Macroscopic Shear Bands Research Articles

Critical state uniqueness of dense granular materials using discrete element method in conjunction with flexible membrane boundary

An explanation of the meso-mechanism of sand granular materials for the uniqueness of critical state is presented by means of the discrete element method (DEM) under flexible boundary loading conditions. A series triaxial drainage shear test (DEM simulations), in conjunction with the flexible boundary technique, of were performed for sand samples subjected to various physical states and with different particle size distributions. After carefully investigating the critical status of the results of the numerical calculation, the macroscopic failure modes and shear band evolution of sand, as well as the velocity vector field due to different initial states, were explored and classified. Furthermore, the evaluation rules and discrepancies between overall void ratios of the specimen and local void ratios within the shear band under the critical state were recorded and analyzed. The results proved that a sample with a small void tends to form a shear band, and the rotation of the particles in the non-shear zone is negligible. Conversely, sandy soil with large initial void ratios exhibited limited development of significant shear bands, and the change in void ratios within the shear region and the non-shear area are not significant. Interestingly, the particle-size distribution exerts minimal influence on the evolution rule which the void ratio converges within the shear band and diverges outside the shear region for both multi-stage and single-stage specimens. The void ratio within the shear band and deviator stress ratio tend to exhibit consistently for the same specimen with different initial physical states, thereby distinguishing the critical state. There is a significantly higher change in void ratio within the shear band compared to outside of it, yet it remains stable within a relatively similar range. Additionally, the invariant of the fabric tensor used to describe the critical state characteristics also demonstrates a high degree of consistency within the shear band. These findings strongly indicate that the critical state exists within the shear failure surface and is highly likely to be unique.

Physical mechanism of the intermittent plastic flow at extremely low temperatures

The physical mechanism of the intermittent plastic flow (IPF) of austenitic steels at extremely low temperatures is explained in the context of microstructure evolution and 3D deformation arising in the area of the shear band. The microstructure change is identified by X-ray diffraction with the use of synchrotron radiation as well as electron backscatter diffraction (EBSD). This complete, global and local, approach reveals the intensive formation of twin boundaries which perform two functions. They become a part of the macroscopic shear band, but also constitute barriers piling up dislocations. Thus, their overcoming results in a stress drop as in the case of breaking the Lomer-Cottrell locks. The investigations show that the rapid displacement is blocked by the intensively produced martensite α’. The main features of the uncovered microstructural evolution are captured in the 3D reconstruction of the shear band obtained with the use of profilometer. Moreover, the effect of the thermodynamic instability is taken into account when assessing the conditions of the IPF. Finally, new kinetics of the IPF is developed. It comprises two functions: B reflecting the surface density of the dislocation pile-ups on the internal lattice barriers, and G corresponding to the surface density of twin boundaries. The breakthrough of the work consists in the discovery of multi-scale mechanism of the IPF, including participation of twin boundaries and the Lomer-Cottrell barriers.

Distinct avalanche dynamics detected in metallic glasses with high energy state revealing the crack-like shear banding mechanism

When a sufficiently high stress is applied to a metallic glass, causing plastic deformation, the material undergoes structural reconfiguration through dissipative slip avalanche events that release local stresses. By utilizing isothermal annealing and cold rolling techniques to tune the energy levels of metallic glasses, it has been observed that structural rejuvenation is accompanied by structural relaxation, as evidenced by distinct changes in avalanche dynamics. We present detailed statistics of the avalanche dynamics during shear band formation in energy-tuned metallic glasses, ranging from structurally relaxed to rejuvenated states. By analyzing shear band characteristics and examining scaling exponents, avalanche durations, and stress relaxation rates, we can establish a connection between the local activation of shear transformation zones and the formation of macroscopic shear bands. The statistics of avalanche duration indicate that an increase in soft zones within metallic glasses can alleviate stress release and stabilize plastic flow, as evidenced by the characteristics of shear bands. We attribute the significant transition of serrated flow, observed at different energy levels (i.e., as-cast, relaxed, and rejuvenated states) to the variations in nucleation and multiplication of shear bands that originate from local weak spots. Analysis of the distinct avalanche dynamics suggests that in lower energy level metallic glasses, the nucleation and propagation of shear bands exhibit localized crack-like behavior, while in higher energy level metallic glasses, they display diffused crack-like characteristics. Indeed, our results strongly support that the decreased avalanches observed in the high energy level metallic glasses originate from the nucleation of numerous small shear bands, which directly compete with the propagation of the main local shear band. These findings deepen our fundamental understanding of the relationship between the microscopic mechanism of slip avalanche dynamics and shear banding, providing a pathway to control the plasticity of metallic glasses.

Effect of Contact Pressure on Strain Distribution during Compression-Type Bulk Forming Processes.

Inhomogeneity of the material properties of workpieces developed during compression-type bulk forming processes (CBFPs) is an important issue. The effect of contact pressure on the workpiece surface on the strain inhomogeneity in the workpiece was investigated to understand and reduce the formation of strain inhomogeneity during CBFPs. Workpieces fabricated via rod caliber rolling, rod flat rolling, plate flat rolling, and rod compression were analyzed and compared. The extent of strain inhomogeneity in a workpiece differs with the forming process, because the occurrence of macroscopic shear bands (MSBs) is dependent on the workpiece shape and tool design. A flat-rolled rod exhibits the maximum strain inhomogeneity, whereas a flat-rolled plate shows the minimum strain inhomogeneity. The occurrence of MSBs was influenced by the distribution of the normal contact pressure or compression stress. The MSBs were stronger when the contact pressure was higher in the edge region of the surface. For example, the flat-rolled plate exhibited weak MSBs due to the relatively uniform or higher contact pressure on the central region. In contrast, strong MSBs appeared in the flat-rolled rod and compressed rod, because the contact pressure in the edge region of these two processes was high. Thus, the strain inhomogeneity in a workpiece fabricated via CBFPs can be reduced by controlling the contact pressure distribution on the workpiece surface.

Inception of macroscopic shear bands during hot working of aluminum alloys

Macroscopic shear bands (MSB) may develop during hot working of metallic materials. They are well-understood as a physical manifestation of flow instability, and the processing regimes wherein they form are well-charted. However, the microstructural transitions that occur between the onset of flow instability and MSB inception are not fully understood. In order to elucidate them, several aluminum alloy specimens were subjected to strip testing in a thermomechanical simulator (Gleeble™) at 298 K and 573 K. Prominent MSB were observed along the diagonals of the strip volume of only Aluminum-6 wt% Magnesium alloy specimen deformed at 573 K. Comparing the experimental grain morphology and crystallographic textures with those from plastic flow models revealed that MSB inception occurred only after ∼0.20 homogeneous plane strain deformation. However, classical flow instability was predicted at much smaller strain. This ‘delay’ was explained experimentally by showing that clusters of neighbouring severely deforming, fragmenting, mostly soft-oriented grains gradually developed due to lattice rotations along the specimen diagonals, and that MSB inception corresponded to the formation of a percolating network of such grains spanning the specimen. Further, clear experimental evidence revealed that differential dynamic recovery between hard- and soft-oriented grains was essential for MSB formation.

Interplay Between Friction and Cohesion: A Spectrum of Retrogressive Slope Failure

AbstractRetrogressive failures occur in slopes consisting of sensitive materials such as snow or quick clay. They can be triggered by a small disturbance at the slope toe, but can cause propagated failure spreading miles away. Understanding the physical mechanism and predicting the retrogressive failure process are particularly important. Previous studies have discussed the failure criteria, the soil properties or the method of numerical modeling of retrogressive slope failure. However, little attention has been paid to the microscopic failure mechanism, especially relating to various possible failure patterns. In this study, multiscale modeling is incorporated to study the physical mechanism of different retrogressive failure patterns, including earth flow, flowslide and spread failure, within a unified framework. Utilizing multiscale analysis, we found that earth flow failure is related to the shear failure of granular materials. In contrast, the development of macroscopic shear bands is accompanied by tensile failure. As shear and tension failures are typical failure mechanisms of frictional and cohesive materials, it is deduced that friction and cohesion effects play key roles in different retrogressive failure patterns. Therefore, the distributions of attractive and repulsive contact forces are explored and a novel parameter η is proposed to quantify the interplay between friction and cohesion. Further analysis proves that η can capture the effect of friction and cohesion and distinguish different retrogressive failure patterns. Finally, a spectrum of retrogressive failures for a granular slope is established, in which the failure mechanism is explained by the changeable dominant effect, that is, frictional or cohesive in soil.

Effect of Elongation of a Rod on Strain Inhomogeneity and Shape Change During the Compression-Type Forming Process

Abstract The influence of elongation on the strain inhomogeneity and shape change in twinning-induced plasticity steel rod is systematically investigated to understand the macroscopic shear band (MSB) formation mechanism and to decrease the strain inhomogeneity during the compression-type forming processes. Specimens fabricated by rod flat rolling with elongation (3D rod) and by plane compression without elongation (2D rod) are compared using both finite element analysis and experiment. Despite the similar final product shape, the 2D rod presents a lower effective strain at the surface region than the 3D rod, leading to a high strain inhomogeneity. The higher effective strain at the surface region of the 3D rod is mainly attributed to the elongation of the 3D rod during the rolling. In contrast, the 2D rod exhibits strong dead metal zones owing to the lack of elongation of the specimen. Therefore, the formation of MSBs or strain inhomogeneity of a specimen can be reduced by increasing the elongation of the specimens during the forming process. Both the contact width and lateral spread of the 3D rod are lower than those of the 2D rod because of the elongation of the 3D rod originating from the slip effect at the rod–roll interface during the rolling process. The small frictional effect at the rod and roll interface increased the elongation of the rod, leading to a decrease in the strain inhomogeneity and lateral spreading in the 3D rod.

Finite-Disorder Critical Point in the Yielding Transition of Elastoplastic Models

Upon loading, amorphous solids can exhibit brittle yielding, with the abrupt formation of macroscopic shear bands leading to fracture, or ductile yielding, with a multitude of plastic events leading to homogeneous flow. It has been recently proposed, and subsequently questioned, that the two regimes are separated by a sharp critical point, as a function of some control parameter characterizing the intrinsic disorder strength and the degree of stability of the solid. In order to resolve this issue, we have performed extensive numerical simulations of athermally driven elastoplastic models with long-range and anisotropic realistic interaction kernels in two and three dimensions. Our results provide clear evidence for a finite-disorder critical point separating brittle and ductile yielding, and we provide an estimate of the critical exponents in 2D and 3D.

Inhomogeneity of microstructure and mechanical properties in compression-type bulk formed specimens

The effect of elongation and spreading in a material on strain and microstructure inhomogeneity during compression-type bulk forming processes was investigated to improve the strain homogeneity of the materials. For systematic investigation, four different compression-type forming processes, namely wire flat rolling, plate flat rolling, wire caliber rolling, and uniaxial compression, were compared. Strain inhomogeneity and macroscopic shear bands (MSBs) were clearly observed during the compression-type bulk forming processes based on a comparative study on the shape change, effective strain and hardness distributions, and microstructural evolution in twinning-induced plasticity steel. The flat-rolled wire had the largest strain inhomogeneity, followed by the caliber-rolled wire, compressed specimen, and flat-rolled plate in the listed order. The inhomogeneity of the strain distribution along the horizontal direction tended to increase with the pure spreading during the forming process. Meanwhile, the inhomogeneity of the strain distribution along the vertical direction increased with decreasing elongation during the process. Therefore, a small pure spreading and large elongation of the specimen result in more homogeneous and higher quality products in the compression-type bulk forming process. For instance, the flat-rolled plate exhibited weak MSBs and low strain inhomogeneity because of the small pure spreading and large elongation during the forming process.

Surface Characterization of Lüders Band on NiTi Shape Memory Alloy

The tensile deformation of NiTi alloy can proceeds in either homogeneous manner, or localized deformation via formation and propagation of macroscopic shear bands, that is commonly known as Lüders-like deformation. The high deformation strain within the localized deformed regions can result in the changes of surface characteristics of NiTi specimen. This paper studies the surface roughening effect associated with Lüders-like deformation of martensitic NiTi alloy, via surface characterization of polished surface and localized deformed region that consists of Lüders bands on tensile specimens, respectively. The surface roughness profile and roughness parameters of surface with Lüders bands are significantly different and higher as compared to the polished surface.

Hardening and toughening effects of intermediate nanosized structures in a confined amorphous alloy film

• Intermediate nanosized structure (termed as “nano shear bands” (NSBs)) between microscopic shear transformation zones (STZs) and macroscopic shear bands (SBs) was observed in the crystalline-layer confined amorphous (CLCA) NiW alloy films. • NSBs promoted multiple and controlled shear banding deformation, hence leading to the enhanced crystallization. • NSBs possessed both hardening and toughening effects for the CLCA films. The plastic deformation of amorphous alloys is well known to be localized into shear bands (SBs), which are believed to stem from the atomic-scale flow defects, i.e., shear transformation zones (STZs). Yet, the bridge between the mesoscopic SBs and the atomic-scale STZs remains poorly understood. In this work, through thermally activating pronounced β relaxations in the well-designed crystalline-layer confined amorphous (CLCA) NiW alloy films, we experimentally captured and observed an intermediate nanosized structure termed as “nano shear bands” (NSBs) with a typical size of 1–2 nm in thickness and 5–10 nm in length. The influences of such NSB structures on the macroscale deformation behavior were systematically investigated. It was found that NSBs lead to both hardening and toughening effects for the CLCA films, as they promote multiple and controlled shear banding deformation, which results in enhanced crystallization. The intermediate NSB structure could connect the microstructural characteristics and macroscopic plasticity in amorphous alloys and may provide new insights for understanding the microscopic deformation mechanism of amorphous alloys as well as tuning/designing their properties.

Strain and strain rate hardening effects on the macroscopic shear bands and deformation shape of a caliber-rolled wire

The effect of the strain hardening exponent (n) and strain rate sensitivity (m) on the strain distribution and surface profile of a workpiece during the caliber rolling process has been investigated to fully understand the influence of material properties on shape change and strain distribution of the caliber-rolled wire. Twinning-induced plasticity (TWIP) steel with high n and low m values and low carbon ferritic steel with low n and high m values were mainly compared using both experimental test and finite element analysis. The macroscopic shear bands were clearly observed in the caliber-rolled wire, indicating that the deformation is highly inhomogeneous during the caliber rolling process. The homogeneity of the strain distribution in caliber-rolled wire increased with increasing n and m values due to the fast propagation of the plastic region to other areas from the first hardened area in a material. The effect of the n value was higher than that of the m value. Consequently, TWIP steel had weaker macroscopic shear bands compared to plain carbon steel. The length of the maximum spread of caliber-rolled wire in the free surface increased with decreasing n and m values because the dead metal zone can be easily formed with decreasing n and m values. Therefore, the caliber roll design applied to plain carbon steel cannot be applied to TWIP steel because the roundness of caliber-rolled TWIP steel was poor in such a case. A roll gap thus needs to be modified with the n and m values of a material to fabricate the desired shape of a product.

The time-dependent behaviour and failure mechanism of dacite under unloading condition

To obtain time-dependent responses of rock in excavation engineering projects, a series of creep tests of dacite are conducted through stepwise unloading of confining pressure. Based on the test results, time-dependent behaviour and unloading failure mechanism of dacite are studied in terms of creep rate, failure mode, long-term strength, and time-dependent damage. According to the creep rate, creep deformation can be divided into decelerating creep stage, steady creep stage, and accelerating creep stage during the last unloading failure process. Time-dependent deformation exhibits an anisotropic characteristic. Local shear bands are found in failed samples, which is markedly different from tensile failure in a short-term unloading test. Time-dependent damage of dacite under unloading condition is the result of combined effects of cracks. Creep failure of dacite under unloading condition caused by concentrating in a local zone and coalescing to form one or more macroscopic shear bands. This study gives an insight to understand the mechanism of unloading creep failure of rock and provides an experimental foundation for developing a time-dependent constitutive model under unloading condition.

Kinematic and thermal characteristic of discontinuous plastic flow in metastable austenitic stainless steels

The strain induced martensitic phase stabilizes the propagation of macroscopic shear band during displacement-controlled uniaxial tensile test of metastable austenitic stainless steels (316 L, 304) at liquid helium temperature (4.2 K). It leads to huge Lüders-type deformation, high hardening and large ductility of the specimen. In Lüders range, shear band develops across the specimen in discontinuous and sequential way, which is reflected by stress oscillation on stress-strain curve and comb-like profile of temperature recorded during tests (so called discontinuous plastic flow - DPF). Based on the time responses of temperature and elongation transducers, a full picture of the localized deformation behaviour of specimen at 4.2 K was obtained, including its both spatial and temporal features. Moreover, the experimental results clarified that DPF has mechanical origin and it is accompanied by thermal effects. The model of temperature distribution during DPF was proposed. The model involves temperature effects driven by the elastocaloric phenomenon (experimentally identified at 4.2 K) and the plastic power dissipation. Based on the model, kinematic and thermal limits of DPF in the austenitic stainless steels were determined.

Influence of Roll Diameter on Material Deformation and Properties during Wire Flat Rolling

The influence of roll diameter on the strain distribution, shape change, contact pressure, and damage value of a workpiece was investigated during wire flat rolling to control the material properties of the flattened wire. The flattened wires fabricated by the four different rolls were compared using finite element analysis. The strain inhomogeneity of the flat-rolled wire increased with the roll diameter; thus, the macroscopic shear bands were strengthened as the roll diameter increased during wire flat rolling. The contact width and lateral spreading of the flattened wire increased with the roll diameter; therefore, the reduction in area decreased with the roll diameter. The contour of the normal contact pressure on the wire surface exhibited a similar pattern regardless of the roll diameter. The contact pressure showed higher values at the entry, edge, and exit zones in the contact area. The distribution of the damage value varied with the roll diameter. The free surface region tended to have the peak damage value during the process; however, the center region exhibited the maximum damage value with the roll diameter. From the perspective of the damage value, the optimum roll diameter was in existence during wire flat rolling. The underlying cause of the different strain distributions, shape changes, and damage values of the flat-rolled wire was the different contact lengths originating from the different roll diameters during wire flat rolling.

Hybrid discrete-continuum modeling of shear localization in granular media

Shear localization is a frequent feature of granular materials. While the discrete element method can properly simulate such a phenomenon as long as the grain representation is accurate, it is computationally intractable when there are a large number of grains. The continuum-based finite element method is computationally tractable, yet struggles to capture many grain-scale effects, e.g., shear band thickness, because of mesh dependence, unless the constitutive model has a length scale. We propose a hybrid discrete-continuum technique that combines the speed of the continuum method with the grain-scale accuracy of the discrete method. In the case of shear localization problems, we start the simulation using the continuum-based material point method. As the simulation evolves, we monitor an adaptation oracle to identify the onset of shear bands and faithfully enrich the macroscopic continuum shear bands into the microscopic-scale grains using the discrete element method. Our algorithm then simulates the shear band region with the discrete method while continuing to simulate the rest of the domain with the continuum method so that the computational cost remains significantly cheaper than a purely discrete solution. We validate our technique in planar shear, triaxial compression, and plate indentation tests for both dry and cohesive granular media. Our method is as accurate as a purely discrete simulation but over 100 times faster than a discrete simulation that would require tens of millions of grains.

Enhanced strain delocalization through formation of dispersive micro shear bands in laminated Ni

Nanocrystalline metals usually exhibit limited ductility due to catastrophic shear fracture caused by the preferential formation of a few premature shear bands during tensile deformation. In this work, the tensile and fracture behavior of laminated Ni with varied degree of difference in grain size of hard and soft layers were investigated. Compared with monolithic nanocrystalline Ni, laminated Ni with a large difference in the grain size of hard and soft layers exhibited enhanced elongation to failure without sacrificing tensile strength, because the formation of dispersive micro shear bands could undertake large plastic strain and lead to strain delocalization. With decreasing the difference in the grain size of hard and soft layers and the corresponding resistance to propagation of micro shear bands, there is a transition of fracture mode from ductile necking fracture accompanied with dispersive micro shear bands to shear fracture dominated by a few macroscopic premature shear bands. A mechanical model based on the transition from grain interior dislocation emission to grain-boundary-mediated deformation within the soft layers was proposed and the critical grain size of the soft layers corresponding to the optimum synergy of high strength and ductile necking fracture was obtained.

Critical microstructures and defects in heterostructured materials and their effects on mechanical properties

Systematic study was conducted on the microstructures and mechanical properties of nickel samples with two distinct types of heterostructures. The first is featured with coarse-grained lamellae embedded in a matrix consisting of a very high density of dislocation structures. The second is featured with coarse-grained zones embedded in the ultrafine-grained matrix. The second type of heterostructures exhibits better strength and ductility, although it has a smaller average grain size than the first type. The zone boundaries in the second type of heterostructures are less prone to cracking than those in the first type. Intersecting micro-shear-bands formed net-like patterns in the second type of heterostructures during tensile deformation. This is the first ever observation of structural micro-shear-bands in a heterostructured material. It supports the claim that heterostructure promotes the formation of dispersive shear bands. In contrast, a macroscopic shear band formed and caused early failure of the sample with the first type of heterostructures. Our results indicate that well-developed ultrafine/nano grained matrix in heterostructured materials are necessary for preventing crack formation and shear band localization. This should be considered as a key factor for optimizing the mechanical properties of heterostructured materials.

Brittle yielding of amorphous solids at finite shear rates

Amorphous solids display a ductile to brittle transition as the kinetic stability of the quiescent glass is increased, which leads to a material failure controlled by the sudden emergence of a macroscopic shear band in quasi-static protocols. We numerically study how finite deformation rates influence ductile and brittle yielding behaviors using model glasses in two and three spatial dimensions. We find that a finite shear rate systematically enhances the stress overshoot of poorly-annealed systems, without necessarily producing shear bands. For well-annealed systems, the non-equilibrium discontinuous yielding transition is smeared out by finite shear rates and it is accompanied by the emergence of multiple shear bands that have been also reported in metallic glass experiments. We show that the typical size of the bands and the distance between them increases algebraically with the inverse shear rate. We provide a dynamic scaling argument for the corresponding lengthscale, based on the competition between the deformation rate and the propagation time of the shear bands.

Design, FEM Analysis and Compression Tests on Energy-Absorbing Property of Disordered Cellular Solids Developed by 3D-Voronoi Division

Lightweight cellular solids have been used as an energy-absorbing material for automotive and aerospace applications. Additive manufacturing is a useful process for the design and manufacture of cellular solids. The present study focuses on the relationship between the energy-absorbing property and the cell structure of additively manufactured porous aluminum alloys. Disordered cell structures with different regularity are designed by the technique of 3D-Voronoi division. Open-cell structures with a porosity of 90% and a strut diameter of 1 mm are generated using commercial 3D CAD software. Explicit FEM analysis including a Johnson-Cook failure model successfully simulates the compressive stress-strain curves of disordered porous aluminum alloys. Experimental results of compression tests were in good qualitative agreement with the FEM analysis. The ordered cell structure with high regularity is disadvantageous for energy-absorbing applications because of the formation of a macroscopic shear band. A slightly low cell regularity in a additively manufactured cellular solid is effective for increasing energy absorption.