step the yield stress is obtained using hardness and Young’s modulus data, followed by the computation of the flow parameters. The computational approach is first validated with data from existing literature. It is observed that hydroxyapatite single crystals exhibit anisotropic mechanical response with a lower yield stress along the [1010] crystallographic direction compared to the [0001] direction. Both work hardening rate and work hardening exponent are found to be higher for indentation along the [0001] crystallographic direction. The stress-strain curves extracted here could be used for developing constitutive models for hydroxyapatite single crystals. PMID:21262492

Zamiri, A.; De, S.

2011-01-01

279

Mechanics of slip and fracture along small faults and simple strike-slip fault zones in granitic rock  

NASA Astrophysics Data System (ADS)

We exploit quasi-static fracture mechanics models for slip along pre-existing faults to account for the fracture structure observed along small exhumed faults and small segmented fault zones in the Mount Abbot quadrangle of California and to estimate stress drop and shear fracture energy from geological field measurements. Along small strike-slip faults, cracks that splay from the faults are common only near fault ends. In contrast, many cracks splay from the boundary faults at the edges of a simple fault zone. Except near segment ends, the cracks preferentially splay into a zone. We infer that shear displacement discontinuities (slip patches) along a small fault propagated to near the fault ends and caused fracturing there. Based on elastic stress analyses, we suggest that slip on one boundary fault triggered slip on the adjacent boundary fault, and that the subsequent interaction of the slip patches preferentially led to the generation of fractures that splayed into the zones away from segment ends and out of the zones near segment ends. We estimate the average stress drops for slip events along the fault zones as ˜1 MPa and the shear fracture energy release rate during slip as 5 × 102 - 2 × 104 J/m2. This estimate is similar to those obtained from shear fracture of laboratory samples, but orders of magnitude less than those for large fault zones. These results suggest that the shear fracture energy release rate increases as the structural complexity of fault zones increases.

Martel, Stephen J.; Pollard, David D.

1989-07-01

280

Characterization of low-cycle multiaxial fatigue by a plastic fracture mechanics model  

SciTech Connect

Most mechanical components and structures are subjected to cyclic service loads. Fatigue cracks may appear due to these cyclic service loads. Currently, there are three major approaches to estimate fatigue life, namely, the stress-based approach, the strain-based approach, and the fracture mechanics approach. Here, the near-tip fields of small cracks in power-law hardening materials are investigated under plane-stress, general yielding, and mixed mode I and II conditions by finite element analyses. The characteristics of the near-tip strain fields suggest that Case multiaxial fatigue theories can be explained by a fracture mechanics crack growth criterion based on the maximum effective strain of the near-tip fields for small cracks under general yielding conditions. The constant effective stress contours representing the intense straining zones near the tip are also presented. The results of the J integral from finite element analyses are used to construct a fatigue crack growth criterion for Case A cracks. Based on the concept of the characterization of fatigue crack growth by the J integral, the trend of constant J integral contours on the {Gamma}-plane for Case A cracks in compared well with those of constant fatigue life and constant crack growth rate obtained from experiments.

Wang, Y.; Pan, J. [Univ. of Michigan, Ann Arbor, MI (United States)

1996-12-01

281

Modeling and additive manufacturing of bio-inspired composites with tunable fracture mechanical properties.  

PubMed

Flaws, imperfections and cracks are ubiquitous in material systems and are commonly the catalysts of catastrophic material failure. As stresses and strains tend to concentrate around cracks and imperfections, structures tend to fail far before large regions of material have ever been subjected to significant loading. Therefore, a major challenge in material design is to engineer systems that perform on par with pristine structures despite the presence of imperfections. In this work we integrate knowledge of biological systems with computational modeling and state of the art additive manufacturing to synthesize advanced composites with tunable fracture mechanical properties. Supported by extensive mesoscale computer simulations, we demonstrate the design and manufacturing of composites that exhibit deformation mechanisms characteristic of pristine systems, featuring flaw-tolerant properties. We analyze the results by directly comparing strain fields for the synthesized composites, obtained through digital image correlation (DIC), and the computationally tested composites. Moreover, we plot Ashby diagrams for the range of simulated and experimental composites. Our findings show good agreement between simulation and experiment, confirming that the proposed mechanisms have a significant potential for vastly improving the fracture response of composite materials. We elucidate the role of stiffness ratio variations of composite constituents as an important feature in determining the composite properties. Moreover, our work validates the predictive ability of our models, presenting them as useful tools for guiding further material design. This work enables the tailored design and manufacturing of composites assembled from inferior building blocks, that obtain optimal combinations of stiffness and toughness. PMID:24700202

Dimas, Leon S; Buehler, Markus J

2014-07-01

282

Materials characterization and fracture mechanics of a space grade dielectric silicone insulation  

NASA Technical Reports Server (NTRS)

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.

Abdel-Latif, A. I.; Tweedie, A. T.

1982-01-01

283

Alteration of Fractured Rocks Due to Coupled Chemical and Mechanical Processes: High-Resolution Simulations and Experimental Observations  

NASA Astrophysics Data System (ADS)

Engineering activities such as enhanced geothermal energy production and improved oil recovery techniques are heavily dependent on the permeability of the subsurface, while others such as CO2 sequestration and nuclear waste disposal rely on the efficiency of rock formations as transport barriers. In either case fractures provide the main pathways for fluid flow and transport, especially in rocks with lower matrix porosity. Laboratory experiments aimed at quantifying the chemo-mechanical responses of fractures have shown a range of results, some of which contradict simple conceptual models. For example, under conditions favoring mineral dissolution, where one would expect an overall increase in permeability, experiments show that permeability increases under some conditions and decreases under others. Recent experiments have attempted to link these core-scale observations to the relevant small-scale processes occurring within fractures. Results suggest that the loss of mechanical strength in asperities due to chemical alteration may cause non-uniform deformation and alteration of fracture apertures. However, due to the lack of direct micro-scale measurements of the coupled chemical and mechanical processes that lead to alteration of contacting fracture surfaces, our ability to predict the long-term evolution of fractures is still limited. To explore the processes that control permeability evolution, I developed a computational model that uses micro-scale surface roughness and explicitly couples dissolution and elastic deformation to calculate local alterations in fracture aperture under chemical and mechanical stresses. A depth-averaged algorithm of fracture flow is used to model reactive transport and chemical alteration of the fracture surfaces. Then, I deform the resulting altered fracture-surfaces using an algorithm that calculates the elastic deformation. The results of the model are compared with flow-through experiments conducted on fractured limestone. The fracture apertures are mapped before and after the experiments. These detailed aperture measurements are used as input to the new coupled model. The experiments cover a wide range of transport and reaction conditions; some exhibited negligible reaction-induced deformation and formation of distinct dissolution channels and others more uniform dissolution and measurable deformation caused by dissolution. Simulation results predict these general trends and the small-scale details in regions of contacting asperities.

Ameli, Pasha

284

Liquid metal embrittlement of T91 and 316L steels by heavy liquid metals: A fracture mechanics assessment  

Microsoft Academic Search

LME of the martensitic T91 and the austenitic 316L steels have been investigated in the CCT geometry in the plane-stress condition. Using such a geometry, premature cracking induced by a liquid metal (PbBi and Hg) can be studied using a fracture mechanics approach based on CTOD, J–?a and fracture assessment diagram. One is able to measure a reduction of the

T. Auger; Z. Hamouche; L. Medinaalmazan; D. Gorse

2008-01-01

285

Vertically Oriented Femoral Neck Fractures: Mechanical Analysis of Four Fixation Techniques  

Microsoft Academic Search

Objective: Femoral neck fractures inyoung individuals are typically high angled shear fractures. These injuries are difficult to stabilize due to a strong varus displacement force across the hip with weight bearing. The purpose of this study was to compare the biomechanical stability of four differing fixation techniques for stabilizing vertical shear femoral neck fractures. Methods: Vertical femoral neck fracture stability

Arash Aminian; Fan Gao; Wasyl W. Fedoriw; Li-Qun Zhang; David M. Kalainov; Bradley R. Merk

2007-01-01

286

Fracture of hard thin films using nanoindentation and nanoscratch techniques: A materials and mechanics approach  

SciTech Connect

Thin films are used in many applications where special properties are needed to insure performance and reliability. Of particular interest are thin tantalum nitride films. They are used extensively in microelectronic applications because of their long term stability and low thermal coefficient of resistance. They are sputter deposited which produces films with a high structural defect content and high compressive residual stresses both of which can alter the physical and mechanical properties of microelectronic thin films. Although these films are strong heat generators, they exhibit no changes in structure or composition of the interface with aluminum oxide substrates that degrade performance or reliability. However, the use of high power density components is driving a move to replace aluminum oxide with aluminum nitride for greater heat transfer. 6 This replacement substrate creates concern as residual stresses and long-term operation could induce detrimental changes along the thin film interface not observed in aluminum oxide devices. As a result, the authors employed nanoindentation and continuous nanoscratch testing to determine the effects of the intrinsic compressive residual stresses on the properties and fracture resistance of the thin tantalum nitride films. These techniques sample small volumes of material while preserving the production configuration of a free surface. Although nanoscratch tests lack a rigorous derivation of stress distributions and strain energy release rates, good approximations for strain energy release rates can be obtained using mechanics-based models for blister formation where residual stresses dominate interfacial fracture behavior. When combined with scanning and transmission electron microscopy, the results define structure-property relationships and resistance to fracture of these hard films.

Moody, N.R.; Medlin, D.; Hwang, R.Q.; Boehme, D. [Sandia National Labs., Livermore, CA (United States); Venkataraman, S. [Applied Materials, Santa Clara, CA (United States); Gerberich, W.W. [Univ. of Minnesota, Minneapolis, MN (United States)

1998-03-19

287

Strain rate effects on the mechanical properties and fracture mode of skeletal muscle.  

PubMed

The present study aimed to characterize the mechanical response of beagle sartorius muscle fibers under strain rates that increase logarithmically (0.1mm/min, 1mm/min and 10mm/min), and provide an analysis of the fracture patterns of these tissues via scanning electron microscopy (SEM). Muscle tissue from dogs' sartorius was excised and test specimens were sectioned with a lancet into sections with nominal length, width, and thickness of 7, 2.5 and 0.6mm, respectively. Trimming of the tissue was done so that the loading would be parallel to the direction of the muscle fiber. Samples were immediately tested following excision and failures were observed under the SEM. No statistically significant difference was observed in strength between the 0.1mm/min (2.560±0.37MPa) and the 1mm/min (2.702±0.55MPa) groups. However, the 10mm/min group (1.545±0.50MPa) had a statistically significant lower strength than both the 1mm/min group and the 0.1mm/min group with p<0.01 in both cases. At the 0.1mm/min rate the primary fracture mechanism was that of a shear mode failure of the endomysium with a significant relative motion between fibers. At 1mm/min this continues to be the predominant failure mode. At the 10mm/min strain rate there is a significant change in the fracture pattern relative to other strain rates, where little to no evidence of endomysial shear failure nor of significant motion between fibers was detected. PMID:24863204

Shapiro, Michael; Tovar, Nick; Yoo, Daniel; Sobieraj, Micheal; Gupta, Nikhil; Branski, Ryan C; Coelho, Paulo G

2014-06-01

288

A New Approximate Fracture Mechanics Analysis Methodology for Composites with a Crack or Hole  

NASA Technical Reports Server (NTRS)

A new approximate theory which links the inherent flaw concept with the theory of crack tip stress singularities at a bi-material interface was developed. Three assumptions were made: (1) the existence of inherent flaw (i.e., damage zone) at the tip of the crack, (2) a fracture of the filamentary composites initiates at a crack lying in the matrix material at the interface of the matrix/filament, and (3) the laminate fails whenever the principal load-carrying laminae fails. This third assumption implies that for a laminate consisting of 0 degree plies, cracks into matrix perpendicular to the 0 degree filaments are the triggering mechanism for the final failure. Based on this theory, a parameter bar K sub Q which is similar to the stress intensity factor for isotropic materials but with a different dimension was defined. Utilizing existing test data, it was found that bar K sub Q can be treated as a material constant. Based on this finding a fracture mechanics analysis methodology was developed. The analytical results are correlated well with test results. This new approximate theory can apply to both brittle and metal matrix composite laminates with crack or hole.

Tsai, H. C.; Arocho, A.

1990-01-01

289

A hybrid-dimensional approach for an efficient numerical modeling of the hydro-mechanics of fractures  

NASA Astrophysics Data System (ADS)

of subsurface fluid flow requires accounting for hydro-mechanical coupling between fluid-pressure variations and rock deformation. Particularly, flow of a compressible fluid along compliant hydraulic conduits, such as joints, fractures, or faults, is strongly affected by the associated deformation of the surrounding rock. We investigated and compared two alternative numerical modeling approaches that describe the transient fluid-pressure distribution along a single deformable fracture embedded in a rock matrix. First, we analyzed the coupled hydro-mechanical problem within the framework of Biot's poroelastic equations. Second, in a hybrid-dimensional approach, deformation characteristics of the surrounding rock were combined with a one-dimensional approximation of the fluid-flow problem to account for the high aspect ratios of fractures and the associated numerical problems. A dimensional analysis of the governing equations reveals that the occurring physical phenomena strongly depend on the geometry of the hydraulic conduit and on the boundary conditions. For the analyzed geometries, hydro-mechanical coupling effects dominate and convection effects can be neglected. Numerical solutions for coupled hydro-mechanical phenomena were obtained and compared to field data to characterize the fractured rock in the vicinity of an injection borehole. Either approach captures convection, diffusion, and hydro-mechanical effects, yet the hybrid-dimensional approach is advantageous due to its applicability to problems involving high-aspect-ratio features. For such cases, the modeling of pumping tests by means of the hybrid-dimensional approach showed that the observed inverse-pressure responses are the result of the coupling between the fluid flow in the fracture and the rock deformation caused by fluid-pressure variations along the fracture. Storage capacity as a single parameter of a fracture is insufficient to address all aspects of the coupling.

Vinci, C.; Renner, J.; Steeb, H.

2014-02-01

290

Finite element analysis of surface cracks in the Wilkins Ice Shelf using fracture mechanics  

NASA Astrophysics Data System (ADS)

Ice shelves, located between the warming atmosphere and the ocean, are sensitive elements of the climate system. The Wilkins Ice Shelf is situated in the south-western part of the Antarctic Peninsula, a well known hot spot of global warming. Recent break-up events exemplified the potential of disintegration of the ice shelf. A multi interdisciplinary project consisting of remote sensing, modeling of the ice dynamics and fracture mechanics intends to improve the understanding of the impacts of temperature increase on ice shelf stability. As a part of this project the aim of this presentation is to demonstrate the fracture mechanical approach using finite elements and configurational forces. For fracture mechanical purposes the material behavior of ice is treated as a brittle solid, and linear fracture mechanics is used. Crucial to all methods in linear fracture mechanics is the evaluation of the stress intensity factor K which is a measure for the load concentration at the crack tip and which depends on the geometry of the body and on the applied loading. The computed value of K can be compared to the critical stress intensity factor Kc, a material property obtained from experimental examinations, to judge whether a crack will propagate. One very effective procedure to obtain the stress intensity factor takes advantage of configurational forces, which can be easily obtained in the finite element analysis. An initial investigation is based on a 2-dimensional analysis of a single crack with a mode-I load type using a static plane strain model in the finite element analysis software COMSOL and additional routines to compute and evaluate the configurational forces. Analytical solutions of simple geometry and load cases are called on in comparison. The application to the Wilkins Ice Shelf follows by using material parameters, geometries and loading situations, which are obtained from literature values, remote sensing data analysis and modeling of the ice dynamics, respectively. The examined 2-dimensional geometry describes a vertical cut in a horizontally endless ice shelf creating a rectangular plate fragment. The crack, located in the middle of the plate is opened at the ice/atmosphere transition. The crack opening is small compared to the length of the plate. The plate is loaded on the lateral boundaries through a uniform tensile load resulting from the dynamical analysis and the hydrostatic pressure of the ice mass. The finite element analysis aims to examine the influence of the depth of the crack in connection with the inhomogeneous material properties of the different ice layers, which can be found in the ice shelf. Furthermore the influence of water inside the crevasse, resulting in an additional loading of the crack faces, is discussed. The results of these parameter studies are then used to obtain a measure for the crack induced compliance reduction of the ice to be used as an input for the modeling of the ice dynamics.

Plate, Carolin; Müller, Ralf; Gross, Dietmar; Humbert, Angelika; Braun, Matthias

2010-05-01

291

Computational mechanics  

SciTech Connect

The Computational Mechanics thrust area is a vital and growing facet of the Mechanical Engineering Department at Lawrence Livermore National Laboratory (LLNL). This work supports the development of computational analysis tools in the areas of structural mechanics and heat transfer. Over 75 analysts depend on thrust area-supported software running on a variety of computing platforms to meet the demands of LLNL programs. Interactions with the Department of Defense (DOD) High Performance Computing and Modernization Program and the Defense Special Weapons Agency are of special importance as they support our ParaDyn project in its development of new parallel capabilities for DYNA3D. Working with DOD customers has been invaluable to driving this technology in directions mutually beneficial to the Department of Energy. Other projects associated with the Computational Mechanics thrust area include work with the Partnership for a New Generation Vehicle (PNGV) for ''Springback Predictability'' and with the Federal Aviation Administration (FAA) for the ''Development of Methodologies for Evaluating Containment and Mitigation of Uncontained Engine Debris.'' In this report for FY-97, there are five articles detailing three code development activities and two projects that synthesized new code capabilities with new analytic research in damage/failure and biomechanics. The article this year are: (1) Energy- and Momentum-Conserving Rigid-Body Contact for NIKE3D and DYNA3D; (2) Computational Modeling of Prosthetics: A New Approach to Implant Design; (3) Characterization of Laser-Induced Mechanical Failure Damage of Optical Components; (4) Parallel Algorithm Research for Solid Mechanics Applications Using Finite Element Analysis; and (5) An Accurate One-Step Elasto-Plasticity Algorithm for Shell Elements in DYNA3D.

Raboin, P J

1998-01-01

292

Hydrologic mechanisms governing fluid flow in partially saturated, fractured, porous tuff at Yucca Mountain  

SciTech Connect

In contrast to the saturated zone where fluid moves rapidly along fractures, the fractures (with apertures large relative to the size of matrix pores) will desaturate first during drainage process and the bulk of fluid flow would be through interconnected pores in the matrix. Within a partially drained fracture, the presence of a relatively continuous air phase will produce practically an infinite resistance to liquid flow in the direction parallel to the fracture. The residual liquid will be held by capillary force in regions around fracture contact areas where the apertures are small. Normal to the fracture surfaces, the drained portion of the fractures will reduce the effective area for liquid flow from one matrix block to another matrix block. A general statistical theory is constructed for flow along the fracture and for flow between the matrix blocks to the fractures under partially saturated conditions. Results are obtained from an aperture distribution model for fracture saturation, hydraulic conductivity, and effective matrix-fracture flow areas as functions of pressure. Drainage from a fractured tuff column is simulated. The parameters for the simulations are deduced from fracture surface characteristics, spacings and orientations based on core analyses, and from matrix characteristics curve based on laboratory measurements. From the cases simulated for the fractured, porous column with discrete vertical and horizontal fractures and porous matrix blocks explicitly taken into account, it is observed that the highly transient changes from fully saturated conditions to partially saturated conditions are extremely sensitive to the fracture properties. However, the quasi-steady changes of the fluid flow of a partially saturated, fractured, porous system could be approximately simulated without taking the fractures into account. 22 references, 16 figures.

Wang, J.S.Y.; Narasimhan, T.N.

1984-10-01

293

Aspects of equivalence between contact mechanics and fracture mechanics: theoretical connections and a life-prediction methodology for fretting-fatigue  

Microsoft Academic Search

We identify aspects of quantitative equivalence between contact mechanics and fracture mechanics via asymptotic matching. An analogy is invoked between the geometry of the near-tip regions of cracked specimens and that of the sharp-edged contact region between two contacting surfaces. We then demonstrate that the asymptotic elastic stress and strain fields around the rim of the contact region, as derived

A. E. Giannakopoulos; T. C. Lindley; S. Suresh

1998-01-01

294

Fatigue Properties and Fracture Mechanism of Steel Coated with Diamond-Like Carbon Films  

NASA Astrophysics Data System (ADS)

Diamond-like carbon (DLC) films have attracted much attention in many industrial fields because of their excellent tribological properties, high hardness, chemical inertness and biocompatibility. In order to examine the fatigue properties and to clear the fracture mechanism of DLC coated materials, AISI4140 steel coated with DLC films by using unbalanced magnetron sputtering method was prepared and two types of fatigue test were carried out by using a tension and compression testing machine with stress ratio -1 and a bending testing machine with stress ratio -1 with a focused on the fatigue crack behavior in detail. The fracture origin changed from the slip deformation to micro defects at surface whose size didn't affect the fatigue crack initiation behavior in the case of Virgin series because the hard coating like DLC films make the defect sensitivity of coated material higher. However, DLC series indicated higher fatigue strengths in finite life region and fatigue limit compared with Virgin series. From the continuously observation by using a plastic replicas technique, it is clear that there are no noticeable differences on fatigue crack propagation rate between the Virgin and DLC series, however the fatigue crack initiation of DLC series was delayed significantly by existence of DLC films compared with Virgin series.

Akebono, Hiroyuki; Kato, Masahiko; Sugeta, Atsushi

295

Fundamental Mechanisms of Tensile Fracture in Aluminum Sheet Unidirectionally Reinforced with Boron Filament. Ph.D. Thesis - Virginia Polytechnic Inst.  

NASA Technical Reports Server (NTRS)

Results are presented from an experimental research effort to gain a more complete understanding of the physics of tensile fracture in unidirectionally reinforced B-Al composite sheet. By varying the degree of filament degradation resulting from fabrication, composite specimens were produced which failed in tension by the cumulative mode, the noncumulative mode, or by any desired combination of the two modes. Radiographic and acoustic emission techniques were combined to identify and physically describe a previously unrecognized fundamental fracture mechanism which was responsible for the noncumulative mode. The tensile strength of the composite was found to be severely limited by the noncumulative mechanism which involved the initiation and sustenance of a chain reaction of filament fractures at a relatively low stress level followed by ductile fracture of the matrix. The minimum average filament stress required for initiation of the fracture mechanism was shown to be approximately 170 ksi, and appeared to be independent of filament diameter, number of filament layers, and the identity of the matrix alloy.

Herring, H. W.

1971-01-01

296

Mechanical Behavior of Small-Scale Channels in Acid-etched Fractures  

E-print Network

are assigned a conductivity because of small-scale roughness features using a correlation obtained from laboratory measurements of acid fracture conductivity as a function of closure stress. The overall conductivity of the fracture is then obtained...

Deng, Jiayao

2011-02-22

297

Fracture mechanics characterization of welds: Fatigue life analysis of notches at welds: J(sub Ic) fracture toughness tests for weld metal  

Microsoft Academic Search

In this report two methods of fracture analysis of welds will be emphasized, one addressing fatigue life testing and analysis of notches at welds, and the other addressing the final fracture of the welded component and the fracture toughness tests used to characterize final fracture. These fatigue and fracture methods will be described by referring to recent work from the

John H. Underwood

1995-01-01

298

International Journal of Rock Mechanics & Mining Sciences 44 (2007) 739757 Computer simulation of hydraulic fractures  

E-print Network

of hydraulic fractures J. Adachia , E. Siebritsb , A. Peircec,Ã?, J. Desrochesd a Schlumberger Data of hydraulic fracturing models for use in the petroleum and other industries. We discuss scaling laws and the propagation regimes that control the growth of hydraulic fractures from the laboratory to the field scale. We

Peirce, Anthony

299

A micromodel investigation of two-phase matrix-fracture transfer mechanisms  

Microsoft Academic Search

Micromodels employing a two-dimensional representation of pore space were used to observe directly (via microscope) water imbibition into a matrix and matrix-fracture interactions between wetting and nonwetting fluids. Within a single field of view, some pores are responsible for the uptake of water, whereas immediately adjacent pores expel nonwetting phase into the fracture. When water flow through fractures is relatively

E. R. Rangel-German; A. R. Kovscek

2006-01-01

300

Fracture Mechanical Analysis of Frost Wedging in Ice Shelves as Break-Up Mechanism  

NASA Astrophysics Data System (ADS)

Disintegration events in ice shelves have been the subject of extensive investigations in the past years, however comprehensive explanations applicable to a majority of events are still missing. A popular assumption made by Scambos et al. (2000) [1] links disintegration events to a general thinning of the ice shelf in conjunction with growing melt-water ponds leading to hydro fractures. This explanation seems reasonable for break-up events that happened in Antarctic summers. Large parts of the Wilkins Ice Shelf, however broke-up in fall and winter periods. Therefore, the aim of the present study is to analyse the possibility of frost wedging of water filled surface crevasses in an ice shelf as a source of break-up events. Configurational forces are used to assess crack criticality. The simulations are performed on a 2-dimensional single crack with a mode-I type load, body forces and additional crack-face pressure due to freezing of the water. Depth-dependent density profiles are considered. The relevant parameters, Young's modulus, Poisson's ratio and external loading are obtained from literature, remote sensing data analysis and modelling of the ice dynamics. The investigation is performed using the finite element software COMSOL. The simulations show that in comparison to water filled crevasses without ice, thin layers of frozen water may lead to a decreasing criticality at the crack tip as long as the ice 'bridge' is allowed to take tensile loads. An increasing crack criticality can be seen for thicker layers of ice. The results are compared to findings from previous finite element analyses of dry and water filled cracks as presented in Plate et al. (2012) [2]. [1] Scambos, T., Hulbe, C., Fahnestock, M., & Bohlander, J. (2000). The link between climate warming and break-up of ice shelves in the Antarctic Peninsula. Journal of Glaciology, 46(154), 516-530. [2] Plate, C., Müller, R., Humbert, A., & Gross, D. (2012). Evaluation of the criticality of cracks in ice shelves using finite element simulations. The Cryosphere, 6(5), 973-984.

Plate, Carolin; Humbert, Angelika; Gross, Dietmar; Müller, Ralf

2013-04-01

301

Integral Equations with Hypersingular Kernels -- Theory and Applications to Fracture Mechanics  

Microsoft Academic Search

Hypersingular integrals of the type $I_{\\\\alpha}(T_n,m,r) = \\\\int_{-1}^{1} \\\\hpsngAbs \\\\frac{T_n(s)(1-s^2)^{m-{1\\/2}}}{(s-r)^\\\\alpha}ds |r|<1$ and $I_{\\\\alpha}(U_n,m,r) = \\\\int_{-1}^{1} \\\\hpsngAbs \\\\frac{U_n(s)(1-s^2)^{m-{1\\/2}}}{(s-r)^\\\\alpha}ds |r| 4$ and $m>3$ is provided. The integrals are also evaluated as $|r|>1$ in order to calculate stress intensity factors (SIFs). Examples involving crack problems are given and discussed with emphasis on the linkage between mathematics and mechanics of fracture. The examples include

Youn-Sha Chan; Albert C. Fannjiang; Glaucio H. Paulino

1999-01-01

302

Fracture mechanics of matrix cracking and delamination in glass/epoxy laminates  

NASA Technical Reports Server (NTRS)

This study focused on characterizing matrix cracking and delamination behavior in multidirectional laminates. Static tension and tension-tension fatigue tests were conducted on two different layups. Damage onset, accumulation, and residual properties were measured. Matrix cracking was shown to have a considerable influence on residual stiffness of glass epoxy laminates, and could be predicted reasonably well for cracks in 90 deg piles using a simple shear lag analysis. A fracture mechanics analysis for the strain energy release rate associated with 90 deg ply-matrix crack formation was developed and was shown to correlate the onset of 90 deg ply cracks in different laminates. The linear degradation of laminate modulus with delamination area, previously observed for graphite epoxy laminates, was predicted for glass epoxy laminates using a simple rule of mixtures analysis. The strain energy release rate associated with edge delamination formation under static and cyclic loading was difficult to analyze because of the presence of several contemporary damage phenomena.

Caslini, M.; Zanotti, C.; Obrien, T. K.

1986-01-01

303

A probabilistic fracture mechanics approach for structural reliability assessment of space flight systems  

NASA Astrophysics Data System (ADS)

A probabilistic fracture mechanics approach for predicting the failure life distribution due to subcritical crack growth is presented. A state-of-the-art crack propagation method is used in a Monte Carlo simulation to generate a distribution of failure lives. The crack growth failure model expresses failure life as a function of stochastic parameters including environment, loads, material properties, geometry, and model specification errors. A stochastic crack growth rate model that considers the uncertainties due to scatter in the data and mode misspecification is proposed. The rationale for choosing a particular type of probability distribution for each stochastic input parameter and for specifying the distribution parameters is presented. The approach is demonstrated through a probabilistic crack growth failure analysis of a welded tube in the Space Shuttle Main Engine. A discussion of the results from this application of the methodology is given.

Sutharshana, S.; Creager, M.; Ebbeler, D.; Moore, N.

304

Recursive distribution method for probabilistic fracture mechanics and its application to evaluation of LWR piping  

SciTech Connect

The present authors have previously developed a new method for Probabilistic Fracture Mechanics (PFM), which they call Recursive Distribution (RD) method. The method is based on the construction of the Lebesgue-Stieltjes measure through a deterministic mapping defining a crack growth process. In the present paper, its theoretical background is first discussed, and the Lebesgue decomposition of the measure is given. Then a numerical example of a Light Water Reactor (LWR)`s piping problem is solved by the present method, and the results are compared with those of the Monte Carlo (MC) method. In addition to leakage probability, a variation in stress cycles of the marginal distribution of an aspect ratio of a semi-elliptical surface crack is calculated, which will be used in a study on LBB evaluation.

Akiba, Hiroshi; Suzuki, Masabumi [Allied Engineering Corporation, Tokyo (Japan); Yoshimura, Shinobu; Yagawa, Genki [Univ. of Tokyo (Japan)

1996-12-01

305

DEFORMATION AND FRACTURE OF POORLY CONSOLIDATED MEDIA - Borehole Failure Mechanisms in High-Porosity Sandstone  

SciTech Connect

We investigated failure mechanisms around boreholes and the formation of borehole breakouts in high-porosity sandstone, with particular interest to grain-scale micromechanics of failure leading to the hitherto unrecognized fracture-like borehole breakouts and apparent compaction band formation in poorly consolidated granular materials. We also looked at a variety of drilling-related factors that contribute to the type, size and shape of borehole breakouts. The objective was to assess their effect on the ability to establish correlations between breakout geometry and in situ stress magnitudes, as well as on borehole stability prediction, and hydrocarbon/water extraction in general. We identified two classes of medium to high porosity (12-30%) sandstones, arkosic, consisting of 50-70% quartz and 15 to 50% feldspar, and quartz-rich sandstones, in which quartz grain contents varied from 90 to 100%. In arkose sandstones critical far-field stress magnitudes induced compressive failure around boreholes in the form of V-shaped (dog-eared) breakouts, the result of dilatant intra-and trans-granular microcracking subparallel to both the maximum horizontal far-field stress and to the borehole wall. On the other hand, boreholes in quartz-rich sandstones failed by developing fracture-like breakouts. These are long and very narrow (several grain diameters) tabular failure zones perpendicular to the maximum stress. Evidence provided mainly by SEM observations suggests a failure process initiated by localized grain-bond loosening along the least horizontal far-field stress springline, the packing of these grains into a lower porosity compaction band resembling those discovered in Navajo and Aztec sandstones, and the emptying of the loosened grains by the circulating drilling fluid starting from the borehole wall. Although the immediate several grain layers at the breakout tip often contain some cracked or even crushed grains, the failure mechanism enabled by the formation of the compaction band is largely non-dilatant, a major departure from the dilatant mechanism observed in Tablerock sandstone. The experimental results suggest that unlike our previous assertion, the strength of grain bonding and the mineral composition, rather than the porosity, are major factors in the formation of compaction bands and the ensuing fracture-like breakouts. Some breakout dimensions in all rocks were correlatable to the far-field principal stresses, and could potentially be used (in conjunction with other information) as indicators of their magnitudes. However, we found that several factors can significantly influence breakout geometry. Larger boreholes and increased drilling-fluid flow rates produce longer fracture-like breakouts, suggesting that breakouts in field-scale wellbores could reach considerable lengths. On the other hand, increased drilling-fluid weight and increased drill-bit penetration rate resulted in a decrease in breakout length. These results indicate that breakout growth can be controlled to some degree by manipulating drilling variables. Realizing how drilling variables impact borehole breakout formation is important in understanding the process by which breakouts form and their potential use as indicators of the far-field in situ stress magnitudes and as sources of sand production. As our research indicates, the final breakout size and mechanism of formation can be a function of several variables and conditions, meaning there is still much to be understood about this phenomenon.

Bezalel c. Haimson

2005-06-10

306

Determination of Some Parameters for Fatigue Life in Welded Joints Using Fracture Mechanics Method  

NASA Astrophysics Data System (ADS)

In this work, the parameters stress intensity factor (SIF), initial and final crack lengths ( a i and a f), crack growth parameters ( C and m), and fatigue strength (FAT) are investigated. The determination of initial crack length seems to be the most serious factor in fatigue life and strength calculations for welded joints. A fracture mechanics approach was used in these calculations based on SIF which was calculated with the finite element method (FEM). The weld toe crack was determined to be equal to 0.1 mm, whereas the weld root crack's length was varied depending on the degree of the weld penetration. These initial crack length values are applicable for all types of joints which have the same crack phenomenon. As based on the above calculated parameters, the new limits of FAT for new geometries which are not listed yet in recommendations can be calculated according to the current approach.

Al-Mukhtar, A. M.; Biermann, H.; Hübner, P.; Henkel, S.

2010-12-01

307

Stress and Fracture Mechanics Analyses of Boiling Water Reactor and Pressurized Water Reactor Pressure Vessel Nozzles  

SciTech Connect

This paper describes stress analysis and fracture mechanics work performed to assess boiling water reactor (BWR) and pressurized water reactor (PWR) nozzles located in the reactor pressure vessel (RPV) adjacent to the core beltline region. Various RPV nozzle geometries were investigated: 1. BWR recirculation outlet nozzle; 2. BWR core spray nozzle3 3. PWR inlet nozzle; ; 4. PWR outlet nozzle; and 5. BWR partial penetration instrument nozzle. The above nozzle designs were selected based on their proximity to the core beltline region, i.e., those nozzle configurations that are located close enough to the core region such that they may receive sufficient fluence prior to end-of-license (EOL) to require evaluation as part of establishing the allowed limits on heatup, cooldown, and hydrotest (leak test) conditions. These nozzles analyzed represent one each of the nozzle types potentially requiring evaluation. The purpose of the analyses performed on these nozzle designs was as follows: To model and understand differences in pressure and thermal stress results using a two-dimensional (2-D) axi-symmetric finite element model (FEM) versus a three-dimensional (3-D) FEM for all nozzle types. In particular, the ovalization (stress concentration) effect of two intersecting cylinders, which is typical of RPV nozzle configurations, was investigated; To verify the accuracy of a selected linear elastic fracture mechanics (LEFM) hand solution for stress intensity factor for a postulated nozzle corner crack for both thermal and pressure loading for all nozzle types; To assess the significance of attached piping loads on the stresses in the nozzle corner region; and To assess the significance of applying pressure on the crack face with respect to the stress intensity factor for a postulated nozzle corner crack.

Yin, Shengjun [ORNL; Bass, Bennett Richard [ORNL; Stevens, Gary [U.S. Nuclear Regulatory Commission; Kirk, Mark [NRC

2011-01-01

308

The possibilities of fracture mechanics as applied to problems of strength, service life, and substantiation of safe operation of heat-generating and mechanical equipment  

NASA Astrophysics Data System (ADS)

The fundamentals of fracture mechanics are briefly outlined, and its possibilities as a tool for estimating the longevity and bearing capacity of equipment components the metal of which contains cracklike flaws are pointed out. Experimentally obtained kinetic crack resistance diagrams lying at the heart of methods for determining the survivability of structures are given for some steels. Practical application of the methods of fracture mechanics is demonstrated on a particular example of substantiating temporary operability of a boiler drum having cracklike flaws near the holes for water downtake pipes.

Grin', E. A.

2013-01-01

309

Homogenized finite element analysis on effective elastoplastic mechanical behaviors of composite with imperfect interfaces.  

PubMed

A three-dimensional (3D) representative volume element (RVE) model was developed for analyzing effective mechanical behavior of fiber-reinforced ceramic matrix composites with imperfect interfaces. In the model, the fiber is assumed to be perfectly elastic until its tensile strength, and the ceramic material is modeled by an elasto-plastic Drucker-Prager constitutive law. The RVE model is then used to study the elastic properties and the tensile strength of composites with imperfect interfaces and validated through experiments. The imperfect interfaces between the fiber and the matrix are taken into account by introducing some cohesive contact surfaces. The influences of the interface on the elastic constants and the tensile strengths are examined through these interface models. PMID:25522170

Jiang, Wu-Gui; Zhong, Ren-Zhi; Qin, Qing H; Tong, Yong-Gang

2014-01-01

310

Homogenized Finite Element Analysis on Effective Elastoplastic Mechanical Behaviors of Composite with Imperfect Interfaces  

PubMed Central

A three-dimensional (3D) representative volume element (RVE) model was developed for analyzing effective mechanical behavior of fiber-reinforced ceramic matrix composites with imperfect interfaces. In the model, the fiber is assumed to be perfectly elastic until its tensile strength, and the ceramic material is modeled by an elasto-plastic Drucker-Prager constitutive law. The RVE model is then used to study the elastic properties and the tensile strength of composites with imperfect interfaces and validated through experiments. The imperfect interfaces between the fiber and the matrix are taken into account by introducing some cohesive contact surfaces. The influences of the interface on the elastic constants and the tensile strengths are examined through these interface models. PMID:25522170

Jiang, Wu-Gui; Zhong, Ren-Zhi; Qin, Qing H.; Tong, Yong-Gang

2014-01-01

311

Mechanics of slip and fracture along small faults and simple strike-slip fault zones in granitic rock  

SciTech Connect

We exploit quasi-static fracture mechanics models for slip along pre-existing faults to account for the fracture structure observed along small exhumed faults and small segmented fault zones in the Mount Abbot quadrangle of Calfornia and to estimate stress drop and shear fracture energy from geological field measurements. Along small strike-slip faults, cracks that splay from the faults are common only near fault ends. In contrast, many cracks splay from the boundary faults at the edges of a simple fault zone. Except near segment ends, the cracks preferentially splay into a zone. We infer that shear displacement discontinuities (slip patches) along a small fault propagated to near the fault ends and caused fraturing there. Based on elastic stress analyses, we suggest that slip on one boundary fault ends and caused fracturing there. Based on elastic stress analyses, we suggest that slip patches preferentially led to the generation of fractures that splayed into the zones away from segment ends and out of the zones near segment ends. We estimate the average stress drops for slip events along the fault zones as /similar to/1 MPa and the shear fracture energy release rate during slip as 5/times/10/sup 2//minus/2/times/10/sup 4/ J/m/sup 2/. This estimate is similar to those obtained from shear fracture of laboratory samples, but orders of magnitude less than those for large fault zones. These results suggest that the shear fracture energy release rate increases as the structural complexity of fault zones increases. /copyright/ American Geophysical Union 1989

Martel, S. J.; Pollard, D. D.

1989-07-10

312

Structure-Property-Fracture Mechanism Correlation in Heat-Affected Zone of X100 Ferrite-Bainite Pipeline Steel  

NASA Astrophysics Data System (ADS)

Structural performance of a weld joint primarily depends on the microstructural characteristics of heat-affected zone (HAZ). In this regard, the HAZ in X100 ferrite-bainite pipeline steel was studied by separating the HAZ into intercritically reheated coarse-grained (ICCG) HAZ containing and non-containing regions. These two regions were individually evaluated for Charpy impact toughness and characterized by electron back-scattered diffraction (EBSD). Low toughness of ~50 J was obtained when the notch of impact specimen encountered ICCGHAZ and high toughness of ~180 J when the notch did not contain ICCGHAZ. Fracture surface was ~60 pct brittle in the absence of ICCGHAZ, and 95 pct brittle (excluding shear lip) in the presence of ICCGHAZ in the impact tested samples. The underlying reason is the microstructure of ICCGHAZ consisted of granular bainite and upper bainite with necklace-type martensite-austenite (M-A) constituent along grain boundaries. The presence of necklace-type M-A constituent notably increases the susceptibility of cleavage microcrack nucleation. ICCGHAZ was found to be both the initiation site of the whole fracture and cleavage facet initiation site during brittle fracture propagation stage. Furthermore, the study of secondary microcracks beneath CGHAZ and ICCGHAZ through EBSD suggested that the fracture mechanism changes from nucleation-controlled in CGHAZ to propagation-controlled in ICCGHAZ because of the presence of necklace-type M-A constituent in ICCGHAZ. Both fracture mechanisms contribute to the poor toughness of the sample contained ICCGHAZ.

Li, Xueda; Ma, Xiaoping; Subramanian, S. V.; Misra, R. D. K.; Shang, Chengjia

2015-01-01

313

Performance of multiple fractured horizontal wells in shale gas reservoirs with consideration of multiple mechanisms  

NASA Astrophysics Data System (ADS)

Gas flow in shales is believed to result from a combination of several mechanisms, including desorption, diffusion, viscous flow and the effect of stress-sensitivity of reservoir permeability. However, little work has been done in literature to simultaneously incorporate all these mechanisms in well testing models for shale gas reservoirs. This paper presents a new well testing model for multiple fractured horizontal wells (MFHW) in shale gas reservoirs with consideration of desorption, diffusive flow, viscous flow and stress-sensitivity of reservoir permeability. Comparing with current well testing models for MFHW, the model presented here takes into consideration more mechanisms controlling shale gas flow, which is more in line with the actual reservoir situation. Laplace transformation, point source function, perturbation method, numerical discrete method and Gaussian elimination method are employed to solve the well testing model. The pressure transient responses are then inverted into real time space with Stehfest numerical inversion algorithm. Type curves are plotted, and different flow regimes in shale gas reservoirs are identified. The effects of relevant parameters are analyzed as well. The presented model can be used to interpret pressure data more accurately for shale gas reservoirs and provide more accurate dynamic parameters which are important for efficient reservoir development.

Wang, Hai-Tao

2014-03-01

314

A fracture mechanics analysis of the effects of material properties and geometries of components on various types of package cracks  

Microsoft Academic Search

Various package cracks observed in service, with different locations and propagation directions, in plastic packages of surface mount technology (SMT) are analyzed in terms of the energy release rate in fracture mechanics. It has been reported that the types of the cracks observed in the package depend on the chip size, relative thickness of the epoxy resin on the chip

Hyouk Lee; Youn Young Earmme

1996-01-01

315

THERMO-HYDRO-MECHANICAL MODELING OF WORKING FLUID INJECTION AND THERMAL ENERGY EXTRACTION IN EGS FRACTURES AND ROCK MATRIX  

SciTech Connect

Development of enhanced geothermal systems (EGS) will require creation of a reservoir of sufficient volume to enable commercial-scale heat transfer from the reservoir rocks to the working fluid. A key assumption associated with reservoir creation/stimulation is that sufficient rock volumes can be hydraulically fractured via both tensile and shear failure, and more importantly by reactivation of naturally existing fractures (by shearing), to create the reservoir. The advancement of EGS greatly depends on our understanding of the dynamics of the intimately coupled rock-fracture-fluid-heat system and our ability to reliably predict how reservoirs behave under stimulation and production. Reliable performance predictions of EGS reservoirs require accurate and robust modeling for strongly coupled thermal-hydrological-mechanical (THM) processes. Conventionally, these types of problems have been solved using operator-splitting methods, usually by coupling a subsurface flow and heat transport simulators with a solid mechanics simulator via input files. An alternative approach is to solve the system of nonlinear partial differential equations that govern multiphase fluid flow, heat transport, and rock mechanics simultaneously, using a fully coupled, fully implicit solution procedure, in which all solution variables (pressure, enthalpy, and rock displacement fields) are solved simultaneously. This paper describes numerical simulations used to investigate the poro- and thermal- elastic effects of working fluid injection and thermal energy extraction on the properties of the fractures and rock matrix of a hypothetical EGS reservoir, using a novel simulation software FALCON (Podgorney et al., 2011), a finite element based simulator solving fully coupled multiphase fluid flow, heat transport, rock deformation, and fracturing using a global implicit approach. Investigations are also conducted on how these poro- and thermal-elastic effects are related to fracture permeability evolution.

Robert Podgorney; Chuan Lu; Hai Huang

2012-01-01

316

Mechanisms of hyoid bone fracture after modelling: evaluation of anthropological criteria defining two relevant models.  

PubMed

Several studies have attempted to describe the morphology of the hyoid bone, while other authors have focused on discovering the role of this bone in the occurrence of fractures. Hyoid fractures are known to be dependent on the force applied against the bone, or on the location at which the force is applied. We wished to assess the value of defining one or more models of the hyoid bone by analyzing variations in the size and angle of the various component parts of the bone relative to the sex and morphology of an individual (height and weight) in a sample of 72 bones obtained during forensic autopsy at our institution. Statistical analyses were developed using SAS software (Statistical Analysis System, version 9.2). We observed that the length of the hyoid bone and the angle between the greater horns differed significantly between men and women. Length was significantly greater in men (38.20 ± 4.67 mm) than in women (30.49 ± 7.90 mm) and the angle between the greater horns of the hyoid bone was larger in women (36.46 ± 13.77°) than in men (27.56 ± 13.02°). There was also a statistically significant correlation between the body mass index of an individual and the length of the hyoid bone. As weight increased, the hyoid bone was found to be longer. The weight of an individual was also significantly correlated with the angle of the hyoid bone, with lower weight resulting in larger angles of the bone. Furthermore, hierarchical classification enabled the hyoid bone to be differentiated into two groups or clusters according to anthropometric measurements. ROC curves were used to determine threshold values of length, width and angle to classify the hyoid bones in these two clusters: the first was composed of individuals with longer hyoid bones, and the second of individuals with greater hyoid bone widths and wider angles. Logistic regression showed male gender was more frequently associated with the first group. The morphology of the hyoid bone can be differentiated according to the gender and corpulence of an individual because these parameters are correlated. These findings are crucial in establishing a protocol for modelling the mechanism of fracture of the hyoid bone in strangulation. Two models of the hyoid bone appear to be needed to meet the practical requirements that are the purpose of these biomechanical studies. PMID:21764532

Pollard, J; Piercecchi-Marti, M D; Thollon, L; Bartoli, C; Adalian, Pascal; Bécart-Robert, A; Tournel, G; Hédouin, V; Panuel, M; Gosset, D; Leonetti, G

2011-10-10

317

Dynamic tensile response of Zr-based bulk amorphous alloys: Fracture morphologies and mechanisms  

NASA Astrophysics Data System (ADS)

Plate impact experiments were conducted to examine the dynamic tensile response of Zr-based bulk amorphous alloys (BAAs) having a nominal composition of Zr56.7Cu15.3Ni12.5Nb5.0Al10.0Y0.5. The experimental configuration used in our work permitted soft recovery of the samples to allow a careful examination of the fractured samples along with real-time measurements of the sample free-surface velocity (FSV) histories. Tensile loading was preceded by elastic compressive loading to peak stresses in the 3.6 to 6.0 GPa range. Tensile damage in the recovered samples was examined using optical and electron microscopy. The microscopy results showed that the BAA samples exhibit a brittle behavior (as a glass) at the macroscopic level and a ductile behavior (as a metal) at the microscopic level; in addition, corrugations and bumps are observed at the nanoscale. The observed fracture morphologies are related to three key features present in our spall experiments: preceding compressive stress (3.6-6.0 GPa), high tensile loading rate (˜106/s), high mean tensile stress (˜2.3 GPa); and are intrinsically related to the amorphous glassy structure of the BAAs (free volume content). We propose that the compressive stress depletes the free volume content. With increasing compressive stress, the available free volume decreases causing a suppression of shear stresses during tension. Thus, the mean tensile component becomes more dominant at higher stresses. Consequently, the observed surface morphology results from brittle cleavage, causing an increased damage localization in the recovered samples spalled at higher stresses. These observations support the inferences made from measurements of FSV histories. The high tensile loading rate is proposed to be responsible for cracking by multiple shear band propagation and interception, rendering the observed serrated surface morphology. Finally, we proposed that the corrugations are created due to a succession of arrest and propagation of mode I cracks. A subsequent dilatation, due to the effect of the tensile mean stress, caused the corrugations to evolve to bump-type features with sizes in the range of 10-100 nm. Our proposed mechanisms, although qualitative, constitute a systematic attempt to provide an explanation for the fracture morphologies observed in spalled BAA samples.

Escobedo, J. P.; Gupta, Y. M.

2010-06-01

318

Mechanisms of deformation localization at the tips of shear fractures: Findings from analogue experiments and field evidence  

NASA Astrophysics Data System (ADS)

Using analogue experiments on polymethylmethaacrylate (PMMA) models, we investigated the process of deformation localization at the tips of preexisting planar shear cracks. Experiments show that this can take place in any of the following four principal mechanisms. Mechanism A: Brittle deformation is the dominant process and forms a pair of long tensile fractures at the crack tips. The tensile fractures propagate along the compression direction and transgress the entire model thickness, causing model failure at a small bulk strain (3%). Mechanism B: It involves both brittle and ductile (plastic) strain localization, where the tensile fractures grow to a limited length and incipient ductile zones appear at the tips. Mechanism C: Deformation localization is characterized by an association of macroscale shear bands and short, opened-out tensile fissures (cf. wingfractures). Mechanism D: Ductile strain localizes in the form of a pair of shear bands at each tip. Fracture failure does not occur in this case. The transition from Mechanism A to Mechanism D is a continuous phenomenon in the experimental conditions, which we show as a function of initial crack angle (? angle between the crack and the far-field compression direction) and crack length (l). Mechanism A tends to be replaced by Mechanism D with decreasing ? (60° to 20°) and/or l. Using a finite element method (FEM), we calculated the maximum principal tensile stress (?1max) and the maximum second stress invariant (I2max) of the stress field in the neighborhood of a sliding crack within a linearly elastic medium and analyzed the brittle-ductile transitions observed in physical experiments. The calculations show that ?1max is directly proportional to l and attains a peak value for ? = 45°, promoting Mechanism A. On the other hand, I2max occurs at ? < 45°, favoring nucleation of ductile shear bands (Mechanism D). When ? and l are increased simultaneously, ?1max takes its peak value at ? = 60°. This analysis explains the dominance of Mechanism A for ? > 45° in physical models with simultaneously varying crack length and orientation. We also demonstrate probable interactions between plastic strain localization and tensile fracturing at the crack tips. FEM results indicate that a plastic zone lowers the magnitude of tensile stress concentration at wing cracks and thereby dampen their growth when ? < 45°. We finally complement our study with field examples.

Misra, Santanu; Mandal, Nibir; Dhar, Rajib; Chakraborty, Chandan

2009-04-01

319

Mechanical properties and fracture characteristics of CNTs-doped W-Nb alloys  

NASA Astrophysics Data System (ADS)

W-based alloys are currently considered promising candidates for high heat flux components in future fusion reactors. In this work, the strength and fracture toughness of CNTs-doped W-Nb alloys fabricated by hot pressing were measured by three-point bending tests on smooth and single edge notched beams, respectively, and the fracture characteristics were observed by scanning electron microscopy. The result indicated that the bending strength and fracture toughness increased with increasing the Nb content. Combining with the microstructure analysis on the fracture surface, it was found that the increased Nb content could refine the microstructure and form the oriented Nb(W) solid solutions, which increases the number of crack deflections and total fracture paths and enhanced the strength and fracture toughness.

Sha, J. J.; Hao, X. N.; Li, J.; Dai, J. X.; Yang, X. L.; Yoon, H. K.

2014-12-01

320

Experimental investigation of the effect of coupled processes (Thermo-Hydro-Mechanical-Chemical) on CO2 flow through fractured caprock  

NASA Astrophysics Data System (ADS)

In order for geological storage of CO2 to be effective in mitigation of anthropogenically induced climate change, the occurrence of CO2 leakage from the storage reservoir to the surface must be negligible. One of the key potential pathways for CO2 leakage is escape through macroscopic fractures or faults in caprock overlying the storage reservoir. The laboratory investigation of fracture permeability to CO2 under a range of in-situ temperature and pressure conditions, using typical caprock samples, will improve understanding of the flow and transport properties of CO2 under typical geological storage conditions. Knowledge gained from such laboratory investigations will be critical for development of accurate models of CO2 flow and transport in the subsurface, which in turn will inform on leakage risk and site suitability. Experimental equipment has been designed and built at the University of Edinburgh that allows investigation of CO2 flow through 38mm diameter rock samples under realistic reservoir/overburden conditions of temperature (up to 80°C) and pressure (up to 60MPa). The fractured rock sample is contained within a Hassler-type uniaxial pressure cell and CO2 flow through the sample is controlled via high precision, high pressure syringe pumps. Fluid pressures upstream and downstream of the sample, and the confining pressure, are continuously monitored. Single phase CO2 flow experiments have been conducted on artificial planar fractures within a variety of representative caprock samples. The effect on fracture permeability of varying: (i) effective stress (confining pressure); (ii) temperature; and (iii) fluid pressure has been investigated. The impact of mineralogy has also been considered through comparison of results obtained from samples of differing rock type. Initial results and findings from the experiments are presented. A discussion of the use of the experimental results for development of coupled process (thermo-hydro-mechanical and chemical) single fracture benchmark models is also included. Proposed future experimental work, which includes investigation of natural fractures, is outlined.

McCraw, Claire; Edlmann, Katriona; McDermott, Christopher

2013-04-01

321

Mechanics of two interacting magma-driven fractures: A numerical study  

NASA Astrophysics Data System (ADS)

understand magma focusing from broad melting zones in the crust, propagation in brittle rocks of two interacting dikes ascending from a single deep source has been modeled using a time-dependent plane strain hydraulic fracturing model. The source is assumed to generate a constant influx rate to feed the dike growth, which is also aided by buoyancy effects. In contrast to uncoupled model results, the simultaneous parallel growth of two dikes to a certain distance is found to occur provided that the two dikes are initially of different heights, which might be produced either by previous magma intrusion or during nucleation. The shorter dike will chase the longer one and they can either progressively merge or continue subparallel growth at a reduced spacing, depending on the deviator stress between the vertical and horizontal stresses and the initial lateral separation, with parameters given in dimensionless forms. Numerical results reveal the mechanics for simultaneous ascents of two subparallel dikes, in that dike interaction can produce a low-stress field, which is just above the tip of the shorter dike, favorable to growth of the shorter dike. The energy analysis indicates that the energy required for two subparallel dikes is less than that for a single dike during the late-time buoyancy-viscosity propagation stage where considerable ascent occurs. This growth behavior might provide the mechanism for simultaneous, instead of sequential, growth of various dikes in a single set.

Zhang, Xi; Bunger, Andrew P.; Jeffrey, Robert G.

2014-11-01

322

Mechanical Properties and Fracture Behavior of Electroless Ni-Plated Short Carbon Fiber Reinforced Geopolymer Matrix Composites  

NASA Astrophysics Data System (ADS)

Electroless Ni-plated short carbon fiber reinforced geopolymer matrix composites with various carbon fiber/matrix interface coating thicknesses have been successfully fabricated. The influences of coating thickness on the mechanical properties and fracture behavior have been investigated by three-point bending test and scanning electron microscopy. The flexural strength and Young's modulus of Ni-plated short carbon fiber reinforced geopolymer composites exhibit maximums as the average fiber coating thickness increases, but the work of fracture has a sharp decrease, and the fracture manner changes from ductile to brittle. This is mainly attributed to the fact that the carbon fibers favor breakage rather than pulling-out during loading because of the higher interface bonding strength of fiber/matrix, and pliability of the carbon fibers decreases with the increase of the coating thickness.

Lin, Tiesong; Jia, Dechang

323

Liquid metal embrittlement of T91 and 316L steels by heavy liquid metals: A fracture mechanics assessment  

NASA Astrophysics Data System (ADS)

LME of the martensitic T91 and the austenitic 316L steels have been investigated in the CCT geometry in the plane-stress condition. Using such a geometry, premature cracking induced by a liquid metal (PbBi and Hg) can be studied using a fracture mechanics approach based on CTOD, J-? a and fracture assessment diagram. One is able to measure a reduction of the crack tip blunting and a reduction of the energy required for crack propagation induced by the liquid metal. In spite of some limitations, this qualitative evaluation shows that liquid metals do not induce strong embrittlement on steels in plane-stress condition. Rather, the effect of the liquid metal seems to promote a fracture mode by plastic collapse linked with strain localization. It indicates that the materials, in spite of a potential embrittlement, should still be acceptable in terms of safety criteria.

Auger, T.; Hamouche, Z.; Medina-Almazàn, L.; Gorse, D.

2008-06-01

324

Modeling Fractures in Thermal Systems: Thermal-Mechanical Feedback and Vein Formation  

NSDL National Science Digital Library

Barb Dutrow, Department of Geology & Geophysics, Louisiana State University Topic: Mineralogy, Petrology, fracture formation Course type: Upper level undergraduate course Description In many geologic systems, ...

325

A micromodel investigation of two-phase matrix-fracture transfer mechanisms  

NASA Astrophysics Data System (ADS)

Micromodels employing a two-dimensional representation of pore space were used to observe directly (via microscope) water imbibition into a matrix and matrix-fracture interactions between wetting and nonwetting fluids. Within a single field of view, some pores are responsible for the uptake of water, whereas immediately adjacent pores expel nonwetting phase into the fracture. When water flow through fractures is relatively slow and fluid transfer from the fracture is relatively rapid, imbibition is microscopically cocurrent and micromodel observations teach that uptake of the wetting phase by the matrix correlates directly with the volume of water injected. This mode of transfer is coined a filling fracture. On the other hand, when fractures fill with water quickly relative to the rate of matrix-fracture transfer, the mass of water imbibed scales with the square root of time. Here imbibition is found to be countercurrent at the pore level. In the countercurrent mode, significant channeling of the nonwetting phase through the continuous wetting phase is observed that reduces the efficiency of water infiltration. Overall, it is found that the rate of water uptake from a fracture into an unsaturated matrix and the pore-level pattern of water infiltration depend critically on the rate of water infiltration through fractures.

Rangel-German, E. R.; Kovscek, A. R.

2006-03-01

326

Melt Impregnation, Strain Localization, and Deformation Mechanisms in a Fossil Oceanic Fracture Zone (Ingalls Ophiolite)  

NASA Astrophysics Data System (ADS)

A steep mantle shear zone that deforms ultramafic rocks of the Jurassic Ingalls ophiolite is inferred to preserve a record of the interplay of melt impregnation, strain localization, and switching deformation mechanisms in a fossil oceanic fracture zone. This ~2-km-wide, E-W-striking shear zone separates harzburgite and dunite on the south from lherzolite and cpx-rich harzburgite on the north. Geochemical data from the lherzolite, which contains veinlets of plagioclase and cpx, suggest impregnation by infiltrating basaltic melt. The shear zone reworks the lherzolite unit, but also contains widespread plagioclase peridotite and hornblende peridotite, and shear-zone mylonites are less depleted than the adjacent units. Olivine is reduced in average grain size from 1.5-3.0 mm in the lherzolites to 50 µm in some mylonites. In the mylonites, opx and cpx porphyroclasts are set in a mosaic of olivine, cpx, opx +/- hornblende +/- plagioclase. Lattice preferred orientations (LPO) determined by EBSD indicate that olivine in the dunite-harzburgite and lherzolite units deformed by glide on [100] (010), a common mechanism for dislocation creep in the upper mantle. In contrast, olivine in the mylonites has much weaker, poorly defined LPOs. This weakening of the LPO and the microstructures are compatible with dynamic recrystallization and grain-size reduction resulting from dislocation creep leading to a change to a grain-size-sensitive deformation mechanism. We suggest that impregnation by infiltrating melts may have helped localize strain, and the formation of multiple phases, in part as a result of impregnation, may have stabilized the small olivine grain size.

Miller, R. B.; Gordon, S. M.

2010-12-01

327

Effect of roughness and material strength on the mechanical properties of fracture replicas  

SciTech Connect

This report presents the results of 11 rotary shear tests conducted on replicas of three hollow cylinders of natural fractures with JRC values of 7.7, 9.4 and 12.0. The JRC values were determined from the results of laser profilometer measurements. The replicas were created from gypsum cement. By varying the water-to-gypsum cement ratio from 30 to 45%, fracture replicas with different values of compressive strength (JCS) were created. The rotary shear experiments were performed under constant normal (nominal) stresses ranging between 0.2 and 1.6 MPa. In this report, the shear test results are compared with predictions using Barton`s empirical peak shear strength equation. observations during the experiments indicate that only certain parts of the fracture profiles influence fracture shear strength and dilatancy. Under relatively low applied normal stresses, the JCS does not seem to have a significant effect on shear behavior. As an alternative, a new procedure for predicting the shear behavior of fractures was developed. The approach is based on basic fracture properties such as fracture surface profile data and the compressive strength, modulus of elasticity, and Poisson`s ratio of the fracture walls. Comparison between predictions and actual shear test results shows that the alternative procedure is a reliable method.

Wibowo, J.; Amadei, B.; Sture, S. [Univ. of Colorado, Boulder, CO (United States)] [and others

1995-08-01

328

A mechanism for the production of electromagnetic radiation during fracture of brittle materials  

Microsoft Academic Search

The feasibility of the charge separation model as the source of electromagnetic radiation received from the fracture of brittle materials is demonstrated. An analytical expression and a finite difference method enable the calculation of currents flowing around the tip of a crack. Results from the model compare favourably with fracture events recorded from ice samples. Changes in material conductivity affect

Steven G. O'Keefe; David V. Thiel

1995-01-01

329

Hydromechanical interactions in a fractured carbonate reservoir inferred from hydraulic and mechanical measurements  

E-print Network

Hydromechanical interactions in a fractured carbonate reservoir inferred from hydraulic, France Abstract Hydromechanical coupled processes in a shallow fractured carbonate reservoir rock were of hydraulic loading/unloading of a water reservoir in which fluid flow occurs mainly inside a heterogeneous

Paris-Sud XI, Université de

330

Shear Fracture Precipitated by Strain Softening as a Mechanism of Dry Slab Avalanche Release  

Microsoft Academic Search

.Snow stratigraphy studies at the fracture lines of dry slab avalanches often indicate a situation that consists of a harder thick layer overlying a thin weak layer. Prefracture stress conditions and progressive fracture of snow slabs are discussed here from the point of view that slow shear failure in the weak layer results in Griffith-like critical lengths there, resulting in

David M. McClung

1979-01-01

331

The fracture energy and some mechanical properties of a polyurethane elastomer.  

NASA Technical Reports Server (NTRS)

The energy required to form a unit of new surface in the fracture of a polyurethane elastomer is determined. The rate sensitivity of the material has been reduced by swelling it in toluene. This paper primarily describes the experimental work of measuring the lower limit of the fracture energy. With this value and the creep compliance as a basis, the rate dependence of fracture energy for the unswollen material has been determined. It is thus shown that the dependence of the fracture energy on the rate of crack propagation can be explained by energy dissipation around the tip of the crack. Good agreement between the theoretically and experimentally determined relationships for the rate-sensitive fracture energy is demonstrated.

Mueller, H. K.; Knauss, W. G.

1971-01-01

332

Micro-mechanical investigation for effects of helium on grain boundary fracture of austenitic stainless steel  

NASA Astrophysics Data System (ADS)

Effects of helium (He) on grain boundary (GB) fracture of austenitic stainless steel were investigated by micro-tensile tests. Micro-bicrystal tensile specimens were fabricated for non-coincidence site lattice boundaries of He ion-irradiated 316 stainless steel by focused ion beam (FIB) micro-processing. Micro-tensile tests were conducted in a vacuum at room temperature in the FIB system. Specimens containing more than 2 at.% He fractured at GBs. The criteria for brittle fracture occurrence on GBs were: (1) He concentrations higher than 2 at.%; (2) formation of He bubbles on the GBs with less than a 5 nm spacing; and (3) matrix hardening to more than 4.6 GPa (nano-indentation hardness). The fracture stress of GB brittle fracture was lower for a specimen with higher He concentration while the size and areal density of the GB He bubbles were the same. The specimens that contained 10 at.% He and had been annealed at 923 K after irradiation fractured at the GB nominally in a brittle manner; however the inter-bubble matrix at the GB experienced ductile fracture. The annealing caused He bubbles to grow but decreased the areal density so that the spacing of the GB He bubbles widened and the hardness decreased, therefore the fracture mode changed from brittle to ductile. The findings revealed that He promotes GB fracture by weakening the GB strength and hardening the matrix due to the formation of He bubbles both on GBs and in the matrix. In addition, the findings suggested that GB segregated He atoms may have a role in GB fracture.

Miura, Terumitsu; Fujii, Katsuhiko; Fukuya, Koji

2015-02-01

333

Development of fracture mechanics data for two hydrazine APU turbine wheel materials  

NASA Technical Reports Server (NTRS)

The effects of high temperature, high pressure ammonia were measured on the fracture mechanics and fatigue properties of Astroloy and Rene' 41 turbine wheel materials. Also, the influence of protective coatings on these properties was investigated. Specimens of forged bar stock were subjected to LCF and HCF tests at 950 K (1250 F) and 3.4 MN/sq m (500 psig) pressure, in ammonia containing about 1.5 percent H2O. Aluminized samples (Chromizing Company's Al-870) and gold plated test bars were compared with uncoated specimens. Comparison tests were also run in air at 950 K (1250 F), but at ambient pressures. K sub IE and K sub TH were determined on surface flawed specimens in both the air and ammonia in both uncoated and gold plated conditions. Gold plated specimens exhibited better properties than uncoated samples, and aluminized test bars generally had lower properties. The fatigue properties of specimens tested in ammonia were higher than those tested in air, yet the K sub TH values of ammonia tested samples were lower than those tested in air. However, insufficient specimens were tested to develop significant design data.

Curbishley, G.

1975-01-01

334

Low Magnitude Mechanical Signals Reduce Risk-Factors for Fracture during 90-Day Bed Rest  

NASA Technical Reports Server (NTRS)

Long duration spaceflight leads to multiple deleterious changes to the musculoskeletal system, where loss of bone density, an order of magnitude more severe than that which follows the menopause, combined with increased instability, conspire to elevate the risk of bone fracture due to falls on return to gravitational fields. Here, a ground-based analog for spaceflight is used to evaluate the efficacy of a low-magnitude mechanical intervention, VIBE (Vibrational Inhibition of Bone Erosion), as a potential countermeasure to preserve musculoskeletal integrity in the face of disuse. Twenty-six subjects consented to ninety days of six-degree head-down tilt bed-rest. 18 completed the 90d protocol, 8 of which received daily 10-minute exposure to 30 Hz, 0.3g VIBE, applied in the supine position using a vest elastically coupled to the vibrating platform. The shoulder harness induced a load of 60% of the subjects body weight. At baseline and 90d, Qualitative Ultrasound Scans (QUS) of the calcaneus and CT-scans of the hip and spine were performed to measure changes in bone density. Postural control (PC) was assessed through center of pressure (COP) recordings while subjects stood on a force platform for 4 minutes of quiet stance with eyes closed, and again with eyes opened. As compared to control bedrest subjects,

Muir, J. W.; Xia, Y.; Holquin, N.; Judex, S.; Qin, Y.; Evans, H.; Lang, T.; Rubin, C.

2007-01-01

335

Scaling of strength and lifetime probability distributions of quasibrittle structures based on atomistic fracture mechanics  

PubMed Central

The failure probability of engineering structures such as aircraft, bridges, dams, nuclear structures, and ships, as well as microelectronic components and medical implants, must be kept extremely low, typically <10?6. The safety factors needed to ensure it have so far been assessed empirically. For perfectly ductile and perfectly brittle structures, the empirical approach is sufficient because the cumulative distribution function (cdf) of random material strength is known and fixed. However, such an approach is insufficient for structures consisting of quasibrittle materials, which are brittle materials with inhomogeneities that are not negligible compared with the structure size. The reason is that the strength cdf of quasibrittle structure varies from Gaussian to Weibullian as the structure size increases. In this article, a recently proposed theory for the strength cdf of quasibrittle structure is refined by deriving it from fracture mechanics of nanocracks propagating by small, activation-energy-controlled, random jumps through the atomic lattice. This refinement also provides a plausible physical justification of the power law for subcritical creep crack growth, hitherto considered empirical. The theory is further extended to predict the cdf of structural lifetime at constant load, which is shown to be size- and geometry-dependent. The size effects on structure strength and lifetime are shown to be related and the latter to be much stronger. The theory fits previously unexplained deviations of experimental strength and lifetime histograms from the Weibull distribution. Finally, a boundary layer method for numerical calculation of the cdf of structural strength and lifetime is outlined. PMID:19561294

Bažant, Zden?k P.; Le, Jia-Liang; Bazant, Martin Z.

2009-01-01

336

A review of path-independent integrals in elastic-plastic fracture mechanics, task 4  

NASA Technical Reports Server (NTRS)

The path independent (P-I) integrals in elastic plastic fracture mechanics which have been proposed in recent years to overcome the limitations imposed on the J integral are reviewed. The P-I integrals considered herein are the J integral by Rice, the thermoelastic P-I integrals by Wilson and Yu and by Gurtin, the J* integral by Blackburn, the J sub theta integral by Ainsworth et al., the J integral by Kishimoto et al., and the delta T sub p and delta T* sub p integrals by Atluri et al. The theoretical foundation of these P-I integrals is examined with emphasis on whether or not path independence is maintained in the presence of nonproportional loading and unloading in the plastic regime, thermal gradients, and material inhomogeneities. The similarities, differences, salient features, and limitations of these P-I integrals are discussed. Comments are also made with regard to the physical meaning, the possibility of experimental measurement, and computational aspects.

Kim, K. S.

1985-01-01

337

Comparative study on the mechanical and fracture properties of acrylic bone cements prepared with monomers containing amine groups.  

PubMed

In this work, the effect of the incorporation of comonomers containing amine groups on the mechanical and fracture properties of acrylic bone cements was studied. Cements were prepared with either diethyl amino ethyl acrylate (DEAEA), dimethyl amino ethyl methacrylate (DMAEM) or diethyl amino ethyl methacrylate (DEAEM) as comonomer in the liquid phase. It was found that strength and modulus decreased with increasing comonomer content in the bending and compressive tests. It was also observed that fracture toughness (K(IC)) and the critical strain energy release rate (G(IC)) increase with increasing comonomer concentration and are significantly higher compared to the control formulation. The mechanical and fracture properties of cements were also evaluated after soaking the specimens in Simulated Body Fluid (SBF) for 3 and 6 months. It was found that the mechanical properties of cements decreased when the samples were stored in SBF, although the impact strength increased in the first 3 months and then decreased. SEM micrographs were in agreement with the results obtained during mechanical characterization as the increase in toughness was confirmed by the appearance of ductile tearing pattern which is associated with plastic deformation. PMID:22301178

May-Pat, Alejandro; Herrera-Kao, Wilberth; Cauich-Rodríguez, Juan V; Cervantes-Uc, José M; Flores-Gallardo, Sergio G

2012-02-01

338

Anisotropy in Mechanical Properties and Fracture Behavior of an Oxide Dispersion Fe20Cr5Al Alloy  

NASA Astrophysics Data System (ADS)

Anisotropy of fracture toughness and fracture behavior of Fe20Cr5Al oxide dispersion-strengthened alloy has been investigated by means of compression tests, hardness tests, and wedge splitting test. The results show a small effect of the compression direction on yield strength (YS) and strain hardening. The YS is minimum for longitudinal direction and maximum for the tangential direction. The transverse plastic strain ratio is similar for tangential and longitudinal directions but very different from that in normal direction. Hardness depends on the indentation plane; it is lower for any plane parallel to the L-T plane and of similar magnitude for the other orthogonal planes, i.e., the L-S and T-S planes. Macroscopically, two failure modes have been observed after wedge-splitting tests, those of LS and TS specimens in which fracture deviates along one or two branches normal to the notch plane, and those of LT, TL, SL, and ST specimens in which fracture propagates along the notch plane. Besides LT and TL specimens present delaminations parallel to L-T plane. Both, the fracture surface of branching cracks and that of the delaminations, show an intergranular brittle fracture appearance. It is proposed that the main cause of the delamination and crack branching is the alignment in the mesoscopic scale of the ultrafine grains structure which is enhanced by the <110>-texture of the material and by the presence in the grain boundaries of both yttria dispersoids and impurity contaminations. An elastoplastic finite element analysis was performed to study what stress state is the cause of the branches and delaminations. It is concluded that the normal to the crack branches and/or the shear stress components could determine the crack bifurcation mechanism, whereas the delamination it seems that it is controlled by the magnitude of the stress component normal to the delamination plane.

Chao, J.; Capdevila, C.

2014-08-01

339

Fracture mechanisms of the Strombus gigas conch shell: II-micromechanics analyses of multiple cracking and large-scale crack bridging  

Microsoft Academic Search

Micromechanics analyses of the dominant energy-dissipating mechanisms responsible for the resistance to catastrophic fracture of the aragonitic shell of the giant Queen conch, Strombus gigas, are presented. The crossed lamellar microstructure of the shell is associated with a work of fracture that is three orders of magnitude higher than that of non-biogenic aragonite [J. Mater. Sci. 6 (1996) 6583]. Previous

Shekhar Kamat; Hannes Kessler; Roberto Ballarini; Maissarath Nassirou; Arthur. H. Heuer

2004-01-01

340

Future target for geothermal development -- Fractal Fracture Mechanics and its application to conceptual HDR reservoir design  

SciTech Connect

A simple model is proposed for water/rock interaction in rock fractures through which geothermal water flows. Water/rock interaction experiments were carried out at high temperature and pressure (200-350 C, 18 MPa) in order to obtain basic solubility and reaction rate data. Based on the experimental data, changes of idealized fracture apertures with time are calculated numerically. The results of the calculations show that the precipitation from water can lead to plugging of the fractures under certain conditions. Finally, the results are compared with the experimental data.

Takahashi, Hideaki; Watanable, Kimio; Hashida, Toshiyuki

1995-01-26

341

Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass  

Microsoft Academic Search

The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr41.2Ti13.8Cu12.5 Ni10Be22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens\\u000a (?7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline\\u000a aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces

C. J. Gilbert; V. Schroeder; R. O. Ritchie

1999-01-01

342

A fracture mechanics study of the influence of moisture on the fatigue behaviour of adhesively bonded aluminium-alloy joints  

Microsoft Academic Search

A fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively bonded joints, which consisted of aluminium-alloy substrates bonded using a toughened-epoxy structural adhesive. The results were plotted in the form of the rate of crack growth per cycle, dadN, versus the maximum strain-energy release rate, Gmax, applied in the fatigue cycle, using logarithmic axes.

M. Fernando; W. W. Harjoprayitno; A. J. Kinloch

1996-01-01

343

Microstructure and mechanical properties of V–Ti–N microalloyed steel used for fracture splitting connecting rod  

Microsoft Academic Search

A new kind of V–Ti–N high strength microalloyed medium carbon steel has been developed, which is used for fracture splitting\\u000a connecting rod. In this article, the characteristics of this carbon steel and its production process were studied. The microstructure,\\u000a precipitated phases and their effects on mechanical properties were investigated by optical microscope, SEM, and TEM. The\\u000a results showed that the

Xianzhong Zhang; Qizhou Cai; Guifeng Zhou; Qingfeng Chen; Yuzhang Xiong

2011-01-01

344

Induced seismicity of a normal blind undetected reservoir-bounding fault influenced by dissymmetric fractured damage zones  

NASA Astrophysics Data System (ADS)

Fluid injection in deep sedimentary porous formations might induce shear reactivation of reservoir bounding faults. Here, we focus on `blind' 1000-m-long normal faults (with shear displacement ?10 m), which can hardly be detected using conventional seismic surveys, but might potentially induce seismicity felt on surface. The influence of the dissymmetry in the internal structure of the fractured damage zone DZ is numerically investigated by using 2-D plane-strain finite-element simulations of a 1500-m-deep fluid injection into a porous reservoir. The problem is solved within the framework of fully saturated isothermal elasto-plastic porous media by both accounting for fault slip weakening and shear-induced degradation of fault core permeability. The numerical results show that the presence of a thick fractured hanging wall's DZ (with Young's modulus decreasing with the distance to the fault core due to the presence of fractures) strongly controls the magnitude M of the seismic event induced by the rupture. In the case modelled, M changed by more than 1.0 unit when the DZ thickness is varied from 5 to 50 m (M ranges from ˜0.1 to ˜1.5, i.e. from a `low' to a `low-to-moderate' seismicity activity). However, further extending DZ up to 90 m has little effect and the relationship reaches a quasi-horizontal plateau. This tendency is confirmed considering other initial conditions and injection scenarios. Finally, the presence of a thicker footwall DZ appears to lower the influence of hanging wall's DZ, but with lesser impact than the degree of fracturing.

Rohmer, J.

2014-04-01

345

Fracture Mechanical Measurements with Commercial Stainless Steels at 4 K and with Cp-Titanium at 173 K  

NASA Astrophysics Data System (ADS)

Using the JETT (J-Evaluation on Tensile Test) technique, measurements have been performed with commercial stainless steels in forged and cast condition for the reason of an assessment for low temperature service down to 4 K. These steels frequently used for industrial applications are designated by German Werkstoff (WNr) 1.4308 and 1.4408 cast stainless steels and a forged material with the number 1.4307. The fracture toughness tests at 4 K with forged material 1.4307 comprised apart from the base metal also the weld zone and additionally the 5% and 8% pre-strained conditions of the base metal. Fracture toughness reduced slightly for cold worked condition gradually as well as for the weld joint. The Reliability of the JETT measurements has been also checked using the ASTM E 1820—99a standard. In addition, to these measurements, commercial pure ASTM grade 2 titanium (WNr 3.7035) has been also examined using the same JETT method for the reason of industrial application and the requirement of minimum fracture toughness of 100 MPa?m was fulfilled at 173 K. Furthermore, test results performed at 7 K of pure titanium plate material (ASTM grade 1) with respect to fracture mechanical JETT method are presented.

Nyilas, A.; Mitterbacher, H.

2010-04-01

346

Fracture mechanics data for 2024-T861 and 2124-T851 aluminum  

NASA Technical Reports Server (NTRS)

The fracture toughness and fatigue flaw growth characteristics of 2024-T861 and 2124-T851 aluminum were evaluated under plane stress conditions. Center cracked tension specimens were employed to evaluate these properties under a number of different test conditions which included variations in specimen thickness, specimen orientation, test environment, and initial flaw size. The effect of buckling was also investigated for all tests of thin gage specimens, and the effect of frequency and stress ratio was evaluated for the cyclic tests. Fracture toughness test results were analyzed and presented in terms of fracture resistance curves; fatigue flaw growth data was analyzed using empirical rate models. The results of the study indicate that both fracture toughness and resistance to fatigue crack growth improve with increasing temperature and decreasing thickness. The presence of buckling during testing of thin gage panels was found to degrade the resistance to fatigue flaw growth only at elevated temperatures.

Pionke, L. J.; Linback, R. K.

1974-01-01

347

[New experimental insights into the mechanism of blow-out fractures].  

PubMed

Model impact tests on 3-dimensional PMMA eye models with a swing type impact tester (Dynstat) were performed. There are three kinds of impact tests; the impact to the eyeball alone, the impact to the eyeball and infraorbital margin, and the impact to the isolated infraorbital margin. All impact tests of eye models were recorded by high speed camera with 3,000 pictures per second. Analysis of impact tests showed, that the primary factor of blow-out fractures was deformation of the eyeball by kinetic energy of impact. Transmission of impact force by deformed eyeball to orbital walls is causing blow-out fractures on the weakest areas of the orbit without fracturing the orbital rim. Against the present accepted theories of "hydraulic force" und "buckling force" this is a new theory for pathomechanism of blow-out fractures. Our experimental results support the clinical investigations on treated patients. PMID:2528872

Schneider, D

1989-01-01

348

Application of small specimens to fracture mechanics characterization of irradiated pressure vessel steels  

Microsoft Academic Search

In this study, precracked Charpy V-notch (PCVN) specimens were used to characterize the fracture toughness of unirradiated and irradiated reactor pressure vessel steels in the transition region by means of three-point static bending. Fracture toughness at cleavage instability was calculated in terms of elastic-plastic K{sub Jc} values. A statistical size correction based upon weakest-link theory was performed. The concept of

M. A. Sokolov; K. Wallin; D. E. McCabe

1996-01-01

349

Mechanical behavior of polycrystalline ceramics: Brittle fracture of SiC-Si3N4 materials  

NASA Technical Reports Server (NTRS)

Research on the fracture behavior of silicon nitride and silicon carbide is reported along with the role of anion impurities in the fabrication and behavior of magnesium oxide. The results of a survey of crack propagation in SiC and Si3N4 are presented. Studies in the following areas are reported: development of a fracture toughness testing technique, constant moment beam, microcrack examination, and etching techniques.

Ceipold, M. H.; Kapadia, C. M.; Kelkar, A. H.

1972-01-01

350

Experimental evidence for hydrologic and mechanical controls on formation of hydraulic fractures in quartz-rich sandstone and siltstone  

NASA Astrophysics Data System (ADS)

Natural hydraulic fractures are inferred to form where pore fluid pressure exceeds the least compressive stress by an amount equal to the tensile strength of the rock (i.e. Pf-?3 ? T). However, consistent production of hydraulic fractures has not, until now, been possible. We report an experimental protocol which produces a pore fluid pressure gradient (?Pf) within a cylindrical sample, parallel to its long axis, such that Pf-?3 ? T is met within the sample. Two-dimensional poroelastic models of these experiments help interpret the timing and location of fracture formation in terms of the evolution and magnitudes of ?, ?, and Pf. The models are parametrized using the drained elastic properties (Poisson's ratio and bulk modulus) and hydrologic properties (permeability) of each sample. Four experiments on samples from the St. Peter Sandstone and two on samples from the Abo Formation were performed under these pore fluid pressure conditions as well as under drained conditions. None of the drained experiments resulted in sample failure, but all of the sandstone and one of the siltstone samples formed hydraulic fractures at elevated Pf. The St. Peter Sandstone samples are fine to medium-grained quartz arenites with spatially variable amounts of cement and primary structures. In the suite of samples studied, permeability ranges from k=10-12 m2 to k=10-15 m2. Two experiments were performed on a high-k sample with well defined cm-scale bedding, one with ?3 and ?Pf perpendicular to bedding and another with ?3 and ?Pf parallel to bedding. Two mechanically and hydrologically isotropic, low-k, samples were deformed. One is compositionally homogeneous and the other exhibits local iron oxide cements. The Abo Formation siltstone is a fluvial quartz-rich red bed with low permeability (k=10-17 m2) and mm-scale bedding laminae defined by grain shape preferred orientation and oxide and clay abundance. Experiments were performed with ?3 and ?Pf both perpendicular and parallel to bedding. The sample with ?3 parallel to bedding did not fail. Samples sustained conditions of Pf-?3 » T for up to 250sec prior to failure. Significant mechanical heterogeneity is a first order control on the fracture location, but is not required for fracture formation. The most isotropic and homogeneous sample failed after sustaining Pf-?3 > 10T for 30sec, with a through-going fracture that formed where Pf-?3 and ?, as determined from the poroelastic models, were greatest. The rest of the samples did not fail in locations of highest Pf-?3 and ?, but were localized along along mechanical heterogeneities. However, these samples still fractured where Pf-?3 » T prior to failure. Typical Mohr-Coulomb and Griffith failure criteria do not explain the large stresses sustained prior to failure. Interestingly, samples did experience similar total strain at failure regardless of differences in mechanical properties and experimental conditions. Our results suggest that in a space-limited system, fracture formation is dependent on the ability of the system to accommodate sufficient strain.

French, M. E.; Goodwin, L. B.; Boutt, D. F.

2009-12-01

351

Development of a balanced experimental-computational approach to understanding the mechanics of proximal femur fractures.  

PubMed

The majority of people who sustain hip fractures after a fall to the side would not have been identified using current screening techniques such as areal bone mineral density. Identifying them, however, is essential so that appropriate pharmacological or lifestyle interventions can be implemented. A protocol, demonstrated on a single specimen, is introduced, comprising the following components; in vitro biofidelic drop tower testing of a proximal femur; high-speed image analysis through digital image correlation; detailed accounting of the energy present during the drop tower test; organ level finite element simulations of the drop tower test; micro level finite element simulations of critical volumes of interest in the trabecular bone. Fracture in the femoral specimen initiated in the superior part of the neck. Measured fracture load was 3760N, compared to 4871N predicted based on the finite element analysis. Digital image correlation showed compressive surface strains as high as 7.1% prior to fracture. Voxel level results were consistent with high-speed video data and helped identify hidden local structural weaknesses. We found using a drop tower test protocol that a femoral neck fracture can be created with a fall velocity and energy representative of a sideways fall from standing. Additionally, we found that the nested explicit finite element method used allowed us to identify local structural weaknesses associated with femur fracture initiation. PMID:24629624

Helgason, B; Gilchrist, S; Ariza, O; Chak, J D; Zheng, G; Widmer, R P; Ferguson, S J; Guy, P; Cripton, P A

2014-06-01

352

The Relationships between Weight Functions, Geometric Functions,and Compliance Functions in Linear Elastic Fracture Mechanics  

SciTech Connect

Linear elastic fracture mechanics is widely used in industry because it established simple and explicit relationships between the permissible loading conditions and the critical crack size that is allowed in a structure. Stress intensity factors are the above-mentioned functional expressions that relate load with crack size through geometric functions or weight functions. Compliance functions are to determine the crack/flaw size in a structure when optical inspection is inconvenient. As a result, geometric functions, weight functions and compliance functions have been intensively studied to determine the stress intensity factor expressions for different geometries. However, the relations between these functions have received less attention. This work is therefore to investigate the intrinsic relationships between these functions. Theoretical derivation was carried out and the results were verified on single-edge cracked plate under tension and bending. It is found out that the geometric function is essentially the non-dimensional weight function at the loading point. The compliance function is composed of two parts: a varying part due to crack extension and a constant part from the intact structure if no crack exists. The derivative of the compliance function at any location is the product of the geometric function and the weight function at the evaluation point. Inversely, the compliance function can be acquired by the integration of the product of the geometric function and the weight function with respect to the crack size. The integral constant is just the unchanging compliance from the intact structure. Consequently, a special application of the relations is to obtain the compliance functions along a crack once the geometric function and weight functions are known. Any of the three special functions can be derived once the other two functions are known. These relations may greatly simplify the numerical process in obtaining either geometric functions, weight functions or compliance functions for new test geometries.

Yuan, Rong

2007-02-06

353

Fracture mechanics analysis of NGV fuel cylinders. Part 1: Steel cylinders  

NASA Astrophysics Data System (ADS)

Compressed natural gas (CNG) cylinders for natural gas vehicles (NGVs) are subject to a combination of pressure cycles, associated with periodic refueling, and a potentially corrosive CNG environment. Under these conditions it has been shown that the life of the cylinder is governed by the corrosion-fatigue crack growth of internal flaws such as voids, pits or folds that may be present after manufacture. For NGV applications, these cylinders are required to operate for at least 15 years and the report, through a detailed fracture mechanics analysis, describes approaches to achieving the desired life. The analysis shows that a 15 year cylinder life can be obtained by using quality control to ensure that no initial defects greater than 0.045 in. X 0.090 in. exist after manufacture. Alternatively, gas drying can be used at the distribution stations to reduce the detrimental effects of the remaining CNG impurities, and thereby, produce long cylinder lives. The analysis also considers the role of in-service inspection/retest and shows that in-service NDE has little advantage, either technically or economically, for ensuring the fitness-for-service of steel NGV cylinders. The analysis also shows that hydrostatic testing of cylinders, either at manufacture or in service, is ineffective for detecting fatigue cracks and therefore should not be implemented as part of a fitness-for-service plan for NGV fuel cylinders. The issue of cylinder geometry was also considered and the analysis shows that improperly designed flat-bottomed CNG cylinders can result in premature fatigue failures originating at the inner wall in the transition region between the cylinder end and sidewall.

Connolly, M. P.; Hudak, S. J.; Roy, S.

1993-02-01

354

A thermo-hydro-mechanical coupled model in local thermal non-equilibrium for fractured HDR reservoir with double porosity  

NASA Astrophysics Data System (ADS)

The constitutive thermo-hydro-mechanical equations of fractured media are embodied in the theory of mixtures applied to three-phase poroelastic media. The solid skeleton contains two distinct cavities filled with the same fluid. Each of the three phases is endowed with its own temperature. The constitutive relations governing the thermomechanical behavior, generalized diffusion and transfer are structured by, and satisfy, the dissipation inequality. The cavities exchange both mass and energy. Mass exchanges are driven by the jump in scaled chemical potential, and energy exchanges by the jump in coldness. The finite element approximation uses the displacement vector, the two fluid pressures and the three temperatures as primary variables. It is used to analyze a generic hot dry rock geothermal reservoir. Three parameters of the model are calibrated from the thermal outputs of Fenton Hill and Rosemanowes HDR reservoirs. The calibrated model is next applied to simulate circulation tests at the Fenton Hill HDR reservoir. The finer thermo-hydro-mechanical response provided by the dual porosity model with respect to a single porosity model is highlighted in a parameter analysis. Emphasis is put on the influence of the fracture spacing, on the effective stress response and on the permeation of the fluid into the porous blocks. The dual porosity model yields a thermally induced effective stress that is less tensile compared with the single porosity response. This effect becomes significant for large fracture spacings. In agreement with field data, fluid loss is observed to be high initially and to decrease with time.

Gelet, R.; Loret, B.; Khalili, N.

2012-07-01

355

Mechanical characterization of a CO2 fractured reservoir by means of microseismicity induced by high pressure injection tests  

NASA Astrophysics Data System (ADS)

Reservoir characterization is an essential issue in geological storage of CO2 in Technological Development Plant (TDP). In particular, hydromechanical characterization of the caprock-reservoir system is crucial, in order to define the maximum suitable injection pressure and the in-situ mechanical properties. Thus, it is possible to conjecture the hydromechanical behavior of the system during CO2 injection. Microseismicity induced by fluid injection may be used as instruments to find out fractured reservoir properties. Indeed, the hydromechanical response is controlled by permeability (k), Young modulus (E) and Poisson ratio (?). In caprock-reservoir systems, reservoir stiffness controls the stress transfer towards the caprock, where failure may occur. Therefore, the location of the microseismic hypocenters could give information on the reservoir stiffness. In this work we propose a simulation and calibration method of the microseismicity induced by high pressure fluid injection in a fractured reservoir. Coupled hydromechanical models are peformed. The methology is applied to a particular case study.

De Simone, Silvia; Soler, Joaquim; Carrera, Jesus; Slooten, Luit Jan; Ortiz, Gema

2014-05-01

356

Fracture mechanics and statistical modeling of ternary blends of polylactide/ethylene-acrylate copolymer /wood-flour composites  

NASA Astrophysics Data System (ADS)

This study examined the mechanisms of toughening the brittle bio-based poly(lactic acid) (PLA) with a biodegradable rubbery impact modifier to develop biodegradable and cost effective PLA/wood-flour composites with improved impact strength, toughness, high ductility, and flexibility. Semicrystalline and amorphous PLA grades were impact modified by melt blending with an ethylene-acrylate copolymer (EAC) impact modifier. EAC content was varied to study the effectiveness and efficiency of the impact modifier in toughening the semicrystalline and amorphous grades of the PLA. Impact strength was used to assess the effectiveness and efficiency of the EAC in toughening the blends, whereas the toughening mechanisms were determined with the phase morphologies and the miscibilities of the blends. Subsequent tensile property analyses were performed on the most efficiently toughened PLA grade. Composites were made from PLA, wood flour of various particle sizes, and EAC. Using two-level factorial design the interaction between wood flour content, wood flour particle size, and EAC content and its effect on the mechanical properties of the PLA/wood-flour composites was statistically studied. Numerical optimization was also performed to statistically model and optimize material compositions to attain mechanical properties for the PLA/wood-flour composites equivalent to at least those of unfilled PLA. The J-integral method of fracture mechanics was applied to assess the crack initiation (Jin) and complete fracture (J f) energies of the composites to account for imperfections in the composites and generate data useful for engineering designs. Morphologies of the fractured surfaces of the composites were analyzed to elucidate the failure and toughening mechanisms of the composites. The EAC impact modifier effectively improved the impact strength of the PLA/EAC blends, regardless of the PLA type. However, the EAC was more efficient in the semicrystalline grades of PLA compared to the amorphous grade. The semicrystalline blends showed decreased tensile strength and modulus with increased impact modifier content. In contrast, the ductility, elongation at break, and energy to break increased significantly. Mechanisms of toughening of PLA with EAC included impact modifier debonding, fibrillization, crack bridging and matrix shear yielding resulting in a ductile behavior. Increasing the EAC content in PLA/wood-flour composites enhanced the impact strength and elongation at break, but reduced the tensile modulus and strength of the composites. Composites with fine wood particles showed greater improvement in elongation at break than those with coarse particles; an opposite trend was observed for impact strength, tensile modulus and tensile strength. Numerical optimization produced two scenarios based on materials compositions to produce composites with similar mechanical properties as unfilled PLA. These optimization solutions were successfully validated experimentally. The crack initiation (Jin) and complete fracture (Jf) energies of unmodified PLA/wood-flour composites showed the deleterious effect of wood fiber incorporation into the plastic matrix by significantly decreasing the fracture toughness of PLA as the wood flour content increased. By contrast, impact modification of wood plastic composites with EAC significantly increased both the resistance to crack initiation (Jin) and complete fracture (Jf). Microscopic morphological studies revealed that the major mechanisms of toughening was through the EAC existing as separate domains in the bulk matrix of the composites which tended to act as stress concentrators that initiated local yielding of the matrix around crack tips and enhanced the toughness of the composites.

Afrifah, Kojo Agyapong

357

Mechanical Properties and Fracture Surfaces of Thixoformed HP9/4/30 Steel  

SciTech Connect

Most work in semi-solid metal processing has been focused on relatively low-melting temperature materials such as aluminium, magnesium, tin, lead and their composites. Because of the high melting temperatures and related measurement difficulties, there is relatively small amount of experimental data available on the thixoforming of high temperature materials such as steels. This paper discusses the relationships between tensile tests data and the fracture surfaces of HP9/4/30 steel samples, in as-received and as-thixoformed conditions. Some of the as-thixoformed samples were subjected to commercial heat treatments before the tests were carried out. The as-received samples showed a typical ductile fracture of 'cup' and 'cone' surface appearance. The thixoformed samples, although showing some significant increase in tensile strength values, failed in a brittle manner, with the fracture surfaces showing a 'cobbled' surface appearance. The application of the commercial heat treatment to the thixoformed samples has significantly improved their ductility.

Omar, M. Z.; Jaharah, A. G. [Department of Mechanical and Materials Engineering, National University of Malaysia, 43600 UKM Bangi, Selangor (Malaysia); Atkinson, H. V. [Department of Engineering, University of Leicester, University Rd., Leicester, LE1 7RH (United Kingdom); Kapranos, P. [Department of Engineering Materials, University of Sheffield, Hadfield Building, Mappin St., Sheffield, S1 3JD (United Kingdom)

2007-04-07

358

A partitioned model order reduction approach to rationalise computational expenses in nonlinear fracture mechanics  

PubMed Central

We propose in this paper a reduced order modelling technique based on domain partitioning for parametric problems of fracture. We show that coupling domain decomposition and projection-based model order reduction permits to focus the numerical effort where it is most needed: around the zones where damage propagates. No a priori knowledge of the damage pattern is required, the extraction of the corresponding spatial regions being based solely on algebra. The efficiency of the proposed approach is demonstrated numerically with an example relevant to engineering fracture. PMID:23750055

Kerfriden, P.; Goury, O.; Rabczuk, T.; Bordas, S.P.A.

2013-01-01

359

Mechanism of low temperature brittle fracture in high nitrogen bearing austenitic steels  

Microsoft Academic Search

Low temperature embrittlement is studied for three high-nitrogen bearing austenitic steels, 18Mn–18Cr–0.5N, 18Mn–18Cr–0.8N and 17Cr–13Ni–2Mo–0.5N (all in mass%) steels. These steels all show ductile-to-brittle transition behavior. They have fractured in a cleavage-like mode when tested at 77K by a Charpy impact tester and the fractured surfaces have shown facets along {111} planes identified by using etch-pits and scanning electron microscopy.

Y. Tomota; Y Xia; K Inoue

1998-01-01

360

Interaction of Age and Mechanical Stability on Bone Defect Healing: An Early Transcriptional Analysis of Fracture Hematoma in Rat  

PubMed Central

Among other stressors, age and mechanical constraints significantly influence regeneration cascades in bone healing. Here, our aim was to identify genes and, through their functional annotation, related biological processes that are influenced by an interaction between the effects of mechanical fixation stability and age. Therefore, at day three post-osteotomy, chip-based whole-genome gene expression analyses of fracture hematoma tissue were performed for four groups of Sprague-Dawley rats with a 1.5-mm osteotomy gap in the femora with varying age (12 vs. 52 weeks - biologically challenging) and external fixator stiffness (mechanically challenging). From 31099 analysed genes, 1103 genes were differentially expressed between the six possible combinations of the four groups and from those 144 genes were identified as statistically significantly influenced by the interaction between age and fixation stability. Functional annotation of these differentially expressed genes revealed an association with extracellular space, cell migration or vasculature development. The chip-based whole-genome gene expression data was validated by q-RT-PCR at days three and seven post-osteotomy for MMP-9 and MMP-13, members of the mechanosensitive matrix metalloproteinase family and key players in cell migration and angiogenesis. Furthermore, we observed an interaction of age and mechanical stimuli in vitro on cell migration of mesenchymal stromal cells. These cells are a subpopulation of the fracture hematoma and are known to be key players in bone regeneration. In summary, these data correspond to and might explain our previously described biomechanical healing outcome after six weeks in response to fixation stiffness variation. In conclusion, our data highlight the importance of analysing the influence of risk factors of fracture healing (e.g. advanced age, suboptimal fixator stability) in combination rather than alone. PMID:25187955

Ode, Andrea; Duda, Georg N.; Geissler, Sven; Pauly, Stephan; Ode, Jan-Erik; Perka, Carsten; Strube, Patrick

2014-01-01

361

Numerical Modeling of Hydraulic Fracture Propagation Using Thermo-hydro-mechanical Analysis with Brittle Damage Model by Finite Element Method.  

E-print Network

??Better understanding and control of crack growth direction during hydraulic fracturing are essential for enhancing productivity of geothermal and petroleum reservoirs. Structural analysis of fracture… (more)

Min, Kyoung

2013-01-01

362

Mechanical test in-situ fracture device for Auger electron spectroscopy  

NASA Technical Reports Server (NTRS)

An in-situ fracture device for Auger spectroscopy was described. The device is designed to handle small tensile specimens or small double cantilever beam specimens and is fully instrumented with load and displacement transducers so that quantitative stress-strain measurements can be made directly. Some initial test results for specimens made from 4130 and 1020 steel were presented.

Moorhead, R. D.

1975-01-01

363

Mechanical behavior of polycrystalline ceramics: Brittle fracture of Si C - Si3N4 materials  

NASA Technical Reports Server (NTRS)

The results are described of the final stage of the research involving the role of anions in the behavior of magnesium oxide, as well as the continued efforts of the fracture behavior of silicon nitride materials. These efforts, particularly the first, are further sub-divided in subsections describing individual types of behavior of materials.

Leipold, M. H.; Kapadia, C. M.; Kelkar, A. H.

1973-01-01

364

Physical and mechanical modeling of the neutron irradiation effect on ductile fracture. Part 2. Prediction of swelling effect on drastic decrease in strength  

NASA Astrophysics Data System (ADS)

A drastic decrease in the ultimate tensile strength of irradiated austenitic steels with high swelling values is considered. The physical-mechanical model proposed in Part 1 of the present paper is applied for the prediction of a drastic decrease in ultimate tensile strength. The mechanism called by the authors the “running collapse mechanism” is used for modeling the material ductile fracture when stresses are less than the yield strength. This ductile mechanism is similar to brittle fracture when crack propagates unstable manner. Running collapse mechanism occurs due to evolution of vacancy voids resulting in irradiation swelling. Nanoscale of vacancy voids (void sizes, distance between voids) results in the possibility of ductile fracture in very small zones whose size is considerably smaller than the grain size.

Margolin, Boris; Sorokin, Alexander

2014-09-01

365

Design characteristics, primary stability and risk of fracture of orthodontic mini-implants: Pilot scan electron microscope and mechanical studies  

PubMed Central

Objectives: Orthodontic mini-implants (OMIs) are increasingly used in orthodontics but can fail for various reasons. This study investigates the effects of OMI design characteristics on the mechanical properties in artificial bone. Material and Methods: Twelve self-drilling OMIs (2 small, 6 medium, 4 large) from 8 manufacturers were tested for their primary stability in simulated medium-high cancellous bone and the risk to fracture in high-density methacrylate blocks. For the assessments of the maximum insertion torque (IT) and torsional fracture (TF) 5 of each OMI were used and for the pull-out strength (POS) 10. The OMIs were inserted with a torque screwdriver (12 sec/360°) until the bottom at 8 mm depth was reached. OMI designs were analyzed with a scan electron microscope (SEM). Results: SEM images revealed a great variation in product refinement. In the whole sample, a cylindrical OMI shape was associated with higher POS (p<0.001) but lower IT (p=0.002) values. The outer and inner OMI diameters were design characteristics well correlated with POS, IT and TF values (ranging from 0.601 to 0.961). Greater thread depth was related to greater POS values (r= 0.628), although OMIs with similar POS values may have different IT values. Thread depth and pitch had some impact on POS. TF depended mainly on the OMI inner (r= 0.961) and outer diameters (r=0.892). A thread depth to outer diameter ratio close to 40% increased TF risk. Conclusions: Although at the same insertion depth the OMI outer and inner diameters are the most important factors for primary stability, other OMI design characteristics (cylindrical vs. conical, thread design) may significantly affect primary stability and torsional fracture. This needs to be considered when selecting the appropriate OMI for the desired orthodontic procedures. Key words:Orthodontic mini-implants, primary stability, insertion torque, pullout strength, torsional fracture. PMID:23722125

Walter, André; Winsauer, Heinz; Marcé-Nogué, Jordi; Mojal, Sergi

2013-01-01

366

Modeling Shear-Enhanced Permeability as the Mechanism for Fluid Flow in Fractured Reservoirs - A Promising Improvement to Predicting Reservoir Production  

NASA Astrophysics Data System (ADS)

An accurate geomechanical reservoir model including constraints on stress magnitudes and orientations, mechanical rock properties, and the orientations and characteristics of natural fractures is essential to understanding reservoir response to stimulation and production in low permeability reservoirs such as crystalline basement geothermal or oil and gas reservoirs. In these low permeability reservoirs, stimulation response is controlled largely by the properties of natural and induced fracture networks which are in turn controlled by the in situ stresses, the fracture distribution and the hydraulic behavior of the fractures. These hydraulic properties of the fractures, their width, stiffness and strength are often difficult to quantify, leading to large uncertainties in predicted response to stimulation of fractured reservoirs. A well-constrained and calibrated fracture model makes it possible not only to predict reservoir response to stimulation, including the shape and orientation of the stimulated region, but also to predict the required stimulation pressure. Such a model also makes it possible to predict the change in flow properties during production due to depletion, resulting in better predictions of production rate and ultimate recovery. As part of the evaluation process of a compartmentalized fractured basement reservoir, wellbore image and other data were used to develop a 3D geomechanical model of stress and natural fractures through the reservoir volume. Although the results clearly defined the optimal directions in which to drill wells to exploit pre-existing natural fractures, large uncertainties in the models resulted in significant uncertainties in predictions of stimulation response. Because the pre-existing natural fractures were insufficiently permeable and operational constraints precluded the use of hydraulic fracturing to stimulate the reservoir, an innovative approach was taken to determine the extent to which injection at pressures below the least principal stress could enhance natural fracture conductivity. This approach involves injecting small amounts of water at successively higher rates, followed by a period of lower-rate injection, to determine pre- and post-stimulation fracture permeability and the required stimulation pressure. Additional successively higher rate injectivity tests were designed and conducted to verify reservoir permeability. These tests allowed assessment of the likely effects of stimulation on reservoir flow properties and helped to reduce uncertainties in the prediction of the optimal well trajectory for enhanced reservoir production. The approach applied in this basement reservoir is applicable to any field in which fractures provide the primary flow paths but are too impermeable in their natural state to make wells economic.

Barton, C.; Moos, D.

2011-12-01

367

Micromechanics, Fracture Mechanics and Gas Permeability of Composite Laminates for Cryogenic Storage Systems  

NASA Technical Reports Server (NTRS)

A micromechanics method is developed to investigate microcrack propagation in a liquid hydrogen composite tank at cryogenic temperature. The unit cell is modeled using square and hexagonal shapes depends on fiber and matrix layout from microscopic images of composite laminates. Periodic boundary conditions are applied to the unit cell. The temperature dependent properties are taken into account in the analysis. The laminate properties estimated by the micromechanics method are compared with empirical solutions using constituent properties. The micro stresses in the fiber and matrix phases based on boundary conditions in laminate level are calculated to predict the formation of microcracks in the matrix. The method is applied to an actual liquid hydrogen storage system. The analysis predicts micro stresses in the matrix phase are large enough to cause microcracks in the composite. Stress singularity of a transverse crack normal to a ply-interface is investigated to predict the fracture behavior at cryogenic conditions using analytical and finite element analysis. When a transverse crack touches a ply-interface of a composite layer with same fiber orientation, the stress singularity is equal to 1/2. When the transverse crack propagates to a stiffer layer normal to the ply-direction, the singularity becomes less than 1/2 and vice versa. Finite element analysis is performed to predict the fracture toughness of a laminated beam subjected to fracture loads measured by four-point bending tests at room and cryogenic temperatures. As results, the fracture load at cryogenic temperature is significantly lower than that at room temperature. However, when thermal stresses are taken into consideration, for both cases of room and cryogenic temperatures, the difference of the fracture toughness becomes insignificant. The result indicates fracture toughness is a characteristic property, which is independent to temperature changes. The experimental analysis is performed to investigate the effect of cryogenic cycling on permeability for various composite material systems. Textile composites have lower permeability than laminated composites even with increasing number of cryogenic cycle. Nano-particles dispersed in laminated composites do not show improvement on permeability. The optical inspection is performed to investigate the microcrack propagation and void content in laminated composites and compared the microscopic results before and after cryogenic cycling.

Choi, Sukjoo; Sankar, Bhavani; Ebaugh, Newton C.

2005-01-01

368

Micromechanisms of brittle fracture  

Microsoft Academic Search

Mechanical processes operating in materials on the scale of the microstructure have come to be called “micromechanisms.” The\\u000a fundamental science and the micromechanisms of brittle fracture are reviewed here, with particular emphasis on cleavage and\\u000a intergranular fracture. Extant micromechanisms for these fracture types are evaluated. The role of solutes, particularly in\\u000a intergranular fracture, is also discussed in terms of the

Anthony W. Thompson; John F. Knott

1993-01-01

369

A simple fracture-mechanical model for the evolution of seismicity  

NASA Astrophysics Data System (ADS)

A simple model is presented in which a fault is represented by a one-dimensional array of elements to which a remote stress is applied. The fracture toughnesses of the fault elements are initially generated by a random walk function, but the fracture system quickly becomes organized into a fractal set characterized by a (box-counting) fractal dimension. The slope, b, of the frequency-magnitude distribution and the fractal dimension of the crack population are measured at each step, and found to vary in a similar manner to the seismic b-value and fractal dimension of earthquake epicentres. The model also predicts the observed variation of b-value of acoustic emissions in laboratory experiments.

Henderson, Jeremy; Main, Ian

1992-02-01

370

Fracture mechanisms of the Strombus gigas conch shell: implications for the design of brittle laminates  

Microsoft Academic Search

Flexural strength, crack-density evolution, work of fracture, and critical strain energy release rates were measured for wet and dry specimens of the Strombus gigas conch shell. This shell has a crossed-lamellar microarchitecture, which is layered at five distinct length scales and can be considered a form of ceramic “plywood”. The shell has a particularly high ceramic (mineral) content (99.9 wt%),

L. T. Kuhn-Spearing; H. Kessler; E. Chateau; R. Ballarini; A. H. Heuer; S. M. Spearing

1996-01-01

371

Fracture Mechanisms of Bone: A Comparative Study between Antler and Bovine Femur  

Microsoft Academic Search

In this study, fracture toughness of North American elk (Cervus elaphus canadensis) antler and bovine femur were measured using four-point bending tests on single-edge notched compact samples (ASTM C1421). Tests were conducted on crack growth directions longitudinal and transverse to the long axis of antler and bone in both dry and hydrated conditions to study the effects of fiber orientation

P. Y. Chen; F. A. Sheppard; J. M. Curiel; J. McKittrick

372

An investigation of the edge-sliding mode in fracture mechanics  

NASA Technical Reports Server (NTRS)

A boundary collocation procedure has been applied to the Williams stress function to determine the elastic stress distribution for the crack tip region of a finite, edge-cracked plate subjected to mode II loading at the crack tips. The asymmetric specimen selected was particularly suitable for the determination of plane strain fracture toughness for mode II loading. Numerical solutions for stress intensity factors for the edge-sliding mode obtained by the boundary collocation method were in close agreement with values obtained from photoelastic experiments. Fracture tests of several compact shear specimens of 2024-T4 aluminum were conducted in order to experimentally investigate the behavior of the edge-sliding mode. In each case a brittle shear failure was observed and mode II fracture toughness values were obtained. The average value for K-IIc obtained from two tests was 39.5 ksi radical (in). No K-Ic data for 2024-T4 were available for comparison purposes; however, K-Ic values for a similar alloy, 2024-T351, have been reported as 34 ksi radical (in) which is only about 15 per cent below the corresponding K-IIc value.

Jones, D. L.; Chisholm, D. B.

1975-01-01

373

Mechanisms of bone fragility in a glucocorticoid-treated mouse model of rheumatoid arthritis – implications for insufficiency fracture risk  

PubMed Central

Objective Glucocorticoid (GC) therapy is associated with increased fracture risk in rheumatoid arthritis (RA) patients. To elucidate the cause of this increased risk, we examined the effects of chronic inflammatory-erosive arthritis and GC treatment on bone quality, structure, and biomechanical properties in a murine model. Methods Transgenic mice expressing human TNF-?-transgene (TNF-tg) with established arthritis and wild-type (WT) littermates were continually treated with GC (subcutaneous prednisolone controlled-release pellet; 5 mg/kg/day) or placebo for 14, 28 and 42 days. Microstructure, biomechanical properties, chemical composition, and morphology of tibiae and lumbar vertebral bodies were assessed by micro-CT, biomechanical testing, Raman spectroscopy, and histology, respectively. Serum markers of bone turnover were also determined. Results TNF-tg and GC treatment additively decreased mechanical strength and stiffness in both tibiae and vertebral bodies. GC treatment in the TNF-tg mice increased the ductility of tibiae under torsional loading. These changes were associated with significant alterations in the biochemical and structural composition of the mineral and organic components of the bone matrix, a decrease in osteoblast activity and bone formation, and an increase in osteoclastic activity. Conclusions Our findings indicate that the concomitant decrease in bone strength and increase in ductility associated with chronic inflammation and GC therapy, coupled with the significant changes in the bone quality and structure, may increase the susceptibility of the bone to failure under low energy loading. This may explain the mechanism of symptomatic insufficiency fractures in patients with RA receiving GC therapy without radiographic manifestation of fracture. PMID:22832945

Takahata, Masahiko; Maher, Jason R.; Juneja, Subhash C.; Inzana, Jason; Xing, Lianping; Schwarz, Edward M.; Berger, Andrew J.; Awad, Hani A.

2013-01-01

374

Assessment of strength-limiting flaws in ceramic heat exchanger components INEL support: Fracture mechanics and nondestructive evaluation technology. Final report, June 1, 1986--May 31, 1993  

SciTech Connect

An examination of a siliconized SiC material, CS101K, has been performed to determine if linear fracture mechanics concepts can be used to characterize and predict the behavior of this material. Phase II of this project showed that a value that appeared to represent the true fracture toughness could be measured using small specimens with a machined notch, if the notch root radius was less than 75 {mu}m. Methods to produce sharply cracked specimens were then investigated to verify this hypothesis. A new technique, called the {open_quotes}beam support{close_quotes} precracking method, was subsequently developed and used to make sharply cracked SE(B) specimens. Tests of these specimens showed a slightly rising R-curve-type of behavior, with elevated values of plane strain fracture toughness. Interference of the crack surfaces in the precrack wake was hypothesized as the most likely cause of these phenomena. Subsequent testing with various precrack lengths provided preliminary verification of the hypothesis. Test results show that, for fracture mechanics-based design and assessment, adequate values of fracture toughness can be obtained from EDM-notched specimens, instead of the more costly precracked specimens. These results imply that, for the Si-SiC material tested, caution is warranted when using any of the methods of assessing fracture toughness that use a sharp precrack. It is also reasoned that these results may generally be more applicable to the coarser-grained structural ceramics that exhibit a rougher fracture surface. Based on results of testing EDM-notched bend specimens in 1250{degrees}C air, no degradation of material properties were observed for exposures, under applied stress, up to 900 h. Instead, some increase in fracture toughness was measured for these conditions. These same tests indicated that the threshold stress intensity factor for stress corrosion cracking (static fatigue) in the hot air environment was the same as the fracture toughness.

Lloyd, W.R.; Reuter, W.G.

1993-06-01

375

Mechanisms of plastic deformation, hardening, and fracture in single crystals of nitrogen-containing austenitic stainless steels  

NASA Astrophysics Data System (ADS)

We have carried out a systematic investigation of the mechanisms for solid-solution hardening by nitrogen atoms and dispersion hardening by nitride particles in single crystals of austenitic stainless steels with different stacking fault (SF) energies ?SF=0.02 0.2 J/m2. We show that alloying with nitrogen CN=0 0.7 mass % and precipitation of dispersed particles leads to the appearance of an orientation dependence of the critical shearing stresses ?cr, asymmetry phenomena, an orientation dependence of the slip and twinning deformation mechanisms, superelasticity, and transition from ductile fracture to brittle fracture. We develop dislocation models for solid-solution hardening by interstitial atoms, the orientation dependence and the asymmetry of ?cr, based on taking into account the effect of the external stress field on the splitting of a/2<110> dislocations into partial Schockley a/6<211> dislocations and the change in the position of the interstitial atoms from octahedral interstitial sites to tetrahedral sites with a shift of the twinning a/6<211> dislocations by a Burgers vector. We establish the role of strain localization, splitting of gliding dislocations, twinning, and a high stress level in creation of strain hardening, plastic flow instabilities, and the conditions for the "brittle-ductile" transition.

Chumlyakov, Yu. I.; Kireeva, I. V.; Korotaev, A. D.; Litvinova, E. I.; Zuev, Yu. L.

1996-03-01

376

Understanding the Degradation of Silicon Electrodes for Lithium-Ion Batteries Using Acoustic Emission and Fracture Mechanics  

NASA Astrophysics Data System (ADS)

Silicon is a promising anode material for lithium-ion battery application due to its high specific capacity, low cost, and abundance. However, when silicon is lithiated at room temperature, it can undergo a volume expansion in excess of 280%, which leads to an extensive fracturing. This is thought to be a primary cause of the rapid decay in cell capacity routinely observed. We have developed a special cell design which allows us to monitor acoustic emissions stemming from mechanical events in the cell and allow for detailed structural analysis using X-ray diffraction with an internal standard. The combined result from acoustic emissions and X-ray diffraction allow for a first of its kind detailed look at how silicon anodes degrade and together with presented theories of fracture mechanics enable a material engineering approach to optimize its long term behavior. In collaboration with Kevin Rhodes and Sergiy Kalnaus.[4pt] Parts of this research were performed at the High Temperature Materials Laboratory, a national user facility sponsored by the same office.

Daniel, Claus

2012-02-01

377

High-Temperature Mechanical Behavior and Fracture Analysis of a Low-Carbon Steel Related to Cracking  

NASA Astrophysics Data System (ADS)

Cracking in continuously cast steel slabs has been one of the main problems in casting for decades. In recent years, the use of computational models has led to a significant improvement in caster performance and product quality. However, these models require accurate thermomechanical properties as input data, which are either unreliable or nonexistent for many alloys of commercial interest. A major reason for this lack of reliable data is that high-temperature mechanical properties are difficult to measure. Several methods have been developed to assess the material strength during solidification, especially for light alloys. The tensile strength during solidification of a low carbon aluminum-killed (LCAK; obtained from Tata Steel Mainland Europe cast at the DSP plant in IJmuiden, the Netherlands) has been studied by a technique for high-temperature tensile testing, which was developed at Sumitomo Metal Industries in Japan. The experimental technique enables a sample to melt and solidify without a crucible, making possible the accurate measurement of load over a small solidification temperature range. In the current study, the tensile test results are analyzed and the characteristic zero-ductility and zero-strength temperatures are determined for this particular LCAK steel grade. The fracture surfaces are investigated following tensile testing, which provides an invaluable insight into the fracture mechanism and a better understanding with respect to the behavior of the steel during solidification. The role of minor alloying elements, like sulfur, in hot cracking susceptibility is also discussed.

Santillana, Begoña; Boom, Rob; Eskin, Dmitry; Mizukami, Hideo; Hanao, Masahito; Kawamoto, Masayuki

2012-12-01

378

Fracture mechanics of brick masonry: size effects and snap-back analysis  

Microsoft Academic Search

Fracture energy,G\\u000a F, and the critical value of stress-intensity factor,K\\u000a IC, are determined for brick masonry specimens tested in bending with different notch depths. The experimental results are compared\\u000a with numerical simulations, obtained through a cohesive crack model developed originally for concrete. Theoretical and experimental\\u000a load-deflection curves present very similar softening branches. In some cases, a snap-back instability is predicted

Pietro Bocca; Alberto Carpinteri; Silvio Valente

1989-01-01

379

Development of spiral notch torsion test: A new Fracture mechanics approach to determination of KISCC  

SciTech Connect

SNTT utilizes an extremely innovative concept of testing round-rod specimens having a V-grooved spiral notch line with a 45 a-pitch angle. The paper discusses the validity of SNTT in determining the fracture toughness, KIC, as established at ORNL. The paper also presents preliminary results of a collaborative research program of Monash University, NRL, ORNL and DSTO, for development and use of the novel technique of Spiral Notch Torsion Test (SNTT) for determination of threshold stress intensity for stress corrosion cracking, i.e., KISCC. SNTT experiments have been carried out in chloride and air environments, using fatigue pre-cracked SNTT specimens of Al-alloy, 7075.

Wang, Jy-An John [ORNL; Singh, R. K. [Monash University, Australia; Bayles, Robert [Naval Research Laboratory, Washington, D.C.; Knight, S. P. [Monash University, Australia; Hinton, B. R.W. [Defence Science and Technology Organisation, Australia; Muddle, B. C. [Monash University, Australia

2007-11-01

380

Internal architecture of calcaneus: correlations with mechanics and pathoanatomy of calcaneal fractures  

Microsoft Academic Search

Background  Available studies on internal architecture of the calcaneus are cursory and contradictory. Present study focused on elaborate\\u000a descriptions of the different trabecular groups and their correlation with the fractures of this bone.\\u000a \\u000a \\u000a \\u000a Method  To study the internal architecture, 50 dry adult human calcanei were sectioned in various planes and grossly dissected.\\u000a \\u000a \\u000a \\u000a Results  Six different groups (A–F) of lamellae were identified. Based on

Sunita Arvind Athavale; Subhash D. Joshi; Sharda S. Joshi

2010-01-01

381

Proceedings of the Joint IAEA/CSNI Specialists` Meeting on Fracture Mechanics Verification by Large-Scale Testing held at Pollard Auditorium, Oak Ridge, Tennessee  

SciTech Connect

This report contains 40 papers that were presented at the Joint IAEA/CSNI Specialists` Meeting Fracture Mechanics Verification by Large-Scale Testing held at the Pollard Auditorium, Oak Ridge, Tennessee, during the week of October 26--29, 1992. The papers are printed in the order of their presentation in each session and describe recent large-scale fracture (brittle and/or ductile) experiments, analyses of these experiments, and comparisons between predictions and experimental results. The goal of the meeting was to allow international experts to examine the fracture behavior of various materials and structures under conditions relevant to nuclear reactor components and operating environments. The emphasis was on the ability of various fracture models and analysis methods to predict the wide range of experimental data now available. The individual papers have been cataloged separately.

Pugh, C.E.; Bass, B.R.; Keeney, J.A. [comps.] [Oak Ridge National Lab., TN (United States)

1993-10-01

382

Contaminant transport in fractured porous media: analytical solution for a single fracture  

Microsoft Academic Search

A general analytical solution is developed for the problem of contaminant transport along a discrete fracture in a porous rock matrix. The solution takes into account advective transport in the fracture, longitudinal mechanical dispersion in the fracture, molecular diffusion in the fracture fluid along the fracture axis, molecular diffusion from the fracture into the matrix, adsorption into the face of

D. H. Tang; E. O. Frind; E. A. Sudicky

1981-01-01

383

Further study on the mechanism of the ductile-to-brittle fracture transition in C-Mn base and weld steel  

SciTech Connect

In the present study, the crack opening displacement (COD) tests of specimens of C-Mn base and weld steel were carried out in the ductile-brittle transition temperature region. The majority of the specimens were fractured and others were unloaded prior to fracture after ductile fracture initiated and extended. The cavities and cleavage microcracks located in the vicinities of tips of fibrous cracks of the unloaded specimens were observed in detail. The finite element method (FEM) calculations of the stress and strain distribution ahead of the tip of an extending fibrous crack were completed. The mechanism of the ductile-to-brittle fracture transition was further investigated. It was revealed that in the ductile-brittle transition temperature region, the ductile fracture process was independent of temperature. The ductile-to-brittle fracture transition was triggered by initiating a catastrophic extension of a cleavage crack ahead of the fibrous crack tip, which occurred in a condition satisfying a combined criterion composed of three items, i.e., {var_epsilon}{sub p} {ge} {var_epsilon}{sub pc} for initiating a crack nucleus; {sigma}{sub m}/{bar {sigma}} {ge} {Tc} for preventing the crack nucleus from blunting; and {sigma}{sub yy} {ge} {sigma}{sub f} for propagating the crack nucleus. For a specimen in which a fibrous crack occurred and propagated, the critical event for initiating a brittle cleavage fracture was the propagation of a ferrite grain-sized crack into neighboring grains. With extension of a fibrous crack, the behavior of the ductile-to-brittle fracture transition could be analyzed by the effect of the size of an active zone on the initiation of the brittle cleavage fracture.

Wang, G.Z.; Chen, J.H. [Gansu Univ. of Technology, Lanzhou, Gansu (China). Welding Research Inst.; Li, Z.H. [Xian Jiaotong Univ. (China). Mechanics Dept.

1997-08-01

384

Coupled quantum mechanical\\/molecular mechanical modeling of the fracture of defective carbon nanotubes and graphene sheets  

Microsoft Academic Search

Coupled quantum mechanical\\/molecular mechanical (QM\\/MM) calculations were used to study the effects of large defects and cracks on the mechanical properties of carbon nanotubes and graphene sheets. The semi-empirical method PM3 was used to treat the QM subdomains and a Tersoff-Brenner potential was used for the molecular mechanics; some of the QM calculations were also done using density functional theory

Roopam Khare; Steven L. Mielke; Jeffrey T. Paci; Sulin Zhang; Roberto Ballarini; George C. Schatz; Ted Belytschko

2007-01-01

385

Prediction of the Time Course of Callus Stiffness as a Function of Mechanical Parameters in Experimental Rat Fracture Healing Studies - A Numerical Study  

PubMed Central

Numerous experimental fracture healing studies are performed on rats, in which different experimental, mechanical parameters are applied, thereby prohibiting direct comparison between each other. Numerical fracture healing simulation models are able to predict courses of fracture healing and offer support for pre-planning animal experiments and for post-hoc comparison between outcomes of different in vivo studies. The aims of this study are to adapt a pre-existing fracture healing simulation algorithm for sheep and humans to the rat, to corroborate it using the data of numerous different rat experiments, and to provide healing predictions for future rat experiments. First, material properties of different tissue types involved were adjusted by comparing experimentally measured callus stiffness to respective simulated values obtained in three finite element (FE) models. This yielded values for Young’s moduli of cortical bone, woven bone, cartilage, and connective tissue of 15,750 MPa, 1,000 MPa, 5 MPa, and 1 MPa, respectively. Next, thresholds in the underlying mechanoregulatory tissue differentiation rules were calibrated by modifying model parameters so that predicted fracture callus stiffness matched experimental data from a study that used rigid and flexible fixators. This resulted in strain thresholds at higher magnitudes than in models for sheep and humans. The resulting numerical model was then used to simulate numerous fracture healing scenarios from literature, showing a considerable mismatch in only 6 of 21 cases. Based on this corroborated model, a fit curve function was derived which predicts the increase of callus stiffness dependent on bodyweight, fixation stiffness, and fracture gap size. By mathematically predicting the time course of the healing process prior to the animal studies, the data presented in this work provides support for planning new fracture healing experiments in rats. Furthermore, it allows one to transfer and compare new in vivo findings to previously performed studies with differing mechanical parameters. PMID:25532060

Wehner, Tim; Steiner, Malte; Ignatius, Anita; Claes, Lutz

2014-01-01

386

Joint location and source mechanism inversion of microseismic events: benchmarking on seismicity induced by hydraulic fracturing  

NASA Astrophysics Data System (ADS)

Seismic monitoring can greatly benefit from imaging events with a low signal-to-noise ratio (SNR) as the number of the events with a low signal grows exponentially. One way to detect weaker events is improvement of a SNR by migration-type stacking of waveforms from multiple stations. We have developed a new method of location of seismic events that involves stacking of seismic phases and amplitudes along diffraction traveltime curves to suppress noise and detect seismic events with a SNR lower than that on individual receivers. The stacking includes polarity correction based on a simultaneous seismic moment tensor inversion and detection algorithm on the stack function. We applied this method to locate microseismicity induced by hydraulic fracturing. First we calibrated the velocity model by locating perforation shots at known locations. Then we processed 3 d of data from microseismic monitoring of shale stimulation and benchmarked migration-type locations of the largest events that were manually located. The detected and located events induced by hydraulic fracturing in this case study are mostly shear events forming narrow bands along the maximum horizontal stress direction approximately 100 m above the injection intervals. The proposed technique is fully automated and feasible for real-time seismic monitoring.

Anikiev, D.; Valenta, J.; Stan?k, F.; Eisner, L.

2014-07-01

387

Probabilistic fracture mechanics by Galerkin meshless methods - part I: rates of stress intensity factors  

NASA Astrophysics Data System (ADS)

This is the first in a series of two papers generated from a study on probabilistic meshless analysis of cracks. In this paper (Part I), a Galerkin-based meshless method is presented for predicting first-order derivatives of stress-intensity factors with respect to the crack size in a linear-elastic structure containing a single crack. The method involves meshless discretization of cracked structure, domain integral representation of the fracture integral parameter, and sensitivity analysis in conjunction with a virtual crack extension technique. Unlike existing finite-element methods, the proposed method does not require any second-order variation of the stiffness matrix to predict first-order sensitivities, and is, consequently, simpler than existing methods. The method developed herein can also be extended to obtain higher-order derivatives if desired. Several numerical examples related to mode-I and mixed-mode problems are presented to illustrate the proposed method. The results show that first-order derivatives of stress-intensity factors using the proposed method agree very well with reference solutions obtained from either analytical (mode I) or finite-difference (mixed mode) methods for the structural and crack geometries considered in this study. For mixed-mode problems, the maximum difference between the results of proposed method and finite-difference method is less than 7 . Since the rates of stress-intensity factors are calculated analytically, the subsequent fracture reliability analysis can be performed efficiently and accurately.

Rao, B. N.; Rahman, S.

388

A fracture mechanics analysis of adhesive failure in a single lap shear joint.  

NASA Technical Reports Server (NTRS)

Discussion of adhesive fracture of single lap shear joints in terms of a maximum stress criterion and an energy balance. The Goland and Reissner (1944) analysis is used to determine the stress distribution in the adhesive assembly, and the results obtained are introduced into an energy balance to determine the initiation of adhesive fracture. In the stress analysis the loads at the edges of the joint are first determined. This is a problem in which the deformation of the joint sheets must be taken into account and is solved by using the finite-deflection theory of cylindrically bent plates. Then the stress in the joint due to applied loads is determined. This problem is formulated as one in plane strain consisting of two rectangular sheets of equal thickness and unit width. With the aid of this stress analysis and the stresses obtained from the conditions of equilibrium the contributions to the energy change with crack length are calculated. The analysis performed is then compared with a maximum stress criterion for a lap joint.

Devries, K. L.; Williams, M. L.; Chang, M. D.

1972-01-01

389

A unified enrichment scheme for fracture Safdar Abbas  

E-print Network

of fracture Types of fracture Strain Ductile Fracture Brittle Fracture Stress Brittel Ductile From: FractureA unified enrichment scheme for fracture problems Safdar Abbas Thomas-Peter Fries AICES, RWTH XFEM in fracture mechanics Numerical examples (cohesionless cracks) Numerical examples (cohesive cracks

390

Seismic imaging of hydraullically-stimulated fractures: A numerical study of the effect of the source mechanism  

E-print Network

We present a numerical study of seismic imaging of hydraulically stimulated fractures using a single source from an adjacent fracturing-process. The source is either a point force generated from the perforation of the ...

Shabelansky, Andrey Hanan

2012-01-01

391

Cleavage Fracture Modeling of Pressure Vessels Under Transient Thermo-Mechanical Loading  

SciTech Connect

Abstract The next generation of fracture assessment procedures for nuclear reactor pressure vessels (RPVs) will combine nonlinear analyses of crack-front response with stochastic treatments of crack size, shape, orientation, location, material properties and thermal-pressure transients. The projected computational demands needed to support stochastic approaches with detailed 3-D, nonlinear stress analyses of vessels containing defects appear well beyond current and near-term capabilities. In the interim, 2-D models be-come appealing to approximate certain classes of critical flaws in RPVs, and have computational demands within reach for stochastic frameworks. The present work focuses on the capability of 2-D models to provide values for the Weibull stress fracture parameter with accuracy comparable to those from very detailed 3-D models. Weibull stress approaches provide one route to connect nonlinear vessel response with fracture toughness values measured using small laboratory specimens. The embedded axial flaw located in the RPV wall near the cladding-vessel interface emerges from current linear-elastic, stochastic investigations as a critical contributor to the conditional probability of initiation. Three different types of 2-D models reflecting this configuration are subjected to a thermal-pressure transient characteristic of a critical pressurized thermal shock event. The plane-strain, 2-D models include: the modified boundary layer (MBL) model, the middle tension (M(T)) model, and the 2-D RPV model. The 2-D MBL model provides a high quality estimate for the Weibull stress but only in crack-front regions with a positive T-stress. For crack-front locations with low constraint (T-stress < 0), the M(T) specimen provides very accurate Weibull stress values but only for pressure load acting alone on the RPV. For RPVs under a combined thermal-pressure transient, Weibull stresses computed from the 2-D RPV model demonstrate close agreement with those computed from the corresponding crack-front locations in the 3-D RPV model having large negative T-stresses. Applications of this family of 2-D models provide Weibull stress values in excellent agreement with very detailed 3-D models while retaining practical levels of computational effort.

Qian, Xudong [National University of Singapore; Dodds, Robert [University of Illinois; Yin, Shengjun [ORNL; Bass, Bennett Richard [ORNL

2008-01-01

392

Mechanism of sintering and fracture of superfluxed iron-ore sinters  

NASA Astrophysics Data System (ADS)

Mineral formation in the binders of a commercial sinter with a basicity of 1.6 at the Magnitogorsk Metallurgical Integrated Works has been found to be determined by the crystallization of two morphological forms of high-iron aluminosilicoferrite (namely, dendritic and lamellar forms) from the melt. In a sintering zone, an association of dendritic aluminosilicoferrite crystals and dicalcium silicate Ca2SiO4 forms in high-calcium melt regions separated from magnetite grains. This association leads to the fracture of the finished product as a result of the phase transformation of Ca2SiO4 from the ? into the ? modification during sinter cooling. Lamellar aluminosilicoferrites forming in high-iron sinter volumes serve as a high-strength binder for ore grains.

Malysheva, T. Ya.; Gibadulin, M. F.; Mansurova, N. R.; Lekin, V. P.

2007-06-01

393

Breakdown of Linear Elastic Fracture Mechanics near the Tip of a Rapid Crack Ariel Livne, Eran Bouchbinder, and Jay Fineberg  

E-print Network

on polyacrylamide gels, brittle elastomers whose fracture dynamics mirror those of typical brittle amorphous on recent experimental results in the dynamic fracture of brittle polyacrylamide gels. In these soft materials, fracture dynamics are identical to those observed in standard brittle amorphous materials

Fineberg, Jay

394

Fracture mechanics analysis of cast duplex stainless steel elbows containing a surface crack  

SciTech Connect

EDF, in cooperation with the French Atomic Energy Commission (CEA) and Framatome, has conducted a research program on the fracture behavior of aged cast duplex stainless steel elbows. The main task of this program consisted in testing two large diameter aged cast elbows under in-plane closure bending at 420 C. This paper, after a short presentation of the experimental results, presents the ductile fracture analyses performed. Both elbows contained a semi-elliptical notch machined on the outer surface of one flank, oriented either in the longitudinal direction (first test) or in the circumferential one (second test). The crack was submitted to tensile stresses, so it may initiate and subsequently grow by ductile tearing. Despite the low toughness of the steel, the crack extension remained stable up to the end of the tests, the final crack extension reaching 8 mm (first test) and 13 mm (second one). The test analyses were performed using elastic-plastic finite element calculations, with a model built up with 20-node elements and containing about 12,000 nodes. Due to the importance of the ovalization phenomenon in the elbows, the calculations were made under the large deformation hypothesis, requiring the development of a new formulation for the energy release rate parameter G. The first purpose of these calculations was to show their ability to simulate accurately the tests, by comparison with the measurements. The second purpose was to conduct a crack growth analysis by comparing calculated J curves (accounting for different crack depths) to the material J-R curve obtained on CT specimens. The accuracy of this type of analysis is satisfactory, considering the scatter of the J material data.

Le Delliou, P.; Semete, P. [Electricite De France, Moret S/Loing (France); Ignaccolo, S. [Electricite De France (EDF), Villeurbanne (France)

1996-12-01

395

The effect of friction stir processing on the microstructure, mechanical properties and fracture behavior of investment cast titanium aluminum vanadium  

NASA Astrophysics Data System (ADS)

The use of investment cast titanium components is becoming increasingly common in the aerospace industry due to the ability to produce large, one-piece components with complex geometries that were previously fabricated by mechanically fastening or welding multiple smaller parts together. However, the coarse, fully lamellar microstructure typical of investment cast alpha + beta titanium alloys results in relatively poor fatigue strength compared to forged titanium products. As a result, investment castings are not considered for use in fatigue limited structures. In recent years, friction stir processing has emerged as a solid state metalworking technique capable of substantial microstructure refinement in aluminum and nickel-aluminum-bronze alloys. The purpose of the present study is to determine the feasibility of friction stir processing and assess its effect on the microstructure and mechanical properties of the most widely used alpha + beta titanium alloy, Ti-6Al-4V. Depending on processing parameters, including tool travel speed, rotation rate and geometry, the peak temperature in the stir zone was either above or below the beta transus. The resulting microstructures consisted of either ˜1 mum equiaxed a grains, ˜25 mum prior beta grains containing a colony alpha + beta microstructure or a combination of 1 mum equiaxed alpha and fine, acicular alpha + beta. The changes in microstructure were characterized with scanning and transmission electron microscopy and electron backscatter diffraction. The texture in the stir zone was nearly random for all processing conditions, however, several components of ideal simple shear textures were observed in both the hexagonal close packed alpha and the body centered cubic beta phases which provided insight into the operative grain refinement mechanisms. Due to the relatively small volume of material affected by friction stir processing, conventionally sized test specimens were unable to be machined from the stir zone. Thus, the mechanical properties were investigated using micropillar compression and microtensile specimens. The effect of friction stir processing on crack initiation resistance was assessed using high cycle fatigue tests conducted in four-point bend which put only the stir zone in maximum tension. The results indicated that at constant stress amplitude, there was greater than an order of magnitude increase in fatigue life after friction stir processing. In addition, the fatigue strength of the investment cast material was improved between 20 pct. and 60 pct. by friction stir processing. These improvements have been verified with a statistically significant number of tests. Finally, the wide range of microstructures created by friction stir processing provided an opportunity to study the effect of underlying microstructure on the fracture behavior of alpha + beta titanium alloys. For this purpose, high resolution fractography coupled with quantitative tilt fractography and electron backscatter diffraction was used to provide a direct link between microstructure, crystallography and fracture topography. These techniques have been used extensively to study the early stages of post-initiation crack growth in Ti-6Al-4V, especially at low stress intensity ranges (DeltaK) in the as-cast material. A limited number of experiments were also performed on Ti-6Al-4V specimens in other microstructural conditions to assess the generality of the detailed results obtained for the fully lamellar material. The results show that fracture topography depends strongly on DeltaK and microstructural length scale. In addition, many of the features observed on the fracture surface were directly related to the underlying crystallographic orientation.

Pilchak, Adam L.

396

In situ studies and modeling of the deformation and fracture mechanism for wrought Zircaloy-4 and Zircaloy-2 as a function of stress-state  

NASA Astrophysics Data System (ADS)

In situ deformation and fracture studies were performed on wrought Zircaloy-4 and Zircaloy-2 to evaluate the mechanism for fracture initiation and propagation over a range of stress-state defined as triaxiality. High values of triaxiality (?) were achieved using the pre-cracked CT specimen (? ? 1.7), intermediate values (? = 0.8-1.2) were produced for a symmetric notched specimen, lower values were initially observed for a smooth tensile specimen (? = 0.33), while the lowest initial triaxiality values were produced for the dual keyhole specimens (? = 0.0-0.16). Unstable crack extension is shown to occur under plastic constraint by a process of void nucleation, growth, and coalescence under all stress-states tested. A micromechanical model is developed for the mechanism of ductile tearing by void growth and coalescence. Excellent agreement between the model and experimental measurements of the critical strain for failure initiation and strain for fracture is observed.

Cockeram, B. V.; Chan, K. S.

2013-03-01

397

Inhibition of GSK-3? Rescues the Impairments in Bone Formation and Mechanical Properties Associated with Fracture Healing in Osteoblast Selective Connexin 43 Deficient Mice  

PubMed Central

Connexin 43 (Cx43) is the most abundant gap junction protein in bone and is required for osteoblastic differentiation and bone homeostasis. During fracture healing, Cx43 is abundantly expressed in osteoblasts and osteocytes, while Cx43 deficiency impairs bone formation and healing. In the present study we selectively deleted Cx43 in the osteoblastic lineage from immature osteoblasts through osteocytes and tested the hypothesis that Cx43 deficiency results in delayed osteoblastic differentiation and impaired restoration of biomechanical properties due to attenuated ?-catenin expression relative to wild type littermates. Here we show that Cx43 deficiency results in alterations in the mineralization and remodeling phases of healing. In Cx43 deficient fractures the mineralization phase is marked by delayed expression of osteogenic genes. Additionally, the decrease in the RankL/ Opg ratio, osteoclast number and osteoclast size suggest decreased osteoclast bone resorption and remodeling. These changes in healing result in functional deficits as shown by a decrease in ultimate torque at failure. Consistent with these impairments in healing, ?-catenin expression is attenuated in Cx43 deficient fractures at 14 and 21 days, while Sclerostin (Sost) expression, a negative regulator of bone formation is increased in Cx43cKO fractures at 21 days, as is GSK-3?, a key component of the ?-catenin proteasomal degradation complex. Furthermore, we show that alterations in healing in Cx43 deficient fractures can be rescued by inhibiting GSK-3? activity using Lithium Chloride (LiCl). Treatment of Cx43 deficient mice with LiCl restores both normal bone formation and mechanical properties relative to LiCl treated WT fractures. This study suggests that Cx43 is a potential therapeutic target to enhance fracture healing and identifies a previously unknown role for Cx43 in regulating ?-catenin expression and thus bone formation during fracture repair. PMID:24260576

Loiselle, Alayna E.; Lloyd, Shane A. J.; Paul, Emmanuel M.; Lewis, Gregory S.; Donahue, Henry J.

2013-01-01

398

On the symbolic manipulation and code generation for elasto-plastic material matrices  

NASA Technical Reports Server (NTRS)

A computerized procedure for symbolic manipulations and FORTRAN code generation of elastoplastic material matrix for finite element applications is presented. Special emphasis is placed on expression simplifications during intermediate derivations, optimal code generation, and interface with the main program. A systematic procedure is outlined to avoid redundant algebraic manipulations. Symbolic expressions of the derived material stiffness matrix are automatically converted to RATFOR code which is then translated into FORTRAN statements through a preprocessor. To minimize the interface problem with the main program, a template file is prepared so that the translated FORTRAN statements can be merged into the file to form a subroutine (or a submodule). Three constitutive models; namely, von Mises plasticity, the Drucker-Prager model, and a concrete plasticity model, are used as illustrative examples.

Chang, T. Y.; Saleeb, A. F.; Wang, P. S.; Tan, H. Q.

1986-01-01

399

Numerical Analysis of Algorithms for Infinitesimal Associated and Non-Associated Elasto-Plasticity.  

E-print Network

??The thesis studies nonlinear solution algorithms for problems in infinitesimal elastoplasticity and their numerical realization within a parallel computing framework. New algorithms like Active Set… (more)

Sauter, Martin

2010-01-01

400

About contacts of adhesive, elasto-plastic, frictional powders Stefan Luding  

E-print Network

, adhesion, and rolling-resistance. From contacts to many-particle behavior The behavior of particulate media between pairs of particles involve both normal and tangential direction and the resultant torques (as well as torques connected to rolling and torsion). Since the exact calculation of the deformations

Luding, Stefan

401

Theoretical Development of an Orthotropic Elasto-Plastic Generalized Composite Material Model  

NASA Technical Reports Server (NTRS)

The need for accurate material models to simulate the deformation, damage and failure of polymer matrix composites is becoming critical as these materials are gaining increased usage in the aerospace and automotive industries. While there are several composite material models currently available within LSDYNA (Livermore Software Technology Corporation), there are several features that have been identified that could improve the predictive capability of a composite model. To address these needs, a combined plasticity and damage model suitable for use with both solid and shell elements is being developed and is being implemented into LS-DYNA as MAT_213. A key feature of the improved material model is the use of tabulated stress-strain data in a variety of coordinate directions to fully define the stress-strain response of the material. To date, the model development efforts have focused on creating the plasticity portion of the model. The Tsai-Wu composite failure model has been generalized and extended to a strain-hardening based orthotropic yield function with a nonassociative flow rule. The coefficients of the yield function, and the stresses to be used in both the yield function and the flow rule, are computed based on the input stress-strain curves using the effective plastic strain as the tracking variable. The coefficients in the flow rule are computed based on the obtained stress-strain data. The developed material model is suitable for implementation within LS-DYNA for use in analyzing the nonlinear response of polymer composites.

Goldberg, Robert; Carney, Kelly; DuBois, Paul; Hoffarth, Canio; Harrington, Joseph; Rajan, Subramaniam; Blankenhorn, Gunther

2014-01-01

402

Load transmission by elastic, elasto-plastic or fully plastic deformation of rough interface asperities  

Microsoft Academic Search

The normal contact between two nominally flat, parallel rough surfaces is analysed on the microgeometric scale from a theoretical point of view. The microgeometry is described using a probabilistic model based on the quantification of the variability in the summit altitude and the asperity radius. This model takes for its basic data six standardised parameters for roughness and waviness defects

F. Robbe-Valloire; B. Paffoni; R. Progri

2001-01-01

403

Development of the program SPECEL: A special element for elasto-plastic crack tip analysis  

NASA Technical Reports Server (NTRS)

Theory is stated in terms of increments of displacement and stress, and of instantaneous or accumulated values of stress. The equilibrium equations, in the absence of body forces, are written, along with constitutive relations for plane strain.

Swedlow, J. L.

1974-01-01

404

Elasto-plastic stress and strain behaviour at notch roots under monotonic and cyclic loadings  

E-print Network

direction for plane strain conditions î Poisson's ratio r notch tip radius ó notch root stress ó1, ó2 first plastic deformation takes place at the notch root. When a notched component is subjected to cyclic loading, however, cyclic plastic deformation in the area of stress and strain concentrations can severely reduce

Fatemi, Ali

405

Fracture Mechanics of Thin, Cracked Plates Under Tension, Bending and Out-of-Plane Shear Loading  

NASA Technical Reports Server (NTRS)

Cracks in the skin of aircraft fuselages or other shell structures can be subjected to very complex stress states, resulting in mixed-mode fracture conditions. For example, a crack running along a stringer in a pressurized fuselage will be subject to the usual in-plane tension stresses (Mode-I) along with out-of-plane tearing stresses (Mode-III like). Crack growth and initiation in this case is correlated not only with the tensile or Mode-I stress intensity factor, K(sub I), but depends on a combination of parameters and on the history of crack growth. The stresses at the tip of a crack in a plate or shell are typically described in terms of either the small deflection Kirchhoff plate theory. However, real applications involve large deflections. We show, using the von-Karman theory, that the crack tip stress field derived on the basis of the small deflection theory is still valid for large deflections. We then give examples demonstrating the exact calculation of energy release rates and stress intensity factors for cracked plates loaded to large deflections. The crack tip fields calculated using the plate theories are an approximation to the actual three dimensional fields. Using three dimensional finite element analyses we have explored the relationship between the three dimensional elasticity theory and two dimensional plate theory results. The results show that for out-of-plane shear loading the three dimensional and Kirchhoff theory results coincide at distance greater than h/2 from the crack tip, where h/2 is the plate thickness. Inside this region, the distribution of stresses through the thickness can be very different from the plate theory predictions. We have also explored how the energy release rate varies as a function of crack length to plate thickness using the different theories. This is important in the implementation of fracture prediction methods using finite element analysis. Our experiments show that under certain conditions, during fatigue crack growth, the presence of out-of-plane shear loads induces a great deal of contact and friction on the crack surfaces, dramatically reducing crack growth rate. A series of experiments and a proposed computational approach for accounting for the friction is discussed.

Zehnder, Alan T.; Hui, C. Y.; Potdar, Yogesh; Zucchini, Alberto

1999-01-01

406

Integrated vapor pressure, hygroswelling, and thermo-mechanical stress modeling of QFN package during reflow with interfacial fracture mechanics analysis  

Microsoft Academic Search

In this paper, a comprehensive and integrated package stress model is established for quad flat non-lead package with detailed considerations of effects of moisture diffusion, heat transfer, thermo-mechanical stress, hygro-mechanical stress and vapor pressure induced during reflow. The critical plastic materials, i.e., moldcompound and die attach are characterized for hygroswelling and moisture properties, which are not easily available from material

Tong Yan Tee; Zhaowei Zhong

2004-01-01

407

FRACTURE MECHANICS AND STRENGTH OF MATERIALS: ACHIEVEMENTS AND PROGRESS MECHANIKA PORUŠENÍ A PEVNOST MATERIÁL?: DOSA?ENÍ A PERSPEKTIVY  

Microsoft Academic Search

The main stages of research development into fracture mechanics and strength of materials in the second part of the XX century have been considered. The principal attention was paid to the analysis of the calculation models of limiting equilib- rium ofdeformed solids, containing sharp stress concentrators (cracks), to the development of the methods of the stress intensity factor calculations, to

Volodymyr PANASYUK

408

Dynamic problems of the mechanics of brittle fracture of materials with initial stresses for moving cracks. 2. Cracks of normal separation (mode 1)  

Microsoft Academic Search

Introduction. Within the framework of a version [3, 6, 7] of the mechanics of brittle fracture of materials with initial stresses acting only along cracks, a general dynamic problem was formulated in [9] for a plane crack of finite width that moves at a constant velocity for compressible and incompressible bodies with an arbiWary elastic-potential structure. Also in [9], formulas

1999-01-01

409

Experimental and numerical study on the relationship between creep crack growth properties and fracture mechanisms  

NASA Astrophysics Data System (ADS)

Evaluation of creep crack growth properties taking microscopic aspects into account is effective for developing more accurate life prediction of structural components. The present study investigated the relationship between creep crack growth properties and microscopic fracture aspects for austenitic alloy 800H and 316 stainless steel. The growth rate of wedge-type intergranular and transgranular creep crack could be characterized by creep ductility. Creep damages formed ahead of the void-type crack tip accelerated the crack growth rate. Based on these experimental results, a three-dimensional finite element method (FEM) code, which simulates creep crack growth, has been developed. The effect of creep ductility on da/dt vs C* relations could be simulated based on the critical strain criteria. The diffusion of vacancies toward crack tip would accelerate the crack growth under creep conditions. The change of vacancy concentration during creep was computed for a three-dimensional compact-type (CT) specimen model by solving the diffusive equation under multiaxial stress field. The experimental results that crack growth was accelerated by creep damages formed ahead of the crack tip could be successfully simulated.

Tabuchi, Masaaki; Ha, Jechang; Hongo, Hiromichi; Watanabe, Takashi; Yokobori, Toshimitsu

2004-06-01

410

Advances in Fatigue and Fracture Mechanics Analyses for Metallic Aircraft Structures  

NASA Technical Reports Server (NTRS)

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.

Newman, J. C., Jr.

2000-01-01

411

Statistics and thermodynamics of fracture  

NASA Technical Reports Server (NTRS)

A probabilistic model of the fracture processes unifying the phenomenological study of long term strength of materials, fracture mechanics and statistical approaches to fracture is briefly outlined. The general framework of irreversible thermodynamics is employed to model the deterministic side of the failure phenomenon. The stochastic calculus is used to account for thg failure mechanisms controlled by chance; particularly, the random roughness of fracture surfaces.

Chudnovsky, A.

1984-01-01

412

Mechanical Testing of Polymers in Pressurized Hydrogen: Tension, Creep and Ductile Fracture  

Microsoft Academic Search

The paper aims at characterizing some key features of the mechanical behavior of two semi-crystalline polymers in weakly pressurized\\u000a hydrogen. The opportunity to use hydrogen as an alternative energy strengthens the need for reliable data on possible coupling\\u000a effects between gas diffusion and mechanical properties, especially for safe design purpose. However, such effects have not\\u000a been really quantified in polymers.

S. Castagnet; J.-C. Grandidier; M. Comyn; G. Benoît

413

Relationship between microstructure, toughening mechanisms, and fracture toughness of reinforced silicon nitride ceramics  

Microsoft Academic Search

Different microstructures in SiâNâ ceramics containing YâOâ and AlâOâ as sintering additives were prepared by two-step sintering. Pull-out and elastic bridging were most frequently observed as the toughening mechanisms in samples with fine-grained microstructures having needlelike β-SiâNâ grains with diameters of < 1 μm. Crack deflection was the main toughening mechanism observed in samples with coarse-grained microstructures having grains with

Pavol Sajgalik; M. J. Hoffmann; J. Dusza

1995-01-01

414

Fracture Mechanics of Collagen Fibrils: Influence of Natural Cross-Links  

PubMed Central

Tendons are important load-bearing structures, which are frequently injured in both sports and work. Type I collagen fibrils are the primary components of tendons and carry most of the mechanical loads experienced by the tissue, however, knowledge of how load is transmitted between and within fibrils is limited. The presence of covalent enzymatic cross-links between collagen molecules is an important factor that has been shown to influence mechanical behavior of the tendons. To improve our understanding of how molecular bonds translate into tendon mechanics, we used an atomic force microscopy technique to measure the mechanical behavior of individual collagen fibrils loaded to failure. Fibrils from human patellar tendons, rat-tail tendons (RTTs), NaBH4 reduced RTTs, and tail tendons of Zucker diabetic fat rats were tested. We found a characteristic three-phase stress-strain behavior in the human collagen fibrils. There was an initial rise in modulus followed by a plateau with reduced modulus, which was finally followed by an even greater increase in stress and modulus before failure. The RTTs also displayed the initial increase and plateau phase, but the third region was virtually absent and the plateau continued until failure. The importance of cross-link lability was investigated by NaBH4 reduction of the rat-tail fibrils, which did not alter their behavior. These findings shed light on the function of cross-links at the fibril level, but further studies will be required to establish the underlying mechanisms. PMID:23746520

Svensson, Rene B.; Mulder, Hindrik; Kovanen, Vuokko; Magnusson, S. Peter

2013-01-01

415

3-D Numerical Simulation of Bending Fracture of Concrete Beam Based on Meso-mechanics Damage  

Microsoft Academic Search

A method for establishing 3-D random aggregate and random mechanical parameters model on meso-level was proposed. Two numerical models of concrete beam specimens with ANSYS were established. By the flexural numerical simulation of two models, the gradual damage process, the aris