Plastic Shear Strain Research Articles

Dynamic links among rock damage, erosion, and strain during orogenesis

Abstract We provide model evidence for a previously unexplored positive feedback between tectonic strain and fluvial erosion by considering rock erodibility as a function of shear damage. Plastic shear strain permanently damages the upper crust within planar shear zones, providing a greater ease for detachment and transport by fluvial processes. The subsequent rapid erosion of exposed shear zones reforms the topographic stress field in a way that encourages continued accommodation of strain, a positive feedback response that becomes more prominent with greater shear damage. Based on model results and natural examples, rock strength heterogeneity plays a major role in the evolution of drainage network patterns and the capability for valley-scale strain localization in active orogens.

A finite element model to simulate the physical mechanisms of wear and crack initiation in wheel/rail contact

The presented work describes firstly the wear and crack initiation mechanism using metallographic investigations of deformed surfaces by rolling sliding wheel/rail contact. Secondly, a multi-scale finite element model is presented which allows the simulation of the deformation process near the surface of a rail under rolling sliding contact. It is necessary to model the roughness of the surfaces of wheel and rail to obtain a realistic deformation state which is comparable to experiments. Furthermore, the realistic stick-slip behaviour of a rolling sliding wheel along a rail is considered. Regarding these aspects of wheel/rail contact a realistic deformation picture with near quantitative amounts of plastic shear strains from micrometre to millimetre range can be predicted. The numerical results obtained using the multi-scale model can be compared to metallographic observations and deliver a satisfying match.

A finite element framework for distortion gradient plasticity with applications to bending of thin foils

A novel general purpose Finite Element framework is presented to study small-scale metal plasticity. A distinct feature of the adopted distortion gradient plasticity formulation, with respect to strain gradient plasticity theories, is the constitutive inclusion of the plastic spin, as proposed by Gurtin (2004) through the prescription of a free energy dependent on Nye’s dislocation density tensor. The proposed numerical scheme is developed by following and extending the mathematical principles established by Fleck and Willis (2009). The modeling of thin metallic foils under bending reveals a significant influence of the plastic shear strain and spin due to a mechanism associated with the higher-order boundary conditions allowing dislocations to exit the body. This mechanism leads to an unexpected mechanical response in terms of bending moment versus curvature, dependent on the foil length, if either viscoplasticity or isotropic hardening are included in the model. In order to study the effect of dissipative higher-order stresses, the mechanical response under non-proportional loading is also investigated.

Numerical modelling of submarine landslides with sensitive clay layers

A large-deformation finite-element (FE) modelling technique is presented to model submarine landslides. A strain softening model for undrained shear strength of marine clay is incorporated in the FE modelling. The development of large plastic shear strain concentrated zones (shear bands) and their propagation with displacement of soil mass are simulated. FE simulations show that the existence of a weak layer might result in the initiation and propagation of shear bands leading to large-scale progressive landslides. Such progressive development of failure planes cannot be simulated using the limit equilibrium method of slope stability analysis. Depending upon the geometry and soil properties, a number of failure patterns are identified which are comparable with morphologic features seen in field observations. Based on this type of FE analysis and compared with seabed morphology, the developed failure planes could be identified where the shear strengths are expected to be lower because of pre-shearing than the shear strengths of the soil outside these zones, which could then be implemented in the modelling of seabed for offshore development projects in the areas where failure occurred in the past.

The Coffin-Manson formula for unleaded Sn/3.0Ag/0.5Cu solder

ABSTRACTThis study explores Coffin-Manson equation and its feasibility for tin-silver-copper (Sn/3.0Ag/0.5Cu) solder. Most of the Sn/Ag/Cu solder studies were performed by experiment or the finite element method. In this study, MARC, a finite element analysis package, is applied for simulation. In the simulation, the material properties are temperature-dependent and the boundary conditions are time-dependent. Complying with JEDEC standard, the derivation of fatigue life formula for solder is built from experimental data. Using the experimental results of the BGA ball shear test, the amplitude of equivalent plastic shear strain at 1/4 cycle is obtained by finite element method. Meanwhile, substituting the CSP thermal cycle test results into Weibull life distribution, the amplitude of equivalent plastic shear strain at 3085 cycle is solved. The numerical results of the equivalent plastic shear strain amplitude are used to derive the Coffin-Manson equation proposed. To verify the feasibility and reliability of the formula derived, the result of an isothermal low cycle fatigue test is analyzed by the finite element method. It is observed that the proposed equation has a good agreement with the experimental data.

Modeling wear and rolling contact fatigue: Parametric study and experimental results

A recently developed simulation model is used to study wear and rolling contact fatigue crack initiation. The model is capable of predicting the damage pattern observed in a full-scale test-rig experiment with respect to crack initiation and wear under conformal contact conditions. The crack initiation model assesses the propensity for rolling contact fatigue crack initiation at the surface of rails based on the combined assessment of the rolling contact stresses and the plastic shear strain distribution in a near-surface layer. Crack initiation is not necessarily linked to low wear rates. If favorable microstructural crack paths away from the surface exist, crack initiation can also take place in parts of the rail where high wear rates are observed. The parametric study shows that an increasing angle of attack of the wheel and an increasing coefficient of friction lead to an increase in the profile height change at the gauge corner of the rail. Likewise, the effective stress for crack initiation at the gauge corner of the rail increases with both an increasing angle of attack and an increasing coefficient of friction, but it is insensitive with respect to small changes in the rail inclination under test rig conditions.

Depiction of interfacial characteristic changes during impact welding using computational methods: Comparison between Arbitrary Lagrangian - Eulerian and Eulerian simulations

This paper investigates the development and evolution of various interfacial features during high speed impact welding processes using numerical simulations. Arbitrary Lagrangian - Eulerian (ALE) and Eulerian computations are performed and compared with experimental features of welded joints. ALE method reveals poor mesh quality while the interface experiences severe shear deformation. Therefore, ALE simulations cannot accurately capture the morphological transition toward a wavy shape and cannot compute the successive jet formation from the interface. But, the Eulerian method becomes a good alternative to overcome those limitations of the ALE method. Computed interfacial features using Eulerian simulations are agreed well with the experimental observations of various interfacial phenomena. The actual kinematics along the whole length of the weld including wavy pattern, localized plastic deformation and shear strain orientation are clearly reproduced. Eulerian simulation also provides understanding for the potential cause of defects that corresponds to the experimental observations, in terms of morphology and site occurrence.

Cutting Edge Geometry Effect on Plastic Deformation of Titanium Alloy

The paper presents experimental studies of OT4 titanium alloy machining with cutting edges of various geometry parameters. Experiments were performed at a low speed by the scheme of free cutting. Intensity of plastic shear strain was set for defining of cutting edge geometry effect on machining. Images of chip formed are shown. Estimation of strain magnitude was accomplished with digital image correlation method. Effect of rake angle and cutting edge angle has been studied. Depth of deformed layer and the area of the plastic strain is determine. Results showed that increasing the angle of the cutting edge inclination results in a change the mechanism of chip formation.

COMPUTATIONAL STUDY ON MICROSTRUCTURE-SENSITIVE HIGH CYCLE FATIGUE DISPERSIVITY

Empirical approaches to characterize the variability of high cycle fatigue have been widely used. However, little is understood about the intrinsic relationship of randomness of microstructure attributes on the overall variability in high cycle fatigue. The ability of quantifying the dispersivity of high cycle fatigue with physics based computational methods has great potential in design of minimum life and can aid in the improvement of fatigue resistance. To investigate the effects between microstructure attributes and high cycle fatigue dispersivity, the microstructure-sensitive extreme value probabilistic framework is introduced. First, the Voronoi algorithm is used to construct random polycrystalline microstructure representative volume elements. Different kinds of periodic boundary conditions are proposed to simulate the interior and surface constraints in polycrystalline microstructure representative volume elements. Then mechanical responses of both interior and surface microstructure representative volume elements under different strain amplitudes are simulated by internal state variable based crystal plasticity. The fatigue indicator parameter is introduced to characterize the driving force for fatigue crack formation dominated by maximum shear plastic strain amplitude. By computing a limited number of random polycrystalline microstructure representative volume elements, the distributions of fatigue indicator parameter under different strain amplitudes are obtained and analyzed with extreme value probability theory. The study with a kind of titanium alloy with material grade TC4 supports that the high cycle fatigue dispersivity increases with the decrease of the strain *国家重点基础研究发展计划资助项目2015CB057400 收到初稿日期: 2015-06-23,收到修改稿日期: 2015-12-01 作者简介:韩世伟,男, 1987年生,博士生 DOI: 10.11900/0412.1961.2015.00322 第289-297页 pp.289-297

Finite element modeling of lateral pipeline–soil interactions in dense sand

Finite element (FE) analyses of pipeline–soil interaction for pipelines buried in dense sand subjected to lateral ground displacements are presented in this paper. Analysis is performed — using the Arbitrary Lagrangian–Eulerian (ALE) method available in Abaqus/Explicit FE software — in the plane strain condition using the Mohr–Coulomb (MC) and modified Mohr–Coulomb (MMC) models. The MMC model considers a number of important features and properties of stress–strain and volume change behaviour of dense sand including the nonlinear pre- and post-peak behaviour with a smooth transition and the variation of the angle of internal friction and dilation angle with plastic shear strain, loading conditions (triaxial or plane strain), density, and mean effective stress. Comparing FE and experimental results, it is shown that the MMC model can better simulate the force–displacement response for a wide range of lateral displacements of the pipe for different burial depths, although the peak force on the pipe could be matched using the MC model. Examining the progressive development of zones of large inelastic shear deformation (shear bands), it is shown that the mobilized angle of internal friction and dilation angle vary along the length of the shear band; however, constant values are used in the MC model. A comprehensive parametric study is also performed to investigate the effects of pipeline diameter, burial depth, and soil properties. Many important aspects in the force–displacement curves and failure mechanisms are explained using the present FE analyses.

Modeling surface rolling contact fatigue crack initiation taking severe plastic shear deformation into account

A new model for the prediction of rolling contact fatigue crack initiation at the surface of railway rails and wheels has been developed, which takes contact-related plasticity effects into account. The model assesses the propensity towards fatigue crack initiation based on microstructural crack paths in the severely shear-deformed, anisotropic material near the surface. The key to differentiate between situations favoring crack initiation accompanied by wear and situations where only wear prevails, is the distribution of plastic shear strain in combination with stress in a crack initiation layer at the surface, rather than the maximum values of the plastic strain or stress. The model predicts the formation of head checks at the gauge corner of rails and the corresponding damage pattern on wheels. It is parameterized based on results from a full-scale test rig experiment. The model can be coupled to multi-body systems simulations of railway vehicles to account for the effect of plastic shear deformation on rolling contact fatigue crack initiation in such simulations. This allows systematic studies of contact conditions, material properties and railway vehicle dynamics behavior with regard to rolling contact fatigue crack initiation.

Development of a novel severe plastic deformation method: friction stir equal channel angular pressing

In this research, a novel severe plastic deformation (SPD) method has been proposed which imposes a higher amount of the plastic shear strain during the modified equal channel angular pressing (ECAP) process. The deformation behavior of this novel process was analyzed through the finite element method, and the numerical simulations were validated by experimental data. The results showed that this process can impose higher effective strain during only one pass in comparison with the conventional ECAP even at high numbers of passes. According to the mean stress analyses of the process, it is obtained that this novel method leads to produce products with lower possibility of crack growth in comparison with the common method. At the second pass, the final product of the friction stir equal channel angular pressing (FS-ECAP) process exhibits better strain homogeneity at its surface along the length in comparison with that of the first FS-ECAP pass.

Coupled infrared laser-thermo-mechanical response of RDX-PCTFE energetic aggregates

A computational approach is developed to investigate the coupled phenomena of high frequency electromagnetic (EM) wave propagation, laser heat absorption, thermal conduction, and inelastic dynamic thermo-mechanical deformation in heterogeneous energetic materials. The method is used to study hot spot formation in RDX-PCTFE aggregates subjected to high strain rate loads and infrared laser irradiation. The approach couples Maxwell's equations with a dislocation density-based crystal plasticity formulation within a nonlinear finite-element approach to predict and understand thermo-mechanical response due to the interrelated effects of dielectric heating, adiabatic heating, thermal decomposition, and heat conduction. RDX crystalline interfaces and orientations, polymer binder, inelastic strains, dislocation-density evolution, and voids significantly affected the coupled EM-thermo-mechanical response. EM and thermo-mechanical mismatches at interfaces between RDX crystals, binder, and voids resulted in localized regions with high electric field and laser heat generation rates, which subsequently led to hot spot formation. It is predicted that incident laser intensity and plastic shear strain localization are the dominant mechanisms that lead to hot spot formation.

Seismic and aseismic deformations and impact on reservoir permeability: The case of EGS stimulation at The Geysers, California, USA

AbstractIn this paper, we use the Seismicity‐Based Reservoir Characterization approach to study the spatiotemporal dynamics of an injection‐induced microseismic cloud, monitored during the stimulation of an enhanced geothermal system, and associated with the Northwest Geysers Enhanced Geothermal System (EGS) Demonstration project (California). We identified the development of a seismically quiet domain around the injection well surrounded by a seismically active domain. Then we compare these observations with the results of 3‐D Thermo‐Hydro‐Mechanical simulations of the EGS, which accounts for changes in permeability as a function of the effective normal stress and the plastic strain. The results of our modeling show that (1) the aseismic domain is caused by both the presence of the injected cold water and by thermal processes. These thermal processes cause a cooling‐stress reduction, which prevent shear reactivation and favors fracture opening by reducing effective normal stress and locally increasing the permeability. This process is accompanied by aseismic plastic shear strain. (2) In the seismic domain, microseismicity is caused by the reactivation of the preexisting fractures, resulting from an increase in injection‐induced pore pressure. Our modeling indicates that in this domain, permeability evolves according to the effective normal stress acting on the shear zones, whereas shearing of preexisting fractures may have a low impact on permeability. We attribute this lack of permeability gain to the fact that the initial permeabilities of these preexisting fractures are already high (up to 2 orders of magnitude higher than the host rock) and may already be fully dilated by past tectonic straining.

Considerations of rock dilation on modeling failure and deformation of hard rocks—a case study of the mine-by test tunnel in Canada

For the compressive stress-induced failure of tunnels at depth, rock fracturing process is often closely associated with the generation of surface parallel fractures in the initial stage, and shear failure is likely to occur in the final process during the formation of shear bands, breakouts or V-shaped notches close to the excavation boundaries. However, the perfectly elastoplastic, strain-softening and elasto-brittle-plastic models cannot reasonably describe the brittle failure of hard rock tunnels under high in-situ stress conditions. These approaches often underestimate the depth of failure and overestimate the lateral extent of failure near the excavation. Based on a practical case of the mine-by test tunnel at an underground research laboratory (URL) in Canada, the influence of rock mass dilation on the depth and extent of failure and deformation is investigated using a calibrated cohesion weakening and frictional strengthening (CWFS) model. It can be found that, when modeling brittle failure of rock masses, the calibrated CWFS model with a constant dilation angle can capture the depth and extent of stress-induced brittle failure in hard rocks at a low confinement if the stress path is correctly represented, as demonstrated by the failure shape observed in the tunnel. However, using a constant dilation angle cannot simulate the nonlinear deformation behavior near the excavation boundary accurately because the dependence of rock mass dilation on confinement and plastic shear strain is not considered. It is illustrated from the numerical simulations that the proposed plastic shear strain and confinement-dependent dilation angle model in combination with the calibrated CWFS model implemented in FLAC can reasonably reveal both rock mass failure and displacement distribution in vicinity of the excavation simultaneously. The simulation results are in good agreement with the field observations and displacement measurement data.

Large deformation finite-element modelling of progressive failure leading to spread in sensitive clay slopes

The occurrence of large landslides in sensitive clays, such as spreads, can be modelled by progressive development of large inelastic shear deformation zones (shear bands). The main objective of the present study is to perform large deformation finite-element modelling of sensitive clay slopes to simulate progressive failure and dislocation of failed soil mass using the coupled Eulerian Lagrangian (CEL) approach available in Abaqus finite-element software. The degradation of undrained shear strength with plastic shear strain (strain-softening) is implemented in Abaqus CEL, which is then used to simulate the initiation and propagation of shear bands due to river bank erosion. The formation of horsts and grabens and dislocation of soil masses that lead to large-scale landslides are simulated. This finite-element model explains the displacements of different blocks in the failed soil mass and also the remoulding of soil around the shear bands. The main advantages of the present finite-element model over other numerical models available in the literature are that it can simulate the whole process of progressive failure leading to spread. The finite-element results are consistent with previous conceptual models proposed from field observations. The parametric study shows that, depending upon geometry and soil properties, toe erosion could cause three types of shear band formation: (a) only a horizontal shear band without any global failure; (b) global failure of only one block of soil; (c) global failure of multiple blocks of soil in the form of horsts and grabens.

Deformation patterns in cross-sections of twisted bamboo-structured Au microwires

In order to investigate an almost pure extrinsic size effect we propose an experimental approach to investigate the deformation structure within single crystalline cross-sections of twisted bamboo-structured Au microwires. The cross-sections of individual 〈100〉 oriented grains of 25μm thick Au microwires have been characterized by Laue microdiffraction. The diffraction data were used to calculate the misorientation of each data point with respect to the neutral fiber in the center of the cross-section as well as the kernel average misorientation to map the global and local deformation structure as function of the imposed maximum plastic shear strain. The study is accompanied by crystal plasticity simulations which yield the equivalent plastic strain distributions in the cross-section of the wire. The global deformation structures are directly related to the activated slip systems, resulting from the real orientations of the investigated grains. When averaging the degree of deformation along ring segments, an almost continuous but non-linear increase of misorientation from the center toward the surface is observed, reflecting the overall strain gradient imposed by torsion. For the local deformation structure, pronounced and graded deformation traces are observed which often pass over the neutral fiber of the twisted wire and which are obviously reflecting domains of high geometrically necessary dislocations content.

Wear of Carbon Steel (0.65%C) in Rolling-Sliding Contact with Creep Ratio

In rolling-sliding contact, wear will occur when the accumulated plastic shear strain of the material at the surface exceeds its critical shear strain for failure. During rolling-sliding contact, the difference in relative velocities of two contacting components can cause slip in the contact (known as creep). The higher creep ratio may increase the severity of wear. In this work, the wear rate of the material and the behaviour of material just below the contact surface in rolling-sliding contact with various creep ratios were investigated. The carbon steel (about 0.65% C) was chosen as the test material and wear test was conducted using disc-to-disc contact testing machine with the maximum contact pressure of 1000 MPa and with various creep ratio of 1%, 5% and 7%. The results show the higher creep ratio causes the material to accumulate critical shear strain more quickly, resulting in the increase of wear (i.e., from about 0.0047μm/cycle for creep ratio of 1% up to about 0.0077μm/cycle for creep ratio of 7%). .

Investigation on the Effect of Linear Kinematic Hardening Model on Plasticity Prediction of Reciprocating Sliding Contact

This paper discusses a finite element analysis of cylinder on flat contact configuration subjected to constant normal load and reciprocating tangential displacement with linear kinematic hardening models based on bi-modal Ti-6Al-4V cyclic stress-strain curves. The predicted evolution of plastic deformation such as the equivalent plastic strain, tangential plastic strain and shear plastic strain distributions on the contact region has been studied along with its respective predicted stress distributions. The effect of applied forward and backward sliding displacement movements on predicted stress and strain distributions have also been looked at. It is found that the stress distributions predicted for kinematic hardening model is similar for forward and backward movements while the predicted plastic strain distribution is increasing with reciprocating sliding movement. The predicted value keep increasing when it moves forward, backward and finally moves forward again. This is due to large strain effect of the model and its dependant on the displacement movement amount.

Coupled hydro-mechanical processes and fault reactivation induced by Co2 Injection in a three-layer storage formation

The interaction between mechanical deformation and fluid flow in fault zones gives rise to a host of coupled hydro-mechanical processes fundamental to fault instability, induced seismicity, and associated fluid migration. Fault stability is studied in the context of the Heletz site which was chosen as a test site for CO2 injection experiment in the framework of the EU- MUSTANG project. The potential reservoir for CO2 storage at the Heletz site consists of three sandstone layers that are approximately one, two and nine meters in thickness, separated by impermeable shale layers of various thicknesses, and overlaid by a five-meter limestone and a thick impermeable shale, which serves as caprock. The storage formation is intersected by two pre-existing sub-vertical normal faults (F1 and F2) on two opposite sides of the injection point. A hydro-mechanical model was developed to study the interaction between mechanical deformation and fluid flow in the two faults during CO2 injection and storage. We evaluate the consequences caused by potential fault reactivation, namely, the fault slip and the CO2 leakage through the caprock. The difference in the results obtained by considering the three-layer storage formation as an equivalent single-layer storage formation is analysed. It was found that for the two cases the pore pressure evolution is similar, but the differences in the evolution of CO2 saturation are significant, which is attributed to the differences in CO2 spread in a single and three-layer storage. No fault reactivation was observed in either case. A sensitivity analysis was made to study the influence of the fault dip angle, the ratio between the horizontal and vertical stresses, the offset of the layers across fault F2, the initial permeability of the fault and the permeability of the confinement formations. Results show that reactivation of faults F1 and F2 is most sensitive to the stress ratio, the initial permeability of the faults and the permeability of the confinement formations. The offset of the layers across the fault F2 was also found to be an important parameter, mainly because an offset leads to an increase in CO2 leakage. Changes in permeability were found to be small because plastic shear strains induced by the reactivation of the faults and associated increase in volumetric strains and permeability, occur mainly in a fault section of only 10 m length, which is the approximate total thickness of the storage layers.