Shear Strain Accumulation Research Articles

Microstructure, texture, and mechanical properties of asymmetrically cold-rolled Ni1.5FeCrCu0.5 high-entropy alloy

This study deals with the microstructural evolution, texture development, and mechanical properties of the cold-rolled and annealed Ni1.5FeCrCu0.5 high-entropy alloy (HEA). In addition to symmetric rolling (SR), asymmetric rolling (AR) was performed with different velocity ratios (VRs) between the working rolls. Electron backscatter diffraction (EBSD) measurements showed that the grain size of the annealed samples was drastically reduced by increasing the VR of the rolls and imposing shear strain. Moreover, annealing was associated with forming a high volume fraction of annealing twins. Recovery was accelerated by increased accumulated shear strain through the thickness of the ARed strips. The X-ray diffraction (XRD) method was employed to investigate the crystallographic texture in the processed high-entropy alloy. Macrotexture data revealed that the intensity of E {111} <110> and F {111} <112> shear texture components were remarkably higher in the ARed sheets. However, the intensities of S {123} <634> and Copper {112} <111> orientations were decreased in favor of the Brass {011} <211> component, indicating the pure-metal to alloy type texture transition in low stacking fault energy (SFE) materials. Compared to the homogenized sample, the Cube {001} <100> component was dramatically weakened by the annealing of the cold-rolled specimens. In terms of the mechanical properties, the pronounced grain refinement of the post-roll annealed strips enhanced the yield strength (YS) and ultimate tensile strength (UTS) from 332 to 503 in the homogenized condition to 625 and 709 MPa in the asymmetrically rolled (ARed) sample with a velocity ratio of 8. Finally, plastic anisotropy analysis demonstrated that the sheet drawability was gradually improved by increasing the velocity ratio due to the increased mean r-values and reduced planar anisotropy (Δr) values.

Microstructurally short crack growth simulation combining crystal plasticity with extended finite element method

Accurate description of microstructurally short crack growth remains a long-standing challenging task, due to the fluctuation of crack growth rate induced by grain size and orientation, and grain boundary impediment. Combining crystal plasticity with extended finite element method, a novel framework for short crack growth prediction is proposed, where a new criterion for short crack propagation is established in terms of accumulated shear strain on specific slip system. Compared with three classic empirical models, the proposed method exhibits substantial advantage on the prediction of the fluctuation in FGH96 superalloy, serving as a promising tool for precise estimation of short crack growth rate in polycrystalline materials.

Predicting accumulated deformation of offshore large-diameter monopile using hollow cylinder apparatus

Offshore wind turbine structures are commonly subjected to cyclic lateral loading, from waves and wind. The cyclic loading can cause an accumulated deformation of monopile. This study investigates cyclic accumulated deformation of offshore monopiles by using soil cyclic shear tests and numerical simulation. A series of soil axial-torsional bi-directional cyclic load tests are conducted to obtain the cyclic behavior of sand with different relative densities. The test results are analyzed in terms of soil secant modulus from accumulated shear strain, and the results are then used to improve a degradation stiffness method. This improved method was verified by comparing with a pile test. Finally, the cyclic accumulated deformation of large-diameter monopile was studied in loose over dense sand, medium-dense over dense sand and dense sand by using this improved method. The results show that the secant modulus decreases rapidly in the initial cycles of loading, and then the rate of decrease diminishes and remains below an asymptote. The depth of the max bending moment increases with increasing the cyclic number.

Monazite as a monitor of shear strain in orogenic crust

The Ramsay-Graham heterogeneous simple shear zone model revolutionized our understanding of finite strain variation in orogenic belts. Many deeply exhumed (>20 km depths) shear zones in orogenic continental crust are a consequence of sub-simple shear strain accumulation during contraction and displacement at high temperature. Here we explore the micro-scale record of this process using syn-kinematic monazite in two shear zones in a >20,000 km2 terrane of exhumed lower continental crust. Field observations combined with high-spatial resolution X-ray mapping, Th–U-Pb electron microprobe petrochronology, and electron backscatter diffraction analysis demonstrate that monazite grains can provide an important textural and temporal record of shear strain in mylonite and ultramylonite gneisses. Monazite grains exhibit aspect ratios of 1.7–4.6 parallel to the stretching lineation and parallel or oblique (<30°) to the penetrative foliation. Monazite EBSD data for grain orientation spread, i.e., average misorientation relative to the mean, are consistent with no significant dislocation-accommodated internal deformation. We conclude that dissolution precipitation creep of monazite produced compositionally-defined geometries compatible with the dextral sense of shear observed in outcrop and thin section. Syn-kinematic rims on monazite grains that develop in low strain domains during accumulation of shear strain can provide absolute ages for μm-to km-scale kinematics in continental orogens.

Effect of strain history on the monotonic and cyclic response of natural and reconstituted silts

The effect of strain history on monotonic and cyclic response of intact and reconstituted, low and high plasticity silt deposits have been investigated through a series of constant-volume, staged, stress- and strain-controlled cyclic direct simple shear tests. In many cases the specimens subjected to stress-controlled loading exhibited a progressive increase in cyclic resistance due to beneficial effects of increased density following post-cyclic reconsolidation, apparent pseudo-overconsolidation and presumably increased lateral stresses. Such effects outweighed the detrimental effects of fabric destructing as a result of the prior strain history. However, some specimens exhibited an inconsistent evolution of cyclic resistance when assessed using different shear strain cyclic failure criteria. Symmetric accumulation of shear strains in earlier shearing stages did not consistently result in an increasing trend in the post-cyclic resistance; additionally, increases or decreases in cyclic resistance can occur in subsequent shearing stages depending on the amplitude of the maximum shear strain. Staged, strain-controlled tests were used to investigate the cyclic soil response to small and large shear strains, the latter of which caused a reduction in the cyclic resistance of overconsolidated (OC) silt specimens in the following loading event, confirmed using shear wave velocity, Vs, measurements which indicated substantial fabric disturbance following the large strain event. In contrast, normally-consolidated (NC) and OC specimens subjected to multiple stages of small cyclic shear strain exhibited progressive increases in Vs, the magnitude of which varied with stress history. The increase in cyclic resistance in the NC specimens was related to increased pseudo-overconsolidation following reconsolidation. The monotonic undrained shear strength of all silt specimens subjected to staged cyclic loading increased following post-cyclic reconsolidation; the increase in strength and changes in volumetric tendencies are governed by the magnitude of post-cyclic reconsolidation and changes in soil fabric inferred from Vs, respectively.

A simultaneously improved strength and ductility on medium Mn TRIP steel designed by pre-twinning strategy based on SFE controlling

In this work, we developed a strategy based on SFE controlling to improve the mechanical properties of a medium Mn steel. Two successive cycles of deformation were conducted in the stacking fault energy ranges dominated by dislocation slip and twinning deformation, which developed steel with high density dislocations and nanotwins. Dislocation strengthening results in high yield strength. Meanwhile, martensitic transformation within nanotwins provides continuous hardening capacity and consequently high strength and acceptable ductility via alleviating shear strain accumulation and retarding crack initiation.

Strain localization and crack initiation behavior of a PM Ni‐based superalloy: SEM‐DIC characterization and crystal plasticity simulation

AbstractThe strain localization and crack initiation behavior of a powder metallurgy (PM) nickel‐based superalloy FGH4098 under low cycle fatigue (LCF) loads at room temperature is investigated by combining scanning electron microscope (SEM)‐digital image correlation (DIC) characterization with crystal plasticity simulation. The elastic anisotropy near annealing twin boundaries contributes to strain localization and results in high propensity of LCF failure. The effect of local microstructure on the fatigue crack initiation at {111} slip bands and tortuous crack propagation is revealed. Meanwhile, local crystal plasticity finite element cyclic deformation simulation is carried out based on explicit microstructure reconstructed from electron backscatter diffraction (EBSD) characterization. Four metrics of accumulated shear strain, effective plastic strain, shear strain energy dissipation density, and Fatemi–Socie parameter are selected as fatigue indicator parameters to predict crack initiation site. Fatemi–Socie parameter is proved to be a promising metric for crack initiation prediction.

Observed responses of saturated sand under constant deviatoric stress path considering influence of loading amplitude

The soil in embankment dams and slopes may accumulate irreversible deformation due to water level variations. During this process, the soil skeleton experiences a constant deviatoric stress path, along with cyclic mean effective stress. In this work, triaxial drained tests were conducted on saturated sand under a low rate of strain to investigate the strain response along such stress paths, considering the influence of loading amplitude. The test results demonstrated that the volumetric strain of the saturated sand varied when the consolidation stress conditions were different. When the consolidation deviatoric stress was increased from 300 kPa to 900 kPa, the effect of cyclic load amplitude on the volumetric strain was strengthened, while the cumulative effect was weakened. The shear strain curves under different loading amplitudes were parallel to each other regardless of the consolidation stress condition. However, the cumulative shear strain showed great differences. The accumulation of shear strain was increased with the enhancement of loading amplitude. When the consolidation deviatoric stress was held constant at a low stress level, the effect of loading amplitude on volumetric strain was greater than that on shear strain. However, with the enhancement of stress level, the effect on shear strain was significantly enhanced.

Dynamic deformation and fault locking of the Xianshuihe Fault Zone, Southeastern Tibetan Plateau: implications for seismic hazards

The Xianshuihe Fault Zone is one of the most historically seismically active regions in mainland China. However, the seismicity along this fault zone has been quiescent for the past 40 years, since the Daofu M6.9 earthquake in 1981. Understanding its current deformation patterns and fault coupling characteristics is of great significance to estimate the potential risk of strong earthquakes. In this study, we analyzed the dynamic deformation and fault coupling characteristics along the Xianshuihe Fault Zone using Global Navigation Satellite System (GNSS) data for 1999–2007 and 2016–2020. The results show that the deformation pattern of the Xianshuihe fault zone underwent a dynamic adjustment after the 2008 Wenchuan Mw7.9 and 2013 Lushan Mw6.6 earthquakes, i.e., the maximum shear strain accumulation rates of the Luhuo and Daofu sections significantly decreased from 6.0 × 10–8/a to 3.2 × 10–8/a, while that of the southeastern segment (i.e., Kangding and Moxi sections) increased from 4.5 × 10–8/a to 6.2 × 10–8/a. Additionally, the slip rate and deformation width of the Xianshuihe Fault Zone also changed during these two periods. Combined with the near-field cross-fault observation data, we suggest that the surrounding strong earthquakes Wenchuan and Lushan had evident differential impacts on the deformation pattern of the Xianshuihe Fault Zone. The fault-coupling inversion results show that the locking degree of the Xianshuihe Fault Zone continued to increase after the Wenchuan and Lushan earthquakes, especially the Qianning and Moxi sections increased significantly, with an average coupling coefficient of greater than 0.9 and left-lateral slip-rate deficits of ~ 5 mm/a and ~ 8 mm/a, respectively. In contrast, the locking degree of the Kangding section decreased with almost no slip-rate deficit, which is in a state of creeping status. The analysis of the recent rupture history and strain accumulation characteristics of the Xianshuihe Fault Zone indicates that both the Qianning and Moxi sections have a high seismic potential for the next strong earthquake in the Xianshuihe Fault Zone.Graphical

Mechanisms of shear strain accumulation in laboratory experiments on sands exhibiting cyclic mobility behavior

The factors and mechanisms controlling the accumulation of shear strains of clean uniform sands exhibiting cyclic mobility behavior under level-ground conditions are examined. This phenomenon is investigated through a series of constant-volume cyclic direct simple shear (DSS) tests subjected to uniform and irregular loading conditions, and undrained cyclic element tests collected from the literature. Experimental data show that the rate of shear strain accumulation per loading cycle depends on the relative density, cyclic stress amplitude, and effective overburden stress. Mechanisms of shear strain accumulation are investigated by decoupling the shear strain developed in each loading cycle in two components: γ0, developed at near-zero effective stress, and γd, developed during dilation. Results show that γ0 mostly depends on the shear strain history, while γd depends on the cyclic stress amplitude and relative density. These dependencies of γd and γ0 are used to provide an explanation for the gradual decrease of the rate of shear strain accumulation that is observed while increasing the number of post-triggering loading cycles in tests performed on dense specimens.

Slippage during high-pressure torsion of commercially pure titanium and application of accumulative HPT to it

A new efficient method was used to find that in the case of high-pressure torsion of commercially pure titanium, accumulation of shear strain in Ti does not occur due to slippage of anvils. Despite this, micro-hardness increases as the number of turns n increases, and Ti structure is refined more intensively. High-pressure torsion is accompanied by a high-pressure ω-phase formation. However, the content of ω-phase changes non-monotonously with an increase in the number of turns. First, while number of turns is less than n=5, the ω-phase content reaches 50%. Upon further deformation, the ω-phase content decreases to 15% for n=20. A new accumulative high-pressure torsion method is applied to commercially pure titanium for the first time. Accumulative high-pressure torsion leads to the strongest transformation of the structure and an increase in hardness, since stronger real deformation occurs due to composition of compression and torsion strain cycles.

A simultaneously improved strength and ductility on carbide free bainite steel via novel ausrolling and twinning process based on SFE controlling

In this paper, we developed a strategy to circumvent the strength-ductility trade-off in high-strength carbide-free nanobainite steel. Two successive cycles of deformation were designed in the corresponding stacking fault energy ranges of supercooled austenite and retained austenite, which are respectively dominated by dislocation slip and twinning mechanism. The results indicate that the ausrolling and twinning process yielded a high density of dislocations and nanotwins in steel. A high density of dislocations in bainite ferrite trapped more carbon atoms, reducing the carbon content in retained austenite and the corresponding stability. Therefore, martensitic transformation of retained austenite was accelerated, during the tensile process, resulting in an excellent work hardening capacity and consequently ultrahigh strength in the resultant material. Meanwhile, multistage γ (face-centered cubic) → ε (hexagonal close-packed) → α' (body-centered cubic) martensitic transformation and interaction of nanotwins with dislocations provide superior ductility by alleviating shear strain accumulation and retarding crack initiation. The excellent strength (2.4 GPa) and ductility (15.7%) of the ausrolling and twinning steel indicates that the proposed strategy has potential applications in the development of high-strength, high-ductility materials.

Effect of slip to roll ratio on cyclic plastic deformation response at subsurface during rolling contact fatigue

The objective of this investigation is to evaluate the cyclic plastic deformation response at the subsurface during rolling contact fatigue (RCF) under the conditions of pure rolling and rolling-sliding. The finite element simulation results indicate that the slip-to-roll ratio (SRR)has an immense effect on the cyclic plastic deformation evolution at the subsurface during RCF. At SRR of 0% (pure rolling), symmetric shear stress cycling is evident at the subsurface, and as a result plastic shakedown takes place. On the other hand, at SRR of 5% (rolling-sliding), asymmetric shear stress cycling is noticed at the subsurface, and consequently ratcheting i.e. progressive accumulation of shear strain is evident. Rolling-sliding results in the shear flow of the subsurface material along the sliding direction.

Molecular dynamics simulation of vacancy and void effects on strain-induced martensitic transformations in Fe-50 at.% Ni model concentrated solid solution alloy

Strain-induced martensitic transformations can improve the strength and ductility of some face centered cubic (FCC) metals and alloys. Irradiation-induced defects such as vacancies, dislocation loops, and voids activate martensitic transformations over a wider range of mechanical loading conditions than in pristine material. However, the mechanisms underlying irradiation-enabled martensite transformations remain unclear. In this work, we use molecular dynamics simulations to study the effect of vacancies and voids on strain-induced martensitic transformations in a model concentrated solid solution alloy Fe-50 at.% Ni. It has been found that single vacancies have no resolvable effect on the transformation because they reduce the stacking fault energy by a relatively insignificant margin and do so only if the vacancy is located on the stacking fault plane. Voids, however, activate the martensite transformation through shear strain accumulation around the void due to dislocation pileup. The larger the void, the more pronounced this effect.

Evaluation of post-liquefaction volumetric strain of reconstituted samples based on soil compressibility

In this study, we performed undrained cyclic–torsional–shear tests on clean and silty sands to examine post-liquefaction volumetric strain. Accordingly, we proposed a novel density index—ultimate volumetric strain—to evaluate the density of clean sand and silty soils; the proposed density index is based on soil compressibility. We discussed the test results with respect to relative density, skeleton void ratio, and ultimate volumetric strain. Consequently, we confirmed that ultimate volumetric strain showed good correlation with post-liquefaction volumetric strain irrespective of soil type, whereas values for relative density and skeleton void ratio were discordant. Further, we observed that, compared to maximum shear strain, accumulated shear strain was a better index for representing the effect of loading history. From these results therefore, we proposed a chart to evaluate post-liquefaction volumetric strain based on ultimate volumetric strain and accumulated shear strain.

Thermomechanical Microstructural Predictions of Fracture Nucleation of Zircaloy-4 Alloys With δ and ɛ Hydride Distributions

Abstract A crystalline dislocation-density formulation that was incorporated with a nonlinear finite-element (FE) method was utilized to understand and to predict the thermomechanical behavior of an hexagonal closest packed (h.c.p.) zircaloy system with hydrides with either face-centered cubic (f.c.c.) or body-centered cubic (b.c.c.) hydrides. This formulation was then used with a recently developed fracture methodology that is adapted for finite inelastic strains and multiphase crystalline systems to understand how different microstructurally based fracture modes nucleate and propagate. The interrelated microstructural characteristics of the different crystalline hydride and matrix phases with the necessary orientation relationships (ORs) have been represented, such that a detailed physical understanding of fracture nucleation and propagation can be predicted for the simultaneous thermomechanical failure modes of hydride populations and the matrix. The effects of volume fraction, morphology, crystalline structure, and orientation and distribution of the hydrides on simultaneous and multiple fracture modes were investigated for radial, circumferential, and mixed distributions. Another key aspect was accounting for temperatures changes due to the effects of thermal conduction and dissipated plastic work and their collective effects on fracture. For hydrided aggregates subjected to high temperatures, thermal softening resulted in higher ductility due to increased dislocation-density activity, which led to higher shear strain accumulation and inhibited crack nucleation and growth. The predictions provide validated insights into why circumferential hydrides are more fracture-resistant than radial hydrides for different volume fractions and thermomechanical loading conditions.

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.

Evolution of granular media under constant-volume multidirectional cyclic shearing

By means of the three-dimensional discrete element method, we study the long-time evolution toward liquefaction state in granular materials composed of spherical particles under multidirectional cyclic shearing at constant volume. Extensive simulations were carried out along 1-D linear, 2-D linear, circular/oval, and 8-like shear paths, and the evolution of the system was analyzed in terms of pore pressure, shear strain, and granular texture. The macroscopic stress path and stress–strain response agree well with laboratory experiments. We find that the liquefaction resistance, i.e., the number of cycles necessary to reach the liquefaction state, is generally lower under multidirectional loading as compared to unidirectional loading. As the transient vanishing of mean stress does not occur for all stress paths, we introduce a shear strain-based liquefaction criterion that can be consistently applied to all strain paths. The granular texture is monitored through the coordination number, particle connectivity, force and fabric anisotropies, and friction mobilization. In particular, a particle-void descriptor, named centroid distance, is found to be closely related to the shear strain accumulation. We show that the force anisotropy tensors become almost proportional to the deviatoric stress tensor more quickly than the fabric anisotropy tensor, which takes most of the pre-liquefaction period to follow the external loading. The relationship between deviatoric stress ratio and the force and fabric anisotropies, known to hold in monotonic triaxial loading, also holds with high accuracy in the studied multidirectional cyclic shearing paths; the contributing weights of the anisotropies level off in the post-liquefaction period and do not depend on the shear path.

Multiple ballistic impacts on 2024-T4 aluminum alloy by spheres: Experiments and modelling

• First investigation on multiple ballistic impacts of 2024Al-T4 alloy. • Crater parameters exhibit power-law increase with the number of impacts. • Twin-like deformation bands and macro-deformation twins coexist around crater walls. • FEM modelling reveals accumulated shear strain leads to deformation twinning. Multiple ballistic impacts are carried out on a 2024-T4 aluminum alloy by spherical steel projectiles (5-mm diameter) at ∼ 400 m s − 1 , to investigate its dynamic deformation and damage. The ballistic impact process is captured with high-speed photography. Postmortem samples are characterized with optical imaging, three-dimensional laser scanning, microhardness testing and electron backscatter diffraction. With increasing number of impacts, crater diameter increases slightly, but crater depth and crater volume increase significantly, and strain accumulation leads to microhardness increase overall. Crater parameters all follow power-law relations with the number of impacts. Twin-like deformation bands and macroscopic deformation twins are produced by impact as a result of spontaneous dislocation self-pinning under high strain rate, large shear deformation. Under multiple impacts, shear strain accumulation in the arc-shaped region of the crater induces deformation twinning when it exceeds a critical value ( ∼ 1.1 − 1.6 ). It is highly possible that the deformation twins are related to deformation bands, since they both share one set of the { 111 } pole with the initial matrix grain. A finite element method model is optimized to reproduce experimental observations and interpret deformation mechanisms.

Molecular Dynamics Simulation of Vacancy and Void Effects on Strain-Induced Martensitic Transformations in an FeNi Alloy

Strain-induced martensitic transformations can improve the strength and ductility of some face centered cubic (FCC) metals and alloys. Irradiation-induced defects such as vacancies, dislocation loops, and voids activate martensitic transformations over a wider range of mechanical loading conditions than in pristine material. However, the mechanisms underlying irradiation-enabled martensite transformations remain unclear. In this work, we use molecular dynamics simulations to study the effect of vacancies and voids on strain-induced martensitic transformations. It has been found that single vacancies have no resolvable effect on the transformation because they reduce the stacking fault energy by a relatively insignificant margin and do so only if the vacancy is located on the stacking fault plane. Voids, however, activate the martensite transformation through shear strain accumulation around the void due to dislocation pileup. The larger the void, the more pronounced this effect.