Large Shear Strains Research Articles

Experimental Investigations on Creep Behavior of Coal under Combined Compression and Shear Loading

The creep behavior of rock has received much attention for analyzing the long-term response and stability of underground rock engineering structures. Numerous studies have been carried out on the creep properties of various rocks under pure compression conditions. However, little attention has been paid to the creep behavior of rocks in a combined compression-shear loading state. In this work, a novel combined compression and shear test (C-CAST) system was used to carry out inclined uniaxial compression tests and creep tests for various inclination angles (0°, 5°, 10°, and 15°). The results revealed that the peak strength of the coal decreased with the inclination angle of the specimen, which could provide the basis for setting up a creep test scheme. Multistage compression-shear creep tests were carried out on specimens with different inclination angles. Based on the analysis of the creep test data, the creep behavior of the coal in a combined compression-shear state was studied. It was found that the specimen inclination affected the time-dependent deformation, long-term strength (LTS), and time to failure. Compared with the specimen under pure compression, the inclination specimens tend to produce large shear strain with time, while they were more prone to shear failure. The reduction of the long-term strength was closely associated with the increase of the specimen inclination angle when the angle was more than 5°. Moreover, the ratio of the peak strength to the LTS was not affected by the specimen inclination, which is considered an inherent characteristic. We anticipate that the results obtained will assist in pillar design and long-term stability analysis.

A two-surface plasticity model for clay; numerical implementation and applications to large deformation coupled problems of geomechanics

This paper presents numerical treatments of elastoplasticity relations for a previously developed critical state two-surface plasticity model and its implementation into a bespoke finite element code as well as the commercial software Abaqus via the user defined subroutine, UMAT. The model is implemented with an efficient integration scheme based on the explicit modified Euler scheme with automatic substepping and error control. Validation of the implemented model is assessed through simulation of several laboratory tests comprising a variety of initial states and loading conditions followed by a study on the accuracy and efficiency of the integration scheme. Subsequently, the model is employed to analyse some sophisticated boundary value problems involving finite deformations, inertia effects, soil-structure interactions and saturated porous media. The constitutive model captures particular features of clay behaviour, such as the prediction of strain rate dependence, small-strain stiffness degradation, the development of residual strength at very large shear strains and stress anisotropy. The effects of these features on the behaviour of two important geotechnical problems – including pipe-seabed interaction under lateral motion and dynamically installed anchors – are specifically investigated using two advanced finite deformation schemes: one based on the Arbitrary Lagrangian Eulerian (ALE) method and another on the Particle Finite Element Method (PFEM). The study illustrates the robustness of the proposed integration scheme and successful application of the soil model, indicating that the use of such complex soil models may be useful for realistic analyses of geotechnical problems.

Harnessing shape optimization techniques to develop novel methods to determine shear properties in PMCs

Shape optimization techniques are utilized to develop an optimal shear specimen for polymer matrix composites (PMCs) in which shear stresses result from simple uniaxial extension. An expanding array of composite applications means certification and damage tolerance are critical design factors. This methodology improves the ease of characterization for classically challenging material properties, such as shear failure. Numerical simulations were conducted using IM7/977-3 unidirectional prepreg mechanical properties. Large shear strain concentrations in the region of interest led to predicted failure in the region of interest under conditions consistent with shear. The geometry generated by the shape optimization should continue to be explored due to its capability generating a geometry which can measure the shear modulus and shear strength of a laminate. The shape optimization method’s ability to tailor a sample geometry for a desired strain state and failure mode leads the authors to recommend exploration of additional optimized test geometries for other failure modes and materials, in addition to the experimental investigation of the geometry generated for shear.

Rheological properties of artificial boluses of cereal foods enriched with legume proteins

The properties of artificial food bolus are studied by dynamic oscillatory and capillary rheometry as functions of bolus water content (WC), in the usual range of saliva hydration, for four cereal products: sponge cake, extruded flat bread and their counterpart enriched in legume proteins. All boluses followed the same rheological behavior characterized by (1) solid -like in the linear viscoelastic domain and (2) Herschel-Bulkley model for large shear strain. Hence, four characteristic rheological properties are determined: modulus at viscoelastic plateau, characteristic stress at transition to flow, yield stress and consistency in the flow regime. The decrease of these properties with WC was fitted by an exponential decay function, from which was extracted a coefficient α (5 = α ≤ 30), defined as a coefficient of interaction of the food with water. The values of α are of the same order of magnitude as the plasticization coefficient of starch by water. They were larger for the extruded pea based (EFP, α ≥ 15), and were lower for the sponge cake (SC, α ≤ 15). The variations for the different rheological properties are discussed in terms of matter state, envisioning bolus as a suspension of soft swellable particles. The comparison of these values with those encountered for real boluses from similar foods suggests that these results contribute to define a coefficient of interaction of food with saliva.

Compact forced simple shear specimen: Design improvement based on quasi-static experiments and finite element simulations

Determination of the large-strain shear response of materials is important for an improved understanding of shear driven failure mechanisms as well as constitutive modeling efforts. While many shear specimen designs have been proposed in the literature, few systematic studies have examined the effects of varying sample geometry on the homogeneity of the resultant stress state and strain path. Here we propose modifications to the compact forced simple shear specimen of Gray et al. (2016), and use both digital image correlation and finite element analysis to determine the effect of specimen geometry on simple shear deformation at quasi-static rates in high purity wrought tantalum. For the specimens considered here, where a notch is used to create the shear zone, it is found that a notch inclination angle of 45 degrees provides a substantial improvement in the deformation behavior compared to a horizontally oriented notch. A parametric study of notch inclination angle is conducted using FEA simulation.

Applications of micropolar SPH in geomechanics

A smoothed particle hydrodynamics code based on micropolar continua for geomaterials is developed for problems involving large deformation and shear strain localization. Two typical geotechnical problems, i.e., biaxial compression test and sand column collapse, are simulated using classical and micropolar model to demonstrate the performance of the newly proposed method. A parameter study is given on the scale effect in the micropolar continua.

Coupled Eulerian-Lagrangian simulation of a modified direct shear apparatus for the measurement of residual shear strengths

The simulation of large-strain geotechnical laboratory tests with conventional Lagrangian finite element method (FEM) techniques is often problematic due to excessive mesh distortion. The multiple reversal direct shear (MRDS) test can be used to measure the residual shear strength of soils in a laboratory setting. However, modelling and simulation generally require advanced numerical methods to accommodate the large shear strains concentrated in the shear plane. In reality, when the standard direct shear (DS) apparatus is used, the MRDS method is prone to two major sources of measurement error: load cap tilting and specimen loss. These sources of error make it difficult or even impossible to correctly determine the residual shear strength. This paper presents a modified DS apparatus and multi-reversal multi-stage test method, simulated using the coupled Eulerian-Lagrangian (CEL) method in a finite element environment. The method was successful in evaluating equipment and preventing both load cap tilting and specimen loss, while modelling large-deformation behaviour that is not readily simulated with the conventional FEM or arbitrary Lagrangian-Eulerian (ALE) analysis. Thereafter, a modified DS apparatus was created for the purpose of analysing mixtures of organic materials found in an Australian clay. The results obtained from the modified DS CEL model in combination with laboratory tests show a great improvement in the measured residual shear strength profiles compared to those from the standard apparatus. The modified DS setup ensures that accurate material residual shear strengths are calculated, a factor that is vital to ensure appropriate soil behaviour is simulated for numerical analyses of large-scale geotechnical projects.

Finite Element Model of Equal Channel Angular Extrusion of Ultra High Molecular Weight Polyethylene

Abstract Ultra-high molecular weight polyethylene (UHMWPE) used in biomedical applications, e.g., as a bearing surface in total joint arthroplasty, has to possess superior tribological properties, high mechanical strength, and toughness. Recently, equal channel angular extrusion (ECAE) was proposed as a processing method to introduce large shear strains to achieve higher molecular entanglement and superior mechanical properties of this material. Finite element analysis (FEA) can be utilized to evaluate the influence of important manufacturing parameters such as the extrusion rate, temperature, geometry of the die, back pressure, and friction effects. In this paper, we present efficient FEA models of ECAE for UHMWPE. Our studies demonstrate that the choice of the constitutive model is extremely important for the accuracy of numerical modeling predictions. Three considered material models (J2-Plasticity, Bergstrom-Boyce, and the three-network model) predict different extrusion loads, deformed shapes, and accumulated shear strain distributions. The work has also shown that the friction coefficient significantly influences the punch force and that the two-dimensional (2D) plane strain assumption can become inaccurate in the presence of friction between the billet and the extrusion channel. Additionally, a sharp corner in the die can lead to the formation of the so-called “dead zone” due to a portion of the material lodging into the corner and separating from the billet. Our study shows that the presence of this material in the corner substantially affects the extrusion force and the resulting distribution of accumulated shear strain within the billet.

High load sliding, deformation microstructures, strength, and hardening for cutting and metal forming

The plowing action of ridge and wedge shaped micro asperities under large sliding loads is utilized to induce extremely high hardness and strain levels in the subsurface regions of Cu. A unique apparatus was designed and employed to apply high normal stresses (loads) that ranged herein from 0.4 to 3.7 times the yield stress, at either ambient (293 K) or cryogenic (77 K) temperatures. A steep gradient in the size scale of the induced deformation microstructures is coupled to an unprecedented average size (crystallite) scale of 5 nm that includes a high dislocation density near the surface. This structural size is graded in size from very fine at the surface to large with increasing depth and decreasing strain. Due to these size scales, quantitative and statistical measurements of the subsurface deformation microstructures, as a function of depth, load, and temperature were performed with high resolution transmission, scanning electron, and microprobe microscopy. The observed deformation microstructures follow the principal of grain subdivision creating a hierarchical structure, but with a finer size scale, that is similar to deformation structures developing under different deformation modes. The measured structure parameters include the inverse spacing, 1/DavGNB, of deformation induced boundaries (geometrically necessary boundaries GNBs), that have evolved with increasing strain to have medium to high misorientation angles like grain boundaries, and the dislocation density, ρav, in between the GNBs. Dislocation mechanisms were demonstrated to dominate the subsurface deformation at all size scales based on the high density of dislocations, the average refinement that extends to 5 nm, and universal scaling. Subsurface stress estimates use the measured parameters in a linear addition of the classic Hall-Petch formulation for the barrier strength contribution of 1/DavGNB, plus Taylor forest dislocation hardening for the contribution of ρav. A further unification of these parameters is made since ρav is directly proportional to 1/DavGNB. Flow stress estimates indicate that the nearest subsurface layers are extremely hard, 1.7–2.8 GPa with hardness and subsurface refinement increasing with increasing normal load and decreasing temperature. At the highest load and 77 K, the depth at which large shear strains are observed is increased by a factor of about six, i.e. eighty times the average platen asperity height. A quantitative comparison of friction and deformation energies is made together with the stress and strain estimates. Friction and deformation energies increase with increasing the normal loads. At the same time, coefficients of friction decrease due to the creation of a harder surface at higher loads. Large sliding or rubbing loads are present during cutting and metal forming creating subsurface deformation. These loads largely contribute to high friction forces, and strongly influence surface finishes and subsurface hardness.

Dynamic shearing resistance of hydroxyl-terminated polybutadiene (HTPB)

Hydroxyl-terminated polybutadiene (HTPB) is a widely used polymeric binder in polymer-bonded explosives (PBXs) and solid rocket propellants. Even though used in small fractions, the elastomeric binder absorbs much of the impact energy and therefore requires careful modeling of its mechanical behavior to accurately simulate the response of PBXs when they are subjected to large strains and strain rates. While the normal response of HTPB has been characterized under uniaxial stress and uniaxial strain loading, shear strength measurements under large pressures and large shear strain rates have not been made so far. Such measurements are critical for modeling localization and failure in PBXs and accurate prediction hotspot formation. In this study, pressure-shear plate impact (PSPI) experiments have been used to measure the shearing resistance of HTPB at different pressures. The shearing resistance of HTPB is found to be strongly pressure dependent. A quasi-linear viscoelastic model with pressure-dependent shear wave speeds and shearing resistance is used to describe the experimentally observed dynamic response of HTPB. The model consists of an instantaneous elastic response and viscoelastic relaxation of the elastic response.

Flat hardness distribution in AA6061 joints by linear friction welding

It is known that one of the main concerns associated with the conventional welding of precipitation-strengthened Al alloys is the formation of softening regions, resulting in the deterioration of mechanical properties. In this study, we show that linear friction welding (LFW) can completely suppress softening regions in precipitation-strengthened AA6061-T6 alloy by introducing a large shear strain and by controlling the interfacial temperature. We found that the LFW process resulted in an extremely low interfacial temperature; it decreased as the applied pressure increased from 50 to 240 MPa. This approach can essentially suppress both softening and hardening regions, leading to uniform hardness distribution in Al joints. The high-pressure LFW process demonstrated here can thus provide an innovated guidance to obtain high-performance Al alloy joints and be extended to other precipitation-strengthened Al alloys, which undergo high-temperature softening.

Nanomechanical scratching induced local shear deformation and microstructural evolution in single crystal copper

Shear deformation at the nanoscale has practical applications due to the ability of shear strains to significantly change the microstructures and textures of the deforming materials, and for material removal processes at this scale. Here we demonstrate nanomechanical scratching with atomic force microscopy (AFM) as a tool to impose large shear strains on nanoscale material volumes in single-crystal copper. Nano-scratching, with the process parameters and AFM tip geometry used here, resulted in material removal through the cutting mode. This mode enabled the use of stress and strain models, developed for bulk machining, to be applied for AFM cutting as well. The models for bulk-scale were used to predict the strains in the chip (γ ≈ 3.9) and in the surface (γ ≈ 4.6) and the depth of deformed subsurface. Detailed characterization of the scratch region with transmission electron microscopy (TEM) showed microstructural refinement comprising 0.2 μm wide dislocation cells in the chips that resemble features during shear deformation of copper single-crystals at bulk-scale. The subsurface also contained dislocation networks and stacking faults up to ~1.1 μm depth. These results highlight the unique ability of AFM for imparting local shear deformation in materials and studying its effects on the microstructure.

Effects of Nonlinear Soil Behavior on Kappa (κ): Observations from the KiK-Net Database

ABSTRACTSoil nonlinear behavior is often triggered in soft sedimentary deposits subjected to strong ground shaking and has led to catastrophic damage to civil infrastructure in many past earthquakes. Nonlinear behavior in soils is associated with large shear strains, increased material damping ratio, and reduced stiffness. However, most investigations of the high-frequency spectral decay parameter κ, which captures attenuation, have focused on low-intensity ground motions inducing only small shear strains. Because studies of the applicability of the κ model when larger deformations are induced are limited, this article investigates the behavior of κ (both κr per record and site-specific κ0 estimates) beyond the linear-elastic regime. About 20 stations from the Kiban–Kyoshin network database, with time-average shear-wave velocities in the upper 30 m between 213 and 626 m/s, are used in this study. We find that the classification scheme used to identify ground motions that trigger soil nonlinear behavior biases estimates of κ0 in the linear and nonlinear regimes. A hybrid method to overcome such bias is proposed considering proxies for in situ deformation (via the shear-strain index) and ground shaking intensity (via peak ground acceleration). Our findings show that soil nonlinearity affects κr and κ0 estimates, but this influence is station dependent. Most κ0 at our sites had a 5%–20% increase at the onset of soil nonlinear behavior. Velocity gradients and impedance contrasts influence the degree of soil nonlinearity and its effects on κr and κ0. Moreover, we observe that other complexities in the wave propagation phenomenon (e.g., scattering and amplifications in the high-frequency range) impose challenges to the application of the κ0 model, including the estimation of negative values of κr.

A distortional hardening model for finite plasticity

Plastic anisotropy may strongly affect the stress and strain response in metals subjected to multiaxial cyclic loading. This anisotropy evolves due to various microstructural features. We first use simple models to study how such features result in evolving plastic anisotropy. A subsequent analysis of existing distortional hardening models highlights the difference between stress- and strain-driven models. Following this analysis, we conclude that the stress-driven approach is most suitable and propose an improved stress-driven model. It is thermodynamically consistent and guarantees yield surface convexity. Many distortional hardening models in the literature do not fulfill the latter. In contrast, the model proposed in this work has a convex yield surface independent of its parameter values. Experimental results, considering yield surface evolution after large shear strains, are used to assess the model’s performance. We carefully analyze the experiments in the finite strain setting, showing how the numerical results can be compared with the experimental results. The new model fits the experimental results significantly better than its predecessor without introducing additional material parameters.

Mechanical property enhancement in gradient structured aluminum alloy by ultrasonic nanocrystalline surface modification

The large strength difference between hard and soft components in heterogeneous structured materials leads to the evolution of high back-stress hardening, which increases the strength and ductility of materials simultaneously. Moreover, the combination of high shear strain and elevated temperature allows an increase in the strength of low-melting temperature metallic alloys by grain refinement, solute migration, and clustering. In this study, to design a new heterogeneous microstructure in the aluminum alloy, both room temperature (RT) and high-temperature (HT) ultrasonic nanocrystalline surface modification (UNSM) were conducted, and their mechanical properties and microstructural evolutions were investigated. The large shear strain from the UNSM treatment reduces the grain size at the sample surface and creates a gradient structure. The combination of shear strain and elevated temperature during UNSM treatment induces solute migration at a certain depth of the specimens, resulting in the nano-sized Mg-rich particles at the surface region. Both grain refinement and precipitation at the surface region of the HT sample provide strong back-stress hardening in the early stages of deformation that enhances the strength and ductility of materials. Therefore, a high shear strain and control of processing temperature allow the design of a unique heterogeneous microstructure in low-melting temperature metallic alloys, which is a good strategy for enhancing the mechanical properties of sheet or thin metallic products.

Outstanding mechanical properties of ultrafine-grained Al7075 alloys by high-pressure torsion

High frictional die and continuous rotation during high-pressure torsion provide not only a large shear strain but also a large amount of frictional heat that facilitates the generation of complex microstructural changes, including grain refinement, dynamic recovery, and solute migration. Since metallic alloys with low melting temperatures are sensitive to heat energy, the mechanical properties and microstructural evolution of high-pressure torsion-processed aluminum 7075 alloys are investigated in this study. The large shear strain resulting from high-pressure torsion induces both grain refinement and dislocation cell formation, leading to strength enhancement and ductility degradation of the alloy. The deformation and frictional heat become a driving force for solute migration, including brittle intermetallic compound dissolution and nano-sized precipitation generation in the matrix. These solute migration activities contribute toward enhancing both the strength and ductility by inducing precipitation hardening and dissolution of the brittle phase simultaneously. Consequently, both the strength and ductility of the high-pressure torsion-processed aluminum 7075 alloy become larger than that of the initial state. This result shows that the microstructural changes of the severe plastic deformation-processed low-melting-temperature metallic alloys induce a significant mechanical property enhancement of nanocrystalline materials.

Experiment investigation and seismic performance evaluation of hybrid isolation bearing with uncoupled compression and shear mechanism

SummaryLead rubber bearings (LRBs) are widely used in the seismic isolated structures. However, the lateral and vertical stiffness of rubber bearing would degrade under high pressure and larger shear strain, which has influence on the structural safety under great earthquakes. This paper proposes a novel hybrid isolation bearing (HIB) with uncoupled compression and shear mechanism. The HIB consists of sliding bearing and rubber springs, carrying superstructural weight and providing lateral stiffness, respectively. The configuration and mechanical model of the HIB are introduced, and an experiment investigation on two sizes of HIB was completed. Through the model test, the deformation mechanism and the mechanical model were verified. A numerical simulation of a frame–shear wall structure was performed, and the variation of structural responses is illustrated. Compared with conventional LRB isolated structure, the HIB isolated structural has similar response variations under earthquakes from frequent to rare levels. Under extremely rare earthquakes, the HIB has more stable mechanical behavior.

Coupled SPH-DEM simulations of liquefaction-induced flow failure

A fully Lagrangian particle-based method for coupled fluid-particle interaction is utilized to evaluate flow liquefaction of saturated granular soils overlain by an impermeable crust. The solid particles are modelled as spherical particles using the discrete element method (DEM). The smoothed particle hydrodynamics (SPH) is employed to model the interstitial fluid using an averaged form of Navier-Stokes equations that accounts for the presence of the solid phase. The coupling between SPH and DEM is achieved through local averaging techniques and well-established semi-empirical formulas for fluid-particle interaction. The responses of loose and dense level granular deposits overlain by an impermeable crust to a seismic excitation are first analyzed. The loose deposit exhibited significant pore pressure development and liquefaction while the dense deposit barely showed any considerable buildup of pore pressure and did not liquefy. The formation of a water film was visible at the interface between the top crust and the underlying liquefied soil. When the same deposits were tilted to form an infinite slope, the loose deposit exhibited flow lgiquefaction at the location immediately underneath the impermeable crust and large shear strains accumulated within a thin layer immediately below the crust. Flow liquefaction was marked by dilative behavior at the surface of the liquefied sand and large lateral spreading that continued post-shaking. Void redistribution was observed along the loose deposit in the form of dilation for a thin layer near sand surface and contraction for the deeper depth. The dense sloping deposit did not liquefy and flow liquefaction was not observed.

Huge local elastic strains in bulk nanostructured pure zirconia materials

From the liquid state to room temperature, two successive solid-state phase transitions occur in pure zirconia. It is well-known that the last one (tetragonal to monoclinic) is martensitic and induces large volume variations and shear strains. Elastic and inelastic behaviors of zirconia-based materials are strongly influenced by this transition and the associated strain fields that it induces. Knowledge of strain and stress at the crystal scale is thus a crucial point. Using fully dense pure zirconia polycrystals obtained by a fuse casting process, we have determined at a sub-micrometric scale, by X-ray Laue microdiffraction, the strains map at room temperature in as-cast specimens and after a post elaboration high temperature thermal treatment. We observed that the fluctuation of deviatoric elastic strain is huge, the standard deviation of normal component being in the range of 1–2%. The heat treatment tends to even further increase this range of fluctuation, despite the development of a multiscale crack network formed during the cooling. Correspondingly, the associated stress level is also huge. It lies in the 5 GPa range with stress gradient amounting 1 GPa μm−1.

Numerical modeling of soil liquefaction and lateral spreading using the SANISAND-Sf model in the LEAP experiments

Laboratory element and centrifuge tests from LEAP-UCD-2017 and LEAP-Asia-2019 were used for model calibration and evaluation in a dynamic coupled analysis of a saturated and gently sloped deposit of sand subjected to base excitation. The paper focuses on using a recently proposed novel constitutive ingredient for modeling the post-liquefaction large cyclic shear strains in sands. An existing critical state compatible, bounding surface plasticity reference model is used, with and without this new constitutive ingredient, to explore its improved modeling capabilities. The constitutive model was first calibrated based on available laboratory element tests on Ottawa-F65 sand, and compared to the reference model showed significantly improved performance in capturing the strain-based liquefaction strength curve of a series of undrained hollow cylinder cyclic torsional shear tests. The calibrated models were used in Class-C prediction of the slope surface deformation in five centrifuge tests on a mildly sloping liquefiable ground of the same soil subjected to dynamic loading, in the three-dimensional finite difference program FLAC3D. The simulation results were compared with experiments in terms of acceleration history, spectral response, excess pore water pressure development, and horizontal displacement evolution at specified control points. The vectors and contours of displacements at the end of shaking also revealed a pattern of slope deformation consistent with that of a gently sloped liquefiable ground. Following the insights from the performance of the models in simulation of the slope response, the calibration was adjusted to more realistically account for the impact of initial static shear stress on the development of post-liquefaction shear strains. The models were again used for Class-C1 prediction of the slope deformation of the same centrifuge tests. The overall assessment revealed the capabilities and limitations of the models in simulating the soil liquefaction strength and its post-liquefaction response.