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1

Elasto-plastic fracture mechanics of crack growth in soil  

NASA Astrophysics Data System (ADS)

A predominant variable in soil structure formation and degradation is crack propagation. Empirical models exist to predict fracture but these do not describe the underlying physical processes. Theoretical fracture mechanics models have been applied to soil, but most are not applicable when soil is in a wet, plastic state. Since the onset of crack formation in soil tends to occur in this condition, physically sound elasto-plastic fracture mechanics approaches are long overdue. We address this weakness by applying a new elasto-plastic fracture mechanics approach to describe crack formation in plastic soil. Samples are fractured using a deep-notch (modified 4-point) bend test, with data on load transmission, sample bending, crack growth, and crack mouth opening collected to assess the crack opening angle (COA), the crack tip opening angle (CTOA) and the plastic energy dissipation rate (Dpl). These are all material properties that can be used directly to predict and describe crack propagation. CTOA will be used to discuss the results here, although a full description of the other parameters will be provided in the conference presentation. It provides a powerful parameter for describing soil cracking since CTOA is induced by soil shrinkage (an easily measured parameter) and can be used to describe elasto-plastic fracture in finite element modelling packages. The test variables we have studied to date are clay platelet orientation, soil texture, clay mineralogy, and pore water salinity. All samples were formed by consolidating a soil slurry with a 120 kPa vertical stress. Tests on pure kaolinite showed that platelet orientation did not affect CTOA which was 0.23 ± 0.02 for both conditions. Soil texture did have a marked influence, however, with silica sand:kaolinite mixes of 20:80 and 40:60 reducing CTOA to 0.14 ± 0.02 and 0.12 ± 0.01 respectively. These lower values of CTOA indicate that less strain is required to induce fracture when the amount of clay is lowered. Replacing kaolinite with smectite in the 40:60 sample caused CTOA to increase significantly to 0.63 ± 0.01, indicating that higher strain levels are required to fracture smectite rich soils. CTOA had a positive correlation with the plastic and liquid consistency limits for all samples. Other mechanical properties were correlated to CTOA if the kaolinite samples were examined independently of smectite. Salinity (3% NaCl) caused a reduction in the CTOA of pure kaolinite from 0.23 ± 0.02 to 0.17 ± 0.03. Finite element analysis showed good agreement between modelled and measured crack opening morphology, and predicted the stress distribution in the vicinity of an advancing crack. The approach describes elasto-plastic fracture in soil well, and is being extended to consider the influence of biological exudates, plant roots and fungal hyphae. CTOA analysis has already been applied to finite element modelling of soil crack formation.

Hallett, P. D.; Newson, T. A.

2003-04-01

2

Effects of mechanical properties and surface friction on elasto-plastic sliding contact  

E-print Network

Effects of mechanical properties and surface friction on elasto-plastic sliding contact S and many recent computational studies have established quantitative relationships between elasto-plastic systematically quantified the effect of the plastic deformation characteristics on the frictional sliding

Suresh, Subra

3

Fracture Model of Bimaterial under Delamination of Elasto-Plastic Structured Media  

NASA Astrophysics Data System (ADS)

Delamination of bimaterial composed of two structured materials is considered. A crack is located at the interface between two media. Under tension applied at infinity, I mode fracture is implemented. The improved Leonov-Panasyuk-Dugdale model (LPD model) is proposed to be applied in combination with the Neuber-Novozhilov approach. The case when elastic material characteristics are identical and strength ones essentially differ is analyzed in detail. Analytical description of plotting the fracture diagram of quasi-brittle bimaterial for the plane stress state is given. Numerical modeling of the plasticity zone in bimaterial under quasi-static loading has been performed. The updated Lagrange formulation of solid-state mechanics equations is used in a numerical model. This formulation is most preferable for modeling of bodies made from elasto-plastic material subjected to large strain. Using the finite element method, a plastic zone in the vicinity of a crack tip has been described. It is shown that the shape of the plastic zone in bimetal essentially differs from that in a homogenous medium. Numerical experiments are in good agreement with the proposed analytical model of the pre-fracture zone in the weakest material.

Kornev, V. M.; Kurguzov, V. D.; Astapov, N. S.

2013-04-01

4

A physical mechanism based investigation on the elasto-plastic-damage behavior of anisotropic aluminum alloys under finite deformation  

NASA Astrophysics Data System (ADS)

The elasto-plastic-damage behavior of anisotropic aluminum alloys is investigated under finite deformation using a physical mechanism based constitutive model. With an application to the structural calculation, the present model is used to describe and analyze the mechanical response of anisotropic 6260-T6 aluminum alloy extrusions. For the tensile specimens extracted along three different material orientations from the extruded aluminum profile, twelve simulations are carried out covering four different specimen geometries. The simulation results in force-displacement response and central logarithmic axial strain evolution are compared with experimental results. From the comparisons, it can be concluded that the present model has the capacity to describe the behavior of anisotropic material. From the force-displacement curves, the anisotropy is observed in different material orientations, and the physical mechanism of anisotropy is analyzed.

Chen, Cen; Liang, NaiGang; Liu, Fang; Fu, Qiang

2014-03-01

5

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

NASA Astrophysics Data System (ADS)

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 known and understood. In this study we extend these models by investigating the case of a contracting rock body (caused by declining pore pressure) embedded within a linear-elastic half space, but with the addition of a visco-elasto-plastic layer between the contracting reservoir and the free surface. The problem is of growing importance as the exploitation of hydrocarbon reservoirs beneath salt bodies occurs at ever greater depths in the deepwater Gulf of Mexico. This is because the creep properties of salt are strongly temperature dependent, so that as depths increase, and hence ambient temperatures, creep can occur at a rate that is impossible to ignore over the reservoir lifecycle. The models are explored using a finite element approach and make use of sophisticated salt constitutive models and large-deformation three-dimensional geomechanical simulation codes; the reservoir deformations are governed by either poro-elastic or cap plasticity constitutive laws. However, a general behaviour pattern can be observed: the visco-elasto-plastic salt layer tends to decouple the deformation fields from the free surface with stress and displacements accentuated below the salt. The magnitude of the increased horizontal displacements below the salt layer are relatively independent of the layer thickness. The accentuated vertical displacements though are more strongly dependent on the thickness of the salt layer. This work was performed at Sandia National Laboratories funded by the US DOE under Contract DE-AC04-94AL85000. Sandia is a multiprogam laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration.

Mossop, A.; Fredrich, J. T.

2004-12-01

6

A combined experimental-numerical approach for elasto-plastic fracture of individual grain boundaries  

NASA Astrophysics Data System (ADS)

The parameters for a crystal plasticity finite element constitutive law were calibrated for the aluminum-lithium alloy 2198 using micro-column compression testing on single crystalline volumes. The calibrated material model was applied to simulations of micro-cantilever deflection tests designed for micro-fracture experiments on single grain boundaries. It was shown that the load-displacement response and the local deformation of the grains, which was measured by digital image correlation, were predicted by the simulations. The fracture properties of individual grain boundaries were then determined in terms of a traction-separation-law associated with a cohesive zone. This combination of experiments and crystal plasticity finite element simulations allows the investigation of the fracture behavior of individual grain boundaries in plastically deforming metals.

Kupka, D.; Huber, N.; Lilleodden, E. T.

2014-03-01

7

A possible mechanism for the initiation of the Yinggehai Basin: A visco-elasto-plastic model  

NASA Astrophysics Data System (ADS)

The Yinggehai Basin, lying along the trace of the Red River fault zone in the South China Sea, has been related to the movements of the Red River fault zone and the rotation of the Indochina block. However, the tectonic origin of the Yinggehai Basin remains unclear. This paper explores the initiation of the Yinggehai Basin using a visco-elasto-plastic model. This model concentrates on two main aspects: lateral viscosity variations (mainly controlled by temperature) in the lithosphere and internal friction angles of the pre-existing fault zone. Modeling results show that: (1) for a layered viscosity, en echelon faults would likely form when there is no pre-existing strike-slip fault; whereas, the basin would be narrow and deep, if any, when there is a pre-existing fault; (2) for moderate lateral viscosity variations, a large basin forms, even without a pre-existing fault zone; and (3) for strong lateral viscosity variations, a major rift could form over geologic time. Our results indicate that the initiation of the Yinggehai Basin requires moderate lateral viscosity variations (i.e., a pre-existing gentle upwelling of the Mohorovicic discontinuity) but no pre-existing fault. In addition, the initial extension predicted is NE-SW and is generated by the rotation of the Indochina block. This differs from the NW-SE extension that resulted from the movements of the Red River fault zone. This indicates that the left-lateral displacements of the Red River fault zone and the spreading of the South China Sea only influence the basin evolution after its initiation.

Wang, Xinguo; He, Jiankun; Ding, Lin; Gao, Risheng

2013-09-01

8

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

9

Damage analysis and dynamic response of elasto-plastic laminated composite shallow spherical shell under low velocity impact  

Microsoft Academic Search

Based on the elasto-plastic mechanics, the damage analysis and dynamic response of an elasto-plastic laminated composite shallow spherical shell under low velocity impact are carried out in this paper. Firstly, a yielding criterion related to spherical tensor of stress is proposed to model the mixed hardening orthotropic material, and accordingly an incremental elasto-plastic damage constitutive relation for the laminated shallow

Fu Yiming; Mao Yiqi; Tian Yanping

2010-01-01

10

Elasto-Plastic FEM Stress Analysis and Mechanical Characteristics of Pipe Flange Connections with Non-Asbestos Gaskets under Internal Pressure  

NASA Astrophysics Data System (ADS)

The effects of the nominal diameter of pipe flange connections with non-asbestos spiral wound gaskets(SWG) under internal pressure on the mechanical characteristics such as the contact gasket stress distribution which governs the sealing performance, the load factor and the hub stress of the connections were evaluated. The stresses in the connections with the nominal diameters from 3” to 24” under internal pressure are analyzed using the elasto-plastic(EP) FEM analysis taking account the hysteresis and non-linearity of deformation behavior of the non-asbestos SWG. As a result, it is found that the variations in the contact gasket stress distributions are substantial due to the flange rotation in the connections with the larger nominal diameter. Leakage tests were conducted to measure the axial bolt forces (the load factor) and the hub stress. The results obtained from the EP-FEM analyses are fairly consistent with the experimental results concerning the variation in the axial bolt forces (the load factor) and the hub stress. Using the obtained contact gasket stress distributions and the fundamental relationship between the amount of leakage and the contact gasket stress, the amount of the leakage of the connections is estimated. It is observed that the sealing performance of the connections with larger nominal diameter is worse than that of the connection with smaller nominal diameter because of the flange rotation. The estimated results are in a fairly good agreement with the measured results. The difference in the hub stress between the EP-FEM and ASME code is demonstrated and the differences in the load factor and the sealing performance of the connections are shown between the asbestos and non-asbestos gaskets.

Takagi, Yoshio; Omiya, Yuya; Kobayashi, Takashi; Sawa, Toshiyuki

11

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

12

Elasto-plastic modeling of volcano ground deformation  

NASA Astrophysics Data System (ADS)

Elasto-plastic models for pressure sources in heterogeneous domain were constructed to describe, assess, and interpret observed deformation in volcanic regions. We used the Finite Element Method (FEM) to simulate the deformation in a 3D domain partitioned to account for the volcano topography and the heterogeneous material properties distribution. Firstly, we evaluated the extent of a heated zone surrounding the magmatic source calculating the temperature distribution by a thermo-mechanical numerical model. Secondly, we included around the pressurized magma source an elasto-plastic zone, whose dimension is related to the temperature distribution. This elasto-plastic model gave rise to deformation comparable with that obtained from elastic and viscoelastic models, but requiring a geologically satisfactory pressure. We successfully applied the method to review the ground deformation accompanying the 1993-1997 inflation period on Mt Etna. The model considerably reduces the pressure of a magma chamber to a few tens of MPa to produce the observed surface deformation. Results suggest that the approach presented here can lead to more accurate interpretations and inferences in future modeling-based assessments of volcano deformation.

Currenti, Gilda; Bonaccorso, Alessandro; Del Negro, Ciro; Scandura, Danila; Boschi, Enzo

2010-08-01

13

Experimental and finite element analysis of fracture criterion in general yielding fracture mechanics  

Microsoft Academic Search

Efforts made over the last three decades to understand the fracture behaviour of structural materials in elastic and elasto-plastic\\u000a fracture mechanics are numerous, whereas investigations related to fracture behaviour of materials in thin sheets or general\\u000a yielding fracture regimes are limited in number. Engineering simulative tests are being used to characterize formability and\\u000a drawability of sheet metals. However, these tests

D. M. Kulkarni; Ravi Prakash; A. N. Kumar

2002-01-01

14

Parameter sensitivity of anisotropic elasto-plastic shell  

Microsoft Academic Search

1.Abstract A continuum-based design sensitivity analysis (DSA) applied to the thickness design of the anisotropic elasto-plastic shell structures is developed. Since main interest lies in developing path-following algorithms for solving sensitivity problems of elasto-plasticity, such an incremental or a step-by-step formalism will be employed to implementation into the structural analysis program. Because of the response sensitivity at a given time

Sami Holopainen

15

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

Microsoft Academic Search

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

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

2004-01-01

16

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

17

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

18

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

19

Thermo-elasto-plastic finite element analysis of quasi-state processes in Eulerian reference  

E-print Network

Thermo-elasto-plastic finite element analysis of quasi-state processes in Eulerian reference frames ­ Incremental scheme ­ Fine mesh along entire heat source path ­ Lengthy computer runs · Elasto-Plasticity at times ti and ti-1, respectively. Penn State University 5 #12;Elasto-Plasticity Equilibrium: r(r, t) + b

Michaleris, Panagiotis

20

Ecient elasto-plastic simulation ICA, University of Stuttgart, Germany  

E-print Network

EÃ?cient elasto-plastic simulation C. Wieners ICA, University of Stuttgart, Germany Summary. In this paper we describe a method for the construction of radial re- turn algorithms to the plasticity models plasticity, the constitutive equations are determined by the free energy functional and a monotone function

Wieners, Christian

21

The method of lines in three-dimensional fracture mechanics  

NASA Technical Reports Server (NTRS)

A review is presented of recent developments in the calculation of design parameters for fracture mechanics by the method of lines (MOL). Three-dimensional elastic and elasto-plastic formulations are examined and results are reported from previous and current research activities. The application of MOL to the appropriate partial differential equations of equilibrium leads to coupled sets of simultaneous ordinary differential equations. Solutions of these equations are obtained by the Peano-Baker and by the recurrence relations methods. The advantages and limitations of both solution methods from the computational standpoint are summarized.

Gyekenyesi, J.; Berke, L.

1980-01-01

22

The Effect of Scale-Dependent Hardness on ElastoPlastic Asperity Contact between Rough Surfaces  

Microsoft Academic Search

Statistical methods are used to model elasto-plastic contact between two rough surfaces using a recent finite element model of elasto-plastic hemispherical contact and also recent advances in strain gradient modeling. The elasto-plastic hemispherical contact model used to model individual asperities accounts for a varying hardness effect due to deformation of the contact geometry that has been documented by other works.

Robert L. Jackson

2006-01-01

23

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

24

Nonlinear elasto-plastic model for dense granular flow  

E-print Network

This work proposes a model for granular deformation that predicts the stress and velocity profiles in well-developed dense granular flows. Recent models for granular elasticity (Jiang and Liu 2003) and rate-sensitive plastic flow (Jop et al. 2006) are reformulated and combined into one universal granular continuum law, capable of predicting flowing regions and stagnant zones simultaneously in any arbitrary 3D flow geometry. The unification is performed by justifying and implementing a Kroner-Lee elasto-plastic decomposition, with care taken to ensure certain continuum physical principles are necessarily upheld. The model is then numerically implemented in multiple geometries and results are compared to experiments and discrete simulations.

Ken Kamrin

2009-05-07

25

An accurate elasto-plastic frictional tangential forcedisplacement model for granular-flow  

E-print Network

An accurate elasto-plastic frictional tangential force­displacement model for granular for both elastic and plastic deformations together with interfacial friction occurring in collisions of spherical particles. This elasto-plastic frictional TFD model, with its force-driven version presented in [L

Vu-Quoc, Loc

26

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

E-print Network

ARMA 10-473 Comparison between elasto-plastic and rigid-plastic cohesive surface elements- plastic (EP) and rigid-plastic (RP) cohesive surface finite element (CSE) implementations of cracking). For the CSE, we will consider elasto-plastic and rigid-plastic for- mulations, where the rigid-plastic

Regueiro, Richard A.

27

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

28

Modeling Elasto-Plastic Behavior of Polycrystalline Grain Structure of Steels at Mesoscopic Level  

E-print Network

Modeling Elasto-Plastic Behavior of Polycrystalline Grain Structure of Steels at Mesoscopic Level. The constitutive model of crystal grains utilizes anisotropic elasticity and crystal plasticity. Commercially be considered macroscopically homogeneous. Elastic and rate independent plastic deformation modes are considered

Cizelj, Leon

29

Elasto-plastic response of reversibly crosslinked biopolymer bundles.  

PubMed

We study the response of F-actin bundles to driving forces through a simple analytical model. We consider two filaments connected by reversibly bound crosslinks and driven by an external force. Two failure modes under load can be defined. Brittle failure is observed when crosslinks suddenly and collectively unbind, leading to catastrophic loss of bundle integrity. During ductile failure, on the other hand, bundle integrity is maintained, however at the cost of crosslink reorganization and defect formation. We present phase diagrams for the onset of failure, highlighting the importance of the crosslink stiffness for these processes. Crossing the phase boundaries, force-deflection curves display (frequency-dependent) hysteresis loops, reflecting the first-order character of the failure processes. We evidence how the introduction of defects can lead to complex elasto-plastic relaxation processes, once the force is switched off. Depending on, both the time-scale for defect motion and the crosslink stiffness, bundles can remain in a quasi-permanent plastically deformed state for a very long time. PMID:24965158

Sadhukhan, Poulomi; Schumann, Ole; Heussinger, Claus

2014-06-01

30

Elasto-plastic response of reversibly crosslinked biopolymer bundles  

E-print Network

We study the response of F-actin bundles to driving forces through a simple analytical model. We consider two filaments connected by reversibly bound crosslinks and driven by an external force. Two failure modes under load can be defined. \\textit{Brittle failure} is observed when crosslinks suddenly and collectively unbind, leading to catastrophic loss of bundle integrity. During \\textit{ductile failure}, on the other hand, bundle integrity is maintained, however at the cost of crosslink reorganization and defect formation. We present phase diagrams for the onset of failure, highlighting the importance of the crosslink stiffness for these processes. Crossing the phase boundaries, force-deflection curves display (frequency-dependent) hysteresis loops, reflecting the first-order character of the failure processes. We evidence how the introduction of defects can lead to complex elasto-plastic relaxation processes, once the force is switched off. Depending on, both, the time-scale for defect motion as well as the crosslink stiffness, bundles can remain in a quasi-permanent plastically deformed state for a very long time.

Poulomi Sadhukhan; Ole Schuman; Claus Heussinger

2014-05-19

31

Elasto-plastic deformation of compositionally graded metal-ceramic composites  

SciTech Connect

The elasto-plastic deformation due to thermal and mechanical loading of layered metal-ceramic composites with compositionally graded interfaces is analyzed using detailed finite element models. The model material considered is a Ni-Al{sub 2}O{sub 3} layered system with a compositionally graded interface. The analyzes consider planar geometries with perfectly periodic arrangements of the constituent phases, by recourse of new classes of square-packing and hexagonal-packing unit cell formulations for the graded material. Also considered are graded phase arrangements within which large numbers of microstructural units of the two phases are randomly placed within the unit cell. It is found that square-packing arrangements provide the best possible bounds for the thermal strains and coefficient of thermal expansion (CTE) of the graded multi-layer, among the different unit cell models examined; however, no unique bounds could be identified for mechanical loading. The numerical predictions of thermal and mechanical response are compared with those provided by the mean-field approach involving an incremental Mori-Tanaka analysis and by the simple rule-of-mixture approximations. The former method provides a stiffer mechanical response than the finite-element unit cell models. The finite element predictions of bending CTE due to thermal excursions match the overall trends observed experimentally for the Ni-Al{sub 2}O{sub 3} graded system, and further provide a quantitative prediction of the temperature for the onset of plastic flow in the graded material.

Weissenbek, E.; Pettermann, H.E.; Suresh, S. [Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Materials Science and Engineering] [Massachusetts Inst. of Tech., Cambridge, MA (United States). Dept. of Materials Science and Engineering

1997-08-01

32

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

33

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

34

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

35

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

36

Elasto-plastic flow of a foam around an obstacle F. Boulogne  

E-print Network

Elasto-plastic flow of a foam around an obstacle F. Boulogne Institute of Mathematics and Physics to this evidence of plasticity, a foam's rheology is dominated by elasticity at low strains and viscous flow at high strain-rates: they are elasto-visco-plastic fluids [4]. A common probe of foam rheology

Cox, Simon

37

An elasto-plastic damage model cast in a co-rotational kinematic framework for large  

E-print Network

analysis of laminated composite shells Choon L. Tham, Zhe Zhang, Arif Masud * Department of Civil framework for damage analysis in laminated plates and shells undergoing large deflections. Numerical and shells can undergo elastic­brittle damage as well as elasto-plastic damage. Various continuum 0045

Masud, Arif

38

Fracture mechanics  

NASA Technical Reports Server (NTRS)

The application of fracture mechanics to the design of ceramic structures will require the precise measurement of crack growth and fracture resistance of these materials over their entire range of anticipated service temperatures and standardized test methods for making such measurements. The development of a standard test for measuring the plane strain fracture toughness is sought. Stress intensity factor coefficients were determined for three varieties of chevron-notch specimens, and fracture toughness measurements were made on silicon nitrides, silicon carbides, and aluminum oxides to assess the performance of each specimen variety. It was determined that silicon nitride and silicon carbides have flat crack growth resistance curves, but aluminum oxide does not. Additionally, batch-to-batch differences were noticed for the aluminum oxide. Experiments are continuing to explain the rising crack growth resistance and batch-to-batch variations for the aluminum oxide.

Shannon, John L., Jr.

1986-01-01

39

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

40

A 6-node co-rotational triangular elasto-plastic shell element  

NASA Astrophysics Data System (ADS)

A 6-node co-rotational triangular elasto-plastic shell element is developed. The local coordinate system of the element is defined by the vectors directing from one vertex to the other two vertices and their cross product. Based on such a co-rotational framework, the element rigid-body rotations are excluded in calculating the local nodal variables from the global nodal variables. The two smallest components of each nodal orientation vector are defined as rotational variables, resulting in the desired additive property for all nodal variables in a nonlinear incremental solution procedure. Different from other existing co-rotational finite element formulations, both the element tangent stiffness matrices in the local and in the global coordinate systems are symmetric owing to the commutativity of the nodal variables in calculating the second derivatives of the strain energy with respect to the local nodal variables and, through chain differentiation, with respect to the global nodal variables. For elasto-plastic analysis, the Maxwell-Huber-Hencky-von Mises yield criterion is employed together with the backward-Euler return-mapping method for the evaluation of the elasto-plastic stress state, where a consistent tangent modulus matrix is employed. To overcome locking problems, the assumed linear membrane strains and shear strains are obtained by using the line integration method proposed by MacNeal, and the assumed higher-order membrane strains are obtained by enforcing the stationarity of the mixed displacement-strain canonical functional, these assumed strains are then employed to replace the corresponding conforming strains. The reliability and convergence of the present 6-node triangular shell element formulation are verified through two elastic plate patch tests as well as two elastic and five elasto-plastic plate/shell problems undergoing large displacements and large rotations.

Li, Zhongxue; Xiang, Yu; Izzuddin, Bassam A.; Vu-Quoc, Loc; Zhuo, Xin; Zhang, Chuanjie

2015-03-01

41

Dynamic analysis of an elasto-plastic sandwich subjected to low velocity impact  

Microsoft Academic Search

Purpose – The purpose of this paper is to analyse the dynamic behaviour of an elasto-plastic sandwich subjected to low velocity impact. Design\\/methodology\\/approach – A numerical model is developed with the assumption that the plastic deformation is confined under the contact area. The structure is analyzed using the in-house finite element code with an appropriate contact law. During the impact

Mondher Wali; Moez Abdennadher; Tahar Fakhfakh; Mohamed Haddar

2011-01-01

42

Finite element modelling of anisotropic elasto-plastic timber composite beams with openings  

Microsoft Academic Search

In this paper, constitutive equations to model anisotropic elasto-plastic timber composite beams with openings were formulated and implemented into the finite element (FE) package ABAQUS, via a user-defined subroutine. The Tsai–Hill criterion was applied to judge failure of Oriented Strand Board (OSB) and timber in tension. Both OSB and timber in tension were modelled as linear orthotropic elastic materials, and

Z. W. Guan; E. C. Zhu

2009-01-01

43

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

NASA Astrophysics Data System (ADS)

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

1993-12-01

44

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

45

Numerical Procedure for ElastoPlastic No-Tension Model  

Microsoft Academic Search

In the present work a generalization of the classical elastic no-tension model accounting for irreversible crushing strains is proposed. The model considers limited tensile and compressive strength; in particular, fracture strain arises when the tensile strength is reached, while plastic strain can be developed because of the limited compressive strength. As a consequence, inelastic strains are partitioned into the sum

Sonia Marfia; Elio Sacco

2005-01-01

46

A new multilayered visco-elasto-plastic experimental model to study strike-slip fault seismic cycle  

NASA Astrophysics Data System (ADS)

Nowadays, technological advances in satellite imagery measurements as well as the development of dense geodetic and seismologic networks allow for a detailed analysis of surface deformation associated with active fault seismic cycle. However, the study of earthquake dynamics faces several limiting factors related to the difficulty to access the deep source of earthquake and to integrate the characteristic time scales of deformation processes that extend from seconds to thousands of years. To overcome part of these limitations and better constrain the role and couplings between kinematic and mechanical parameters, we have developed a new experimental approach allowing for the simulation of strike-slip fault earthquakes and analyze in detail hundreds of successive seismic cycle. Model rheology is made of multilayered visco-elasto-plastic analog materials to account for the mechanical behavior of the upper and lower crust and to allow simulating brittle/ductile coupling, postseismic deformation phase and far-field stress transfers. The kinematic evolution of the model surface is monitored using an optical system, based on subpixel spectral correlation of high-resolution digital images. First, results show that the model succeed in reproducing the deformation mechanisms and surface kinematics associated to the main phases of the seismic cycle indicating that model scaling is satisfactory. These results are comforted by using numerical algorithms to study the strain and stress distribution at the surface and at depth, along the fault plane. Our analog modeling approach appears, then, as an efficient complementary approach to investigate earthquake dynamics.

Caniven, Y.; Dominguez, S.; Soliva, R.; Cattin, R.; Peyret, M.; Marchandon, M.; Romano, C.; Strak, V.

2015-02-01

47

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

48

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

49

Application of the Sinc method to a dynamic elasto-plastic problem  

NASA Astrophysics Data System (ADS)

This paper presents the application of Sinc bases to simulate numerically the dynamic behavior of a one-dimensional elastoplastic problem. The numerical methods that are traditionally employed to solve elastoplastic problems include finite difference, finite element and spectral methods. However, more recently, biorthogonal wavelet bases have been used to study the dynamic response of a uniaxial elasto-plastic rod [Giovanni F. Naldi, Karsten Urban, Paolo Venini, A wavelet-Galerkin method for elastoplasticity problems, Report 181, RWTH Aachen IGPM, and Math. Modelling and Scient. Computing, vol. 10, 2000]. In this paper the Sinc-Galerkin method is used to solve the straight elasto-plastic rod problem. Due to their exponential convergence rates and their need for a relatively fewer nodal points, Sinc based methods can significantly outperform traditional numerical methods [J. Lund, K.L. Bowers, Sinc Methods for Quadrature and Differential Equations, SIAM, Philadelphia, 1992]. However, the potential of Sinc-based methods for solving elastoplasticity problems has not yet been explored. The aim of this paper is to demonstrate the possible application of Sinc methods through the numerical investigation of the unsteady one dimensional elastic-plastic rod problem.

Abdella, K.; Yu, X.; Kucuk, I.

2009-01-01

50

Numerical investigations on the fatigue failure of forging tools due to thermo-mechanical cyclic loading  

Microsoft Academic Search

During service hot forging dies are exposed to a combination of cyclic thermo-mechanical, tribological and chemical loads.\\u000a Besides abrasive and adhesive wear on the die surface, fatigue crack initiation with subsequent fracture is one of the most\\u000a frequent cause of failure. Fatigue cracks in forging dies are caused by local elasto-plastic strains due to cyclic mechanical\\u000a and thermal loads. Aiming

B.-A. Behrens; A. Bouguecha; H. W. Raedt; M. Sc. T. Hadifi

2010-01-01

51

FRACTALS IN THERMO-ELASTO-PLASTIC MATERIALS (to appear in JoMMS J. Mech. Mater. Struct.)  

E-print Network

FRACTALS IN THERMO-ELASTO-PLASTIC MATERIALS (to appear in JoMMS ­ J. Mech. Mater. Struct.) J. LI AND M. OSTOJA-STARZEWSKI Fractal patterns are observed in computer simulations of elastic-plastic transitions in linear, locally isotropic thermoelastic-hardening plastic heterogeneous materials. The models

Ostoja-Starzewski, Martin

52

A single hardening elasto-plastic model for Kaolin clay with loading-history-dependent plastic potential function  

E-print Network

A single hardening elasto-plastic model for Kaolin clay with loading-history- dependent plastic and failure criteria are found to be strongly dependent on the principal stress rotation angle () and plastic work. A unique plastic potential function determined solely by the current stress state

Prashant, Amit

53

Elastic and Elasto-Plastic Analysis of Gravity Dam-Foundation System without and with Shear Seams  

E-print Network

Elastic and Elasto-Plastic Analysis of Gravity Dam-Foundation System without and with Shear Seams hydro-power potential has been harnessed. Dams form a major component of a hydro-power project and apply. The interaction of the dam with the foundation, especially one with difficult or troublesome geology, needs

Kumar, M. Jagadesh

54

Degenerate Dirichlet Problems Related to the Invariant Measure of Elasto-Plastic Oscillators  

SciTech Connect

A stochastic variational inequality is proposed to model a white noise excited elasto-plastic oscillator. The solution of this inequality is essentially a continuous diffusion process for which a governing diffusion equation is obtained to study the evolution in time of its probability distribution. The diffusion equation is degenerate, but using the fact that the degeneracy occurs on a bounded region we are able to show the existence of a unique solution satisfying the desired properties. We prove the ergodic properties of the process and characterize the invariant measure. Our approach relies on extending Khasminskii's method (Stochastic Stability of Differential Equations, Sijthoff and Noordhoff, 1980), which in the present context leads to the study of degenerate Dirichlet problems with nonlocal boundary conditions.

Bensoussan, Alain [University of Texas at Dallas, International Center for Decision and Risk Analysis, ICDRiA, School of Management (United States)], E-mail: Alain.Bensoussan@utdallas.edu; Turi, Janos [University of Texas at Dallas, Programs in Mathematical Sciences (United States)], E-mail: turi@utdallas.edu

2008-08-15

55

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 an elasto-plastic 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, 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.

1991-01-01

56

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

57

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

58

Model Experiments and Elasto-Plastic Finite Element Analysis about Seepage Failure of Sand Behind Fixed Sheet Pile  

NASA Astrophysics Data System (ADS)

Constructing civil engineering structures, temporaly or permanent water proofing sheet piles often are used. The stability of seepage failure are carefully applied to those sheet piles, although many troubles of seepage failure were reported. On this problem the predictive method of the deformation and critical water head is required. In this study we carried out the model experiments which were designed for studying the seepage failure of soil behind fixed sheet piles and our elasto-plastic finite element method was applied to verify the effectiveness. Terzaghi method is very famous method for this problem and Terzaghi method was investigated by experiments data. As a result, it was confirmed that elasto-plastic finite element method was effectiveness and Terzaghi method was useful for this problem.

Okajima, Kenji; Tanaka, Tadatsugu; Zhang, Shanji; Komatsu, Takahiro

59

A thermo-elasto-plastic constitutive law for inhomogeneous materials based on an incremental Mori–Tanaka approach  

Microsoft Academic Search

The behavior of a composite consisting of aligned thermo-elastic reinforcements embedded in a thermo-elasto-plastic matrix is described by an incremental Mori–Tanaka mean field approach. The matrix phase behavior is described by incremental J2 plasticity and the breakdown of isotropy of the matrix phase upon yielding is accounted for. The proposed method is implemented as a constitutive material model for a

H. E. Pettermann; A. F. Plankensteiner; H. J. Böhm; F. G. Rammerstorfer

1999-01-01

60

A novel approach to elasto-plastic finite element analysis of beam structures using the concept of incremental secant stiffness  

Microsoft Academic Search

This paper presents an incremental secant stiffness formulation for materially non-linear analysis of planar beam structures under monotonically increasing external loads. To describe the elasto-plastic behaviour of a typical beam member, a set of non-dimensional plasticity coefficients are introduced to progressively deteriorate the elastic stiffness properties over an incremental load history. The proposed method is developed to provide the accuracy

Y. Wen; Q. Y. Zeng

2010-01-01

61

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

62

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

63

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

64

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

65

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

66

Comparison of isotropic elasto-plastic models for the plastic metric tensor $C_p=F_p^T\\, F_p$  

E-print Network

We discuss in detail existing isotropic elasto-plastic models based on 6-dimensional flow rules for the positive definite plastic metric tensor $C_p=F_p^T\\, F_p$ and highlight their properties and interconnections. We show that seemingly different models are equivalent in the isotropic case.

Patrizio Neff; Ionel-Dumitrel Ghiba

2014-10-08

67

Mechanical and petrophysical study of fractured shale materials  

NASA Astrophysics Data System (ADS)

Understanding of the mechanical and physical properties of shales is of major importance in many fields such as faults hydro-mechanical behavior, cap-rock and unconventional reservoir studies or nuclear waste disposal. In particular, relationships between fluid transport properties, applied stress and textural anisotropy are critical both in intact and fractured shales. Therefore, these relations need to be investigated in the laboratory in order to have a better understanding on in-situ mechanisms. Hence, the mechanical behavior and the petrophysical properties of Toarcian shale of the Tournemire underground laboratory (France) have been investigated. The petrophysical properties have been measured along a 20 meters core drilled through a fault zone from the Tournemire tunnel. Along the core, P and S waves velocity and anisotropy, as well as magnetic susceptibility anisotropy and porosity were measured. In addition, conventional triaxial tests have been performed in order to determine the elasto-plastic yield envelope on three sets of samples with different orientations relative to bedding (0°, 45°, and 90° to the vertical axe). For each set, six experiments were carried out at increasing confining pressures (2.5, 5, 10, 20, 40, 80MPa). Experiments were performed in dry conditions, at a strain rate of 5x10-7 s-1 up to failure. During each experiment, P and S wave elastic velocities were continuously measured along different directions, in order to assess both P wave anisotropy and shear wave splitting and their evolutions with deformation. Our results show that brittle failure is preceded by the development of P wave anisotropy and shear wave splitting, due to crack re-opening and crack growth. However, the orientation of principal stress components relative to the bedding plane plays an important role on both the brittle strength, as well as on the magnitude of shear-enhanced P wave velocity anisotropy and S wave splitting. Our perspective is now to perform fluid injections, both at the field and laboratory scale and measure the evolution of hydraulic diffusivity by tracking the fluid front by both passive and active seismic measurements.

Bonnelye, A.; Schubnel, A.; David, C.; Henry, P.; Guglielmi, Y.; Gout, C.; Dick, P.

2013-12-01

68

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

69

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

70

Lecture Notes on Fracture Mechanics  

NSDL National Science Digital Library

This book by a member of Cornell's Department of Theoretical and Applied Mechanics provides an introduction to fracture mechanics, that branch of mechanics that deals with the behavior of cracked bodies subjected to stresses and strains. Materials include mechanics models for crack tip fields and energy flows, a discussion of how these results affect observed fracture behavior, a brief discussion of computational fracture methods, and fracture toughness testing and fracture criteria. A student who has worked through the notes should be able to read and understand research articles or monographs on advanced aspects of fracture and should be able to apply this knowledge to real-world problems.

Dr. Alan T. Zehnder

71

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

72

Linear elastic fracture mechanics primer  

NASA Technical Reports Server (NTRS)

This primer is intended to remove the blackbox perception of fracture mechanics computer software by structural engineers. The fundamental concepts of linear elastic fracture mechanics are presented with emphasis on the practical application of fracture mechanics to real problems. Numerous rules of thumb are provided. Recommended texts for additional reading, and a discussion of the significance of fracture mechanics in structural design are given. Griffith's criterion for crack extension, Irwin's elastic stress field near the crack tip, and the influence of small-scale plasticity are discussed. Common stress intensities factor solutions and methods for determining them are included. Fracture toughness and subcritical crack growth are discussed. The application of fracture mechanics to damage tolerance and fracture control is discussed. Several example problems and a practice set of problems are given.

Wilson, Christopher D.

1992-01-01

73

Plane elasto-plastic analysis of v-notched plate under bending by boundary integral equation method. Ph.D. Thesis  

NASA Technical Reports Server (NTRS)

A method of solution is presented, which, when applied to the elasto-plastic analysis of plates having a v-notch on one edge and subjected to pure bending, will produce stress and strain fields in much greater detail than presently available. Application of the boundary integral equation method results in two coupled Fredholm-type integral equations, subject to prescribed boundary conditions. These equations are replaced by a system of simultaneous algebraic equations and solved by a successive approximation method employing Prandtl-Reuss incremental plasticity relations. The method is first applied to number of elasto-static problems and the results compared with available solutions. Good agreement is obtained in all cases. The elasto-plastic analysis provides detailed stress and strain distributions for several cases of plates with various notch angles and notch depths. A strain hardening material is assumed and both plane strain and plane stress conditions are considered.

Rzasnicki, W.

1973-01-01

74

Mechanisms of intergranular fracture  

SciTech Connect

The authors present a study of the atomistic mechanisms of crack propagation along grain boundaries in metals and alloys. The failure behavior showing cleavage crack growth and/or crack-tip dislocation emission is demonstrated using atomistic simulations for an embedded-atom model. The simulations follow the quasi-equilibrium growth of a crack as the stress intensity applied increases. Dislocations emitted from crack tips normally blunt the crack and inhibit cleavage, inducing ductile behavior. When the emitted dislocations stay near the crack tip (sessile dislocations), they do blunt the crack but brittle cleavage can occur after the emission of a sufficient number of dislocations. The fracture process occurs as a combination of dislocation emission/micro-cleavage portions that are controlled by the local atomistic structure of the grain boundary. The grain boundary is shown to be a region where dislocation emission is easier, a mechanism that competes with the lower cohesive strength of the boundary region.

Farkas, D. [Virginia Polytechnic Inst. and State Univ., Blacksburg, VA (United States). Dept. of Materials Science and Engineering

1999-08-01

75

Inverse extraction of interfacial tractions from elastic and elasto-plastic far-fields by nonlinear field projection  

NASA Astrophysics Data System (ADS)

Determining the tractions along a surface or interface from measurement data in the far-fields of nonlinear materials is a challenging inverse problem which has significant engineering and nanoscience applications. Previously, a field projection method was established to identify the crack-tip cohesive zone constitutive relations in an isotropic elastic solid (Hong and Kim, 2003. J. Mech. Phys. Solids 51, 1267). In this paper, the field projection method is further generalized to extracting the tractions along interfaces bounded by nonlinear materials, both with and without pre-existing cracks. The new formulation is based on Maxwell-Betti's reciprocal theorem with a reciprocity gap associated with nonlinear materials. We express the unknown normal and shear tractions along the interface in terms of the Fourier series, and use specially constructed analytical auxiliary fields in the reciprocal theorem to extract the unknown Fourier coefficients from far-field data; the reciprocity gap in the formulation is iteratively determined with a set of numerical algorithms. Our detailed numerical experiments demonstrate that this nonlinear field projection method (NFPM) is well-suited for extracting the interfacial tractions from the far-field data of any nonlinear elastic or elasto-plastic material with known constitutive laws. Applications of the NFPM to experiments and atomistic simulations are discussed.

Chew, Huck Beng

2013-01-01

76

Dynamic fracture mechanics  

NASA Technical Reports Server (NTRS)

Dynamic fracture and crack propagation concepts for ductile materials are reviewed. The equations for calculating dynamic stress integrity and the dynamic energy release rate in order to study dynamic crack propagation are provided. The stress intensity factor versus crack velocity relation is investigated. The uses of optical experimental techniques and finite element methods for fracture analyses are described. The fracture criteria for a rapidly propagating crack under mixed mode conditions are discussed; crack extension and fracture criteria under combined tension and shear loading are based on maximum circumferential stress or energy criteria such as strain energy density. The development and use of a Dugdale model and finite element models to represent crack and fracture dynamics are examined.

Kobayashi, A. S.; Ramulu, M.

1985-01-01

77

The frictional sliding response of elasto-plastic materials in contact with a conical indenter  

Microsoft Academic Search

Over the past decade, many computational studies have explored the mechanics of normal indentation. Quantitative relationships have been well established between the load–displacement hysteresis response and material properties. By contrast, very few studies have investigated broad quantitative aspects of the effects of material properties, especially plastic deformation characteristics, on the frictional sliding response of metals and alloys. The response to

S. Bellemare; M. Dao; S. Suresh

2007-01-01

78

Elasto-plastic analysis of a mode I edge crack with application to a surface notch.  

NASA Technical Reports Server (NTRS)

A finite element analysis has been undertaken of the mechanical state of a plate containing a crack subjected to mode I loading. Specifically, a localized, well-defined surface depression (dimple) previously observed on the back surface of plate specimens immediately behind the plane of the crack was examined. The approach taken is concerned with defining more precisely the dimpling behavior as a function of relative flaw depth and applied stress level, as well as the development of the plastic zone at the crack tip.

Francis, P. H.

1971-01-01

79

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

80

Two-temperature hydrodynamics of laser-generated ultrashort shock waves in elasto-plastic solids  

NASA Astrophysics Data System (ADS)

Shock-wave generation by ultrashort laser pulses opens new doors for study of hidden processes in materials happened at an atomic-scale spatiotemporal scales. The poorly explored mechanism of shock generation is started from a short-living two-temperature (2T) state of solid in a thin surface layer where laser energy is deposited. Such 2T state represents a highly non-equilibrium warm dense matter having cold ions and hot electrons with temperatures of 1-2 orders of magnitude higher than the melting point. Here for the first time we present results obtained by our new hybrid hydrodynamics code combining detailed description of 2T states with a model of elasticity together with a wide-range equation of state of solid. New hydro-code has higher accuracy in the 2T stage than molecular dynamics method, because it includes electron related phenomena including thermal conduction, electron-ion collisions and energy transfer, and electron pressure. From the other hand the new code significantly improves our previous version of 2T hydrodynamics model, because now it is capable of reproducing the elastic compression waves, which may have an imprint of supersonic melting like as in MD simulations. With help of the new code we have solved a difficult problem of thermal and dynamic coupling of a molten layer with an uniaxially compressed elastic solid. This approach allows us to describe the recent femtosecond laser experiments.

Ilnitsky, Denis K.; Khokhlov, Viktor A.; Inogamov, Nail A.; Zhakhovsky, Vasily V.; Petrov, Yurii V.; Khishchenko, Konstantin V.; Migdal, Kirill P.; Anisimov, Sergey I.

2014-05-01

81

Contact Problems for Elasto-Plastic Impact in Multi-Body Systems  

NASA Astrophysics Data System (ADS)

Low to moderate speed collision between elastic-plastic bodies results in imperceptible permanent indentation of the contact surfaces if the bodies are hard. Nevertheless in these collisions the contact force-indentation relation is irreversible since internal energy gained from work done by the contact force during the compression is partially trapped in elastic waves, work done in plastic deformation and work done to overcome friction during sliding. These sources of kinetic energy l oss in collisions depend on relative velocity between the bodies at the contact point, material properties, inertia properties related to the impact configuration and geometric constraints on the deformation field. Colliding bodies that can be represented as elastic-plastic or visco-plastic solids have been analyzed using specific models of contact compliance to calculate the energy transformed into irrecoverable forms. These calculations show how elastic waves, plastic deformation and friction affect the energetic coefficient of restitution - a coefficient that is a measure of impact energy loss from internal sources. The calculations indicate that there is considerable difference in the sources of energy loss f or 2D and 3D deformations. Also, that friction little affects the plastic energy losses unless the impact speed is large enough to cause uncontained plastic deformation beneath the contact area. Consequently, for low-speed impact rhonu_3^2 (0) / Y , the energetic coefficient of friction e* is insensitive to friction. In analysing impact of multi-body systems (e.g. mechanisms, kinematic chains or agglomerates of granules), it is crucial to employ these ideas by specifically modelling the contact compliance. During impact on a mechanism, the velocity changes in one area of contact induce small displacements that develop at other compliant contacts where the impacted body is supported or connected to other elements; the force that develops at these secondary contacts (as a consequence of relative displacement at each i ndividual contact) is the means of transmitting the impact process through system. Although the global compliance of elements of the system may be small enough so that vibration energy is negligible, the dynamics of multi-body collisions requires consideration of local compliance in each contact regions. Previously impact on a system of interconnected bodies has been analyzed as either a sequence of separate collisions or a set of simultaneous collisions. In general neither of these assumptions gives an accurate representation of the dynamic behaviour of multi-body systems. Rather, it is necessary to model the compliance of each contact and consider the contact forces which develop since it is these forces which prevent interpenetration or overlap. When applied to impact on multi-body systems such as me chanisms, the impulse-momentum methods used in rigid body dynamics give but one limit of the range of response - a range that depends on the distribution of local compliance at each contact between bodies in the system. An accurate analysis of multi-body system response to impact generally requires consideration of the time-dependent contact forces in a wave of reaction that propagates away from the initial site of impact.

Stronge, W. J.

82

PLAN2D - A PROGRAM FOR ELASTO-PLASTIC ANALYSIS OF PLANAR FRAMES  

NASA Technical Reports Server (NTRS)

PLAN2D is a FORTRAN computer program for the plastic analysis of planar rigid frame structures. Given a structure and loading pattern as input, PLAN2D calculates the ultimate load that the structure can sustain before collapse. Element moments and plastic hinge rotations are calculated for the ultimate load. The location of hinges required for a collapse mechanism to form are also determined. The program proceeds in an iterative series of linear elastic analyses. After each iteration the resulting elastic moments in each member are compared to the reserve plastic moment capacity of that member. The member or members that have moments closest to their reserve capacity will determine the minimum load factor and the site where the next hinge is to be inserted. Next, hinges are inserted and the structural stiffness matrix is reformulated. This cycle is repeated until the structure becomes unstable. At this point the ultimate collapse load is calculated by accumulating the minimum load factor from each previous iteration and multiplying them by the original input loads. PLAN2D is based on the program STAN, originally written by Dr. E.L. Wilson at U.C. Berkeley. PLAN2D has several limitations: 1) Although PLAN2D will detect unloading of hinges it does not contain the capability to remove hinges; 2) PLAN2D does not allow the user to input different positive and negative moment capacities and 3) PLAN2D does not consider the interaction between axial and plastic moment capacity. Axial yielding and buckling is ignored as is the reduction in moment capacity due to axial load. PLAN2D is written in FORTRAN and is machine independent. It has been tested on an IBM PC and a DEC MicroVAX. The program was developed in 1988.

Lawrence, C.

1994-01-01

83

Modelling the graphite fracture mechanisms  

SciTech Connect

In order to define a design criterion for graphite components, it is important to identify the physical phenomena responsible for the graphite fracture, to include them in a more effective modelling. In a first step, a large panel of experiments have been realised in order to build up an important database; results of tensile tests, 3 and 4 point bending tests on smooth and notched specimens have been analysed and have demonstrated an important geometry related effects on the behavior up to fracture. Then, first simulations with an elastic or an elastoplastic bilinear constitutive law have not made it possible to simulate the experimental fracture stress variations with the specimen geometry, the fracture mechanisms of the graphite being at the microstructural scale. That is the reason why a specific F.E. model of the graphite structure has been developed in which every graphite grain has been meshed independently, the crack initiation along the basal plane of the particles as well as the crack propagation and coalescence have been modelled too. This specific model has been used to test two different approaches for fracture initiation: a critical stress criterion and two criteria of fracture mechanic type. They are all based on crystallographic considerations as a global critical stress criterion gave unsatisfactory results. The criteria of fracture mechanic type being extremely unstable and unable to represent the graphite global behaviour up to the final collapse, the critical stress criterion has been preferred to predict the results of the large range of available experiments, on both smooth and notched specimens. In so doing, the experimental observations have been correctly simulated: the geometry related effects on the experimental fracture stress dispersion, the specimen volume effects on the macroscopic fracture stress and the crack propagation at a constant stress intensity factor. In addition, the parameters of the criterion have been related to experimental observations: the local crack initiation stress of 8 MPa corresponds to the non-linearity apparition on the global behavior observed experimentally and the the maximal critical stress defined for the particle of 30 MPa is equivalent to the fracture stress of notched specimens. This innovative combination of crack modelling and a local crystallographic critical stress criterion made it possible to understand that cleavage initiation and propagation in the graphite microstructure was driven by a mean critical stress criterion. (authors)

Jacquemoud, C.; Marie, S. [CEA Saclay, Gif-sur-Yvette, (France); Nedelec, M. [IRSN/PSN-EXP/SES, Fontenay-aux-Roses, (France)

2012-07-01

84

Fracture mechanics and corrosion fatigue.  

NASA Technical Reports Server (NTRS)

Review of the current state-of-the-art in fracture mechanics, particularly in relation to the study of problems in environment-enhanced fatigue crack growth. The usefulness of this approach in developing understanding of the mechanisms for environmental embrittlement and its engineering utility are discussed. After a brief review of the evolution of the fracture mechanics approach and the study of environmental effects on the fatigue behavior of materials, a study is made of the response of materials to fatigue and corrosion fatigue, the modeling of the mechanisms of the fatigue process is considered, and the application of knowledge of fatigue crack growth to the prediction of the high cycle life of unnotched specimens is illustrated.

Mcevily, A. J.; Wei, R. P.

1972-01-01

85

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

86

UK Conference on Computational Mechanics (ACME-UK) Modelling Elasto-plasticity with the Hybrid Meshless Scaled Boundary  

E-print Network

charles.augarde@durham.ac.uk www.dur.ac.uk/charles.augarde Key Words: Meshless or Meshfree Methods} - i [N2 (x, y)]T [N1 (s)] di {usb} = {fm} . (1) A similar result can be found for the scaled (s)]T [N1 (s)]di {usb}+ i [N2 (s)]T [T][D][B1 (x, y)] - [N1 (x, y)] di {^um} = {fsb} . (2) The shape

Augarde, Charles

87

(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

88

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

89

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

E-print Network

Section 3: Damage and fracture mechanics 1 Section 3: Damage and fracture mechanics Organizers ­ effect of distributed crazing Abstract S3.2: Continuum damage mechanics and phase field models Tue, 16 in elastically stressed alloys Abstract #12;2 Section 3: Damage and fracture mechanics 17:20 ­ 17:40: Sebastian

Kohlenbach, Ulrich

90

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

E-print Network

Section 3: Damage and fracture mechanics 1 Section 3: Damage and fracture mechanics Organizers the strain field of existing displacement based, as- #12;2 Section 3: Damage and fracture mechanics sumed to use a continuum damage approach for the continuous behavior up to the critical stress-strain states

Kohlenbach, Ulrich

91

Contact mechanics at nanometric scale using nanoindentation technique for brittle and ductile materials.  

PubMed

In the last years, Nanoindentation or Instrumented Indentation Technique has become a powerful tool to study the mechanical properties at micro/nanometric scale (commonly known as hardness, elastic modulus and the stress-strain curve). In this review, the different contact mechanisms (elastic and elasto-plastic) are discussed, the recent patents for each mechanism (elastic and elasto-plastic) are summarized in detail, and the basic equations employed to know the mechanical behaviour for brittle and ductile materials are described. PMID:22455731

Roa, J J; Rayon, E; Morales, M; Segarra, M

2012-06-01

92

FINITE FRACTURE MECHANICS OF MATRIX MICROCRACKING IN COMPOSITES  

E-print Network

FINITE FRACTURE MECHANICS OF MATRIX MICROCRACKING IN COMPOSITES JOHN A. NAIRN INTRODUCTION damage following complex loading conditions. This chapter describes a fracture mechanics approach to the microcracking problem. A complicating feature of composite fracture mechanics analysis is that laminates often

Nairn, John A.

93

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

94

Fracture mechanics of functionally graded materials  

Microsoft Academic Search

In this paper, after a brief discussion of the elementary concepts of fracture mechanics in nonhomogeneous materials, a number of typical problem areas relating to the fracture of functionally gradient materials (FGMs) are identified. The main topics considered are the investigation of the nature of stress singularity near the tip of a crack fully embedded in a nonhomogeneous medium, the

F. Erdogan

1995-01-01

95

Mechanisms for shrinkage fracturing at Meridiani Planum  

NASA Astrophysics Data System (ADS)

We investigate the role of water in fracturing at Meridiani Planum with the aim of shedding light on the history of densely-fractured outcroppings of light-toned rocks at low-latitudes on Mars. The fractures that occur throughout the inter-crater plains at Meridiani exhibit many characteristics of shrinkage cracks: they have significant width (i.e., not hairline), commonly connect in 90-degree and 120-degree junctions, and exhibit a "hierarchical" organization: i.e., the longest fractures are widest, and narrower fractures terminate against wider fractures at 90-degree junctions (T-shaped). Using the Pancam and Navcam stereo-pair images acquired by the Opportunity rover, we have measured the geometric scaling of fracture networks at Meridiani (e.g., fracture width vs. fracture separation) as well as the total volume change. We have also characterized the diversity of patterns in detail, as well as the modification of fractures and polygonal "tiles" by wind-blown sand abrasion. Identical observations were carried-out for an analogue site where similar fractures are ubiquitous in the playas of Death Valley, California, and where modification processes are also comparable. By also estimating the expected volume change and results from numerical models of shrinkage fracturing, we evaluate the likelihood of three candidate contraction mechanisms: loss of water bound in hydrated minerals (dehydration), loss of water from pore spaces (desiccation), and contraction from cooling (thermal fracturing). The evidence to date favors the second of these (desiccation); this result would have significant implications for the history of Meridiani since the time when sulfate-rich sediments were deposited.

Watters, W. A.; Squyres, S. W.

2009-12-01

96

A Hierarchical Approach to Fracture Mechanics  

NASA Technical Reports Server (NTRS)

Recent research conducted under NASA LaRC's Creativity and Innovation Program has led to the development of an initial approach for a hierarchical fracture mechanics. This methodology unites failure mechanisms occurring at different length scales and provides a framework for a physics-based theory of fracture. At the nanoscale, parametric molecular dynamic simulations are used to compute the energy associated with atomic level failure mechanisms. This information is used in a mesoscale percolation model of defect coalescence to obtain statistics of fracture paths and energies through Monte Carlo simulations. The mathematical structure of predicted crack paths is described using concepts of fractal geometry. The non-integer fractal dimension relates geometric and energy measures between meso- and macroscales. For illustration, a fractal-based continuum strain energy release rate is derived for inter- and transgranular fracture in polycrystalline metals.

Saether, Erik; Taasan, Shlomo

2004-01-01

97

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

98

Mechanisms of fracture of sea ice  

Microsoft Academic Search

To extend fracture mechanics-based solutions to the floe splitting problem requires a more realistic description of the problem if one is to provide information useful for design ice load calculations. The crushing phase is often stable and develops with initially partial and then total stress-relaxation in the vicinity of the structure. In some particular cases, radial crack formation and crack

John P. Dempsey; Samuel J. DeFranco; Denis Blanchet; Anton Prodanovic

1994-01-01

99

Application of infrared thermography to fracture mechanics  

Microsoft Academic Search

We have studied the possibility of applying infrared thermography to the problems of fracture mechanics. First of all we have shown that the stress intensity factor, K, can be analyzed by measuring the distribution of the elastic component of the sum of the principal stress around the crack tip by an infrared stress measuring device, SPATE- 8000. Next, the increase

Nagahisa Ogasawara; Masaki Shiratori

1997-01-01

100

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

101

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

102

Dependence of fracture mechanical and fluid flow properties on fracture roughness and sample size  

Microsoft Academic Search

A parameter study has been carried out to investigate the interdependence of mechanical and fluid flow properties of fractures with fracture roughness and sample size. A rough fracture can be defined mathematically in terms of its aperture density distribution. Correlations were found between the shapes of the aperture density distribution function and the specific fractures of the stress-strain behavior and

Y. W. Tsang; P. A. Witherspoon

1983-01-01

103

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

E-print Network

6. Fracture mechanics lead author: J, R. Rice Division of Applied Sciences, Harvard University. F. Shih, and the ASME/AMD Technical Committee on Fracture Mechanics, pro- vided by A. S. Argon, S. N, W. D. Stuart, and R. Thomson. 6.0 ABSTRACT Fracture mechanics is an active research field

104

Multiple-code benchmark simulation study of coupled THMC processes in the excavation disturbed zone associated with geological nuclear waste repositories  

E-print Network

fracturing using the boundary element method (FRACOM) isboundary element, finite element, finite difference, particle mechanics, and cellular automata methods (boundary element, finite element, and finite difference, particle mechanics, and elasto-plastic cellular automata methods.

2006-01-01

105

LOCAL CRITERIA OF BRITTLE AND DUCTILE FRACTURE AND APPLICATION OF LOCAL APPROACH TO FRACTURE MECHANICS PROBLEMS  

Microsoft Academic Search

New local criteria of brittle and ductile fracture are considered. Models based on the proposed local criteria for prediction of critical fracture mechanics parameters are presented. These models are as follows: (i) a probabilistic model for prediction of the temperature dependence of brittle fracture toughness, named now the Prometey model; (ii) a model for prediction of JR-curves; (iii) a model

B. Z. MARGOLIN; A. G. GULENKO; V. I. KOSTYLEV; V. A. SHVETSOVA

106

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

107

Mechanics of fracture - Fundamentals and some recent developments  

NASA Technical Reports Server (NTRS)

An overview is presented of the fundamental aspects of and recent developments in fracture mechanics. Reference is made to linear elastic fracture mechanics including the state of stresses and displacements in the vicinity of cracks, effects of crack geometry and orientation on stress intensity factors, energy balance of Griffith, Irwin's stress intensity concept, and linear elastic fracture mechanics testing for fracture toughness. Other aspects of this paper include the non-linear behavior of materials and their influence on fracture mechanics parameters, consideration of viscoelasticity and plasticity, non-linear fracture toughness parameters as C.O.D., R-curve and J-integral, and a non-linear energy method, proposed by Liebowitz. Finite element methods applied to fracture mechanics problems are indicated. Also, consideration has been given to slow crack growth, dynamic effects on K(IC), Sih's criterion for fracture, Lee and Liebowitz's criterion relating crack growth with plastic energy, and applications of fracture mechanics to aircraft design. Suggestions are offered for future research efforts to be undertaken in fracture mechanics.

Liebowitz, H.; Subramonian, N.; Lee, J. D.

1979-01-01

108

Patterns and perspectives in applied fracture mechanics  

SciTech Connect

This lecture begins with a overview of applied fracture mechanics pertinent to safety of pressure vessels. It then progresses to a chronological panorama of experimental and analytical results. To be useful and dependable in safety analysis of real structures, new analysis developments must be physically realistic, which means that they must accurately describe physical cause and effect. Consequently, before mathematical modeling can begin, cause and effect must be established from experimental data. This can be difficult and time consuming, but worth the effort. Accordingly, the theme of this paper is that the search for patterns is constant and vital. This theme is illustrated by the development of small, single-specimen, fracture toughness testing techniques. It is also illustrated by the development, based on two different published large-strain, elastic-plastic, three-dimensional finite-element analyses, of a hypothesis concerning three-dimensional loss of constraint. When a generalization of Irwin`s thickness-normalized plastic-zone parameter, reaches a value close to 2{pi}, the through-thickness contraction strain at the apex of the near-tip logarithmic-spiral slip-line region becomes the dominant negative strain accommodating crack opening. Because slip lines passing from the midplane to the stress-free side surfaces do not have to curve, once these slip lines are established, stresses near the crack tip are only elevated by strain hardening and constraint becomes significantly relaxed. This hypothesis, based on published three-dimensional elastic-plastic analyses, provides a potentially valuable means for gaining additional insight into constraint effects on fracture toughness by considering the roles played by the plastic strains as well as the stresses that develop near a crack tip.

Merkle, J.G.

1994-12-31

109

Fundamental mechanisms of fatigue and fracture.  

PubMed

A brief overview is given in this article on the main design philosophies and the resulting description concepts used for components which undergo monotonic and cyclic loading. Emphasis is put on a mechanistic approach avoiding a plain reproduction of empirical laws. After a short consideration of fracture as a result of monotonic loading using fracture mechanics basics, the phenomena taking place as a consequence of cyclic plasticity are introduced. The development of fatigue damage is treated by introducing the physical processes which (i) are responsible for microstructural changes, (ii) lead to crack initiation and (iii) determine crack propagation. From the current research topics within the area of metal fatigue, two aspects are dealt with in more detail because of their relevance to biomechanics. The first one is the growth behaviour of microstructural short cracks, which controls cyclic life of smooth parts at low stress amplitudes. The second issue addresses the question of the existence of a true fatigue limit and is of particular interest for components which must sustain a very high number of loading cycles (very high cycle fatigue). PMID:18376013

Christ, Hans-Jürgen

2008-01-01

110

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 in aquifers overlying the Marcellus formation but asserts that we prematurely ascribed its cause to hydraulic mechanisms were leaky gas well casings and the possibility that hydraulic fracturing might generate new

Jackson, Robert B.

111

A Fracture-Mechanics-Based Approach to Fracture Control in Biomedical Devices Manufactured From Superelastic Nitinol Tube  

E-print Network

A Fracture-Mechanics-Based Approach to Fracture Control in Biomedical Devices Manufactured From: 10.1002/jbm.b.30840 Abstract: Several key fracture-mechanics parameters associated with the onset of subcritical and critical cracking, specifically the fracture toughness, crack-resistance curve, and fatigue

Ritchie, Robert

112

Breakdown of Continuum Fracture Mechanics at the Nanoscale  

PubMed Central

Materials fail by the nucleation and propagation of a crack, the critical condition of which is quantitatively described by fracture mechanics that uses an intensity of singular stress field characteristically formed near the crack-tip. However, the continuum assumption basing fracture mechanics obscures the prediction of failure of materials at the nanoscale due to discreteness of atoms. Here, we demonstrate the ultimate dimensional limit of fracture mechanics at the nanoscale, where only a small number of atoms are included in a singular field of continuum stress formed near a crack tip. Surprisingly, a singular stress field of only several nanometers still governs fracture as successfully as that at the macroscale, whereas both the stress intensity factor and the energy release rate fail to describe fracture below a critically confined singular field of 2–3?nm, i.e., breakdown of fracture mechanics within the framework of the continuum theory. We further propose an energy-based theory that explicitly accounts for the discrete nature of atoms, and demonstrate that our theory not only successfully describes fracture even below the critical size but also seamlessly connects the atomic to macroscales. It thus provides a more universal fracture criterion, and novel atomistic insights into fracture. PMID:25716684

Shimada, Takahiro; Ouchi, Kenji; Chihara, Yuu; Kitamura, Takayuki

2015-01-01

113

Breakdown of continuum fracture mechanics at the nanoscale.  

PubMed

Materials fail by the nucleation and propagation of a crack, the critical condition of which is quantitatively described by fracture mechanics that uses an intensity of singular stress field characteristically formed near the crack-tip. However, the continuum assumption basing fracture mechanics obscures the prediction of failure of materials at the nanoscale due to discreteness of atoms. Here, we demonstrate the ultimate dimensional limit of fracture mechanics at the nanoscale, where only a small number of atoms are included in a singular field of continuum stress formed near a crack tip. Surprisingly, a singular stress field of only several nanometers still governs fracture as successfully as that at the macroscale, whereas both the stress intensity factor and the energy release rate fail to describe fracture below a critically confined singular field of 2-3?nm, i.e., breakdown of fracture mechanics within the framework of the continuum theory. We further propose an energy-based theory that explicitly accounts for the discrete nature of atoms, and demonstrate that our theory not only successfully describes fracture even below the critical size but also seamlessly connects the atomic to macroscales. It thus provides a more universal fracture criterion, and novel atomistic insights into fracture. PMID:25716684

Shimada, Takahiro; Ouchi, Kenji; Chihara, Yuu; Kitamura, Takayuki

2015-01-01

114

Breakdown of Continuum Fracture Mechanics at the Nanoscale  

NASA Astrophysics Data System (ADS)

Materials fail by the nucleation and propagation of a crack, the critical condition of which is quantitatively described by fracture mechanics that uses an intensity of singular stress field characteristically formed near the crack-tip. However, the continuum assumption basing fracture mechanics obscures the prediction of failure of materials at the nanoscale due to discreteness of atoms. Here, we demonstrate the ultimate dimensional limit of fracture mechanics at the nanoscale, where only a small number of atoms are included in a singular field of continuum stress formed near a crack tip. Surprisingly, a singular stress field of only several nanometers still governs fracture as successfully as that at the macroscale, whereas both the stress intensity factor and the energy release rate fail to describe fracture below a critically confined singular field of 2-3 nm, i.e., breakdown of fracture mechanics within the framework of the continuum theory. We further propose an energy-based theory that explicitly accounts for the discrete nature of atoms, and demonstrate that our theory not only successfully describes fracture even below the critical size but also seamlessly connects the atomic to macroscales. It thus provides a more universal fracture criterion, and novel atomistic insights into fracture.

Shimada, Takahiro; Ouchi, Kenji; Chihara, Yuu; Kitamura, Takayuki

2015-02-01

115

Compressive fracture morphology and mechanism of metallic glass  

NASA Astrophysics Data System (ADS)

We quantitatively investigated the fracture morphologies of Zr52.5Cu17.9Ni14.6Al10Ti5 and Pd78Cu6Si16 metallic glasses (MGs) under compression. The characteristic features of the compressive fracture morphology were captured, and the shear vein patterns were found to be not a one-to-one correspondence between two opposing fracture surfaces in an identical sample. This finding experimentally confirms that the compressive failure behaves in a fracture mode of pure shear (mode II). Quantitative measurements show that a ˜1 ?m thickness layer with materials not only inside but also adjacent to the major shear band contributes to the formation of shear vein patterns. The critical shear strain to break a shear band was found to be more than 105% and higher in more ductile MGs under compression than tension. Estimation on the temperature rise at the fracture moment indicates that only ˜5% of the total elastic energy stored in the sample converts into the heat required for melting the layer to form the vein patterns. The mode II fracture toughness was also estimated based on the quantitative measurements of shear vein pattern and found larger than the mode I fracture toughness. Finally, the deformation and fracture mechanisms of MGs under tension and compression were compared and discussed. These results may improve the understanding on the fracture behaviors and mechanisms of MGs and may provide instructions on future design for ductile MGs with high resistance for fracture.

Qu, R. T.; Zhang, Z. F.

2013-11-01

116

Proceedings of the fifteenth national symposium on fracture mechanics  

Microsoft Academic Search

This book compiles the papers presented at a conference on the subject of fracture mechanics. Some of the chapter's titles are: An elastic-plastic finite-element analysis of crack initiation, stable crack growth, and instability; recent advances in crack-arrest-technology; failure assessment-approach for handling combined thermomechanical loading; effect-of Fast-Neutron irradiation on fracture toughness of alloy A-286; microstructural aspects of fracture toughness cleavage fibrous

Sanford

1984-01-01

117

Numerical prediction of slant fracture with continuum damage mechanics  

Microsoft Academic Search

Ductile specimens always exhibit an inclined fracture surface with an angle relative to the loading axis. This paper reports a numerical study on the cup–cone fracture mode in round bar tensile tests and the slant fracture in plane-strain specimens based on continuum damage mechanics. A combined implicit–explicit numerical scheme is first developed within ABAQUS through user defined material subroutines, in

X. Teng

2008-01-01

118

Fracture Mechanics for Composites: State of the Art and Challenges  

NASA Technical Reports Server (NTRS)

Interlaminar fracture mechanics has proven useful for characterizing the onset of delaminations in composites and has been used with limited success primarily to investigate onset in fracture toughness specimens and laboratory size coupon type specimens. Future acceptance of the methodology by industry and certification authorities however, requires the successful demonstration of the methodology on the structural level. In this paper, the state-of-the-art in fracture toughness characterization, and interlaminar fracture mechanics analysis tools are described. To demonstrate the application on the structural level, a panel was selected which is reinforced with stringers. Full implementation of interlaminar fracture mechanics in design however remains a challenge and requires a continuing development effort of codes to calculate energy release rates and advancements in delamination onset and growth criteria under mixed mode conditions.

Krueger, Ronald; Krueger, Ronald

2006-01-01

119

Integration of NDE Reliability and Fracture Mechanics  

SciTech Connect

The Pacific Northwest Laboratory is conducting a four-phase program for measuring and evaluating the effectiveness and reliability of in-service inspection (lSI} performed on the primary system piping welds of commercial light water reactors (LWRs). Phase I of the program is complete. A survey was made of the state of practice for ultrasonic rsr of LWR primary system piping welds. Fracture mechanics calculations were made to establish required nondestrutive testing sensitivities. In general, it was found that fatigue flaws less than 25% of wall thickness would not grow to failure within an inspection interval of 10 years. However, in some cases failure could occur considerably faster. Statistical methods for predicting and measuring the effectiveness and reliability of lSI were developed and will be applied in the "Round Robin Inspections" of Phase II. Methods were also developed for the production of flaws typical of those found in service. Samples fabricated by these methods wilI be used in Phase II to test inspection effectiveness and reliability. Measurements were made of the influence of flaw characteristics {i.e., roughness, tightness, and orientation) on inspection reliability. These measurernents, as well as the predictions of a statistical model for inspection reliability, indicate that current reporting and recording sensitivities are inadequate.

Becker, F. L.; Doctor, S. R.; Heas!er, P. G.; Morris, C. J.; Pitman, S. G.; Selby, G. P.; Simonen, F. A.

1981-03-01

120

To mechanics of deformation, flow, and fracture  

E-print Network

It is stated in the main in essence new approach to mechanics of the stressed state of the solid body from statistically isotropic material and the homogeneous liquid dynamics. The approach essence is in the detected property of the core-shell spontaneous structurization of internal energy of the solid and liquid bodies in its natural state and under action of external forces. The method elements of construction of physically adequate model of the stressed state of the solid and liquid bodies, reproduced exactly its behavior on the stages of elastic and plastic deformation, flow and fracture, are stated. It is adduced a number of the examples of the stressed state construction of the simple form bodies under action of its tension, compression, torsion and at its contact interaction. For the first time it is adduced structure of the principal - normal - stresses in cylindrical bar under action of the torsion moment. The detected property and the developed method is one of necessary bases for construction of physically adequate mathematical model of the stressed state of the body and fluid in contrast to traditional approach.

S. L. Arsenjev

2008-09-23

121

Chemical and Mechanical Alteration of Fractured Caprock Under Reactive Flow  

NASA Astrophysics Data System (ADS)

Permeability evolution of fractures depends on chemical and mechanical processes. Stress perturbations lead to mechanical deformation and fracture propagation that can increase formation permeability. Chemical disequilibrium between fluids and resident minerals leads to dissolution and precipitation that further alter fracture porosity and permeability. The ability to predict whether these coupled chemical and mechanical processes will enhance or diminish fracture permeability remains elusive. Here, we present results from reactive-transport experiments in fractured anhydrite cores, with significant alteration of the rock matrix, where only the flow rate differed. For high flow rate, the transformation of anhydrite to gypsum occurred uniformly within the fracture leading to compaction and a two-order-of-magnitude decrease in permeability. For low flow rate, rock-fluid reactions proceeded to near equilibrium within the fracture with preferential flow paths persisting over the 6-month duration of the experiment and a negligible change in permeability. Anticipating such permeability evolution is critical for successful geologic CO2 sequestration and waste injection. Additionally, reactive alteration of the porous matrix bounding fractures will influence the strength of earthquake fault zones. Comparison of the aperture field before (a) and after (b) the reactive flow-through experiment at low flow rate. a) Aperture field from optical profilometry measurements of the fracture surfaces. b) Inferred aperture from x-ray computed tomography scans. Color scale I (blue) denotes mainly unaltered regions of the fracture and/or aperture < 200 ?m. Color scale II (green/yellow) denotes reacted regions of the fracture surfaces and the matrix adjacent to the fracture. Persistent flow paths are clearly observed in panel (b) (color scale III corresponds to aperture > 200 ?m) leading to negligible change in permeability after a 6-month run.

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

2013-12-01

122

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

123

Linear electro-elastic fracture mechanics of piezoelectric materials  

Microsoft Academic Search

The concepts of linear elastic fracture mechanics, generalized to treat piezoelectric effects, are employed to study the influence of the electrical fields on the fracture behavior of piezoelectric materials. The method of distributed dislocations and electric dipoles, already existing in the literature, is used to calculate the electro-elastic fields and the energy-release rate for a finite crack embedded in an

Y. E. Pak

1992-01-01

124

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

125

Structure, Mechanical Properties and Fracture Behavior of Organosilicate Glass  

E-print Network

of the requirements for the degree of Doctor of Philosophy in the subject of Engineering Sciences Harvard University on the mechanical properties and fracture behavior. We will show that the composition and structure of OSG can

126

“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

127

Elastic-plastic fracture mechanics technology  

NASA Technical Reports Server (NTRS)

Among the topics discussed are: fracture analysis methods evaluation; instability prediction using the K sub R-curve approach; and the deformation failure assessment diagram. Consideration is also given to: instability prediction based on the modified J, J sub M-resistance curve approach; and stable crack growth and instability prediction on the V by means of the V sub R-curve method.

Newman, J. C., Jr. (editor); Loss, F. J. (editor)

1985-01-01

128

Elastic-plastic fracture mechanics of compact bone  

NASA Astrophysics Data System (ADS)

Bone is a composite composed mainly of organics, minerals and water. Most studies on the fracture toughness of bone have been conducted at room temperature. Considering that the body temperature of animals is higher than room temperature, and that bone has a high volumetric percentage of organics (generally, 35--50%), the effect of temperature on fracture toughness of bone should be studied. Single-edged V-shaped notched (SEVN) specimens were prepared to measure the fracture toughness of bovine femur and manatee rib in water at 0, 10, 23, 37 and 50°C. The fracture toughness of bovine femur and manatee rib were found to decrease from 7.0 to 4.3 MPa·m1/2 and from 5.5 to 4.1 MPa·m1/2, respectively, over a temperature range of 50°C. The decreases were attributed to inability of the organics to sustain greater stresses at higher temperatures. We studied the effects of water and organics on fracture toughness of bone using water-free and organics-free SEVN specimens at 23°C. Water-free and organics-free specimens were obtained by placing fresh bone specimen in a furnace at different temperatures. Water and organics significantly affected the fracture toughness of bone. Fracture toughness of the water-free specimens was 44.7% (bovine femur) and 32.4% (manatee rib) less than that of fresh-bone specimens. Fracture toughness of the organics-free specimens was 92.7% (bovine femur) and 91.5% (manatee rib) less than that of fresh bone specimens. Linear Elastic Fracture Mechanics (LEFM) is widely used to study bone. However, bone often has small to moderate scale yielding during testing. We used J integral, an elastic-plastic fracture-mechanics parameter, to study the fracture process of bone. The J integral of bovine femur increased from 6.3 KJ/mm2 at 23°C to 6.7 KJ/mm2 at 37°C. Although the fracture toughness of bovine bone decreases as the temperature increases, the J integral results show a contrary trend. The energy spent in advancing the crack beyond the linear-elastic deformation was much greater than the energy spent in linear-elastic deformation. This could be because bone has at least four toughening mechanisms and a high volumetric percentage of organics (approximately 42% for bovine femur). The J integral is shown to better describe the fracture process of bovine femur and manatee rib.

Yan, Jiahau

129

Mechanical testing - In situ fracture device for Auger electron spectroscopy  

NASA Technical Reports Server (NTRS)

An in situ fracture device for Auger spectroscopy is 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 are presented. Results indicate that impurity segregation at interfaces other than grain boundary may play a significant role in the mechanism of ductile fracture.

Moorhead, R. D.

1976-01-01

130

Modeling elastic tensile fractures in snow using nonlocal damage mechanics  

NASA Astrophysics Data System (ADS)

The initiation and propagation of tensile fractures in snow and ice are fundamental to numerous important physical processes in the cryosphere, from iceberg calving to ice shelf rift propagation to slab avalanche release. The heterogeneous nature of snow and ice, their proximity to the melting temperature, and the varied governing timescales typically lead to nonlinear fracture behavior which does not follow the predictions of Linear Elastic Fracture Mechanics (LEFM). Furthermore, traditional fracture mechanics is formally inapplicable for predicting crack initiation in the absence of a pre-existing flaw or stress concentration. An alternative to fracture mechanics is continuum damage mechanics, which accounts for the material degradation associated with cracking in a numerically efficient framework. However, damage models which are formulated locally (e.g. stress and strain are defined as point properties) suffer from mesh-sensitive crack trajectories, spurious localization of damage and improper fracture energy dissipation with mesh refinement. Nonlocal formulations of damage, which smear the effects of the material heterogeneity over an intrinsic length scale related to the material microstructure, overcome these difficulties and lead to numerically efficient and mesh-objective simulations of the tensile failure of heterogeneous materials. We present the results of numerical simulations of tensile fracture initiation and propagation in cohesive snow using a nonlocal damage model. Seventeen beam bending experiments, both notched and unnotched, were conducted using blocks of cohesive dry snow extracted from an alpine snowpack. Material properties and fracture parameters were calculated from the experimental data using beam theory and quasi-brittle fracture mechanics. Using these parameters, a nonlocal isotropic damage model was applied to two-dimensional finite element meshes of the same scale as the experiments. The model was capable of simulating the propagation of a tensile crack from an existing stress concentration as well as the initiation of a crack from a smooth boundary using the same model parameters and boundary conditions. Sensitivity analyses were conducted on the most uncertain model parameters. For the optimally paramaterized model, the simulated load-displacement curves agreed well with the experimental data, with the primary discrepancy related to the loss of elastic stability following peak load in the experiments. The spatial distribution of strain and damage is shown to support a quasi-brittle interpretation of the fracture physics for both crack initiation and crack propagation problems. These results provide a foundation for future predictive modeling applications related to the tensile fractures which release slab avalanches. We conclude by discussing the applicability of the nonlocal damage approach to a viscous or viscoelastic framework for simulating iceberg calving and the initiation and propagation of ice shelf rifts.

Borstad, C. P.; McClung, D. M.

2011-12-01

131

Fracture mechanics of monolayer molybdenum disulfide.  

PubMed

Molecular dynamics (MD) modeling is used to study the fracture toughness and crack propagation path of monolayer molybdenum disulfide (MoS2) sheets under mixed modes I and II loading. Sheets with both initial armchair and zigzag cracks are studied. The MD simulations predict that crack edge chirality, tip configuration and the loading phase angle influence the fracture toughness and crack propagation path of monolayer MoS2 sheets. Furthermore, under all loading conditions, both armchair and zigzag cracks prefer to extend along a zigzag path, which is in agreement with the crack propagation path in graphene. A remarkable out-of-plane buckling can occur during mixed mode loading which can lead to the development of buckling cracks in addition to the propagation of the initial cracks. PMID:25834943

Wang, Xiaonan; Tabarraei, Alireza; Spearot, Douglas E

2015-05-01

132

RSRM nozzle actuator bracket/lug fracture mechanics qualification test  

NASA Astrophysics Data System (ADS)

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

1993-07-01

133

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

134

Fracture mechanics and its relevance to botanical structures.  

PubMed

In the field of fracture mechanics, an analytical framework has been established for understanding the mechanical failure of any structure made of inherently flawed materials. In the context of botany, this includes an extraordinarily wide variety of turgid and/or woody structures made of cellulose-based tissues, the diverse soils penetrated by their roots, and a multitude of plant-based commodities and foodstuffs. The goal of this article is to provide an overview of the theory of engineering fracture mechanics and to identify some special characteristics of wood and other plant-based materials that require further development in this area. PMID:21642092

Farquhar, Tony; Zhao, Yong

2006-10-01

135

Fracture mechanics analysis of vertical root fracture from condensation of gutta-percha.  

PubMed

A two-dimensional fracture mechanics analysis of vertical root fracture (VRF) in single-canal roots from apical condensation of gutta-percha (gp) is developed. The resulting analytic relation for apical load causing VRF agrees with major trends reported in in-vitro tests on roots subjected to either continuous or, the more clinically relevant, repeating vertical condensation of gp. The model explicitly exposes the role of root canal morphology and dentin fracture toughness on VRF. Ovoid and irregular canals are prone to fracture while the effect of mean root canal radius is modest. Canal taper and instrumentation details may affect VRF only marginally and indirectly. The model predicts dentinal cracks to occur following root canal instrumentation and obturation, which may pose long-term threats to tooth integrity. PMID:22503579

Chai, Herzl; Tamse, Aviad

2012-06-01

136

Experimental unsaturated soil mechanics  

E-print Network

In this general report, experimental systems and procedures of investigating the hydro-mechanical behaviour of unsaturated soils are presented. The water retention properties of unsaturated soils are commented and linked to various physical parameters and properties of the soils. Techniques of controlling suction are described together with their adaptation in various laboratory testing devices. Some typical features of the mechanical behaviour of unsaturated soils are presented within an elasto-plastic framework. An attempt to describe the numerous and significant recent advances in the investigation of the behaviour of unsaturated soils, including the contributions to this Conference, is proposed.

Delage, Pierre

2008-01-01

137

From damage to fracture mechanics and conversely: A combined approach  

Microsoft Academic Search

Fracture mechanics and damage mechanics are two correlated theories. In some instances, e.g., for large specimens, crack propagation may be viewed equivalently as a sudden localization of damage. Relationships based on thermodynamic considerations between the two theories are presented in this paper. They lead to the definition of the equivalent crack concept, in passing from a damage zone to a

Jacky Mazars; Gilles Pijaudier-Cabot

1996-01-01

138

Statistical fracture mechanics approach to the strength of brittle rock  

SciTech Connect

Statistical fracture mechanics concepts used in the past for rock are critically reviewed and modifications are proposed which are warranted by (1) increased understanding of fracture provided by modern fracture mechanics and (2) laboratory test data both from the literature and from this research. Over 600 direct and indirect tension tests have been performed on three different rock types; Stripa Granite, Sierra White Granite and Carrara Marble. In several instances assumptions which are common in the literature were found to be invalid. A three parameter statistical fracture mechanics model with Mode I critical strain energy release rate as the variant is presented. Methodologies for evaluating the parameters in this model as well as the more commonly employed two parameter models are discussed. The experimental results and analysis of this research indicate that surfacially distributed flaws, rather than volumetrically distributed flaws are responsible for rupture in many testing situations. For several of the rock types tested, anisotropy (both in apparent tensile strength and size effect) precludes the use of contemporary statistical fracture mechanics models.

Ratigan, J.L.

1981-06-01

139

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

140

Fracture mechanics criteria for turbine engine hot section components  

NASA Technical Reports Server (NTRS)

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

Meyers, G. J.

1982-01-01

141

Relating Cohesive Zone Model to Linear Elastic Fracture Mechanics  

NASA Technical Reports Server (NTRS)

The conditions required for a cohesive zone model (CZM) to predict a failure load of a cracked structure similar to that obtained by a linear elastic fracture mechanics (LEFM) analysis are investigated in this paper. This study clarifies why many different phenomenological cohesive laws can produce similar fracture predictions. Analytical results for five cohesive zone models are obtained, using five different cohesive laws that have the same cohesive work rate (CWR-area under the traction-separation curve) but different maximum tractions. The effect of the maximum traction on the predicted cohesive zone length and the remote applied load at fracture is presented. Similar to the small scale yielding condition for an LEFM analysis to be valid. the cohesive zone length also needs to be much smaller than the crack length. This is a necessary condition for a CZM to obtain a fracture prediction equivalent to an LEFM result.

Wang, John T.

2010-01-01

142

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

143

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

144

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

145

Algorithms for elasto-plastic-creep postbuckling  

NASA Technical Reports Server (NTRS)

This paper considers the development of an improved constrained time stepping scheme which can efficiently and stably handle the pre-post-buckling behavior of general structure subject to high temperature environments. Due to the generality of the scheme, the combined influence of elastic-plastic behavior can be handled in addition to time dependent creep effects. This includes structural problems exhibiting indefinite tangent properties. To illustrate the capability of the procedure, several benchmark problems employing finite element analyses are presented. These demonstrate the numerical efficiency and stability of the scheme. Additionally, the potential influence of complex creep histories on the buckling characteristics is considered.

Padovan, J.; Tovichakchaikul, S.

1984-01-01

146

The Elasto-Plastic Stability of Plates  

NASA Technical Reports Server (NTRS)

This article explains results developed from the following research: 'The Stability of Plates and Shells beyond the Elastic Limit.' A significant improvement is found in the derivation of the relations between the stress factors and the strains resulting from the instability of plates and shells. In a strict analysis, the problem reduces to the solution of two simultaneous nonlinear partial differential equations of the fourth order in the deflection and stress function, and in the approximate analysis to a single linear equation of the Bryan type. Solutions are given for the special cases of a rectangular plate buckling into a cylindrical form, and of an arbitrarily shaped plate under uniform compression. These solutions indicate that the accuracy obtained by the approximate method is satisfactory.

Ilyushin, A. A.

1947-01-01

147

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

148

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

149

3-D thermo-mechanical laboratory modeling of plate-tectonics: modeling scheme, technique and first experiments  

Microsoft Academic Search

We present an experimental apparatus for 3-D thermo-mechanical analogue modeling of plate tectonic processes such as oceanic and continental subductions, arc-continent or continental collisions. The model lithosphere, made of temperature-sensitive elasto-plastic analogue materials with strain softening, is submitted to a constant temperature gradient causing a strength reduction with depth in each layer. The surface temperature is imposed using infrared emitters,

D. Boutelier; O. Oncken

2011-01-01

150

Fracture mechanics: proceedings of the 12th national symposium on fracture mechanics  

SciTech Connect

The conference proceedings contains 30 papers of which five are abstracted separately. The subjects covered include fatigue crack growth in aircraft materials, fractographic measurements, fatigue cracks in nylon 66 blends, cyclic inelastic deformation aspects, prestressing, tensile cracks in creeping solids, creep-crack-growth in 304 stainless steel, high-temperature fatigue, parallel impact loading, numerical fracture dynamic code, J-resistance curves, specimen geometry effects, reactor piping systems, temperature dependence of fracture toughness, small-scale yielding, compact specimens, power hardening materials, semi-empirical fracture analyses, pipeline girth welds, wrought steels, and A36 bridge steels.

Not Available

1980-01-01

151

Mechanics of tidally driven fractures in Europa's ice shell  

Microsoft Academic Search

A fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden. It is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on Europa. Such penetration is found

Sunwoong Lee; Robert T. Pappalardo; Nicholas C. Makris

2005-01-01

152

Mechanical fatigue and fracture of Nitinol S. W. Robertson*1  

E-print Network

Mechanical fatigue and fracture of Nitinol S. W. Robertson*1 , A. R. Pelton1 and R. O. Ritchie2 Nitinol, a near equiatomic intermetallic of nickel and titanium, is the most widely known and used shape transformation, Nitinol displays recoverable strains that are more than an order of magnitude greater than

Ritchie, Robert

153

The Biomechanics of Chewing and Plant Fracture: Mechanisms and Implications  

Microsoft Academic Search

Herbivores that chew on the leaves of flowering plants have had to evolve mechanisms to propagate fractures through structures that are chemically and physically heterogeneous. Associations exist between mandibular morphology and associated musculature and the nature of the plant diet, with species that feed on grasses sharing a suite of characteristics that differ from the features found in dicot-feeders. This

Fiona J. Clissold

2007-01-01

154

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

155

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

156

Elastic plastic fracture mechanics methodology for surface cracks  

Microsoft Academic Search

The Elastic Plastic Fracture Mechanics Methodology has evolved significantly in the last several years. Nevertheless, some of these concepts need to be extended further before the whole methodology can be safely applied to structural parts. Specifically, there is a need to include the effect of constraint in the characterization of material resistance to crack growth and also to extend these

Hugo A. Ernst; D. M. Lambert

1994-01-01

157

Elastic plastic fracture mechanics methodology for surface cracks  

Microsoft Academic Search

The Elastic Plastic Fracture Mechanics (EPFM) Methodology has evolved significantly in the last several years. Nevertheless, some of these concepts need to be extended further before the whole methodology can be safely applied to structural parts. Specifically, there is a need to include the effect of constraint in the characterization of material resistance to crack growth and also to extend

Hugo A. Ernst; D. W. Boatwright; W. J. Curtin; D. M. Lambert

1993-01-01

158

FEA Based Tool Life Quantity Estimation of Hot Forging Dies Under Cyclic Thermo-Mechanical Loads  

NASA Astrophysics Data System (ADS)

Hot forging dies are exposed during service to a combination of cyclic thermo-mechanical, tribological and chemical loads. Besides abrasive and adhesive wear on the die surface, fatigue crack initiation with subsequent fracture is one of the most frequent causes of failure. In order to extend the tool life, the finite element analysis (FEA) may serve as a means for process design and process optimisation. So far the FEA based estimation of the production cycles until initial cracking is limited as tool material behaviour due to repeated loading is not captured with the required accuracy. Material models which are able to account for cyclic effects are not verified for the fatigue life predictions of forging dies. Furthermore fatigue properties from strain controlled fatigue tests of relevant hot work steels are to date not available to allow for a close-to-reality fatigue life prediction. Two industrial forging processes, where clear fatigue crack initiation has been observed are considered for a fatigue analysis. For this purpose the relevant tool components are modelled with elasto-plastic material behaviour. The predicted sites, where crack initiation occurs, agree with the ones observed on the real die component.

Behrens, B.-A.; Bouguecha, A.; Schäfer, F.; Hadifi, T.

2011-01-01

159

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

160

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

161

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

162

The fracture properties and toughening mechanisms of bone and dentin  

NASA Astrophysics Data System (ADS)

The mechanical properties of bone and dentin and in particular their fracture properties, are the subject of intense research. The relevance of these properties is increasing as our population ages and fracture incidence impacts the lives of a greater portion of the population. A robust framework is needed to understand the fracture properties of bone and dentin to guide researchers as they attempt to characterize the effects of aging, disease, and pharmaceutical treatments on the properties of these mineralized tissues. In the present work, this framework is provided and applied to human bone, human dentin, and animal bone. In situ electron microscopy was also used to identify the salient toughening mechanisms in bone and dentin. It was found that bone and dentin are extrinsically toughened materials and consequently their fracture properties are best characterized utilizing a crack-growth resistance approach. A description of the different mechanical measurements commonly employed when using small animal models (rats and mice) to evaluate the influence of drug therapies on bone fragility is provided. A study where these properties were measured for a large population of wild-type rats and mice was also conducted. Given my findings, it was determined that for the most complete understanding of small animal bone it was necessary to measure strength and toughness. Strength measurements probe the flaw distribution and toughness measurements to evaluate the resistance to facture in the presence of a single dominant worst-case flaw.

Koester, Kurt John

163

Physical and mechanical modelling of neutron irradiation effect on ductile fracture. Part 1. Prediction of fracture strain and fracture toughness of austenitic steels  

NASA Astrophysics Data System (ADS)

A physical-and-mechanical model of ductile fracture has been developed to predict fracture toughness and fracture strain of irradiated austenitic steels taking into account stress-state triaxiality and radiation swelling. The model is based on criterion of plastic collapse of a material unit cell controlled by strain hardening of a material and criterion of voids coalescence due to channel shearing of voids. The model takes into account deformation voids nucleation and growth of deformation and vacancy voids. For justification of the model experimental data on fracture strain and fracture toughness of austenitic steel 18Cr-10Ni-Ti grade irradiated up to maximal dose 150 dpa with various swelling were used. Experimental data on fracture strain and fracture toughness were compared with the results predicted by the model. It has been shown that for prediction of the swelling effect on fracture toughness the dependence of process zone size on swelling should be taken into account.

Margolin, Boris; Sorokin, Alexander; Smirnov, Valeriy; Potapova, Vera

2014-09-01

164

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

E-print Network

amplitude fatigue limit. First of all, fracture mechanics models are established based on linear elastic fracture mechanics. Two methods are employed to compute the crack growth and fatigue life under variable amplitude fatigue long life loading...

Zhou, Minjian

1993-01-01

165

Fracture mechanics analysis of composite microcracking - Experimental results in fatigue  

NASA Technical Reports Server (NTRS)

The Nairn (1989) variational mechanics analysis, which yields the energy release rate of a microcrack's formation between two existing microcracks, has proven useful in the fracture mechanics interpretation of cross-ply laminates' microcracking. Attention is presently given to the application of this energy release rate analysis to a fracture mechanics-based interpretation of microcrack formation during fatigue loading, for the case of fatigue experiments on three layups of Avimid K/IM6 laminates and four layups of Fiberite 934/T300 laminates. The single master Paris-law plot onto which the data from all layups of a given material system fall is claimed to offer a complete characterization of that system's microcrack-formation resistance during fatigue loading.

Nairn, J. A.; Liu, S.

1990-01-01

166

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

167

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

168

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

169

Fracture mechanisms in monolithic structural ceramics and ceramic composites  

SciTech Connect

The fracture of three structurally distinct materials was characterized at test temperatures ranging from 20 through 1400{degree}C. Two monolithic, polycrystalline ceramic materials, one a high-purity transparent MgAl{sub 2}O{sub 4} spinel and the other a sintered Si{sub 3}N{sub 4} containing a grain-boundary, second-phase, sintering additive, provided for a comparison of the fracture mechanisms as they are governed by the differing microstructures. The third, a SiC whisker-reinforced, polycrystalline Al{sub 2}O{sub 3} matrix composite offered the added mechanism of a high-strength, high-modulus load-carrying element, not available in conventional, monolithic ceramic microstructures. The high-purity MgAl{sub 2}O{sub 4} spinel was found to maintain a constant fracture toughness of approximately 1.7 MPa{center dot}m{sup {1/2}} through 600{degree}C and then experienced only a slight decrease above this temperature to about 1.3 MPa{center dot}{sup {1/2}}. By comparison, the fracture toughness of the Si{sub 3}N{sub 4} decreased from nearly 8.0 MPa{center dot}m{sup {1/2}} at room temperature, to below 2.0 MPa{center dot}{sup {1/2}} 1400{degree}C, as governed by the character of the grain boundary glass phase. The SiC whisker/Al{sub 2}O{sub 3} composite was characterized by a fracture toughness value of about 6.8 MPa{center dot}{sup 1/2} throughout the test temperature range.

White, K.W.

1987-01-01

170

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

171

State-of-the-art report on piping fracture mechanics  

SciTech Connect

This report is an in-depth summary of the state-of-the-art in nuclear piping fracture mechanics. It represents the culmination of 20 years of work done primarily in the US, but also attempts to include important aspects from other international efforts. Although the focus of this work was for the nuclear industry, the technology is also applicable in many cases to fossil plants, petrochemical/refinery plants, and the oil and gas industry. In compiling this detailed summary report, all of the equations and details of the analysis procedure or experimental results are not necessarily included. Rather, the report describes the important aspects and limitations, tells the reader where he can go for further information, and more importantly, describes the accuracy of the models. Nevertheless, the report still contains over 150 equations and over 400 references. The main sections of this report describe: (1) the evolution of piping fracture mechanics history relative to the developments of the nuclear industry, (2) technical developments in stress analyses, material property aspects, and fracture mechanics analyses, (3) unresolved issues and technically evolving areas, and (4) a summary of conclusions of major developments to date.

Wilkowski, G.M.; Olson, R.J.; Scott, P.M. [Battelle, Columbus, OH (United States)

1998-01-01

172

Wide-range displacement expressions for standard fracture mechanics specimens  

NASA Technical Reports Server (NTRS)

Wide-range algebraic expressions for the displacement of cracked fracture mechanics specimens are developed. For each specimen two equations are given: one for the displacement as a function of crack length, the other for crack length as a function of displacement. All the specimens that appear in ASTM Test for Plane-Strain Fracture Toughness of Metallic Materials (E 399) are represented in addition to the crack mouth displacement for a pure bending specimen. For the compact tension sample and the disk-shaped compact tension sample, the displacement at the crack mouth and at the load line are both considered. Only the crack mouth displacements for the arc-shaped tension samples are presented. The agreement between the displacements or crack lengths predicted by the various equations and the corresponding numerical data from which they were developed are nominally about 3 percent or better. These expressions should be useful in all types of fracture testing including fracture toughness, K-resistance, and fatigue crack growth.

Kapp, J. A.; Gross, B.; Leger, G. S.

1985-01-01

173

Mechanical transport and porous media equivalence in anisotropic fracture networks  

SciTech Connect

The objective of this work is to investigate the directional characteristics of hydraulic effective porosity in an effort to understand porous medium equivalence for continuous and discontinuous fracture systems. Continuous systems contain infinitely long fractures. Discontinuous systems consist of fractures with finite lengths. The distribution of apertures (heterogeneity) has a major influence on the degree of porous medium equivalence for distributed continuous and discontinuous systems. When the aperture distribution is narrow, the hydraulic effective porosity is slightly less than the total porosity for continuous systems, and greater than the rock effective porosity for discontinuous systems. However, when heterogeneity is significant, the hydraulic effective porosity is directionally dependent and greater than total porosity for both systems. Non-porous medium behavior ws found to differ for distributed continuous systems and for continuous systems with parallel sets. For the latter systems, hydraulic effective porosity abruptly decreases below total porosity in those particular directions where the hydraulic gradient and the orientation of a fracture set are orthogonal. The results for the continuous systems with parallel sets also demonstrate that a system that behaves like a continuum for fluid flux may not behave like a continuum for mechanical transport. 3 references, 13 figures.

Endo, H.K.; Witherspoon, P.A.

1985-01-01

174

Fracture and Stress Evolution on Europa: New Insights Into Fracture Interpretation and Ice Thickness Estimates Using Fracture Mechanics Analyses  

NASA Technical Reports Server (NTRS)

The work completed during the funding period has provided many important insights into fracturing behavior in Europa's ice shell. It has been determined that fracturing through time is likely to have been controlled by the effects of nonsynchronous rotation stresses and that as much as 720 deg of said rotation may have occurred during the visible geologic history. It has been determined that there are at least two distinct styles of strike-slip faulting and that their mutual evolutionary styles are likely to have been different, with one involving a significant dilational component during shear motion. It has been determined that secondary fracturing in perturbed stress fields adjacent to older structures such as faults is a prevalent process on Europa. It has been determined that cycloidal ridges are likely to experience shear stresses along the existing segment portions as they propagate, which affects propagation direction and ultimately induces tailcracking at the segment tip than then initiates a new cycle of cycloid segment growth. Finally, it has been established that mechanical methods (e.g., flexure analysis) can be used to determine the elastic thickness of the ice shell, which, although probably only several km thick, is likely to be spatially variable, being thinner under bands but thicker under ridged plains terrain.

Kattenhorn, Simon

2004-01-01

175

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

176

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

177

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

178

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

179

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

180

Toughened semicrystalline engineering polymers: Morphology, impact resistance, and fracture mechanisms  

SciTech Connect

Major factors that contribute to maximized impact resistance of semicrystalline engineering polymers include increased matrix molecular weight, minimal rubber glass-transition temperature, and optimal rubber-particle size, which depends on matrix type. Additional constraints are posed by the limitations of available materials and commercial requirements. Expanding the understanding of impact-modified crystalline engineering polymers will further extend their property ranges and utility. This chapter describes the phenomenology of certain factors, illustrates a new technique to measure rubber particle size, and contrasts the fracture mechanisms of poly(butylene terephthalate), nylon 66, and polyacetal. 49 refs., 24 figs.

Flexman, E.A. [E.I. du Pont de Nemours and Co., Wilmington, DE (United States)

1993-12-31

181

(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

182

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

E-print Network

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

Segatti, Antonio

183

Size and Temperature Effects on the Fracture Mechanisms of Silicon Nanowires: Molecular Dynamics  

E-print Network

Size and Temperature Effects on the Fracture Mechanisms of Silicon Nanowires: Molecular Dynamics. The fracture behavior of the NWs depends both on temperature and NW diameter. For NWs of di- ameter larger than 4 nm, cleavage fracture on the transverse (1 1 0) plane are predominantly observed at temperatures

Cai, Wei

184

Z .Mechanics of Materials 28 1998 247262 Experimental and computational study of fracturing in an  

E-print Network

Z .Mechanics of Materials 28 1998 247­262 Experimental and computational study of fracturing, stress- and energy-based fracture criteria lead to fairly similar results. Theoretical studies show the fracture path along different cleavage planes. The experimental observations are compared

Nemat-Nasser, Sia

185

A three-dimensional stochastic rock mechanics model of engineered geothermal systems in fractured crystalline rock  

Microsoft Academic Search

A three-dimensional (3-D) stochastic network model for simulating a hot dry rock (HDR) or hot wet rock (HWR) engineered geothermal system formed in fractured crystalline rock is presented. The model addresses the problems of fracture network characterization from in situ field data, such as fracture orientation, size, spacing, and other mechanical properties. The model can simulate the changes that occur

Z. Jing; J. Willis-Richards; K. Watanabe; T. Hashida

2000-01-01

186

Fluid flow and structural response modeling associated with the mechanics of hydraulic fracturing  

SciTech Connect

A review of hydraulic-fracture modeling is given. Equations governing pertinent fluid-flow, structural, and fracture-mechanics responses are presented. The finite-element method is used to discretize the field equations and to compute the fracture dimensions, fluid leakoff, and stress intensity factors. In addition, the effects of fracture-fluid properties, layered strata, and in-situ stresses are characterized, and numerical examples are presented.

Advani, S.H.; Lee, J.K.; Khattab, H.A.; Gurdogan, O.

1986-06-01

187

NASGRO(registered trademark): Fracture Mechanics and Fatigue Crack Growth Analysis Software  

NASA Technical Reports Server (NTRS)

This viewgraph presentation describes NASGRO, which is a fracture mechanics and fatigue crack growth analysis software package that is used to reduce risk of fracture in Space Shuttles. The contents include: 1) Consequences of Fracture; 2) NASA Fracture Control Requirements; 3) NASGRO Reduces Risk; 4) NASGRO Use Inside NASA; 5) NASGRO Components: Crack Growth Module; 6) NASGRO Components:Material Property Module; 7) Typical NASGRO analysis: Crack growth or component life calculation; and 8) NASGRO Sample Application: Orbiter feedline flowliner crack analysis.

Forman, Royce; Shivakumar, V.; Mettu, Sambi; Beek, Joachim; Williams, Leonard; Yeh, Feng; McClung, Craig; Cardinal, Joe

2004-01-01

188

Coupled Flow and Mechanics in Porous and Fractured Media*  

NASA Astrophysics Data System (ADS)

Numerical models describing subsurface flow through deformable porous materials are important for understanding and enabling energy security and climate security. Some applications of current interest come from such diverse areas as geologic sequestration of anthropogenic CO2, hydro-fracturing for stimulation of hydrocarbon reservoirs, and modeling electrochemistry-induced swelling of fluid-filled porous electrodes. Induced stress fields in any of these applications can lead to structural failure and fracture. The ultimate goal of this research is to model evolving faults and fracture networks and flow within the networks while coupling to flow and mechanics within the intact porous structure. We report here on a new computational capability for coupling of multiphase porous flow with geomechanics including assessment of over-pressure-induced structural damage. The geomechanics is coupled to the flow via the variation in the fluid pore pressures, whereas the flow problem is coupled to mechanics by the concomitant material strains which alter the pore volume (porosity field) and hence the permeability field. For linear elastic solid mechanics a monolithic coupling strategy is utilized. For nonlinear elastic/plastic and fractured media, a segregated coupling is presented. To facilitate coupling with disparate flow and mechanics time scales, the coupling strategy allows for different time steps in the flow solve compared to the mechanics solve. If time steps are synchronized, the controller allows user-specified intra-time-step iterations. The iterative coupling is dynamically controlled based on a norm measuring the degree of variation in the deformed porosity. The model is applied for evaluation of the integrity of jointed caprock systems during CO2 sequestration operations. Creation or reactivation of joints can lead to enhanced pathways for leakage. Similarly, over-pressures can induce flow along faults. Fluid flow rates in fractures are strongly dependent on the effective hydraulic aperture, which is a non-linear function of effective normal stress. The dynamically evolving aperture field updates the effective, anisotropic permeability tensor, thus resulting in a highly coupled multiphysics problem. Two models of geomechanical damage are discussed: critical shear-slip criteria and a sub-grid joint model. Leakage rates through the caprock resulting from the joint model are compared to those assuming intact material, allowing a correlation between potential for leakage and injection rates/pressures, for various in-situ stratigraphies. *This material is based upon work supported as part of 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. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energys National Nuclear Security Administration under Contract DE-AC04-94AL85000.

Martinez, M. J.; Newell, P.; Bishop, J.

2012-12-01

189

Experimental and theoretical fracture mechanics applied to Antarctic ice fracture and surface crevassing  

Microsoft Academic Search

Recent disintegration of ice shelves on the Antarctic Peninsula has highlighted the need for a better understanding of ice shelf fracture processes generally. In this paper we present a fracture criterion, incorporating new experimental fracture data, coupled with an ice shelf flow model to predict the spatial distribution of surface crevassing on the Filchner-Ronne Ice Shelf. We have developed experiments

M. A. Rist; P. R. Sammonds; S. A. F. Murrell; P. G. Meredith; C. S. M. Doake; H. Oerter; K. Matsuki

1999-01-01

190

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

191

Laser micromachining for fatigue and fracture mechanics applications  

NASA Astrophysics Data System (ADS)

A laser micromachining (LMM) method to initiate flaws for fatigue and fracture mechanics applications is successfully demonstrated. Dynamic response of moving energy pulses during LMM of titanium alloy (Ti-3.5Al-2.5V) was numerically simulated by an integrated energy approach using temperature-dependent thermophysical properties and 3D heat transfer code. Stress and strain analyses were performed for a titanium tube of 9.53 mm outer diameter (OD) and 0.81 mm wall thickness (WT) with a 0.23 mm deep and 1.83-mm-long longitudinal laser micro-machined notch, using nonlinear finite element analysis (FEA). For comparison, an electric-discharge-machined (EDM) notched tube with the same notch profile as the laser-prepared tube was also investigated. The calculated hoop stress and strain amplitudes at the notch root of the EDM-prepared tube were approximately 64% and 63% of the stress and strain amplitudes in the laser-prepared tube, respectively, when two tubes were subjected to inner pressures for R ratio of 0.03 and ? P=45, 50, and 55 MPa. Fatigue life due to crack initiation process can be minimized using LMM method. The described LMM method is, therefore, more appropriate than EDM for accomplishing flaw formation to study fatigue and fracture behavior of various materials.

Gupta, M. C.; Li, B.; Gadag, S.; Chou, K. C.

2010-04-01

192

Elastic plastic fracture mechanics methodology for surface cracks  

NASA Technical Reports Server (NTRS)

The Elastic Plastic Fracture Mechanics (EPFM) Methodology has evolved significantly in the last several years. Nevertheless, some of these concepts need to be extended further before the whole methodology can be safely applied to structural parts. Specifically, there is a need to include the effect of constraint in the characterization of material resistance to crack growth and also to extend these methods to the case of 3D defects. As a consequence, this project was started as a 36 month research program with the general objective of developing an EPFM methodology to assess the structural reliability of pressure vessels and other parts of interest to NASA containing defects. This report covers a computer modelling algorithm used to simulate the growth of a semi-elliptical surface crack; the presentation of a finite element investigation that compared the theoretical (HRR) stress field to that produced by elastic and elastic-plastic models; and experimental efforts to characterize three dimensional aspects of fracture present in 'two dimensional', or planar configuration specimens.

Ernst, Hugo A.; Boatwright, D. W.; Curtin, W. J.; Lambert, D. M.

1993-01-01

193

Fracture Mechanics Analysis of LH2 Feed Line Flow Liners  

NASA Technical Reports Server (NTRS)

Inspections of the Space Shuttle Main Engine revealed fatigue cracks growing from slots in the flow liner of the liquid hydrogen (LH2) feed lines. During flight, the flow liners experience complex loading induced by flow of LH2 and the resonance characteristics of the structure. The flow liners are made of Inconel 718 and had previously not been considered a fracture critical component. However, fatigue failure of a flow liner could have catastrophic effect on the Shuttle engines. A fracture mechanics study was performed to determine if a damage tolerance approach to life management was possible and to determine the sensitivity to the load spectra, material properties, and crack size. The load spectra were derived separately from ground tests and material properties were obtained from coupon tests. The stress-intensity factors for the fatigue cracks were determined from a shell-dynamics approach that simulated the dominant resonant frequencies. Life predictions were obtained using the NASGRO life prediction code. The results indicated that adequate life could not be demonstrated for initial crack lengths of the size that could be detected by traditional NDE techniques.

James, Mark A.; Dawicke, David S.; Brzowski, Matthew B.; Raju, Ivatury S.; Elliott, Kenny B.; Harris, Charles E.

2006-01-01

194

Hermite interpolation algorithm for 2D fracture mechanics  

NASA Astrophysics Data System (ADS)

For boundary element fracture analysis hypersingular equations provide an efficient alternative to the multidomain method. This is especially true if the goal is to simulate crack growth, as remeshing for a multidomain calculation then becomes a major problem. For a simple nodal collocation approximation, the numerical implementation of the hypersingular equation must use a differentiable interpolation of the boundary displacement function. This smoothness requirement is not present in standard boundary integral methods. Alternative approaches to Overhauser elements are of interest, and herein a two dimensional Hermite interpolation algorithm for fracture mechanics is presented. In this approach, a C sup 1 interpolation is achieved by explicitly incorporating the nodal tangential derivatives as additional variables in the approximation. These extra unknowns are determined by using the tangential derivative of the boundary integral equation. For a crack problem however, there are not enough equations to solve for the derivatives on both sides of the crack. In the method presented, the extra equations are obtained by collocating at the midpoint of each crack element.

Potyondy, D. O.; Ingraffea, A. R.; Gray, L. J.

1992-03-01

195

Fracture mechanics; Proceedings of the Seventeenth National Symposium, Albany, NY, August 7-9, 1984  

NASA Technical Reports Server (NTRS)

The present conference gives attention to topics in the application of fracture mechanics, subcritical crack growth phenomena, fracture testing methods, ductile fracture behavior, and fracture mechanisms and their analysis. Specific papers treat the resistance curve approach to composite materials characterization, fracture toughness in ductile iron and cast steel, hold-time effects in elevated temperature fatigue crack propagation, creep crack growth under nonsteady conditions, viscoplastic fatigue in a superalloy at elevated temperatures, fracture testing with arc bend specimens, one-point bend impact test application, and a compact mode II fracture specimen. Also discussed are the computation of stable crack growth using the J-integral, the use of plastic energy dissipation to characterize crack growth, the extension of surface cracks under cyclic loading, the minimum time criterion for crack instability in structural materials, dynamic crack propagation and branching under biaxial loading, and boundary layer effects in cracked bodies.

Underwood, J. M. (editor); Chait, R. (editor); Smith, C. W. (editor); Wilhem, D. P. (editor); Andrews, W. A. (editor); Newman, J. C. (editor)

1986-01-01

196

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

197

Microstructural fracture mechanics in high-cycle fatigue  

SciTech Connect

Microstructural Fracture Mechanics principles are used to develop a model of crack growth in long life fatigue. In its simplest form microstructural modelling considers the material as a polycrystal of uniform grain size D, with a crack system divided into three zones: the crack, the plastic zone and the microstructural barrier zone. The solution of the equilibrium equation allows for the calculation of the stresses sustained by the crack wake, plastic zone, barrier zone and elastic enclave, and the crack tip plastic displacement {phi}. Crack growth rate is calculated through a Paris type relationship in terms of {phi}, i.e., da/dN = C{phi}{sup n}. Conditions for crack arrest and instability are established.

Rios, E.R. de los [Univ. of Sheffield (United Kingdom). Dept. of Mechanical Engineering; Navarro, A. [ETSII, Sevilla (Spain). Dept. De Ingenieria Mecanica

1997-12-31

198

Fracture mechanics concepts in reliability analysis of monolithic ceramics  

NASA Technical Reports Server (NTRS)

Basic design concepts for high-performance, monolithic ceramic structural components are addressed. The design of brittle ceramics differs from that of ductile metals because of the inability of ceramic materials to redistribute high local stresses caused by inherent flaws. Random flaw size and orientation requires that a probabilistic analysis be performed in order to determine component reliability. The current trend in probabilistic analysis is to combine linear elastic fracture mechanics concepts with the two parameter Weibull distribution function to predict component reliability under multiaxial stress states. Nondestructive evaluation supports this analytical effort by supplying data during verification testing. It can also help to determine statistical parameters which describe the material strength variation, in particular the material threshold strength (the third Weibull parameter), which in the past was often taken as zero for simplicity.

Manderscheid, Jane M.; Gyekenyesi, John P.

1987-01-01

199

Simple spline-function equations for fracture mechanics calculations  

NASA Technical Reports Server (NTRS)

The paper presents simple spline-function equations for fracture mechanics calculations. A spline function is a sequence of piecewise polynomials of degree n greater than 1 whose coefficients are such that the function and its first n-1 derivatives are continuous. Second-degree spline equations are presented for the compact, three point bend, and crack-line wedge-loaded specimens. Some expressions can be used directly, so that for a cyclic crack propagation test using a compact specimen, the equation given allows the cracklength to be calculated from the slope of the load-displacement curve. For an R-curve test, equations allow the crack length and stress intensity factor to be calculated from the displacement and the displacement ratio.

Orange, T. W.

1979-01-01

200

A fracture-mechanics-based approach to fracture control in biomedical devices manufactured from superelastic Nitinol tube.  

PubMed

Several key fracture-mechanics parameters associated with the onset of subcritical and critical cracking, specifically the fracture toughness, crack-resistance curve, and fatigue threshold, have recently been reported for the superelastic alloy Nitinol, in the product form of the thin-walled tube that is used to manufacture several biomedical devices, most notably endovascular stents. In this study, we use these critical parameters to construct simple decision criteria for assessing the quantitative effect of crack-like defects in such Nitinol devices with respect to their resistance to failure by deformation or fracture. The criteria are based on the (equivalent) crack-initiation fracture toughness and fatigue threshold stress-intensity range, together with the general yield strength and fatigue endurance strength, and are used to construct a basis for design against single-event (overload) failures as well as for time-/cycle-delayed failures associated with fatigue. PMID:17477387

Robertson, S W; Ritchie, R O

2008-01-01

201

A BEM analysis of fracture mechanics in 2D anisotropic piezoelectric solids  

E-print Network

A BEM analysis of fracture mechanics in 2D anisotropic piezoelectric solids Ernian Pan* Department-domain boundary element method (BEM) analysis of fracture mechanics in 2D anisotropic piezoelectric solids¹3/2 ). 1998 Elsevier Science Ltd. All rights reserved Keywords: Boundary element method; Dual BEM

Pan, Ernie

202

A finite element analysis of continuum damage mechanics for ductile fracture  

Microsoft Academic Search

A finite element formulation of an anisotropic theory of continuum damage mechanics for ductile fracture is presented. The formulation is based on a generalized model of anisotropic continuum damage mechanics of elasticity and plasticity proposed earlier by the authors. The validity of the proposed anisotropic damage model and finite element formulation is verified by comparing the predicted fracture load of

C. L. Chow; June Wang

1988-01-01

203

Experimental and theoretical fracture mechanics applied to fracture of the crust of Venus  

Microsoft Academic Search

Mapping of closely spaced, parallel extensional fractures in the Guinevere and Sedna Planitia regions of Venus reveals a concentric pattern of fractures around the edge of the large topographic rise of Western Eistla Regio. We have constructed 13 transects through these closely spaced parallel fractures (CSPF) and find a mean spacing of between 0.8 and 1.2 km. A two-dimensional, nonlayered,

Matthew R. Balme; Peter R. Sammonds; Claudio Vita-Finzi; Jonathon P. Couchman

2004-01-01

204

Effect of thickness on fracture criterion in general yielding fracture mechanics  

Microsoft Academic Search

In this research work, the effect of thickness on fracture criterion is studied for extra deep drawn (EDD) steel sheets. Experimental\\u000a results are generated on fracture toughness of EDD steel sheets using compact tension specimens and a ‘maximum load’ as a\\u000a fracture criterion. Critical crack tip opening displacement (CTOD) is found with the help of three methods: plastic hinge\\u000a model

D. M. Kulkarni; Vikas Chaudhari; Ravi Prakash; A. N. Kumar

2008-01-01

205

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

206

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

207

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, thereby avoiding frictional effects. A F/M interface toughness test based on this configuration would provide data for K(sub I)/K(sub II) ratios of about 0.7 and 1.6 for fiber and radial normal loading, respectively. However, thermal effects must be accounted for in the data analysis.

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

1991-01-01

208

Mechanisms of fast fracture and arrest in steels  

Microsoft Academic Search

Studies of the unstable propagation and arrest of brittle fractures were conducted on four steels: plain carbon steel, 3 pct\\u000a Si steel, A-517, and 4340. Unstable fractures were initiated in double-cantilever-beam test specimens by forcing a wedge between\\u000a the two beams under compression. These fractures propagate at essentially constant wedge opening displacement and can be made\\u000a to arrest within the

R. G. Hoagland; A. R. Rosenfield; G. T. Hahn

1972-01-01

209

Application of elastic-plastic fracture mechanics to marine structures  

E-print Network

, dynamic and qussistatic tensile data are used in conjunction with Charpy data and the theoretical model to predict crack tip opening displacement (CTOD) transition curves. Tensile, Charpy, and fracture toughness tests were conducted on two ship steels...: A131 EH36 steel and HSLA 80 steel. The tensile tests were con- ducted at three strain rates; 0. 0033, 5. 1 and 280 s '. Most of the Charpy and fracture toughness testing was concentrated in the transition region of each steel. Fracture toughness...

Pathi, Amarkumar

1991-01-01

210

Damage mechanics approach to remove the constraint dependence of elastic-plastic fracture toughness  

Microsoft Academic Search

It is now generally agreed that the applicability of a one-parameter J-based ductile fracture approach is limited to so-called high constraint crack geometries, and that the elastic-plastic fracture toughness J(Ic) is not a material constant but strongly specimen geometry constraint-dependent. In this paper, the constraint effect on elastic-plastic fracture toughness is investigated by use of a continuum damage mechanics approach.

T.-J. Wang; Z.-B. Kuang

1995-01-01

211

Fluid flow, structural, and fracture mechanics modeling associated with hydraulic stimulation operations  

SciTech Connect

A summary review of hydraulic fracture modeling is given. The equations governing pertinent fluid flow, structural, and fracture mechanics responses are presented along with salient assumptions. The finite element modeling approach is used to discretize the field equations and compute the fracture dimensions, fluid pressure profile, leak-off, and stress intensity factors. In addition, the effects of frac fluid properties, layered strata, in situ stresses, and bi-material interface characteristics are discussed and numerical examples are presented. 50 refs.

Advani, S.H.; Lee, J.K.; Hamid, M.S.; Gurdogan, O.; Khattab, H.

1982-01-01

212

Fracture criterion for conductive cracks in soda-lime glass under combined mechanical and electrical loading  

Microsoft Academic Search

Fracture tests of electrically conductive cracks on pre-notched four-point bending soda-lime glass samples were conducted\\u000a under combined mechanical and electrical loading. The experimental results show that the critical stress intensity factor\\u000a at fracture is reduced if an electric field is applied, thereby indicating that the electric field makes contributions to\\u000a the fracture of conductive cracks. Base on the charge-free zone

Dong-Jun Yan; Hai-You Huang; Chi-Wai Cheung; Tong-Yi Zhang

2010-01-01

213

Limitations of elastic definitions in Al–Si–Mg cast alloys with enhanced plasticity: linear elastic fracture mechanics versus elastic–plastic fracture mechanics  

Microsoft Academic Search

Linear elastic fracture mechanics describes the fracture behavior of materials and components that respond elastically under loading. This approach is valuable and accurate for the continuum analysis of crack growth in brittle and high strength materials; however it introduces increasing inaccuracies for low-strength\\/high-ductility alloys (particularly low-carbon steels and light metal alloys). In the case of ductile alloys, different degrees of

Diana A. Lados; Diran Apelian

2006-01-01

214

Fracture mechanics; Proceedings of the 22nd National Symposium, Atlanta, GA, June 26-28, 1990. Vols. 1 & 2  

NASA Technical Reports Server (NTRS)

Current research on fracture mechanics is reviewed, focusing on ductile fracture; high-temperature and time-dependent fracture; 3D problems; interface fracture; microstructural aspects of fatigue and fracture; and fracture predictions and applications. Particular attention is given to the determination and comparison of crack resistance curves from wide plates and fracture mechanics specimens; a relationship between R-curves in contained and uncontained yield; the creep crack growth behavior of titanium alloy Ti-6242; a crack growth response in three heat resistant materials at elevated temperature; a crack-surface-contact model for determining effective-stress-intensity factors; interfacial dislocations in anisotropic bimaterials; an effect of intergranular crack branching on fracture toughness evaluation; the fracture toughness behavior of exservice chromium-molybdenum steels; the application of fracture mechanics to assess the significance of proof loading; and a load ratio method for estimating crack extension.

Ernst, Hugo A. (editor); Saxena, Ashok (editor); Mcdowell, David L. (editor); Atluri, Satya N. (editor); Newman, James C., Jr. (editor); Raju, Ivatury S. (editor); Epstein, Jonathan S. (editor)

1992-01-01

215

Fracture Mechanics of Heterogeneous Materials It was the physicist Ludwig Boltzmann who said that there is nothing more  

E-print Network

1 Fracture Mechanics of Heterogeneous Materials It was the physicist Ludwig Boltzmann who said. One particular example is fracture mechanics at bimaterial interfaces as they inevitably occur interface fracture mechanics has discovered small structures as a new field of application, for example

Berlin,Technische Universität

216

Numerical Assessment of the Progressive Rock Fracture Mechanism of Cracked Chevron Notched Brazilian Disc Specimens  

NASA Astrophysics Data System (ADS)

The International Society of Rock Mechanics (ISRM) suggested cracked chevron notched Brazilian disc method falls into a major testing category of rock fracture toughness measurement by virtue of chevron notched rock samples. A straight through crack front during the whole fracturing process is assumed in the testing principle but is never assessed. In this study, the progressive rock fracture mechanism of cracked chevron notched Brazilian disc rock specimens is numerically simulated for the first time. Two representative sample types with distinct geometry of notch ligaments are modelled. The assumption of a straight through crack front for chevron notched fracture samples is critically assessed. The results show that not only the notch tip but also the saw-cut chevron notch cracks during the experiments. The straight through crack front assumption is never satisfied in the realistic rock fracture progress of chevron notched disc samples. In addition, the crack features prominent curved front, far from being straight. In contrast to the sample type with narrow notch ligament, the acoustic emission (AE) of the simulation on the sample with wide notch ligament depicts obvious biased fracturing of the prescript fracturing route of the notch. The numerically observed progressive fracture mechanism calls for more attention on how to accurately calibrate the critical dimensionless stress intensity factor for a better measurement of Mode I fracture toughness via chevron notched samples.

Dai, F.; Wei, M. D.; Xu, N. W.; Ma, Y.; Yang, D. S.

2015-03-01

217

Fracture Mechanics Analyses for Interface Crack Problems - A Review  

NASA Technical Reports Server (NTRS)

Recent developments in fracture mechanics analyses of the interfacial crack problem are reviewed. The intent of the review is to renew the awareness of the oscillatory singularity at the crack tip of a bimaterial interface and the problems that occur when calculating mode mixity using numerical methods such as the finite element method in conjunction with the virtual crack closure technique. Established approaches to overcome the nonconvergence issue of the individual mode strain energy release rates are reviewed. In the recent literature many attempts to overcome the nonconvergence issue have been developed. Among the many approaches found only a few methods hold the promise of providing practical solutions. These are the resin interlayer method, the method that chooses the crack tip element size greater than the oscillation zone, the crack tip element method that is based on plate theory and the crack surface displacement extrapolation method. Each of the methods is validated on a very limited set of simple interface crack problems. However, their utility for a wide range of interfacial crack problems is yet to be established.

Krueger, Ronald; Shivakumar, Kunigal; Raju, Ivatury S.

2013-01-01

218

On a mathematical comparison of the S- and T-criteria in fracture mechanics  

NASA Astrophysics Data System (ADS)

It is demonstrated that the S-criterion of fracture mechanics is incompatible with the discontinuity of the velocity fields in the theory of ideal plasticity in contrast to the T-criterion, which allows a correct study of the fracture problem of an elastic ideally plastic body. A mathematical comparison study focusing on the S- and T-criteria is presented.

Theocaris, P. S.; Panagiotopoulos, P. D.

219

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

220

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

PubMed Central

There is a need to develop synthetic scaffolds for repairing 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) 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 = 86 ± 9 MPa, elastic modulus = 13 ± 2 GPa, and a Weibull modulus = 12 when tested in compression. In flexural loading, the strength, elastic modulus, and Weibull modulus were 11 ± 3 MPa, 13 ± 2 GPa, and 6, respectively. In compression, the as-fabricated scaffolds had a mean fatigue life of ~106 cycles when tested in air at room temperature or in phosphate-buffered saline at 37 °C under cyclic stresses of 1–10 MPa or 2–20 MPa. The compressive strength of the scaffolds decreased markedly during the first 2 weeks 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 2–4 weeks 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

2013-01-01

221

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

222

Subcritical fracture propagation in rocks: An examination using the methods of fracture mechanics and non-destructive testing. Ph.D. Thesis  

NASA Technical Reports Server (NTRS)

An experimental investigation of tensile rock fracture is presented with an emphasis on characterizing time dependent crack growth using the methods of fracture mechanics. Subcritical fracture experiments were performed in moist air on glass and five different rock types at crack velocities using the double torsion technique. The experimental results suggest that subcritical fracture resistance in polycrystals is dominated by microstructural effects. Evidence for gross violations of the assumptions of linear elastic fracture mechanics and double torsion theory was found in the tests on rocks. In an effort to obtain a better understanding of the physical breakdown processes associated with rock fracture, a series of nondestructive evaluation tests were performed during subcritical fracture experiments on glass and granite. Comparison of the observed process zone shape with that expected on the basis of a critical normal principal tensile stress criterion shows that the zone is much more elongated in the crack propagation direction than predicted by the continuum based microcracking model alone.

Swanson, P. L.

1984-01-01

223

Some basic fracture mechanics concepts in functionally graded materials  

Microsoft Academic Search

In this paper, the crack-tip fields in a general nonhomogeneous material are summarized. The fracture toughness and R-curve of functionally graded materials (FGMs) are studied based on the crack-bridging concept and a rule of mixtures. It is shown that the fracture toughness is significantly increased when a crack grows from the ceramic-rich region into the metal-rich region in an alumina-nickel

Z.-H. Jin; R. C. Batra

1996-01-01

224

Spall fracture: methodological aspects, mechanisms and governing factors  

Microsoft Academic Search

The dynamic tensile strength of materials at load durations of a few microseconds or less is studied by analyzing the spall\\u000a phenomena under shock pulse loading. The paper is devoted to discussing the methodology and capabilities of the technique\\u000a to measure spall strength, its error sources, spall fracture of materials of different classes and the factors governing the\\u000a high-rate fracture

G. I. Kanel

2010-01-01

225

Thermo-hydro-mechanical processes in fractured rock formations during glacial advance  

NASA Astrophysics Data System (ADS)

The paper examines the coupled thermo-hydro-mechanical (THM) processes that develop in a fractured rock region within a fluid-saturated rock mass due to loads imposed by an advancing glacier. This scenario needs to be examined in order to assess the suitability of potential sites for the location of deep geologic repositories for the storage of high-level nuclear waste. The THM processes are examined using a computational multiphysics approach that takes into account thermo-poroelasticity of the intact geological formation and the presence of a system of sessile but hydraulically interacting fractures (fracture zones). The modeling considers coupled thermo-hydro-mechanical effects in both the intact rock and the fracture zones due to contact normal stresses and fluid pressure at the base of the advancing glacier. Computational modelling provides an assessment of the role of fractures that can modify the pore pressure generation within the entire rock mass.

Selvadurai, A. P. S.; Suvorov, A. P.; Selvadurai, P. A.

2014-11-01

226

MECHANICAL ADVANTAGES OF A TRUSS-STRUCTURE-BASED FRACTURE FIXATION SYSTEM – A NOVEL FRACTURE FIXATION DEVICE "PINFIX" –  

PubMed Central

ABSTRACT A small, light, ball-joint device called PinFix, which can instantly convert a simple percutaneous cross pin fracture fixation system into a rigid external fracture fixation system based on truss structure, was developed. The purpose of this study was to compare the mechanical load and breaking strength of this truss-structure-based fixation system to that of the conventionally used external cantilever structure-based fixation system. Three types of mechanical loading tests, axial, bending, and torsion, were performed on an artificial fractured bone treated with either three-dimensional PinFix fixation, two-dimensional PinFix fixation, or conventional external fixation. The three- and two-dimensional PinFix fixations showed significantly more stiffness than conventional fixation on all three loading tests. Finite element analysis was next performed to calculate the stress distribution of the parts in PinFix and in the conventional fixator. The applied stress to the rod and connectors of PinFix was much less than that of the conventional external fixator. These results reflected the physical characteristic of truss structure in which applied load is converted to pure tension or compression forces along the members of the PinFix. In conclusion, PinFix is a simple fracture fixation system that has a truss-structure with a high rigidity. PMID:24640174

ARAI, TETSUYA; YAMAMOTO, MICHIRO; IWATSUKI, KATSUYUKI; NATSUME, TADAHIRO; SHINOHARA, TAKAAKI; TATEBE, MASAHIRO; KURIMOTO, SHIGERU; OTA, HIDEYUKI; KATO, SHUICHI; HIRATA, HITOSHI

2013-01-01

227

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

228

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

229

Fracture Mechanics Analyses of the Slip-Side Joggle Regions of Wing-Leading-Edge Panels  

NASA Technical Reports Server (NTRS)

The Space Shuttle wing-leading edge consists of panels that are made of reinforced carbon-carbon. Coating spallation was observed near the slip-side region of the panels that experience extreme heating. To understand this phenomenon, a root-cause investigation was conducted. As part of that investigation, fracture mechanics analyses of the slip-side joggle regions of the hot panels were conducted. This paper presents an overview of the fracture mechanics analyses.

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

2011-01-01

230

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

231

Mechanical Properties of Rock Fractures derived from ultrasonic and numerical Data  

NASA Astrophysics Data System (ADS)

Rock fracture properties including fracture stiffness and strength have a significant influence on many geophysical problems. We have recently developed and implemented a granular particle based numerical scheme for investigating wave propagation across confined unwelded fractures. Simulations predict that fracture stiffness can be determined directly from seismic data. In addition bi-compliant fracture stiffnesses can result in the generation of subtle non-linear wave effects, which are diagnostic of the fractures mechanical properties. We are conducting physical laboratory experiments to test the predictions of this recent numerical work. In particular we will investigate the effects of fracture cohesive properties, roughness and confining stress on fracture stiffness as measured dynamically from ultrasonic data in the laboratory. The laboratory experiment consists of a rock core sample with a single fracture. The ultrasonic transducers being used have a central frequency of 1MHz and the rock samples are between 4cm and 7cm long and 4cm in diameter. One transducer is fitted centrally on each end of the sample. This physical experiment is replicated in terms of rock grain size, external loading conditions and dynamic wave frequencies using the granular numerical scheme, greatly enhancing our interpretation of the physical seismograms.

Moellhoff, M.; Bean, C. J.; Meredith, P.

2003-04-01

232

Correlating laboratory observations of fracture mechanical properties to hydraulically-induced microseismicity in geothermal reservoirs.  

SciTech Connect

To date, microseismicity has provided an invaluable tool for delineating the fracture network produced by hydraulic stimulation of geothermal reservoirs. While the locations of microseismic events are of fundamental importance, there is a wealth of information that can be gleaned from the induced seismicity (e.g. fault plane solutions, seismic moment tensors, source characteristics). Closer scrutiny of the spatial and temporal evolution of seismic moment tensors can shed light on systematic characteristics of fractures in the geothermal reservoir. When related to observations from laboratory experiments, these systematic trends can be interpreted in terms of mechanical processes that most likely operate in the fracture network. This paper reports on mechanical properties that can be inferred from observations of microseismicity in geothermal systems. These properties lead to interpretations about fracture initiation, seismicity induced after hydraulic shut-in, spatial evolution of linked fractures, and temporal evolution of fracture strength. The correlations highlight the fact that a combination of temperature, stressing rate, time, and fluid-rock interactions can alter the mechanical and fluid transport properties of fractures in geothermal systems.

Stephen L. Karner, Ph.D

2006-02-01

233

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

234

Transactions of the JASCOME, Journal of Boundary Element Methods, Paper No. 01_101913 ITERATIVE COUPLING OF BEM AND FEM FOR THE SOLUTION OF  

E-print Network

COUPLING OF BEM AND FEM FOR THE SOLUTION OF ELASTO-PLASTIC FRACTURE MECHANICS PROBLEMS Wael M. Elleithy; Coupling. 1 Introduction For certain categories of problems, neither the boundary element method (BEM) nor for the BEM and FEM sub-domains. The algorithm for constructing an entire equation is highly complicated when

Al-Gahtani, Husain Jubran

235

Spinal fractures in recreational bobsledders: an unexpected mechanism of injury  

PubMed Central

Study design:?Retrospective case series and literature review. Objective:?To report and discuss spinal fractures occurring in recreational bobsledders. Summary of background data:?Spinal fractures have been commonly described following traumatic injury during a number of recreational sports. Reports have focused on younger patients and typically involved high-impact sports or significant injuries. With an aging population and a wider array of recreational sports, spinal injuries may be seen after seemingly benign activities and without a high-impact injury. Methods:?A retrospective review of two patients and review of the literature was performed. Results:?Two patients with spinal fractures after recreational bobsledding were identified. Both patients, aged 57 and 54 years, noticed a simultaneous onset of severe back pain during a routine turn on a bobsled track. Neither was involved in a high-impact injury during the event. Both patients were treated conservatively with resolution of symptoms. An analysis of the bobsled track revealed that potential forces imparted to the rider may be greater than the yield strength of vertebral bone. Conclusions:?Older athletes may be at greater risk for spinal fracture associated with routine recreational activities. Bobsledding imparts large amounts of force during routine events and may result in spinal trauma. Older patients, notably those with osteoporosis or metabolic bone disease, should be educated about the risks associated with seemingly benign recreational sports. PMID:23230417

Severson, Erik P.; Sofianos, Dmitri A.; Powell, Amy; Daubs, Michael; Patel, Rakesh; Patel, Alpesh A.

2012-01-01

236

Effect of rayleigh waves in dynamic fracture mechanics  

Microsoft Academic Search

The effects described hereafter were accidentally discovered in the course of analyzing results obtained from numerical simulations of fast fracture and crack arrest in DCB specimens. The simulations were performed as part of an investigation concerned with the evaluation of the dynamic material toughness KID. To this end the finite difference code presented in [i] was employed while measurements of

M. Shmuely; D. Peretz; M. Perl

1978-01-01

237

Fracture analysis of debonding mechanism for FRP plates  

E-print Network

small potential crack extension. The crack will extend if the energy release rate is greater than the interface fracture energy. Despite the fact that the crack-tip stress field is not amenable to precise analysis, its influence on the energy balance...

Achintha, Paththini Marakkala Mithila

238

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

239

Mechanical properties and fracture toughness of rail steels and thermite welds at low temperature  

NASA Astrophysics Data System (ADS)

Brittle fracture occurs frequently in rails and thermite welded joints, which intimidates the security and reliability of railway service. Railways in cold regions, such as Qinghai-Tibet Railway, make the problem of brittle fracture in rails even worse. A series of tests such as uniaxial tensile tests, Charpy impact tests, and three-point bending tests were carried out at low temperature to investigate the mechanical properties and fracture toughness of U71Mn and U75V rail steels and their thermite welds. Fracture micromechanisms were analyzed by scanning electron microscopy (SEM) on the fracture surfaces of the tested specimens. The ductility indices (percentage elongation after fracture and percentage reduction of area) and the toughness indices (Charpy impact energy A k and plane-strain fracture toughness K IC) of the two kinds of rail steels and the corresponding thermite welds all decrease as the temperature decreases. The thermite welds are more critical to fracture than the rail steel base metals, as indicated by a higher yield-to-ultimate ratio and a much lower Charpy impact energy. U71Mn rail steel is relatively higher in toughness than U75V, as demonstrated by larger A k and K IC values. Therefore, U71Mn rail steel and the corresponding thermite weld are recommended in railway construction and maintenance in cold regions.

Wang, Yuan-qing; Zhou, Hui; Shi, Yong-jiu; Feng, Bao-rui

2012-05-01

240

Fast water infiltration: a mechanism for fracture formation during land subsidence  

NASA Astrophysics Data System (ADS)

A mechanism for fracture generation and for triggering land subsidence is presented. Infiltration through a pre-existing fracture zone into a two-layered system, as well as the deformation of unconsolidated sediments on the land surface, was numerically investigated. The numerical simulation of infiltration is based on a two-phase flow-model concept for porous media, and for the deformation, it is based on a Mohr-Coulomb model concept. Different studies with variations of the fracture parameter and infiltration conditions have been carried out. The infiltration results show that fast infiltration in a partially saturated aquifer leads to land subsidence, extension of pre-existing fractured zones and the generation of new cracks. If the water column is only on the fracture, the clay layer acts like a barrier and inhibits the infiltration through the fracture. If the water column covers the entire surface, the barrier effect is overcome; the infiltration intensity depends on the height of the water column, the fracture permeability and the fracture width. The deformation results show that a strong rainfall event of 2 h leads to deformations that are about 30 % of the vertical and 70 % of the horizontal annual land-subsidence rates.

Martinez, Isaac; Hinkelmann, Reinhard; Savidis, Stavros

2013-06-01

241

HYDRO-MECHANICAL UPSCALING OF A FRACTURED ROCKMASS USING A 3D NUMERICAL APPROACH  

E-print Network

by the relationship: Qi = -Kij Jj (1) To determine all the tensor terms with 3DEC, 3 sets of (hydraulic) boundary / 2 µ L) ij (2) = 2.45 10-7 m/s where: a (= 10-4 m) is the hydraulic aperture of the fracture; µ (= 10HYDRO-MECHANICAL UPSCALING OF A FRACTURED ROCKMASS USING A 3D NUMERICAL APPROACH Thoraval Alain

Paris-Sud XI, Université de

242

Fracture characteristics of Al4 pct Mg mechanically alloyed with SiC  

Microsoft Academic Search

Subcritical crack growth and rapid fracture of the mechanically alloyed aluminum alloy IN-9052* reinforced with SiC particles\\u000a have been investigated. Fatigue crack growth rates for the composite exceed those of the unreinforced alloy, except that the\\u000a threshold stress intensity for growth is higher for the composite. Fracture toughness of the composite is about 9 MPa?m compared\\u000a to a (reported) value

D. L. Davidson

1987-01-01

243

Development and fracture mechanics data for 6Al-6V-2 Sn titanium alloy  

NASA Technical Reports Server (NTRS)

Fracture mechanics properties of 6Al-6V-2Sn titanium in the annealed, solution-treated, and aged condition are presented. Tensile, fracture toughness, cyclic flaw growth, and sustained-load threshold tests were conducted. Both surface flaw and compact tension-specimen geometries were employed. Temperatures and/or environments used were -65 F (220 K) air, ambient, 300 F (422 K) air, and room-temperature air containing 10 and 100% relative humidity.

Fiftal, C. F.; Beck, E. J.

1974-01-01

244

Scaling of crack surfaces and implications for fracture mechanics  

PubMed

The scaling laws describing the roughness development of crack surfaces are incorporated into the Griffith criterion. We show that, in the case of a Family-Vicsek scaling, the energy balance leads to a purely elastic brittle behavior. On the contrary, it appears that an anomalous scaling reflects an R-curve behavior associated with a size effect of the critical resistance to crack growth in agreement with the fracture process of heterogeneous brittle materials exhibiting a microcracking damage. PMID:10970587

Morel; Schmittbuhl; Bouchaud; Valentin

2000-08-21

245

Fracture Mechanisms of Bone: A Comparative Study between Antler and Bovine Femur , F.A. Sheppard2  

E-print Network

Fracture Mechanisms of Bone: A Comparative Study between Antler and Bovine Femur P.Y. Chen1 , F, University of California, San Diego, La Jolla, CA 92093-0411, U.S.A. ABSTRACT In this study, fracture conditions to study the effects of fiber orientation and hydration. Fracture toughness results

McKittrick, Joanna

246

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

247

Analysis of interfacial delamination in IC packaging: Use stress or fracture mechanics?  

Microsoft Academic Search

After encapsulation, thermo-mechanical deformation builds up within the electronic packages due to temperature coefficient of expansion mismatch between the respective materials within the package as it cools to room temperature. The maximum Von Mises stress or principle stress criterion based on stress analysis and maximum energy release rate criterion based on fracture mechanics are two of the most popular criteria

Hu Guojun; Luan Jing-en; Xavier Baraton; Andrew A. O. Tay

2009-01-01

248

Modelling the physical properties of cracked rocks using fracture mechanics and statistical physics  

Microsoft Academic Search

Cracks play a major role in most rocks submitted to crustal conditions. Mechanically, cracks make the rock much more compliant. They also make it much easier for fluid to flow through any rock body. Relying on Fracture Mechanics and Statistical Physics, we introduce a few key concepts which allow to understand and quantify how cracks do modify both the elastic

P. Benson; A. Schubnel; S. Vinciguerra; J. Hazzard; R. Young; P. Meredith

2004-01-01

249

Atomistic and continuum modeling of mechanical properties of collagen: Elasticity, fracture, and self-assembly  

Microsoft Academic Search

We report studies of the mechanical properties of tropocollagen molecules under different types of mechanical loading including tension, compression, shear, and bending. Our modeling yields predictions of the fracture strength of single tropocollagen molecules and polypeptides, and also allows for quantification of the interactions between tropocollagen molecules. Atomistic modeling predicts a persistence length of tropocollagen molecules ? 23.4 nm, close

Markus J. Buehler

2006-01-01

250

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

251

Micromechanical fracture modeling on underground nuclear waste storage: Coupled mechanical, thermal, and hydraulic effects  

NASA Astrophysics Data System (ADS)

Coupling effects between thermal, hydraulic, chemical and mechanical (THCM) processes for rock materials are one of major issues in Geological engineering, Civil engineering, Hydrology, Petroleum engineering, and Environmental engineering. In all of these fields, at least two mechanisms of THCM coupling are considered. For an example, thermal, hydraulic, and mechanical coupling effects are important in Geological engineering and Civil engineering. The THM coupling produces effects on underground structures, since the underground structures are under influences of geothermal gradient, groundwater, gravitational stresses, and tectonic forces. In particular, underground repository of high-level nuclear waste involves all four of the THCM coupling processes. Thermo-hydro-mechanical coupling model for fractured rock media has been developed based on micromechanical fracture model [Kemeny 1991, Kemeny & Cook 1987]. The THM coupling model is able to simulate time- and rate-dependent fracture propagation on rock materials, and quantify characteristics of damage by extensile and shear fracture growth. The THM coupling model can also simulate coupled thermal effects on underground structures such as high-level nuclear waste repository. The results of thermo-mechanical coupling model are used in conducting a risk analysis on the structures. In addition, the THM coupling model is able to investigate variations of fluid flow and hydraulic characteristics on rock materials by measuring coupled anisotropic permeability. Later, effects of chemical coupling on rock materials are investigated and modified in the THM coupling model in order to develop a thermo-hydro-chemo-mechanical coupling model on fractured rocks. The THCM coupling model is compared with thermal, hydraulic, chemical, and mechanical coupling tests conducted at the University of Arizona. The comparison provides a reasonable prediction for the THCM coupling tests on various rock materials. Finally, the THCM coupling model for fractured rocks simulates the underground nuclear waste storage in Yucca Mountain, Nevada, and conducted performance and risk analysis on the repository.

Leem, Junghun

252

Relationship between Microstructure, Material Distribution, and Mechanical Properties of Sheep Tibia during Fracture Healing Process  

PubMed Central

The aim of this study was to investigate the relationship between microstructural parameters, material distribution, and mechanical properties of sheep tibia at the apparent and tissue levels during the fracture healing process. Eighteen sheep underwent tibial osteotomy and were sacrificed at 4, 8, and 12 weeks. Radiographs and micro-computed tomography (micro-CT) scanning were taken for microstructural assessment, material distribution evaluation, and micro-finite element analysis. A displacement of 5% compressive strain on the longitudinal direction was applied to the micro-finite element model, and apparent and tissue-level mechanical properties were calculated. Principle component analysis and linear regression were used to establish the relationship between principle components (PCs) and mechanical parameters. Visible bony callus formation was observed throughout the healing process from radiographic assessment. Apparent mechanical property increased at 8 weeks, but tissue-level mechanical property did not increase significantly until 12 weeks. Three PCs were extracted from microstructural parameters and material distribution, which accounted for 87.592% of the total variation. The regression results showed a significant relationship between PCs and mechanical parameters (R>0.8, P<0.05). Results of this study show that microstructure and material distribution based on micro-CT imaging could efficiently predict bone strength and reflect the bone remodeling process during fracture healing, which provides a basis for exploring the fracture healing mechanism and may be used as an approach for fractured bone strength assessment. PMID:24046532

Gao, Jiazi; Gong, He; Huang, Xing; Fang, Juan; Zhu, Dong; Fan, Yubo

2013-01-01

253

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

254

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

255

Application of fracture mechanics in maintenance of high temperature equipment -- An assessment of critical needs  

SciTech Connect

Extending the operating life of power-plant, chemical reactor and land, sea and air based gas turbine components beyond their original design life has considerable economic advantages. Fracture mechanics is used extensively to predict the remaining life and safe inspection intervals as part of maintenance programs for these systems. The presence of creep deformation and time-dependent damage accumulation in these components present very significant challenges. Therefore, the emphasis of this paper is on the time-dependent fracture mechanics concepts. A critical assessment of the current state-of-the-art of time dependent fracture mechanics concepts, test techniques, analytical procedures and application tools is made to demonstrate the potential of this technology in maintenance engineering. In addition, future developments that are needed to enhance the application of this technology are also described and limits of the current approaches are also discussed.

Saxena, A. [Georgia Inst. of Tech., Atlanta, GA (United States). School of Materials Science and Engineering

1997-12-31

256

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

NASA Astrophysics Data System (ADS)

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.

Šílený, Jan; Hill, David P.; Eisner, Leo; Cornet, Francois H.

2009-08-01

257

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

258

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

259

The Mechanical Benefit of Medial Support Screws in Locking Plating of Proximal Humerus Fractures  

PubMed Central

Background The purpose of this study was to evaluate the biomechanical advantages of medial support screws (MSSs) in the locking proximal humeral plate for treating proximal humerus fractures. Methods Thirty synthetic left humeri were randomly divided into 3 subgroups to establish two-part surgical neck fracture models of proximal humerus. All fractures were fixed with a locking proximal humerus plate. Group A was fixed with medial cortical support and no MSSs; Group B was fixed with 3 MSSs but without medial cortical support; Group C was fixed with neither medial cortical support nor MSSs. Axial compression, torsional stiffness, shear stiffness, and failure tests were performed. Results Constructs with medial support from cortical bone showed statistically higher axial and shear stiffness than other subgroups examined (P<0.0001). When the proximal humerus was not supported by medial cortical bone, locking plating with medial support screws exhibited higher axial and torsional stiffness than locking plating without medial support screws (P?0.0207). Specimens with medial cortical bone failed primarily by fracture of the humeral shaft or humeral head. Specimens without medial cortical bone support failed primarily by significant plate bending at the fracture site followed by humeral head collapse or humeral head fracture. Conclusions Anatomic reduction with medial cortical support was the stiffest construct after a simulated two-part fracture. Significant biomechanical benefits of MSSs in locking plating of proximal humerus fractures were identified. The reconstruction of the medial column support for proximal humerus fractures helps to enhance mechanical stability of the humeral head and prevent implant failure. PMID:25084520

Liu, Yanjie; Pan, Yao; Zhang, Wei; Zhang, Changqing; Zeng, Bingfang; Chen, Yunfeng

2014-01-01

260

Asian Rock Mechanics Symposium, ARMS2012, 15-19 October 2012, Seoul, Korea Determination of equivalent continuum mechanical model for fractured  

E-print Network

1 7th Asian Rock Mechanics Symposium, ARMS2012, 15-19 October 2012, Seoul, Korea Determination Mechanics Symposium, ARMS2012, Seoul : Korea, Republic Of (2012)" #12;2 (a) (b) Figure 1. Fractured EDZ of equivalent continuum mechanical model for fractured EDZ around underground galleries by homogenization A

Paris-Sud XI, Université de

261

Probabilistic model for pressure vessel reliability incorporating fracture mechanics and nondestructive examination  

SciTech Connect

A probabilistic model has been developed for predicting the reliability of structures based on fracture mechanics and the results of nondestructive examination (NDE). The distinctive feature of this model is the way in which inspection results and the probability of detection (POD) curve are used to calculate a probability density function (PDF) for the number of flaws and the distribution of those flaws among the various size ranges. In combination with a probabilistic fracture mechanics model, this density function is used to estimate the probability of failure (POF) of a structure in which flaws have been detected by NDE. The model is useful for parametric studies of inspection techniques and material characteristics.

Tow, D.M.; Reuter, W.G.

1998-03-01

262

Case histories involving fatigue and fracture mechanics; Proceedings of the Symposium, Charleston, SC, Mar. 21, 22, 1985  

NASA Technical Reports Server (NTRS)

Papers are presented on cracking at nozzle corners in the nuclear pressure vessel industry, applied fracture mechanics for assessing defect significance in a crude oil pipeline, failure analysis of a large wind-tunnel compressor blade, analysis of a compressor-wheel failure, and preventing fracture by inspection and analysis. Consideration is also given to the fatigue crack growth predictions of welded aircraft structures containing flaws in the residual stress field, the fatigue and fracture mechanics analysis of a compression loaded aircraft structure, fracture of an aircraft horizontal stabilizer, fatigue life analysis of fuel tank skins under combined loads, and aircraft structural maintenance recommendations based on fracture mechanics analysis. Additional papers discuss an analysis of two metal-forming die failures, an analysis of a failed saw arbor, and the role of fracture mechanics in assessing the effect on fatigue life of design changes in welded fabrications.

Hudson, C. Michael (editor); Rich, Thomas P. (editor)

1986-01-01

263

Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads  

NASA Astrophysics Data System (ADS)

Hydraulic flow and transport (heat and solute) within crystalline rocks is dominated by the fracture systems found within them. In situ stress conditions have a significant impact on the hydraulic, mechanical and thermal coupled processes, and quantification of these processes provides a key to understanding the often transient time-dependent behaviour of crystalline rocks. In this paper, a geomechanical model is presented which describes fracture closure as a function of effective stress and the changes in parameters such as storage, permeability, porosity and aperture. Allowing the fracture closure to be defined by the change in normal effective stress provides a link to the numerical consideration of parametrical changes due to rock stress alterations caused for example by changes in fracture fluid pressure, stress release, tectonic stress, thermal stress, orientation of the natural fracture in the pervasive stress system and local changes in a rock mass due to stress alteration. The model uses geometrical considerations based on a fractal distribution of apertures on the fracture surface, and applies well-established analytical elastic deformation solutions to calculate the deformation response to changes in effective stress. Analysis of the fractal generation method allows a standard normal distribution of fracture apertures to be predicted for all common fractal dimensions relating to a 2D surface. Changes in the fracture aperture are related to hydraulic functions such as permeability, storage and porosity of the fracture. The geomechanical model is experimentally validated against laboratory scale experimental data gained from the closure of a fractured sample recovered at a depth of 3,800 m from the KTB pilot borehole. Parameters for matching the experimental data were established externally, the only fitting parameters applied were the minimum and maximum contact area between the surfaces and the number of allowable contacts. The model provides an insight into the key processes determining the closure of a fracture, and can act as a material input function for numerical models linking the effects of changes in the stress field, hydraulic or thermal conditions, to the flow and transport parameters of a fractured system.

McDermott, Chris; Kolditz, Olaf

2006-04-01

264

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

265

Hip fracture presenting as mechanical low back pain subsequent to a fall: a case study  

PubMed Central

This case chronicles the assessment and clinical management of a 54 year old female patient who presented with post traumatic lower back, hip and lower extremity pain, initially attributed to mechanical low back pain but ultimately diagnosed as a hip fracture. This case study illustrates a number of important issues germane to chiropractic care. These are; the importance of using different assessment procedures, combined with clinical experience, in order to differentiate between those patients with clinical conditions that are amenable to conservative care from those that are not; the usefulness of a tuning fork test as a clinical tool in differentiating between hip fracture and mechanical spinal pain syndromes and; the impact of falls and fractures among older Canadian patients. PMID:17549186

Gleberzon, Brian; Hyde, David

2006-01-01

266

A fracture mechanics view of iceberg calving from large ice shelves  

NASA Astrophysics Data System (ADS)

Iceberg calving, an important mass loss mechanism for marine ice sheets, is an outstanding accounting problem in ice-sheet/ice-shelf models. Various parameterizations have been proposed but there is no consensus regarding relevant physical processes, key parameters and variables, or the applicability the various relationships have to different floating ice geometries. Long rifts at the fronts of Antarctic ice shelves, which become the planes along which icebergs calve, are observed to initiate as fractures along lateral margins of fast-flowing ice or downstream of grounded features. For most of the shelf, inhomogeneities due to structural boundaries limit propagation. Orientations of active fractures are explained by glaciological stresses. In a comparison of fractures observed in small to medium ice shelves around Antarctica, we find that “starter” fractures, derived from upstream, grounded ice, propagate where floating ice advects around a grounded feature or through a narrowing in the ice shelf, as between two grounded features or the embayment walls. This observation follows expectation because it is in such locations that compressive stresses in the transverse direction become relatively large. We use a numerical model of fracture propagation and shelf velocities derived from remote observation to investigate the growth of long fractures in ice shelves. We propose that principal stresses, computed in models of ice shelf flow, can be used to develop a calving parameterization appropriate for embayed marine margins of ice sheets.

Ledoux, C. M.; Hulbe, C. L.

2010-12-01

267

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

268

Influence of wall roughness on the geometrical, mechanical and transport properties of single fractures  

NASA Astrophysics Data System (ADS)

This paper reviews the main features of the transport properties of single fractures. Particular attention is paid to fractures in geological materials which often display a roughness covering a broad range of length scales. Because of the small distance separating the fracture walls, the surface roughness is a key parameter influencing the structure of the void space. Studies devoted to the characterization of the surface roughness are presented as well as works aimed at characterizing the void space geometry. The correlation of the free space is found to be a crucial function of the failure mechanism (brittle, quasi brittle, plastic, etc) and also of possible shear displacements during the failure. The influence of the surface roughness on the mechanical behaviour of fractures under a normal load and a shear stress is also described. Finally, experimental, numerical and theoretical works devoted to the study of the influence of the fracture void geometry on the permeability and on the hydrodynamic dispersion of a dissolved species are discussed.

Auradou, H.

2009-11-01

269

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

270

Application of fracture mechanics concepts to hierarchical biomechanics of bone and bone-like materials  

Microsoft Academic Search

Fracture mechanics concepts are applied to gain some understanding of the hierarchical nanocomposite structures of hard biological tissues such as bone, tooth and shells. At the most elementary level of structural hierarchy, bone and bone-like materials exhibit a generic structure on the nanometer length scale consisting of hard mineral platelets arranged in a parallel staggered pattern in a soft protein

HUAJIAN GAO

271

The use of fracture mechanics in failure analysis in the offshore diamond mining industry  

Microsoft Academic Search

This paper considers two major failures of offshore diamond mining equipment, which should not have occurred had both fracture mechanics aspects and materials behaviour been more fully understood. The two case studies include (a) failure of a wire rope swivel, and (b) failure of a load cell (used for monitoring rope load). In case (a) the swivel shank had failed

R. B. Tait; C. Emslie

2005-01-01

272

A molecular mechanics simulation of cracks and fractures in a sheet of ice  

Microsoft Academic Search

Rectangular, two dimensional sheets of ice molecules are both stressed and compressed. Computer examples compare dynamical responses when the plate has a slot or does not have a slot. The mechanisms for both crack and fracture development are clearly delineated on the molecular level.

Donald Greenspan

2000-01-01

273

Evaluation of hydrogen assisted cracking: the meaning and significance of the fracture mechanics approach  

Microsoft Academic Search

Analysis of hydrogen assisted cracking is performed with emphasis on the validity of the linear elastic fracture mechanics approach. Its soundness is ensured by the uniqueness of the crack growth kinetics curve `crack growth rate v versus stress intensity factor K' for a material–environment system. The ability of this relationship to match the similitude of crack tip events is revised

J. Toribio; V. Kharin

1998-01-01

274

Fracture resistant bones: unusual deformation mechanisms of seahorse armor Michael M Porter1*  

E-print Network

1 Fracture resistant bones: unusual deformation mechanisms of seahorse armor Michael M Porter1 to insect flight, that inspire materials scientists and engineers to develop new, high-performance materials in the seahorse, Hippocampus kuda. #12;2 Seahorses, known for their equine profile and vertical swimming posture

McKittrick, Joanna

275

Analysis of delamination in composite laminates under low-velocity impact. [Using fracture mechanics approach  

Microsoft Academic Search

Composite laminates subjected to foreign object impact often experience extreme damage that may not be visible on the impacted surface. This damage in the form of delamination may lead to significant reduction in compressive strength of the laminate. In this study, the fracture mechanics approach is employed to predict the extent of delamination for a given impact condition. Static linear

Jih Chanjiun

1991-01-01

276

HYDRO-MECHANICAL UPSCALING OF A FRACTURED ROCKMASS USING A 3D NUMERICAL APPROACH  

E-print Network

terms with 3DEC, 3 sets of (hydraulic) boundary conditions have to be defined (figure 2). / / Pi °tHYDRO-MECHANICAL UPSCALING OF A FRACTURED ROCKMASS USING A 3D NUMERICAL APPROACH Thoraval Alain method has been developed using 3D numerical tools (RESOBLOK & 3DEC). This method has been successfully

Paris-Sud XI, Université de

277

3 RESERVOIR PERFORMANCE ANALYSIS 3.1 ANALYSIS OF IMBIBITION MECHANISM IN THE NATURALLY FRACTURED  

E-print Network

89 3 RESERVOIR PERFORMANCE ANALYSIS 3.1 ANALYSIS OF IMBIBITION MECHANISM IN THE NATURALLY FRACTURED reviewed in this section. Based on analysis of fluid saturation of the field, the volumetric sweep. A sensitivity study of the imbibition model after 40 years of waterflooding in the Spraberry Trend with oil

Schechter, David S.

278

Multiscale computational modeling of deformation mechanics and intergranular fracture in nanocrystalline copper  

E-print Network

, University of Oxford, Parks Road, Oxford OX1 3PJ, UK a r t i c l e i n f o Article history: Received 6 as to accurately predict the processes of inter-granular fracture for different GB character distributions by a mechanism in which the grain boundary (GB) character distribution (GBCD) is promoted and con- trols the NC

Sansoz, Frederic

279

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

280

Thermomechanical fatigue life prediction in gas turbine superalloys: A fracture mechanics approach  

Microsoft Academic Search

A model is presented that was developed to predict thermomechanical fatigue crack initiation and estimate mode I crack growth of gas turbine hot section gas path superalloys. The model is based on a strain energy density fracture mechanics approach modified to account for thermal exposure and single crystal anisotropy. Thermomechanical fatigue crack initiation and small crack growth is modeled by

David M. Nissley

1995-01-01

281

Fracture mechanics analysis of anisotropic plates by the boundary element method  

Microsoft Academic Search

This paper presents an analysis of mixed mode fracture mechanics problems arising in anisotropic composite laminates. The boundary element method (BEM) and the Jk integral are presented as accurate techniques to compute the stress intensity factors KI and KII of two dimensional anisotropic bodies. Using function of a complex variable a decoupling procedure is derived to obtain the stress intensity

P. Sollero; M. H. Aliabadi

1993-01-01

282

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

283

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

284

Heavy Section Steel Technology HSST eLib Computational Structural Fracture Mechanics Team  

E-print Network

Heavy Section Steel Technology ­ HSST eLib Computational Structural Fracture Mechanics Team the reports generated by the Heavy-Section Steel Technology (HSST) program (JCN B0119) from the early 1970's welcome the opportunity to discuss your potential applications and ways that the Heavy-Section Steel

285

Fatigue life prediction of circumferentially notched component based on elastic-plastic fracture mechanics  

Microsoft Academic Search

The elastic-plastic fracture mechanics parameters for the 3D surface and circumferential cracks generated in the circumferentially grooved cylinder component were analyzed from the viewpoint of engineering. Then, fatigue life spent during crack growth was estimated for the bluntly and acutely notched cylindrical components. Consequently, it was found that the crack propagation life of the notched cylinder component was successfully predicted

K. Hatanaka; T. Fujimitsu; J. Omori

1992-01-01

286

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

287

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

288

Fracture mechanics models developed for piping reliability assessment in light water reactors: piping reliability project  

Microsoft Academic Search

The efforts concentrated on modifications of the stratified Monte Carlo code called PRAISE (Piping Reliability Analysis Including Seismic Events) to make it more widely applicable to probabilistic fracture mechanics analysis of nuclear reactor piping. Pipe failures are considered to occur as the result of crack-like defects introduced during fabrication, that escape detection during inspections. The code modifications allow the following

D. O. Harris; E. Y. Lim; D. D. Dedhia; H. H. Woo; C. K. Chou

1982-01-01

289

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

290

Contributions to the anisotropic elasto-plastic analysis of shells  

E-print Network

Shells are probably the most widely used structural component in engineering and also in nature due to their high efficiency and excellent performance when properly designed. On the other hand, they can be very sensitive ...

Kim, Do-Nyun

2009-01-01

291

Elasto-plastic flow of a foam around an obstacle  

E-print Network

We simulate quasistatic flows of an ideal two-dimensional monodisperse foam around different obstacles, both symmetric and asymmetric, in a channel. We record both pressure and network contributions to the drag and lift forces, and study them as a function of obstacle geometry. We show that the drag force increases linearly with the cross section of an obstacles. The lift on an asymmetric aerofoil-like shape is negative and increases with its arc length, mainly due to the pressure contribution.

François Boulogne; Simon Cox

2012-02-27

292

Elasto-plastic flow of a foam around an obstacle  

E-print Network

We simulate quasistatic flows of an ideal two-dimensional monodisperse foam around different obstacles, both symmetric and asymmetric, in a channel. We record both pressure and network contributions to the drag and lift forces, and study them as a function of obstacle geometry. We show that the drag force increases linearly with the cross section of an obstacles. The lift on an asymmetric aerofoil-like shape is negative and increases with its arc length, mainly due to the pressure contribution.

Boulogne, François

2012-01-01

293

Elasto-plasticity in wrinkled polymerized lipid membranes  

PubMed Central

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

294

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

295

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

296

Rock mechanics: Issues and research needs in the disposal of wastes in hydraulic fractures  

NASA Astrophysics Data System (ADS)

Proposed rock mechanics studies are outlined which are designed to answer basic questions concerning hydraulic fracturing for waste disposal. These questions are: (1) how can containment be assured for Oak Ridge or other sites; and (2) what is the capacity of a site. The suggested rock mechanics program consists of four major tasks: (1) numerical modeling; (2) laboratory testing; (3) field testing; and (4) monitoring. These tasks are described.

Doe, T.; McClain, W. C.

1984-07-01

297

A fracture mechanics analysis for delamination growth during impact on composite plates  

NASA Technical Reports Server (NTRS)

A fracture mechanics analysis has been developed that describes the progress of delamination damage in composite plates struck by a hard spherical object. The analysis is based on large deflection plate mechanics for circular isotropic plates wherein multiple axisymmetric delaminations grow. Test data show that the analysis predicts the influence of plate thickness, support conditions, and matrix toughness on the onset and propagation of delamination.

Bostaph, G. M.; Elber, W.

1983-01-01

298

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

299

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

300

A three-dimensional stochastic rock mechanics model of engineered geothermal systems in fractured crystalline rock  

NASA Astrophysics Data System (ADS)

A three-dimensional (3-D) stochastic network model for simulating a hot dry rock (HDR) or hot wet rock (HWR) engineered geothermal system formed in fractured crystalline rock is presented. The model addresses the problems of fracture network characterization from in situ field data, such as fracture orientation, size, spacing, and other mechanical properties. The model can simulate the changes that occur within the rock mass during stimulation (i.e., large volume fluid injection at pressures sufficient to allow shear slip on natural fractures). It can also be used to simulate steady state circulation of the heat exchange system thus created and includes provision for predicting tracer response curves and heat extraction history. The model has been applied to data gathered during the stimulation and circulation of a 2.2-km-deep HDR reservoir at Hijiori, Japan. The predicted shape of the stimulated and shear-propped fractures closely matched the distribution of seismic source distribution of acoustic emission (AE), regardless of realization of the fracture network, suggesting that the geometry of the stimulated volume can be robustly predicted from knowledge of the fracture population and in situ stresses. However, hydraulic behavior and tracer tests during the circulation could only be satisfactorily simultaneously reproduced by a small subset of realizations. These selected realizations, obtained by matching initial circulation and tracer data, are considered to give the best prospect of satisfactory long-term thermal modeling. The success in simultaneously modeling diverse data (hydraulic, microseismic, and tracer) lends confidence to the thermal predictions. The results indicate that a large improvement in the long-term thermal performance of the Hijiori reservoir could result from increasing well spacing from 100 to 150 m without major degradation of the hydraulic performance.

Jing, Z.; Willis-Richards, J.; Watanabe, K.; Hashida, T.

2000-10-01

301

Continuum mechanics analysis of fracture progression in the vitrified cryoprotective agent DP6  

PubMed Central

As part of an ongoing effort to study the continuum mechanics effects associated with cryopreservation, the current report focuses on the prediction of fracture formation in cryoprotective agents. Fractures had been previously observed in 1 m? samples of the cryoprotective agent cocktail DP6, contained in a standard 15 m? glass vial, and subjected to various cooling rates. These experimental observations were obtained by means of a cryomacroscope, which has been recently presented by the current research team. High and low cooling rates were found to produce very distinct patterns of cracking. The current study seeks to explain the observed patterns on the basis of stresses predicted from finite element analysis, which relies on a simple viscoelastic constitutive model and on estimates of the critical stress for cracking. The current study demonstrates that the stress which results in instantaneous fracture at low cooling rates is consistent with the stress to initiate fracture at high cooling rate. This consistency supports the credibility of the proposed constitutive model and analysis, and the unified criterion for fracturing, that is, a critical stress threshold. PMID:18412493

Steif, Paul S.; Palastro, Matthew C.; Rabin, Yoed

2008-01-01

302

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

303

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

304

Fracture closure in extension and mechanical interaction of parallel joints  

Microsoft Academic Search

It is well known that in many cases rock permeability depends upon in situ stress conditions and on the pressure of the flowing fluid. Parallel and quasi-parallel joints represent one of the most often observed permeability structures. Frequently, joint sets are closely spaced and although joint mechanical interaction could significantly affect their aperture, the interaction is usually ignored in the

Leonid N. Germanovich; Dmitriy K. Astakhov

2004-01-01

305

Fracture Mechanics Analyses of Subsurface Defects in Reinforced Carbon-Carbon Joggles Subjected to Thermo-Mechanical Loads  

NASA Technical Reports Server (NTRS)

Coating spallation events have been observed along the slip-side joggle region of the Space Shuttle Orbiter wing-leading-edge panels. One potential contributor to the spallation event is a pressure build up within subsurface voids or defects due to volatiles or water vapor entrapped during fabrication, refurbishment, or normal operational use. The influence of entrapped pressure on the thermo-mechanical fracture-mechanics response of reinforced carbon-carbon with subsurface defects is studied. Plane-strain simulations with embedded subsurface defects are performed to characterize the fracture mechanics response for a given defect length when subjected to combined elevated-temperature and subsurface-defect pressure loadings to simulate the unvented defect condition. Various subsurface defect locations of a fixed-length substrate defect are examined for elevated temperature conditions. Fracture mechanics results suggest that entrapped pressure combined with local elevated temperatures have the potential to cause subsurface defect growth and possibly contribute to further material separation or even spallation. For this anomaly to occur, several unusual circumstances would be required making such an outcome unlikely but plausible.

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

2011-01-01

306

Increased fracture risk in patients with type 2 diabetes mellitus: an overview of the underlying mechanisms and the usefulness of imaging modalities and fracture risk assessment tools.  

PubMed

Type 2 diabetes mellitus has recently been linked to an increased fracture risk. Since bone mass seems to be normal to elevated in patient with type 2 diabetes, the increased fracture risk is thought to be due to both an increased falling frequency and decreased bone quality. The increased falling frequency is mainly a result of complications of the disease such as a retinopathy and polyneuropathy. Bone quality is affected through changes in bone shape, bone micro-architecture, and in material properties such as bone mineralization and the quality of collagen. Commonly used methods for predicting fracture risk such as dual energy X-ray absorptiometry and fracture risk assessment tools are helpful in patients with type 2 diabetes mellitus, but underestimate the absolute fracture risk for a given score. New imaging modalities such as high resolution peripheral quantitative computed tomography are promising for giving insight in the complex etiology underlying the fragility of the diabetic bone, as they can give more insight into the microarchitecture and geometry of the bone. We present an overview of the contributing mechanisms to the increased fracture risk and the usefulness of imaging modalities and risk assessment tools in predicting fracture risk in patients with type 2 diabetes. PMID:25192916

de Waard, Ellis A C; van Geel, Tineke A C M; Savelberg, Hans H C M; Koster, Annemarie; Geusens, Piet P M M; van den Bergh, Joop P W

2014-11-01

307

A study of failure in bonded lap joints using fracture mechanics  

NASA Astrophysics Data System (ADS)

Although adhesively bonded lap joint has already been widely applied in structures, the strength prediction of the joint is still a challenge. Among many parameters which affect the failure load of the lap joint, the adhesive layer thickness is a very important one. In this study, fracture mechanics is used to analyze the failure mechanism of the lap joint and evaluate the thickness effect on strength of the lap joint. Generally, the adhesive material is softer than the substrates and the crack in the adhesive layer is constrained by the rigid boundaries. The stress field is changed due to the effect of the rigid boundaries. For linear elastic material, the K-Dominance zone is highly reduced and the traditional constant stress intensity factor prediction over-predicts the failure load of the specimens. Constant effective fracture toughness is proposed for better strength prediction by considering of the non-singular stress term ahead of the crack tip. For elastic-plastic material, CTOA is proven to be insensitive to the constraining of the rigid boundaries and can be used as the failure criterion for fracture analysis. Both experimental and FEA simulation show that constant CTOA criterion well predicts the adhesive thickness effect on strength of the DCB specimens for mode I fracture failure. The failure initiation mode of single lap joint is analyzed and the constant CTOA criterion is also proven to be capable to prediction the strength of single lap joint.

Qian, Haiyang

308

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

309

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

310

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

E-print Network

The conductivity of acid-etched fractures highly depends on spaces along the fracture created by uneven etching of the fracture walls remaining open after fracture closure. Formation heterogeneities such as variations of mineralogy and permeability...

Deng, Jiayao

2011-02-22

311

Probabilistic Fracture Mechanics Evaluation of Selected Passive Components – Technical Letter Report  

SciTech Connect

This report addresses the potential application of probabilistic fracture mechanics computer codes to support the Proactive Materials Degradation Assessment (PMDA) program as a method to predict component failure probabilities. The present report describes probabilistic fracture mechanics calculations that were performed for selected components using the PRO-LOCA and PRAISE computer codes. The calculations address the failure mechanisms of stress corrosion cracking, intergranular stress corrosion cracking, and fatigue for components and operating conditions that are known to make particular components susceptible to cracking. It was demonstrated that the two codes can predict essentially the same failure probabilities if both codes start with the same fracture mechanics model and the same inputs to the model. Comparisons with field experience showed that both codes predict relatively high failure probabilities for components under operating conditions that have resulted in field failures. It was found that modeling assumptions and inputs tended to give higher calculated failure probabilities than those derived from data on field failures. Sensitivity calculations were performed to show that uncertainties in the probabilistic calculations were sufficiently large to explain the differences between predicted failure probabilities and field experience.

Simonen, Fredric A.; Doctor, Steven R.; Gosselin, Stephen R.; Rudland, David L.; Xu, H.; Wilkowski, Gery M.; Lydell, Bengt O.

2007-05-31

312

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

313

Fracture mechanics correlation of boron/aluminum coupons containing stress risers  

NASA Technical Reports Server (NTRS)

The mechanical behavior of boron/aluminum near stress risers has been studied and reported. This effort was directed toward defining the tensile behavior of both unidirectional and (0/ plus or minus 45) boron/aluminum using linear elastic fracture mechanics (LEFM). The material used was 5.6-mil boron in 6061 aluminum, consolidated using conventional diffusion bonding techniques. Mechanical properties are reported for both unidirectional and (0/ plus or minus 45) boron/aluminum, which serve as control data for the fracture mechanics predictions. Three different flawed specimen types were studied. In each case the series of specimens remained geometrically similar to eliminate variations in finite size correction factors. The fracture data from these tests were reduced using two techniques. They both used conventional LEFM methods, but the existence of a characteristic flaw was assumed in one case and not the other. Both the data and the physical behavior of the specimens support the characteristic flaw hypothesis. Cracks were observed growing slowly in the (0/ plus or minus 45) laminates, until a critical crack length was reached at which time catastrophic failure occurred.

Adsit, N. R.; Waszczak, J. P.

1975-01-01

314

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

315

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

E-print Network

artificially fractured hot dry rock (HDR) reservoirs requires three main ingredients: (1) a proper thermoA thermo-hydro-mechanical coupled model in local thermal non-equilibrium for fractured HDR reservoir Rachel Geleta,b , Benjamin Loreta, , Nasser Khalilib aLaboratoire Sols, Solides, Structures, B

Boyer, Edmond

316

A Qualitative Engineering Analysis of Occlusion Effects on Mandibular Fracture Repair Mechanics  

PubMed Central

Objectives. The purpose of this analytical study was to examine and critique the engineering foundations of commonly accepted biomechanical principles of mandible fracture repair. Materials and Methods. Basic principles of static equilibrium were applied to intact and plated mandibles, but instead of the traditional lever forces, the mandibles were subjected to more realistic occlusal forces. Results. These loading conditions produced stress distributions within the intact mandible that were very different and more complex than the customary lever-based gradient. The analyses also demonstrated the entirely different mechanical environments within intact and plated mandibles. Conclusions. Because the loading and geometry of the lever-idealized mandible is incomplete, the associated widely accepted bone stress distribution (tension on top and compression on the bottom) should not be assumed. Furthermore, the stress gradients within the bone of an intact mandible should not be extrapolated to the mechanical environment within the plated regions of a fractured mandible. PMID:22073042

Katona, Thomas R.

2011-01-01

317

A variationally coupled FE-BE method for elasticity and fracture mechanics  

NASA Technical Reports Server (NTRS)

A new method for coupling finite element and boundary element subdomains in elasticity and fracture mechanics problems is described. The essential feature of this new method is that a single variational statement is obtained for the entire domain, and in this process the terms associated with tractions on the interfaces between the subdomains are eliminated. This provides the additional advantage that the ambiguities associated with the matching of discontinuous tractions are circumvented. The method leads to a direct procedure for obtaining the discrete equations for the coupled problem without any intermediate steps. In order to evaluate this method and compare it with previous methods, a patch test for coupled procedures has been devised. Evaluation of this variationally coupled method and other methods, such as stiffness coupling and constraint traction matching coupling, shows that this method is substantially superior. Solutions for a series of fracture mechanics problems are also reported to illustrate the effectiveness of this method.

Lu, Y. Y.; Belytschko, T.; Liu, W. K.

1991-01-01

318

Unusual Mechanism of Injury Resulting in a Thoracic Chance Fracture in a Rodeo Athlete: A Case Report  

PubMed Central

Objective: To introduce the characteristics of a Chance fracture and increase awareness of the mechanism of injury that may occur during athletic activity. Background: A T12 Chance fracture was diagnosed in an 18-year-old male rodeo athlete. The rider was forced into extreme lumbothoracic hyperflexion when the horse bucked within the chute, pinning the rider's legs to his chest. Differential Diagnosis: Burst fracture, abdominal organ rupture, spinal dislocation, spinal cord injury, disk herniation, pars interarticularis fracture, spinal nerve injury, paralysis. Treatment: The patient underwent an open reduction and fixation of the thoracic fracture. Posterior stabilization was obtained with nonsegmental instrumentation. Allograft and autografts were used for posterolateral arthrodesis at T11–T12 and T12–L1. Uniqueness: Motor vehicle crashes with occupants wearing lap-type–only restraints account for nearly all previously reported Chance fractures. When only lap seatbelts are worn, the pelvis is stabilized, and the torso continues moving forward with impact. The stabilized body segment for this individual was reversed. Nearly 3 years after the initial surgery, fixation, and infection, the bareback rider has returned to full participation in rodeo. Conclusions: To our knowledge, this is the first reported diagnosis of a T12 Chance fracture in a rodeo athlete. When animals buck, athletes can be forced into hyperflexion, exposing them to Chance fractures. Therefore, anyone treating rodeo athletes must suspect possible spinal fracture when this mechanism is present and must treat all athletes with early conservative management and hospital referral. PMID:24520836

Boham, Mikaela; O'Connell, Kim

2014-01-01

319

Application of Fracture Mechanics Concepts to Hierarchical Biomechanics of Bone and Bone-like Materials  

Microsoft Academic Search

Fracture mechanics concepts are applied to gain some understanding of the hierarchical nanocomposite structures of hard biological\\u000a tissues such as bone, tooth and shells. At the most elementary level of structural hierarchy, bone and bone-like materials\\u000a exhibit a generic structure on the nanometer length scale consisting of hard mineral platelets arranged in a parallel staggered\\u000a pattern in a soft protein

Huajian Gao

2006-01-01

320

Anticipating paint cracking: The application of fracture mechanics to the study of paint weathering  

Microsoft Academic Search

The relationships between the fracture energy (Gc), film thickness, stress level, and weathering time were investigated for a number of automotive clearcoats. As weathering\\u000a progressed, most clearcoats embrittled due to the combined effects of photooxidation, hydrolysis, and other degradation mechanisms.\\u000a This embrittlement was measured and related to the chemical composition changes that take place in the clearcoat during weathering.\\u000a When

Mark E. Nichols

2002-01-01

321

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

322

A BEM analysis of fracture mechanics in 2D anisotropic piezoelectric solids  

Microsoft Academic Search

This paper presents a single-domain boundary element method (BEM) analysis of fracture mechanics in 2D anisotropic piezoelectric solids. In this analysis, the extended displacement (elastic displacement and electrical potential) and extended traction (elastic traction and electrical displacement) integral equations are collocated on the outside boundary (no-crack boundary) of the problem and on one side of the crack surface, respectively. The

Ernian Pan

1999-01-01

323

Fracture mechanics analysis of a high-pressure hydrogen facility compressor  

NASA Technical Reports Server (NTRS)

The investigation and analysis of a high-pressure hydrogen facility compressor is chronicled, and a life prediction based on fracture mechanics is presented. Crack growth rates in SA 105 Gr II steel are developed for the condition of sustained loading, using a hypothesis of hydrogen embrittlement associated with plastic zone reverse yielding. The resultant formula is compared with test data obtained from laboratory specimens.

Vroman, G. A.

1974-01-01

324

Model for toughness curves in two-phase ceramics: 1. Basic fracture mechanics  

Microsoft Academic Search

A fracture mechanics model is presented for the toughening of ceramics by bridging from second-phase particles, resulting in toughness curve (T-curve) behavior. It is assumed that the second-phase particles are in a state of residual thermal expansion dilatational mismatch relative to the matrix. In the long-crack region, these stresses augment frictional sliding stresses at the interphase boundaries, enhancing the crack

Brian R. Lawn; Nitin P. Padture; Linda M. Braun; Stephen J. Bennison

1993-01-01

325

Three-dimensional elastic--plastic stress and strain analyses for fracture mechanics: complex geometries  

Microsoft Academic Search

The report describes the continuation of research into capability for three-dimensional elastic-plastic stress and strain analysis for fracture mechanics. A computer program, MARC-3D, has been completed and was used to analyze a cylindrical pressure vessel with a nozzle insert. A method for generating crack tip elements was developed and a model was created for a cylindrical pressure vessel with a

Bellucci

1975-01-01

326

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

327

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

328

On the similarity in the formation mechanism of the fracture structure of a rock massif  

NASA Astrophysics Data System (ADS)

Local segments of the Earth's crust reside at the mechanical nonequilibrium and continuously obtain and dissipate mechanical energy. The energy exchange between the structural elements of a geophysical medium determines its state, especially if the medium is fragmented into blocks. The stationary state of a rock differs from its static equilibrium by the fact that the mechanical energy is conserved because the energy input is equal to the energy dissipation. From this point of view, the cracks, the faults, and the block structure are not simply the manifestations of rock destruction, but rather the mode of existence of a medium with large irreversible deformations. Then, the fracture structure, whose formation is actually a response of a rock to large irreversible deformation, becomes, in terms of physics, the characteristic of the state of a geophysical medium; for example, it allows one to assess the parameters of the deformation processes during the period of formation of the fracture structure. The present paper addresses the identification of the features of the fracture structure in geological objects of different scales.

Bagdasar'yan, A. G.; Sytenkov, V. N.; Fedyanina, L. T.; Shemetov, P. A.

2011-04-01

329

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

330

Studying physical properties of deformed intact and fractured rocks by micro-scale hydro-mechanical-seismicity model  

NASA Astrophysics Data System (ADS)

The pore pressure variation in an underground formation during hydraulic stimulation of low permeability formations or CO2 sequestration into saline aquifers can induce microseismicity due to fracture generation or pre-existing fracture activation. While the analysis of microseismic data mainly focuses on mapping the location of fractures, the seismic waves generated by the microseismic events also contain information for understanding of fracture mechanisms based on microseismic source analysis. We developed a micro-scale geomechanics, fluid-flow and seismic model that can predict transport and seismic source behavior during rock failure. This model features the incorporation of microseismic source analysis in fractured and intact rock transport properties during possible rock damage and failure. The modeling method considers comprehensive grains and cements interaction through a bonded-particle-model. As a result of grain deformation and microcrack development in the rock sample, forces and displacements in the grains involved in the bond breakage are measured to determine seismic moment tensor. In addition, geometric description of the complex pore structure is regenerated to predict fluid flow behavior of fractured samples. Numerical experiments are conducted for different intact and fractured digital rock samples, representing various mechanical behaviors of rocks and fracture surface properties, to consider their roles on seismic and transport properties of rocks during deformation. Studying rock deformation in detail provides an opportunity to understand the relationship between source mechanism of microseismic events and transport properties of damaged rocks to have a better characterizing of fluid flow behavior in subsurface formations.

Raziperchikolaee, Samin

331

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

332

Computational implementation of the multi-mechanism deformation coupled fracture model for salt  

SciTech Connect

The Multi-Mechanism Deformation (M-D) model for creep in rock salt has been used in three-dimensional computations for the Waste Isolation Pilot Plant (WIPP), a potential waste, repository. These computational studies are relied upon to make key predictions about long-term behavior of the repository. Recently, the M-D model was extended to include creep-induced damage. The extended model, the Multi-Mechanism Deformation Coupled Fracture (MDCF) model, is considerably more complicated than the M-D model and required a different technology from that of the M-D model for a computational implementation.

Koteras, J.R.; Munson, D.E.

1996-05-01

333

Mechanical stress, fracture risk and beak evolution in Darwin's ground finches (Geospiza).  

PubMed

Darwin's finches have radiated from a common ancestor into 14 descendent species, each specializing on distinct food resources and evolving divergent beak forms. Beak morphology in the ground finches (Geospiza) has been shown to evolve via natural selection in response to variation in food type, food availability and interspecific competition for food. From a mechanical perspective, however, beak size and shape are only indirectly related to birds' abilities to crack seeds, and beak form is hypothesized to evolve mainly under selection for fracture avoidance. Here, we test the fracture-avoidance hypothesis using finite-element modelling. We find that across species, mechanical loading is similar and approaches reported values of bone strength, thus suggesting pervasive selection on fracture avoidance. Additionally, deep and wide beaks are better suited for dissipating stress than are more elongate beaks when scaled to common sizes and loadings. Our results illustrate that deep and wide beaks in ground finches enable reduction of areas with high stress and peak stress magnitudes, allowing birds to crack hard seeds while limiting the risk of beak failure. These results may explain strong selection on beak depth and width in natural populations of Darwin's finches. PMID:20194171

Soons, Joris; Herrel, Anthony; Genbrugge, Annelies; Aerts, Peter; Podos, Jeffrey; Adriaens, Dominique; de Witte, Yoni; Jacobs, Patric; Dirckx, Joris

2010-04-12

334

Transformation of creep stresses in ice shelves for fracture mechanical analyses  

NASA Astrophysics Data System (ADS)

Recent disintegrations of ice shelves and calving at ice fronts require a better understanding of fracture mechanical processes involved in these events. Previous studies on cracks in ice shelves with different stress states, density profiles and material parameters highlighted the dependence of the crack criticality on the choice of boundary conditions and the applied material law. To this end, we present different approaches how to transfer the flow induced viscous stresses into the fracture mechanical analysis for the study of vertical and horizontal crack scenarios. The numerical simulations are performed using Finite Elements and the concept of configurational forces for the evaluation of stress intensity factors. This approach is rather flexible and can account for inhomogeneous material properties, volume forces and crack face loadings, e.g. due to additional water pressure. The methods are applied to 2-dimensional geometries under various loading conditions and material parameters. The methods are validated using data from the literature and the results are compared to fracture events that have been documented using remote sensing data analysis.

Plate, C.; Müller, R.; Humbert, A.; Wilkens, N.; Gross, D.

2012-04-01

335

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

336

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

337

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

338

Correlation of Mechanical Properties with Fracture Surface Features in a Newly Developed Dual-Phase Steel  

NASA Astrophysics Data System (ADS)

Dual-phase (DP) steels were produced by a newly developed method utilizing simple cold-rolling and subsequent short intercritical annealing of a martensite-ferrite duplex starting structure. Tensile testing revealed an excellent strength-elongation balance (UTS × UE ? 110-150 J/cm3) for the DP steels in comparison with the commercially used high strength steels. Fracture surfaces of the tensile specimens were studied by scanning electron microscopy analysis and image processing. Mechanical properties were correlated with fracture surface features. It was found that the variation of the total elongation and strength-elongation balance with the martensite volume fraction could be well correlated with the variation of the average dimple area. The variation of the yield strength and dimple areal density with the martensite volume fraction followed the same trend.

Mazaheri, Y.; Saeidi, N.; Kermanpur, A.; Najafizadeh, A.

2015-04-01

339

Mechanical Modeling of Foods Including Fracture and Simulation of Food Compression  

NASA Astrophysics Data System (ADS)

The purposes of this research are to simulate the swallowing of foods, and to investigate the relationship between the rheological properties of foods and the swallowing. Here we proposed the mechanical modeling of foods, and simulated the compression test using the finite element method. A linear plasticity model was applied as the rheological model of the foods, and two types of computational elements were used to simulate the fracture behavior. The compression tests with a wedged plunger were simulated for tofu, banana, and biscuit, and were compared with the experimental results. Other than the homogeneous food model, the simulations were conducted for the multi-layer models. Reasonable agreements on the behaviors of compression and fracture were obtained between the simulations and the experiments including the reaction forces on the plunger.

Morimoto, Masamichi; Mizunuma, Hiroshi; Sonomura, Mitsuhiro; Kohyama, Kaoru; Ogoshi, Hiro

2008-07-01

340

A nonlocal continuum damage mechanics approach to simulation of creep fracture in ice sheets  

NASA Astrophysics Data System (ADS)

We present a Lagrangian finite element formulation aimed at modeling creep fracture in ice-sheets using nonlocal continuum damage mechanics. The proposed formulation is based on a thermo-viscoelastic constitutive model and a creep damage model for polycrystalline ice with different behavior in tension and compression. In this paper, mainly, we detail the nonlocal numerical implementation of the constitutive damage model into commercial finite element codes (e.g. Abaqus), wherein a procedure to handle the abrupt failure (rupture) of ice under tension is proposed. Then, we present numerical examples of creep fracture under four-point bending, uniaxial tension, and biaxial tension in order to illustrate the viability of the current approach. Finally, we present simulations of creep crack propagation in idealized rectangular ice slabs so as to estimate calving rates at low deformation rates. The examples presented demonstrate the mesh size and mesh directionality independence of the proposed nonlocal implementation.

Duddu, Ravindra; Waisman, Haim

2013-06-01

341

Correlation of Mechanical Properties with Fracture Surface Features in a Newly Developed Dual-Phase Steel  

NASA Astrophysics Data System (ADS)

Dual-phase (DP) steels were produced by a newly developed method utilizing simple cold-rolling and subsequent short intercritical annealing of a martensite-ferrite duplex starting structure. Tensile testing revealed an excellent strength-elongation balance (UTS × UE ? 110-150 J/cm3) for the DP steels in comparison with the commercially used high strength steels. Fracture surfaces of the tensile specimens were studied by scanning electron microscopy analysis and image processing. Mechanical properties were correlated with fracture surface features. It was found that the variation of the total elongation and strength-elongation balance with the martensite volume fraction could be well correlated with the variation of the average dimple area. The variation of the yield strength and dimple areal density with the martensite volume fraction followed the same trend.

Mazaheri, Y.; Saeidi, N.; Kermanpur, A.; Najafizadeh, A.

2015-02-01

342

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

E-print Network

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, Peter; Gallipoli, Domenico; Wheeler, Simon J

2014-01-01

343

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

344

Study on Shear Fatigue Fracture and Delamination Mechanism of Thermal Barrier Coatings After Thermal Loading  

NASA Astrophysics Data System (ADS)

In this study, it is investigated experimentally and analytically that fracture process and fatigue behavior of Thermal Barrier Coatings (TBCs) used in a high temperature component in a land based gas turbine. The effects of the thermal load on TBCs are investigated by measuring residual stress of top coat, finite element method and torsion test. It is found that delamination of top coat under thermal load occurs at a peak of undulation in top coat and bond coat interface. Around the peak, a tearing stress is generated. Fracture and fatigue tests under torsion are carried out by using cylindrical butted specimens. It is found that the mechanical property of bond coat changes after thermal aging and the shear fatigue behavior of TBCs is affected by bond coat strength.

Kaneko, Kenji; Takatou, Satoshi; Enomoto, Kazuki

345

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

346

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

347

Consideration on the Mechanism of Microwave Emission Due to Rock Fracture  

NASA Astrophysics Data System (ADS)

Microwave emission due to rock fracture was found at 300 MHz, 2 GHz, and 22 GHz, and its power was calibrated in laboratory for the first time in the world. The observed waveform is impulsive, and contains correspondent frequency component inside the envelope at each frequency band. At such high frequencies, the electro-magnetic signal power can be calibrated as a radiating wave with high accuracy. Accordingly, it was verified that a substantial power is emitted. The microwave emission phenomena were also observed on occasions of hypervelocity impact, and esteemed as phenomena generally associated with material destruction. Earthquakes and volcanic activities are association with rock fractures so that the microwave is expected to be emitted. Actually, the e emission was confirmed by the data analysis of the brightness temperature obtained by a remote sensing satellite, which flew over great earthquakes of Wuenchan and Sumatra, and great volcanic eruptions of Reventador and Chanten. It is important to show the microwave emission during rock fracture in natural phenomena. Therefore, the field test to detect the microwave due to the collapse of a crater cliff was planned and persecuted at the volcano of Miyake-jima about 100 km south of Tokyo. Volcanic activity may be more convenient than an earthquake because of the known location and time. As a result, they observed the microwave emission which was strongly correlated with the cliff collapses. Despite of the above-mentioned phenomenological fruits, the reason of the microwave emission is not fixed yet. We have investigated the mechanism of the emission in consideration of the obtained data in rock fracture experiments so far and the study results on material destruction by hypervelocity impact. This paper presents the proposal of the hypothesis and resultant discussions. The microwave sensors may be useful to monitor natural hazards such as an earthquake or a volcanic eruption, because the microwave due to rock fracture can penetrate the ionosphere and can be received by a satellite in orbit. However, the relation between a rock fracture and quakes has not been clarified at all, and is left to the future research. Please fill in your abstract text.

Takano, Tadashi; Sugita, Seiji; Yoshida, Shingo; Maeda, Takashi

2010-05-01

348

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

349

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

350

Type II odontoid fracture from frontal impact: case report and biomechanical mechanism of injury.  

PubMed

The authors report a case of Type II odontoid fracture from a frontal impact sustained in the crash of a late-model motor vehicle. They discuss the biomechanical mechanisms of injury after considering patient demographic data, type and use of restraint systems including seatbelt and airbags, crash characteristics, and laboratory-based experimental studies. Multiple factors contributed to the Type II odontoid fracture: the patient's tall stature and intoxicated state; lack of manual three-point seat belt use; obliqueness of the frontal impact; and the most likely preflexed position of the head-neck complex at the time of impact, which led to contact of the parietal region with the A-pillar roof-rail area of the vehicle and resulted in the transfer of the dynamic compressive force associated with lateral bending. Odontoid fractures still occur in individuals involved in late-model motor vehicle frontal crashes, and because this injury occurs secondary to head impact, airbags may not play a major role in mitigating this type of trauma to an unrestrained occupant. It may be more important to use seat belts than to depend on the airbag alone for protection from injury. PMID:15871490

Yoganandan, Narayan; Baisden, Jamie L; Maiman, Dennis J; Pintar, Frank A

2005-04-01

351

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

352

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

PubMed

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. PMID:25298024

Tang, Chun; Dávila, Lilian P

2014-10-31

353

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

354

Confirmation of a thin sheet toughening mechanism and anisotropic fracture in Al-Fe-X alloys  

NASA Astrophysics Data System (ADS)

The fracture toughness, K c, of two rapid-solidification powder-metallurgy Al-Fe-X alloys, Al-8Fe-1.4V-1. 7Si and Al-8Fe-2Mo-1V, has been measured as a function of specimen thickness. The former alloy has been found to exhibit crack-divider delamination and a K IC value independent of specimen thickness, while an opposite behavior is found in the latter alloy. The results confirm the presence of a thin sheet toughening mechanism in the Al-Fe-V-Si alloy. The orientation dependence of K IC and an unusual anisotropy fracture behavior observed in some, but not all, Al-Fe-X alloys have also been studied and their origins identified. It is shown that both thin sheet toughening and anisotropic fracture in Al-Fe-X alloys arise from interface delamination and void nucleation at oxide fragments aligned along prior powder-particle bound-aries (PPB’s).

Chan, Kwai S.

1989-11-01

355

A review on recent contribution of meshfree methods to structure and fracture mechanics applications.  

PubMed

Meshfree methods are viewed as next generation computational techniques. With evident limitations of conventional grid based methods, like FEM, in dealing with problems of fracture mechanics, large deformation, and simulation of manufacturing processes, meshfree methods have gained much attention by researchers. A number of meshfree methods have been proposed till now for analyzing complex problems in various fields of engineering. Present work attempts to review recent developments and some earlier applications of well-known meshfree methods like EFG and MLPG to various types of structure mechanics and fracture mechanics applications like bending, buckling, free vibration analysis, sensitivity analysis and topology optimization, single and mixed mode crack problems, fatigue crack growth, and dynamic crack analysis and some typical applications like vibration of cracked structures, thermoelastic crack problems, and failure transition in impact problems. Due to complex nature of meshfree shape functions and evaluation of integrals in domain, meshless methods are computationally expensive as compared to conventional mesh based methods. Some improved versions of original meshfree methods and other techniques suggested by researchers to improve computational efficiency of meshfree methods are also reviewed here. PMID:24516359

Daxini, S D; Prajapati, J M

2014-01-01

356

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

357

A Review on Recent Contribution of Meshfree Methods to Structure and Fracture Mechanics Applications  

PubMed Central

Meshfree methods are viewed as next generation computational techniques. With evident limitations of conventional grid based methods, like FEM, in dealing with problems of fracture mechanics, large deformation, and simulation of manufacturing processes, meshfree methods have gained much attention by researchers. A number of meshfree methods have been proposed till now for analyzing complex problems in various fields of engineering. Present work attempts to review recent developments and some earlier applications of well-known meshfree methods like EFG and MLPG to various types of structure mechanics and fracture mechanics applications like bending, buckling, free vibration analysis, sensitivity analysis and topology optimization, single and mixed mode crack problems, fatigue crack growth, and dynamic crack analysis and some typical applications like vibration of cracked structures, thermoelastic crack problems, and failure transition in impact problems. Due to complex nature of meshfree shape functions and evaluation of integrals in domain, meshless methods are computationally expensive as compared to conventional mesh based methods. Some improved versions of original meshfree methods and other techniques suggested by researchers to improve computational efficiency of meshfree methods are also reviewed here. PMID:24516359

Daxini, S. D.; Prajapati, J. M.

2014-01-01

358

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

359

Vena Tech LGM filter retrieval 16 years after implantation: piecemeal removal by intentional mechanical fracture.  

PubMed

A 48-year-old man presented with symptomatic inferior vena cava (IVC) occlusion from a chronically thrombosed and embedded Vena Tech LGM filter resulting in exercise intolerance from diminished cardiac preload and postthrombotic syndrome from chronic venous insufficiency. The patient was treated using a new PRIME technique--Piecemeal Removal by Intentional MEchanical fracture--to achieve successful filter retrieval 16 years after implantation. Removal of the obstructing filter permitted endovascular IVC recanalization with restoration of venous outflow and alleviation of venous obstructive symptoms. Cardiac preload was restored, allowing the patient to resume long-distance running, and he successfully completed a half-marathon 3 months after treatment. PMID:24160825

Kuo, William T; Deso, Steven E; Robertson, Scott W

2013-11-01

360

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

361

Fracture mechanics. [review of fatigue crack propagation and technology of constructing safe structures  

NASA Technical Reports Server (NTRS)

Fracture mechanics is a rapidly emerging discipline for assessing the residual strength of structures containing flaws due to fatigue, corrosion or accidental damage and for anticipating the rate of which such flaws will propagate if not repaired. The discipline is also applicable in the design of structures with improved resistance to such flaws. The present state of the design art is reviewed using this technology to choose materials, to configure safe and efficient structures, to specify inspection procedures, to predict lives of flawed structures and to develop reliability of current and future airframes.

Hardrath, H. F.

1974-01-01

362

FFTF irradiation of fracture mechanics specimens for out-of-core structures  

Microsoft Academic Search

The National Program Plan has established data requirements for out-of-core structures for FBRs. Significant FFTF irradiation space with moderate gamma heating levels is required to irradiate relatively large fracture mechanics specimens to total neutron fluences ranging between 5 x 10²¹ and 5 x 10²² n\\/cm² and temperatures which range between 400°C (750°F) and 650°C (1200°F). Priority 1 data on stainless

2010-01-01

363

Simulation of the behavior of the cutting force during ultrasonic rotary machining of materials using structure-time fracture mechanics  

NASA Astrophysics Data System (ADS)

An analytical model is developed for the behavior of the cutting force during ultrasonic rotary polishing, and it is based on the concepts of dynamic fracture mechanics and the solution to the problem of impact surface fracture. The dependence of the threshold fracture energy obtained in the problem of erosion using a structure-time approach is used to construct the cutting force model. The dependences of the cutting force on the material feed rate and the rate of tool rotation are obtained, and the developed model is shown to be efficient to explain the effects observed in experiments.

Gorbushin, N. A.; Volkov, G. A.; Petrov, Yu. V.

2014-06-01

364

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

365

Mechanical Engineering Department SPECIAL SEMINAR SERIES  

E-print Network

) under an ONR SBIR under the supervision of Drs. John Tague and Jan Lindberg. BIO: Nachiket Gokhale simulation of blast effects on buildings, elasto-plastic material model development for Alu- minum subjected

366

Mechanics of Jointed and Faulted Rock, Rossmanith (ed) 0 1995 Balkema, Rotterdam. ISBN 90 54 10 54 7 0 Seismic properties of a general fracture  

E-print Network

, and they control much of the mechanical strength and transport properties of its solid structure. Fractures of fractures and prediction of fluid flow directions is the use of seismic shear-waves (Crampin 1994; Liu et al 7 0 Seismic properties of a general fracture E. Liu British Geological Survey Edinburgh, UK J

Edinburgh, University of

367

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

368

A statistical, physical-based, micro-mechanical model of hydrogen-induced intergranular fracture in steel  

E-print Network

of the containment vessel from a variety of hydrogen-induced/assisted cracking mechanisms, which can resultA statistical, physical-based, micro-mechanical model of hydrogen-induced intergranular fracture 2009 Received in revised form 10 September 2009 Accepted 17 October 2009 Keywords: Hydrogen

Ritchie, Robert

369

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

370

Mechanoanions produced by mechanical fracture of bacterial cellulose: ionic nature of glycosidic linkage and electrostatic charging.  

PubMed

Mechanoanions were produced by heterogeneous scission of the glycosidic linkages of the main chain of bacterial cellulose (BC); scission was induced by mechanical fracture of the BC in a vacuum in the dark at 77 K. The mechanoanions were detected using electron-spin-trapping methods with tetracyanoethylene. The yield of mechanoanions was positively correlated with the absolute value of the change in the Mulliken atomic charge, which was used as a descriptor of the ionic nature of the glycosidic linkage. Homogeneous scission of the glycosidic linkages induced by mechanical fracture generated mechanoradicals, the electron affinity of which was estimated on the basis of the energy of the lowest unoccupied molecular orbital for the model structure of the mechanoradical. It was concluded that the electrostatic charging of BC is caused by electron transfer from mechanoanions to mechanoradicals, which have high electron affinities. The electrostatic charge density of BC in a vacuum in the dark at 77 K was estimated to be 6.00 × 10(-1) C/g. PMID:22978760

Sakaguchi, Masato; Makino, Masakazu; Ohura, Takeshi; Iwata, Tadahisa

2012-10-11

371

Fracture mechanics models developed for piping reliability assessment in light water reactors: piping reliability project  

SciTech Connect

The efforts concentrated on modifications of the stratified Monte Carlo code called PRAISE (Piping Reliability Analysis Including Seismic Events) to make it more widely applicable to probabilistic fracture mechanics analysis of nuclear reactor piping. Pipe failures are considered to occur as the result of crack-like defects introduced during fabrication, that escape detection during inspections. The code modifications allow the following factors in addition to those considered in earlier work to be treated: other materials, failure criteria and subcritical crack growth characteristic; welding residual and vibratory stresses; and longitudinal welds (the original version considered only circumferential welds). The fracture mechanics background for the code modifications is included, and details of the modifications themselves provided. Additionally, an updated version of the PRAISE user's manual is included. The revised code, known as PRAISE-B was then applied to a variety of piping problems, including various size lines subject to stress corrosion cracking and vibratory stresses. Analyses including residual stresses and longitudinal welds were also performed.

Harris, D.O.; Lim, E.Y.; Dedhia, D.D.; Woo, H.H.; Chou, C.K.

1982-06-01

372

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

NASA Astrophysics Data System (ADS)

Characterization 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

373

Thermo-mechanically coupled fracture analysis of shape memory alloys using the extended finite element method  

NASA Astrophysics Data System (ADS)

In this paper, the extended finite element method is used for fracture analysis of shape memory alloys for both cases of super elastic and shape memory effects. Heat generation during the forward and reverse phase transformations can lead to temperature variation in the material because of strong thermo-mechanical coupling, which significantly influences the SMA mechanical behavior. First, the stationary crack mode is studied and the effects of loading rate on material behavior in the crack tip are examined. Then, the crack propagation analysis is performed in the presence of an initial crack by adopting a weighted averaging criterion, where the direction of crack propagation is determined by weighted averaging of effective stresses at all the integration points in the vicinity of the crack tip. Finally, several numerical examples are analyzed and the obtained results are compared with the available reference results.

Hatefi Ardakani, S.; Ahmadian, H.; Mohammadi, S.

2015-04-01

374

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

375

Fracture and Damage Mechanics of Marginal Weakening and Rifting in Pine Island Glacier Ice Shelf  

NASA Astrophysics Data System (ADS)

The southern ice shelf of Pine Island Glacier has experienced at least seven major tabular calving events since 1973. The most recent calving in July 2013 was the culmination of a rift that initiated further upstream than any in the observational record. Here, we apply fracture and damage mechanics to analyze the pattern, location, and relationship between marginal weakening and rifting over time for the ice shelf. We compute spatially variable maps of ice rigidity for the years 1996 and 2007 using an inverse control method implemented in the Ice Sheet System Model (ISSM). We derive a new theoretical relation for the creep of a damaged ice shelf and use this relation, combined with an analytical temperature calculation for the ice shelf, to partition the inverted rigidity into solutions for damage and backstress for the ice shelf. We find that damage in the shear margins has increased in magnitude and spatial extent over time, in agreement with the amount of marginal weakening necessary for analytical centerline velocity calculations to be consistent with observations. We apply Linear Elastic Fracture Mechanics (LEFM) to analyze the conditions under which rifts originate in the ice shelf. All rifts observed over the last 40 years have been located adjacent to or downstream of an ice rumple located along the northern shear margin of the ice shelf. Prior to 2001, the rifts appeared to initiate in the center of the ice shelf and propagate symmetrically outward and transverse to the flow. Since 2001, however, the rifts have initiated from existing marginal fractures in the wake of the ice rumple. This change in behavior is explained by the loss of buttressing from the ice rumple as the ice upstream has thinned and weakened. Simple LEFM considerations predict that rifts may form more frequently in the future, as propagating a fracture from the edge of a plate is easier, for a given stress, than from the middle. Furthermore, given the reduced buttressing near the front of the ice shelf, future rifts are increasingly likely to originate further upstream than observed previously.

Borstad, C. P.; Rignot, E. J.; Larour, E. Y.

2013-12-01

376

Chemical-mechanical interactions during natural fracture growth in tight gas and oil reservoirs: Implications for flow during reservoir charge and production  

NASA Astrophysics Data System (ADS)

Natural fractures in tight sandstone and shale reservoirs are characterized by partial to complete cementation. In all tight-gas sandstone reservoirs and suitable outcrop reservoir analogs, fractures frequently contain crack-seal quartz and carbonate cement that formed during incremental fracture opening. These synkinematic cements may be followed by blocky postkinematic cement occluding any residual fracture porosity. Fluid inclusion microthermometry combined with Raman analyses demonstrate that synkinematic cement formed under conditions close to maximum burial and incipient exhumation under elevated pore fluid pressures and over time spans of 10-50 m.y.. Fracture opening rates, integrated over the kinematic fracture aperture, are on the order of 10 microm/m.y. Based on the textural evidence of synkinematic cement growth, in combination with kinetic models of quartz cementation, we infer that these rates are comparable to rates of dissolution-precipitation reactions in the host rock, and of mass transfer between host rock and fracture. It is thus suggested that dissolution-precipitation creep is a dominant deformation mechanism allowing accommodation of permanent fracture strain under these deep-burial, diagenetically reactive conditions. Synkinematic mineral reactions in the host rock and precipitation of fracture lining cement guarantee that partially cemented natural fractures remain propped open and thus conductive under production conditions. However, cement linings and bridges can inhibit flow between micro-porous host rock and residual fracture porosity resulting in flow barriers. Complex pore geometry in partially cemented fractures may impede multiphase fracture flow and production. In shale, the interface between host rock and fracture cement is frequently mechanically weak potentially allowing fracture reactivation during well completion. Such artificially reactivated fractures may thus increase flow of production fluids even in formation containing otherwise sealed natural fractures.

Eichhubl, Peter; Fall, Andras; Prodanovic, Masa; Tokan-Lawal, Adenike; Lander, Robert; Laubach, Steve

2013-04-01

377

Chemical-mechanical interactions during natural fracture growth in tight gas and oil reservoirs: Implications for flow during reservoir charge and production  

NASA Astrophysics Data System (ADS)

Natural fractures in tight sandstone and shale reservoirs are characterized by partial to complete cementation. In all tight-gas sandstone reservoirs and suitable outcrop reservoir analogs, fractures frequently contain crack-seal quartz and carbonate cement that formed during incremental fracture opening. These synkinematic cements may be followed by blocky postkinematic cement occluding any residual fracture porosity. Fluid inclusion microthermometry combined with Raman analyses demonstrate that synkinematic cement formed under conditions close to maximum burial and incipient exhumation under elevated pore fluid pressures and over time spans of 10-40 m.y.. Fracture opening rates, integrated over the kinematic fracture aperture, are on the order of 10 ?m/m.y. Based on the textural evidence of synkinematic cement growth, in combination with kinetic models of quartz cementation, we infer that these rates are comparable to rates of dissolution-precipitation reactions in the host rock, and of mass transfer between host rock and fracture. It is thus suggested that dissolution-precipitation creep is a dominant deformation mechanism allowing accommodation of permanent fracture strain under these deep-burial, diagenetically reactive conditions. Synkinematic mineral reactions in the host rock and precipitation of fracture lining cement guarantee that partially cemented natural fractures remain propped open and thus conductive under production conditions. However, cement linings and bridges can inhibit flow between micro-porous host rock and residual fracture porosity resulting in flow barriers. Complex pore geometry in partially cemented fractures may impede multiphase fracture flow and production. In shale, the interface between host rock and fracture cement is frequently mechanically weak potentially allowing fracture reactivation during well completion. Such artificially reactivated fractures may thus increase flow of production fluids even in formation containing otherwise sealed natural fractures.

Eichhubl, P.; Fall, A.; Prodanovic, M.; Weisenberger, T.; Ukar, E.; Laubach, S.; Gale, J. F.

2012-12-01

378

Strut fracture mechanisms of the Björk-Shiley convexo-concave heart valve.  

PubMed

Investigations of convexo-concave (C/C) valve outlet strut fractures (OSFs) were initially confounded by knowledge that the strut was subject to bending forces in arresting the opening disc. Pulse duplicator studies subsequently showed that closing loads were all born by the inlet strut, along with an understandable focus on the nature of the welds, where most fractures occurred. As observations of explanted valves accumulated, certain features pointed to unusual closing loads that might be contributory factors, but these hypothetical forces could not be verified. Epidemiological extrapolations and case-matched control studies have shown that certain valve and patient characteristics were each associated independently with increased OSF risk, leading to clinically valuable risk stratification, but little additional understanding of why OSFs continued to occur. Detection of the causative, highly transient (< 0.5 ms), outlet-strut-tip impacts due to closing disc over-rotation that have almost ten times the force of disc opening, and the capability of inducing leg-base bending stresses beyond the strut wire's fatigue endurance limit had to await the development of computer-controlled pulse duplicators and strut-leg strain gaging. Exercised young animals easily achieved such strut loading, but most human patients would probably have more difficulty. The actual OSF mechanism is a long-term, valve-patient interaction that requires the concurrence of susceptible valve geometry and sufficient ventricular contractility potential to develop the isovolumic, high dP/dt needed for forceful disc over-rotation. Critical strut tip loading must then occur often enough to fatigue fracture both strut legs within the patient's lifetime with the valve. PMID:10224582

Wieting, D W; Eberhardt, A C; Reul, H; Breznock, E M; Schreck, S G; Chandler, J G

1999-03-01

379

Stability-based classification for ankle fracture management and the syndesmosis injury in ankle fractures due to a supination external rotation mechanism of injury.  

PubMed

The aim of this thesis was to confirm the utility of stability-based ankle fracture classification in choosing between non-operative and operative treatment of ankle fractures, to determine how many ankle fractures are amenable to non-operative treatment, to assess the roles of the exploration and anatomical repair of the AITFL in the outcome of patients with SER ankle fractures, to establish the sensitivities, specificities and interobserver reliabilities of the hook and intraoperative stress tests for diagnosing syndesmosis instability in SER ankle fractures, and to determine whether transfixation of unstable syndesmosis is necessary in SER ankle fractures. The utility of stability based fracture classification to choose between non-operative and operative treatment was assessed in a retrospective study (1) of 253 ankle fractures in skeletally mature patients, 160 of whom were included in the study to obtain an epidemiological profile in a population of 130,000. Outcome was assessed after a minimum follow-up of two years. The role of AITFL repairs was assessed in a retrospective study (2) of 288 patients with Lauge-Hansen SE4 ankle fractures; the AITFL was explored and repaired in one group (n=165), and a similar operative method was used but the AITFL was not explored in another group (n=123). Outcome was measured with a minimum follow-up of two years. Interobserver reliability of clinical syndesomosis tests (study 3) and the role of syndesmosis transfixation (study 4) were assessed in a prospective study of 140 patients with Lauge-Hansen SE4 ankle fractures. The stability of the distal tibiofibular joint was evaluated by the hook and ER stress tests. Clinical tests were carried out by the main surgeon and assistant, separately, after which a 7.5-Nm standardized ER stress test for both ankles was performed; if it was positive, the patient was randomized to either syndesmosis transfixation (13 patients) or no fixation (11 patients) treatment groups. The sensitivity and specificity of both clinical tests were calculated using the standard 7.5-Nm external rotation stress test as reference. Outcome was assessed after a minimum of one year of follow-up. Olerud-Molander (OM) scoring system, RAND 36-Item Health Survey, and VAS to measure pain and function were used as outcome measures in all studies. In study 1, 85 (53%) fractures were treated operatively using the stability based fracture classification. Non-operatively treated patients reported less pain and better OM (good or excellent 89% vs. 71%) and VAS functional scores compared to operatively treated patients although they experienced more displacement of the distal fibula (0 mm 30% vs. 69%; 0-2 mm 65% vs. 25%) after treatment. No non-operatively treated patients required operative fracture fixation during follow-up. In study 2, AITFL exploration and suture lead to equal functional outcome (OM mean, 77 vs. 73) to no exploration or fixation. In study 3, the hook test had a sensitivity of 0.25 and a specificity of 0.98. The external rotation stress test had a sensitivity of 0.58 and a specificity of 0.9. Both tests had excellent interobserver reliability; the agreement was 99% for the hook test and 98% for the stress test. There was no statistically significant difference in functional scores (OM mean, 79.6 vs. 83.6) or pain between syndesmosis transfixation and no fixation groups (Study 4). Our results suggest that a simple stability-based fracture classification is useful in choosing between non-operative and operative treatment of ankle fractures; approximately half of the ankle fractures can be treated non-operatively with success. Our observations also suggest that relevant syndesmosis injuries are rare in ankle fractures due to an SER mechanism of injury. According to our research, syndesmotic repair or fixation in SER ankle fracture has no influence on functional outcome or pain after minimum one year compared with no fixation. PMID:23205893

Pakarinen, Harri

2012-12-01

380

[Bone fracture and the healing mechanisms. A new method for evaluation of fracture healing by echo tracking].  

PubMed

Assessment of bone healing on radiographs is not necessarily objective. A clinical system was developed to quantify the stiffness of healing fractures of the tibia in patients by the echo tracking (ET) method in a manner similar to a three-point bending test. The lower leg was supported. An ultrasound probe was placed on the proximal and distal fragments and a load of 25 N was applied. With a multiple ET system, two probes measured the displacement of five tracking points on each of the proximal and distal fragments of the tibia, thereby detecting the bending of the two fragments generated by the load. ET angle was defined as the sum of the inclinations of the proximal and distal fragments. Patients with tibial fracture treated by a cast or internal fixation were measured over time. In patients with radiographically normal healing, the bending angle decreased exponentially over time. However, in patients with nonunion, the angle remained the same over time. It was demonstrated that the ET method could be clinically applicable to evaluate fracture healing as a versatile, quantitative and noninvasive technique. PMID:19398836

Ohnishi, Isao; Matsuyama, Juntaro

2009-05-01

381

Pressurized thermal shock probabilistic fracture mechanics sensitivity analysis for Yankee Rowe reactor pressure vessel  

SciTech Connect

The Nuclear Regulatory Commission (NRC) requested Oak Ridge National Laboratory (ORNL) to perform a pressurized-thermal-shock (PTS) probabilistic fracture mechanics (PFM) sensitivity analysis for the Yankee Rowe reactor pressure vessel, for the fluences corresponding to the end of operating cycle 22, using a specific small-break-loss- of-coolant transient as the loading condition. Regions of the vessel with distinguishing features were to be treated individually -- upper axial weld, lower axial weld, circumferential weld, upper plate spot welds, upper plate regions between the spot welds, lower plate spot welds, and the lower plate regions between the spot welds. The fracture analysis methods used in the analysis of through-clad surface flaws were those contained in the established OCA-P computer code, which was developed during the Integrated Pressurized Thermal Shock (IPTS) Program. The NRC request specified that the OCA-P code be enhanced for this study to also calculate the conditional probabilities of failure for subclad flaws and embedded flaws. The results of this sensitivity analysis provide the NRC with (1) data that could be used to assess the relative influence of a number of key input parameters in the Yankee Rowe PTS analysis and (2) data that can be used for readily determining the probability of vessel failure once a more accurate indication of vessel embrittlement becomes available. This report is designated as HSST report No. 117.

Dickson, T.L.; Cheverton, R.D.; Bryson, J.W.; Bass, B.R.; Shum, D.K.M.; Keeney, J.A. [Oak Ridge National Lab., TN (United States)

1993-08-01

382

Characteristics of fracture during the approach process and wear mechanism of a silicon AFM tip.  

PubMed

The wear of an atomic force microscope (AFM) tip is one of the crucial issues in AFM as well as in other probe-based applications. In this work, wear tests under extremely low normal load using an AFM were conducted. Also, in order to understand the nature of silicon tip wear, the wear characteristics of crystal silicon and amorphous silicon oxide layer were investigated by a high-resolution transmission electron microscope (HRTEM). It was found that fracture of the tip readily occurred due to impact during the approach process. Experimental results showed that the impact should be below 0.1 nNs to avoid significant fracture of the tip. Also, it was observed that wear of the amorphous layer, formed at the end of the tip, occurred at the initial stage of the silicon tip damage process. Based on Archard's wear law, the wear coefficient of the amorphous layer was in the range of 0.009-0.014. As for the wear characteristics of the silicon tip, it was shown that wear occurred gradually under light normal load and the wear rate decreased with increase in the sliding distance. As for the wear mechanism of the silicon tip, oxidation wear was identified to be the most significant. It was shown that the degree of oxidation was higher under high normal load and in a nitrogen environment, oxidation of the silicon tip was reduced. PMID:15590139

Chung, Koo-Hyun; Lee, Yong-Ha; Kim, Dae-Eun

2005-01-01

383

Modeling the Progressive Failure of Jointed Rock Slope Using Fracture Mechanics and the Strength Reduction Method  

NASA Astrophysics Data System (ADS)

The fracturing process during the progressive failure of a jointed rock slope is numerically investigated by using fracture mechanics and the strength reduction method (SRM). A displacement discontinuity method containing frictional elements is developed for the calculation of the stress intensity factor (SIF). The failure initiation of the jointed rock slope is analyzed by evaluating the SIF. A new joint model is proposed by combining solid elements with interface elements in the commercial software FLAC3D. These represent the discontinuous planes in a rock mass on which sliding or separation can occur. The progressive failure process is simulated by reducing the shear strength of the rock mass, which includes the process of stress concentration, crack initiation, crack propagation, slip weakening, and coalescence of failure surfaces. The factor of safety (FS) and location of the critical failure surface are determined by the SRM. The influence of the joint inclination is investigated using the FS and the SIF. Laboratory experiments on specimens containing an inclined flaw under compression-shear stress are also conducted to investigate the effect of the angle between the shear direction and the flaw inclination, which provides an experimental explanation for the shear behavior of jointed rock. The results show that the joint inclination dominates the failure behavior of jointed rock slope, and two failure patterns have been classified.

Zhang, Ke; Cao, Ping; Meng, Jingjing; Li, Kaihui; Fan, Wenchen

2015-03-01

384

Microstructure in linear elasticity and scale effects: a reconsideration of basic rock mechanics and rock fracture mechanics  

NASA Astrophysics Data System (ADS)

An account on the role of higher order strain gradients in the mechanical behavior of elastic-perfectly brittle materials, such as rocks, is given that is based on a special grade-2 elasticity theory with surface energy as this was originated by Casal and Mindlin and further elaborated by the authors. The fundamental idea behind the theory is that the effect of the granular and polycrystalline nature of geomaterials (i.e. their microstructural features) on their macroscopic response may be modeled through the concept of volume cohesion forces, as well as surface forces rather than through intractable statistical mechanics concepts of the Boltzmann type. It is shown that the important phenomena of the localization of deformation in macroscopically homogeneous rocks under uniform tractions and of dependence of rock behavior on the specimen's dimensions, commonly known as size or scale effect, can be interpreted by using this 'non-local', higher order theory. These effects are demonstrated for the cases of the unidirectional tension test, and for the small circular hole under uniform internal pressure commonly known as the inflation test. The latter configuration can be taken as a first order approximation of the indentation test that is frequently used for the laboratory or in situ characterization of geomaterials. In addition, it is shown that the solution of the three basic crack deformation modes leads to cusping of the crack tips that is caused by the action of 'cohesive' double forces behind and very close to the tips, that tend to bring the two opposite crack lips in close contact, and further, it is demonstrated that the fracture toughness depends on the size of the crack, and thus it is not a fundamental property of the material. This latter outcome agrees with experimental results which indicate that materials with smaller cracks are more resistant to fracture than those with larger cracks.

Exadaktylos, George E.; Vardoulakis, Ioannis

2001-06-01

385

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

386

Enhancing mechanical strength during early fracture healing via shockwave treatment: an animal study  

Microsoft Academic Search

Objective. This investigation aims to determine (1) whether shockwave treatment helps fracture healing and (2) whether the effect of shockwave treatment on fracture healing is dose dependent.Design. Shockwave was applied over tibial osteotomy in an animal model to assess its effect on the healing of the fracture.Methodology. Bilateral tibial diaphyseal transverse osteotomy was conducted on 42 rabbits, dividing into experimental

Robert Wen-Wei Hsu; Ching-Lung Tai; Chris Yu-Chih Chen; Wei-Hsiu Hsu; Swei Hsueh

2003-01-01

387

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

388

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

389

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

390

A fracture mechanics analysis of impact damage in a thick composite laminate  

NASA Technical Reports Server (NTRS)

Graphite/epoxy filament-wound cases (FWC) for the solid rocket motors of the space shuttle are being made by NASA. The FWC cases are wound with AS4W graphite fiber impregnated with an epoxy resin and are about 1.4 inches or more thick. Graphite-epoxy composite laminates, unlike metals, can be damaged easily by low velocity impacts of objects like dropped tools. The residual tension strength of the FWC laminate, after impact, is being studied at Langley Research Center. The conditions that give minimum visual evidence of damage are being emphasized. A fracture mechanics analysis was developed to predict the residual strength, after impact, using radiographs to measure the size of the damage and an equivalent surface crack to represent the damage.

Poe, C. C., Jr.

1985-01-01

391

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

392

Addendum to the User Manual for NASGRO Elastic-Plastic Fracture Mechanics Software Module  

NASA Technical Reports Server (NTRS)

The elastic-plastic fracture mechanics modules in NASGRO have been enhanced by the addition of of the following: new J-integral solutions based on the reference stress method and finite element solutions; the extension of the critical crack and critical load modules for cracks with two degrees of freedom that tear and failure by ductile instability; the addition of a proof test analysis module that includes safe life analysis, calculates proof loads, and determines the flaw screening 1 capability for a given proof load; the addition of a tear-fatigue module for ductile materials that simultaneously tear and extend by fatigue; and a multiple cycle proof test module for estimating service reliability following a proof test.

Gregg, M. Wayne (Technical Monitor); Chell, Graham; Gardner, Brian

2003-01-01

393

A fracture mechanics based model for cumulative damage assessment as part of fatigue life prediction  

NASA Astrophysics Data System (ADS)

Cumulative damage related phenomena are considered, and a model for fatigue life prediction in the crack initiation stage is presented. The model interprets damage accumulation as a process of crack growth and takes into account load sequence dependent crack opening and closing levels which are controlled by the elastoplastic strain history and damage sum dependent decrease of the fatigue limit. A crack-driving parameter based on elastoplastic fracture mechanics is derived. Experiments based on two steels, an aluminum alloy, and several types of loading spectra revealed the improved accuracy of the model in comparison with previous approaches. It is concluded that the model can successfully predict load interaction effects which control the effective material straining and the phenomenon of a transient endurance limit for variable amplitude loading sequences.

Vormwald, M.; Heuler, P.; Seeger, T.

394

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

395

Carpal fractures.  

PubMed

Carpal fractures are exceedingly rare clinical entities and are often associated with concomitant injuries. In this review, we focus on fractures of the carpus, excluding the scaphoid, and provide an update on the current consensus as to mechanism, diagnosis, management, outcomes, and complications after such injuries. PMID:24679911

Suh, Nina; Ek, Eugene T; Wolfe, Scott W

2014-04-01

396

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

397

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

398

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

2015-03-01

399

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

400

Earthquake Induced Damage Mechanism of Long Period Structures Using Energy Response  

SciTech Connect

This paper presents a method of expounding the damage of RC long period frame structure using energy analysis method. Since the damage of structures usually occurs under major earthquakes, the structure is assumed to be in elasto-plastic state, and degraded Bouc-Wen model is used to describe the hysteretic component of the restoring force. A double index damage criterion defined by the maximum drift and energy absorption is used as the damage criterion. The energy transferring relation in a structure is derived, and both momentary and cumulative energy response is used to reflect the delay of the collapse of a long period structure. The mechanism of collapse delay of the long period structure is suggested through a numerical example combing the energy response and time history response.

Du Yongfeng; Li Hui [Western Engineering Research Center for Disaster Mitigation in Civil Engineering of Ministry of Education, Lanzhou, 730050 (China); Institute of Earthquake Protection and Disaster Mitigation, Lanzhou University of Technology, Lanzhou, 730050 (China)

2008-07-08

401

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

402

PREDICTION OF PROXIMAL FEMORAL FRACTURE BY USING MECHANICAL QUASI-BRITTLE DAMAGE COUPLED WITH ANISOTROPIC BEHAVIOUR LAW  

E-print Network

if if PREDICTION OF PROXIMAL FEMORAL FRACTURE BY USING MECHANICAL QUASI-BRITTLE DAMAGE COUPLED WITH ANISOTROPIC of proximal femur. The model was developed in term of anisotropic coupled behaviour law (strain-quasi-brittle and the dependence of damage growth to the hydrostatic pressure, a quasi-brittle damage law can be expressed by

Boyer, Edmond

403

The effect of carbon nanotubes and graphene on the mechanical properties of multi-component polymeric composites  

NASA Astrophysics Data System (ADS)

Two types of nano-materials (nanotubes and graphene) were incorporated at different concentrations into a bio-compatible polymer matrix, and the mechanical properties of the composite films were studied. Although both nanomaterials improved the mechanical attributes of the polymer, it was found that the composites containing the nanotube-graphene mixture exhibited significantly superior elasto-plastic properties. This work presents a facile technique of fabricating nano-composites that could be scaled up and applied to various types of polymers. These multi-component films have the potential to be used in a wide range of applications including bio-medicine and photovoltaics, as well as the military and automotive industry.

Dervishi, Enkeleda; Hategekimana, Festus; Boyer, Laurent; Watanabe, Fumiya; Mustafa, Thikra; Biswas, Abhijit; Biris, Alexandru R.; Biris, Alexandru S.

2013-12-01

404

On the mechanical interaction between a fluid-filled fracture and the earth's surface  

USGS Publications Warehouse

The mechanical interaction between a fluid-filled fracture (e.g., hydraulic fracture joint, or igneous dike) and the earth's surface is analyzed using a two-dimensional elastic solution for a slit of arbitrary inclination buried beneath a horizontal free surface and subjected to an arbitrary pressure distribution. The solution is obtained by iteratively superimposing two fundamental sets of analytical solutions. For uniform internal pressure the slit behaves essentially as if it were in an infinite region if the depth-to-center is three times greater than the half-length. For shallower slits interaction with the free surface is pronounced: stresses and displacements near the slit differ by more than 10% from values for the deeply buried slit. The following changes are noted as the depth-to-center decreases: 1. (1) the mode I stress intensity factor increases for both ends of the slit, but more rapidly at the upper end; 2. (2) the mode II stress-intensity factor is significantly different from zero (except for vertical slits) suggesting propagation out of the original plane of the slit; 3. (3) displacements of the slit wall are asymmetric such that the slit gaps open more widely near the upper end. Similar changes are noted if fluid density creates a linear pressure gradient that is smaller than the lithostatic gradient. Under such conditions natural fractures should propagate preferentially upward toward the earth's surface requiring less pressure as they grow in length. If deformation near the surface is of interest, the model should account explicitly for the free surface. Stresses and displacements at the free surface are not approximated very well by values calculated along a line in an infinite region, even when the slit is far from the line. As depth-to-center of a shallow pressurized slit decreases, the following changes are noted: 1. (1) displacements of the free surface increase to the same order of magnitude as the displacements of the slit walls, 2. (2) tensile stresses of magnitude greater than the pressure in the slit are concentrated along the free surface. The relative surface displacements over a shallow vertical slit are downward over the slit and upward to both sides of this area. The tensile stress acting parallel to the free surface over a shallow vertical slit is concentrated in two maxima adjacent to a point of very low stress immediately over the slit. The solution is used to estimate the length-to-depth ratio at which igneous sills have gained sufficient leverage on overlying strata to bend these strata upward and form a laccolith. The pronounced mode II stress intensity associated with shallow horizontal slits explains the tendency for some sills to climb to higher stratigraphie horizons as they grow in length. The bimodal tensile stress concentration over shallow vertical slits correlates qualitatively with the distribution of cracks and normal faults which flank fissure eruptions on volcanoes. The solution may be used to analyze surface displacements and tilts over massive hydraulic fractures in oil fields and to understand the behavior of hydraulic fractures in granite quarries. ?? 1979.

Pollard, D.D.; Holzhausen, G.

1979-01-01

405

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

406

Experimental Mechanics manuscript No. (will be inserted by the editor)  

E-print Network

for a better description of the complex kinematics in a bolted assembly [4]. They are compared to 3D models. In many instances, the behavior of the assembled elements remains elastic, even if elasto-plastic [7

Paris-Sud XI, Université de

407

Mechanical properties of the dentinoenamel junction: AFM studies of nanohardness, elastic modulus, and fracture.  

PubMed

The dentinoenamel junction (DEJ) is a complex and poorly defined structure that unites the brittle overlying enamel with the dentin that forms the bulk of the tooth. In addition, this structure appears to confer excellent toughness and crack deflecting properties to the tooth, and has drawn considerable interest as a biomimetic model of a structure uniting dissimilar materials. This work sought to characterize the nanomechanical properties in the region of the DEJ using modified AFM based nanoindentation to determine nanohardness and elastic modulus. Lines of indentations traversing the DEJ were made at 1-2 microm intervals from the dentin to enamel along three directions on polished sagittal sections from three third molars. Nanohardness and elastic modulus rose steadily across the DEJ from bulk dentin to enamel. DEJ width was estimated by local polynomial regression fits for each sample and location of the mechanical property curves for the data gradient from enamel to dentin, and gave a mean value of 11.8 microm, which did not vary significantly with intratooth location or among teeth. Nanoindentation was also used to initiate cracks in the DEJ region. In agreement with prior work, it was difficult to initiate cracks that traversed the DEJ, or to produce cracks in the dentin. The fracture toughness values for enamel of 0.6-0.9 MPa . m(1/2) were in good agreement with recent microindentation fracture results. Our results suggest that the DEJ displays a gradient in structure and that nanoindenation methods show promise for further understanding its structure and function. PMID:11077406

Marshall, G W; Balooch, M; Gallagher, R R; Gansky, S A; Marshall, S J

2001-01-01

408

Formation of conical fractures in sedimentary basins: Experiments involving pore fluids and implications for sandstone intrusion mechanisms  

NASA Astrophysics Data System (ADS)

Large sand intrusions often exhibit conical morphologies analogous to magmatic intrusions such as saucer-shaped or cup-shaped sills. Whereas some physical processes may be similar, we show with scaled experiments that the formation of conical sand intrusions may be favoured by the pore-pressure gradients prevailing in the host rock before sand injection. Our experiments involve injecting air into a permeable and cohesive analogue material to produce hydraulic fractures while controlling the pore pressure field. We control the state of overpressure in the overburden by applying homogeneous basal pore pressure, and then adding a second local pore pressure field by injecting air via a central injector to initiate hydraulic fractures near the injection point. In experiments involving small vertical effective stresses (small overburden, or high pore fluid overpressure), the fracturing pressure ( ?fract) is supralithostatic and two dipping fractures are initiated at the injection point forming a conical structure. From theoretical considerations, we predict that high values of ?fract are due to strong cohesion or high pore fluid overpressure distributed in the overburden. Such conditions are favoured by the pore pressure/stress coupling induced by both pore pressure fields. The dips of cones can be accounted for elastic-stress rotation occurring around the source. Contrary to magmatic chamber models, the aqueous fluid overpressure developed in a parent sandbody (and prevailing before the formation of injectites) may diffuse into the surrounding overburden, thus favouring stress rotation and the formation of inclined sheets far from the parent source. For experiments involving higher vertical effective stresses (thick overburden or low pore fluid overpressure), the fracturing pressure is lower than the lithostatic stress, and a single fracture is opened in mode I which then grows vertically. At a critical depth, the fracture separates into two dilatant branches forming a flat cone. We make use of a P.I.V. (Particle Imaging Velocimetry) technique to analyse plastic deformation, showing that these inclined fractures are opened in mixed modes. Close to the surface, they change into steep shear bands where fluids can infiltrate. The final morphology of the fracture network is very similar to the common tripartite architecture of various injection complexes, indicating that different mechanisms may be involved in the formation of dykes. Feeder dykes under the sill zones may open as tensile fractures, while overlying dykes may be guided by the deformation induced by the growth of sills. These deformation conditions may also favour the formation of fluid escape structures and pockmarks.

Mourgues, R.; Bureau, D.; Bodet, L.; Gay, A.; Gressier, J. B.

2012-01-01

409

Environment enhanced fatigue crack propagation in metals: Inputs to fracture mechanics life prediction models  

NASA Technical Reports Server (NTRS)

This report is a critical review of both environment-enhanced fatigue crack propagation data and the predictive capabilities of crack growth rate models. This information provides the necessary foundation for incorporating environmental effects in NASA FLAGRO and will better enable predictions of aerospace component fatigue lives. The review presents extensive literature data on 'stress corrosion cracking and corrosion fatigue.' The linear elastic fracture mechanics approach, based on stress intensity range (Delta(K)) similitude with microscopic crack propagation threshold and growth rates, provides a basis for these data. Results are presented showing enhanced growth rates for gases (viz., H2 and H2O) and electrolytes (e.g. NaCl and H2O) in aerospace alloys including: C-Mn and heat treated alloy steels, aluminum alloys, nickel-based superalloys, and titanium alloys. Environment causes purely time-dependent accelerated fatigue crack growth above the monotonic load cracking threshold (KIEAC) and promotes cycle-time dependent cracking below (KIEAC). These phenomenon are discussed in terms of hydrogen embrittlement, dissolution, and film rupture crack tip damage mechanisms.

Gangloff, Richard P.; Kim, Sang-Shik

1993-01-01

410

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

PubMed Central

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

Withers, P. J.

2015-01-01

411

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

NASA Astrophysics Data System (ADS)

To 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

412

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

PubMed Central

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

Hohenwarter, A.; Pippan, R.

2013-01-01

413

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

414

Fracture mechanics investigation of oil shale to aid in understanding the explosive fragmentation process. Final technical report, January 1983-July 1984  

SciTech Connect

This report summarizes goals and findings achieved in developing technologies to improve the overall efficiency of oil shale recovery processes. The objectives are to (a) develop theoretical fracture mechanics tools that are applicable to transversely isotropic materials such as sedimentary rock, more particularly oil shale; and (b) develop a fracture mechanics test procedure that can be conveniently used for rock specimens. Such a test procedure would: utilize the geometry of a typical rock core for the test; require a minimum amount of specimen machining; and provide meaningful, reproducible data that corresponds well to test data obtained from conventional fracture mechanics tests. Critical review of the state-of-the-art of fracture mechanics on layered rocks has been completed. Recommendations are made for innovative and promising methods for oil shale fracture mechanics. Numerical and analytical studies of mixed mode fracture mechanics are investigated. Transversely isotropic properties of oil shale are input using isoparametric finite elements with singular elements at the crack tip. The model is a plate with an edge crack whose angle with the edge varies to study the effect of mixed mode fracture under various conditions. The three-dimensional plate is in tension, and stress, energy methods are used in the fracture analysis. Precracked disks of oil shale cored perpendicular to bedding planes are analyzed numerically. Stress intensity factors are determined by (i) strain energy method, and (ii) elliptic simulation method. 47 refs., 12 figs., 1 tab.

Chong, K.P.

1984-09-01

415

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, ...

416

An experimental and analytical study of the mechanisms influencing the fracture behavior of an extruded Al-Li-Zr alloy  

SciTech Connect

The mechanisms influencing the fracture behavior of a Al-2.6Li-0.09Zr alloy were studied by varying the aging condition, the specimen thickness, and the extrusion parameters that affect grain structure, namely extrusion ratio and extrusion geometry. This alloy does not exhibit the classic plane-stress to plane-strain thickness effect typically observed in conventional alloys. The notch sensitivity of the alloy is also considerably less than observed with other alloys. The fracture toughness exhibits some extrusion ratio sensitivity as evidenced by 25% lower values for the higher extrusion ratio product in the peak aged condition. Furthermore, the difference in toughness between the rod and bar extrusions appears to be minimal. Fracture in the higher toughness underaged condition is characterized by crack tunneling and grain-boundary microcracking confined to a zone surrounding the continuous crack surface. The fracture process for the overaged material is distinguished by large, intergranular delamination cracks perpendicular to the main fracture surface and extending deep into the plastic zone. This delamination cracking phenomenon was investigated using linear, two-dimensional finite element simulations.

McKeighan, P.C.

1991-01-01

417

Probability of pipe fracture in the primary coolant loop of a PWR plant. Volume 5. Probabilistic fracture mechanics analysis. Load Combination Program Project I final report  

SciTech Connect

The primary purpose of the Load Combination Program covered in this report is to estimate the probability of a seismic induced LOCA in the primary piping of a commercial pressurized water reactor (PWR). Best estimates, rather than upper bound results are desired. This was accomplished by use of a fracture mechanics model that employs a random distribution of initial cracks in the piping welds. Estimates of the probability of cracks of various sizes initially existing in the welds are combined with fracture mechanics calculations of how these cracks would grow during service. This then leads to direct estimates of the probability of failure as a function of time and location within the piping system. The influence of varying the stress history to which the piping is subjected is easily determined. Seismic events enter into the analysis through the stresses they impose on the pipes. Hence, the influence of various seismic events on the piping failure probability can be determined, thereby providing the desired information.

Harris, D.O.; Lim, E.Y.; Dedhia, D.D.

1981-06-01

418

A Numerical Study on Generation Mechanism of Vertical Cracks in Top Coat of TBCs During APS Deposition  

NASA Astrophysics Data System (ADS)

Clarification of crack generation mechanism in the top coat of the thermal barrier coatings (TBCs) during atmospheric plasma spray process is important to improve the reliability of TBC. In this study, finite element analyses of stress and strain during the deposition process were conducted with layer-by-layer method to understand the cracking behaviors. Stress relaxation by generation of vertical cracks was expressed as an elasto-plastic behavior of the coating. The effects of pre-heating temperature of the substrate and plasma power on crack development were analyzed by changing of the initial and atmospheric temperatures in simulation, respectively. The simulation results of radial strain explained the experimental results of crack monitoring by non-contact laser acoustic emission method.

Ito, K.; Kuriki, H.; Enoki, M.

2015-03-01

419

Effect of crack-growth mechanism on the prediction of fracture load of adhesive joints  

Microsoft Academic Search

This paper presents observations regarding the cracking behavior of tensile-loaded structural adhesive joints. Experiments showed that fracture occurred by the development and propagation of a damage zone rather than a single sharp crack and that the presence of the adhesive spew fillet did not affect the fracture load of the adhesive joints studied. For joints bonded with the mineral-filled epoxy

M. Papini; G. Fernlund; J. K. Spelt

1994-01-01

420

Could the coseismic fractures of a lake ice reflect the earthquake mechanism?  

Microsoft Academic Search

Eyewitness reports that the two moderate earthquakes (3 February 2002) in Afyon, Central Anatolia, Turkey, produced fractures at the icy surface of a partially frozen lake near the reactivated fault scarp. In places along the shoreline, the ice thrusted towards the land. Far from the shoreline, several fractures developed on the approximately 15 cm-thick ice of the lake. Among them,

Tekin Yürür; Onur Köse; Hünkar Demirba?; Ça?lar Özkaymak; Levent Selçuk

2003-01-01

421

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

422

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

Microsoft Academic Search

Hydromechanical coupled processes in a shallow fractured carbonate reservoir rock were investigated through field experiments coupled with analytical and numerical analyses. The experiments consist of hydraulic loading\\/unloading of a water reservoir in which fluid flow occurs mainly inside a heterogeneous fracture network made up of vertical faults and bedding planes. Hydromechanical response of the reservoir was measured using six pressure–normal

F. Cappa; Y. Guglielmi; P. Fénart; V. Merrien-Soukatchoff; A. Thoraval

2005-01-01

423

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

424

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

425

At the moment of occurrence of a fragility hip fracture, men have higher mechanical properties values in comparison with women  

PubMed Central

Background It is well established that males have lower fracture risk in comparison with females, which suggests a higher bone resistance in men. The aim of our study was to find out if in older patients with hip fragility fractures, gender has also an impact on trabecular bone material behaviour, specifically to determine whether trabecular mechanical properties under compressive loading differ between men and women who suffered a fragility hip fracture. Methods Femoral epiphyses were consecutively collected during hip replacement surgery due to proximal femur fragility fracture. Trabecular bone cylinders were drilled and submitted to uniaxial compression tests and mechanical properties were assessed. Results Seventy-three patients, 55 women (mean age 81 years and standard deviation of 7 years) and 18 men (mean age 81 years and standard deviation of 8 years) were evaluated. The ultimate stress of trabecular bone was significantly higher in men than in women: the median values and the interquartile range (IQR) were respectively 8.04(5.35-10.90) MPa vs. 4.46(3.02-7.73) MPa, (p-value?=?0.005). The same difference between male and female was observed in the Young’s modulus: 293.68(166.67-538.18) MPa vs. 174.26(73.07-322.28) MPa, (p-value?=?0.028), and also in the energy to failure: 0.25(0.07-0.42) MJ/m3 vs. 0.11(0.05-0.25) MJ/m3, (p-value?=?0.058). These differences were also verified after adjusting the analysis for age in a multivariate model analysis. Conclusions Our observations demonstrated that, even in a population who suffered a fragility hip fracture, men still have higher trabecular bone mechanical properties in comparison with women. PMID:24131745

2013-01-01

426

Deferoxamine restores callus size, mineralization and mechanical strength in fracture healing after radiotherapy  

PubMed Central

Background Therapeutic augmentation of fracture site angiogenesis with Deferoxamine (DFO) has proven to increase vascularity, callus size and mineralization in long-bone fracture models. We posit that the addition of DFO would enhance pathological fracture healing in the setting of radiotherapy in a model where non-unions are the most common outcome. Methods Sprague-Dawley rats (n = 35) were divided into 3 groups. Fracture (Fx), radiated fracture (XFx) and radiated fracture + DFO (XFxDFO). Groups XFx and XFxDFO received a human equivalent dose of radiotherapy (7 Gy/day × 5 days = 35 Gy) 2 weeks prior to mandibular osteotomy and external fixation. The XFxDFO group received injections of DFO into the fracture callus after surgery. Following a 40-day healing period, mandibles were dissected, clinically assessed for bony-union, imaged with Micro-CT, and tension tested to failure. Results Compared to radiated fractures, metrics of callus size, mineralization and strength in DFO treated mandibles were significantly increased. These metrics were restored to a level demonstrating no statistical difference from control fractures. In addition we observed an increased rate of achieving bony unions in the XFxDFO treated group when compared to XFx (67% vs. 20% respectively). Conclusions Our data demonstrate near total restoration of callus size, mineralization, and biomechanical strength, as well as a 3-fold increase in the rate of union with the use of DFO. Our results suggest that the administration of DFO may have the potential for clinical translation as a new treatment paradigm for radiation induced pathologic fractures. Level of Evidence Animal study, not gradable for level of evidence. PMID:23629110

Donneys, Alexis; Ahsan, Salman; Perosky, Joseph E.; Deshpande, Sagar S.; Tchanque-Fossuo, Catherine N.; Levi, Benjamin; Kozloff, Ken M.; Buchman, Steven R.

2013-01-01

427

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

428

Thermomechanical fatigue life prediction in gas turbine superalloys: A fracture mechanics approach  

SciTech Connect

A model is presented that was developed to predict thermomechanical fatigue crack initiation and estimate mode I crack growth of gas turbine hot section gas path superalloys. The model is based on a strain energy density fracture mechanics approach modified to account for thermal exposure and single crystal anisotropy. Thermomechanical fatigue crack initiation and small crack growth is modeled by employing an initial material defect size. Model capability was quantified by applying the model to two hot section gas path superalloys: uncoated MAR-M509 and MCrAlY overlay coated PWA 1480. Thermomechanical fatigue model stresses were obtained from nonlinear finite element analysis of thermomechanical fatigue specimen strain-temperature history. Nonlinear stress-strain behavior was predicted using unified viscoplastic constitutive models. Model thermomechanical fatigue life predictions were in good agreement with observed uniaxial thermomechanical fatigue specimen lives. Thermomechanical fatigue cracking effects captured by the model included coating thickness, single crystal anisotropy, cycle waveshape, dwell, and thermal exposure. 45 refs.

Nissley, D.M. [United Technologies Corp., East Hartford, CT (United States)

1995-06-01

429

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