Showing papers on "Shear band published in 2007"
TL;DR: The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for metallic glasses based on small-scale molecular dynamics simulations up to 20,000 atoms and thermomechanical analysis as mentioned in this paper.
Abstract: The aged-rejuvenation-glue-liquid (ARGL) shear band model has been proposed for metallic glasses (Acta Mater. 54 (2006) 4293), based on small-scale molecular dynamics simulations up to 20,000 atoms and thermomechanical analysis. The model predicts the existence of a critical lengthscale � 10 nm, above which melting could occur in shear-alienated glass. Large-scale molecular dynamics simulations with up to 5 million atoms have directly verified this prediction. When the applied stress exceeds the glue traction (computed separately before in a shear cohesive zone, or an amorphous-amorphous ‘‘generalized stacking fault energy’’ calculation), we indeed observe maturation of the shear band embryo into bona fide shear crack, accompanied by melting. In contrast, when the applied stress is below the glue traction, the shear band embryo does not propagate, becomes diffuse, and eventually dies. Thus this all-important quantity, the glue traction which is a property of shearalienated glass, controls the macroscopic yield point of well-aged glass. We further suggest that the disruption of chemical short-range order (‘‘chemical softening’’) governs the glue traction microscopically. Catastrophic thermal softening occurs only after chemical alienation and softening in our simulation, after the shear band embryo has already run a critical length. [doi:10.2320/matertrans.MJ200769]
843 citations
TL;DR: In this paper, the effects of external stress and stacking fault energies (SFE) on the width of the stacking faults were analyzed and an excellent correlation between the calculations and actual microstructures examined by scanning electron microscopy was found.
678 citations
TL;DR: In the absence of dislocation-mediated crystallographic slip, room-temperature deformation in metallic glasses occurs in thin shear bands initially only ~10 nm thick as mentioned in this paper.
Abstract: In the absence of dislocation-mediated crystallographic slip, room-temperature deformation in metallic glasses occurs in thin shear bands initially only ~10 nm thick. A sharp drop in viscosity (shear softening) occurs in deformed glassy matter and facilitates additional flow in existing shear bands. This further localization of plastic flow leads to shearing-off failure without any significant macroscopic plasticity. However, whereas most bulk metallic glasses fail in this manner, some undergo surprisingly extensive plastic deformation (in some cases, up to 50% or more) in compression or bending. When this occurs, the flow is “jerky,” as indicated by serrated stress–strain curves. Each serration may correspond to the emission of a shear band that then ceases to operate, at least temporarily, despite the predicted shear softening. As elastic energy is converted to heat during shear, temperatures rise sharply at or near shear bands. This heating may lead to the growth of nanocrystals that then block propagation of shear bands and cracks. The understanding of the dependence of mechanical response of metallic glasses on intrinsic (elastic constants, chemistry) and extrinsic factors (shapes, flaws) is the subject of intense current interest.
387 citations
TL;DR: In this paper, the deformation behavior of nanocrystalline Ni-W alloys is evaluated by nanoindentation techniques for grain sizes of 3-150nm, spanning both the range of classical Hall-Petch behavior as well as the regime where deviations from the Hall-petch trend are observed.
268 citations
TL;DR: In this article, a noncontact strain measurement technique based on digital image correlation (DIC) analysis was used in order to observe PLC band behavior during tensile deformation of AA5754 sheet and subsequently to measure the level of incremental plastic strain carried within the bands.
239 citations
TL;DR: Instrumented contact experiments are performed on three metallic glasses to systematically study shear band formation near a stress concentration as discussed by the authors, and it is shown that the yield strength must be exceeded along the entire length of a viable shear path in order for a shear bands to form.
236 citations
TL;DR: In this article, a mesh-free particle method is proposed to model shear bands as strong displacement discontinuities in a mesh free particle method, where the loss of material stability is used as the criterion for switching from a classical continuum description of the constitutive behaviour to a traction-separation law acting on the discontinuity surface.
Abstract: A simple methodology to model shear bands as strong displacement discontinuities in a mesh-free particle method is presented. The shear band is represented as a set of sheared particles. A sheared particle is developed through enrichment by tangential displacement discontinuities. The representation of the shear band as set of cohesive segments provides a simple and versatile model of shear bands. The loss of material stability is used as the criterion for switching from a classical continuum description of the constitutive behaviour to a traction-separation law acting on the discontinuity surface. The method is implemented for two and three dimensions. Examples of shear band progression in rate-dependent and rate-independent materials are presented, including the Kalthoff problem, where the transition from brittle fracture to shear banding is studied. Copyright © 2006 John Wiley & Sons, Ltd.
215 citations
TL;DR: In this paper, a discrete element analysis of a two-dimensional, densely-packed, cohesionless granular assembly subject to quasistatic, boundary-driven biaxial compression is presented.
Abstract: Force chain buckling, leading to unjamming and shear banding, is examined quantitatively via a discrete element analysis of a two-dimensional, densely-packed, cohesionless granular assembly subject to quasistatic, boundary-driven biaxial compression. A range of properties associated with the confined buckling of force chains has been established, including: degree of buckling, buckling modes, spatial and strain evolution distributions, and relative contributions to non-affine deformation, dilatation and decrease in macroscopic shear strength and potential energy. Consecutive cycles of unjamming–jamming events, akin to slip–stick events arising in other granular systems, characterize the strain-softening regime and the shear band evolution. Peaks in the dissipation rate, kinetic energy and local non-affine strain are strongly correlated: the largest peaks coincide with each unjamming event that is evident in the concurrent drops in the macroscopic shear stress and potential energy. Unjamming nucleates from...
204 citations
TL;DR: It is shown that partially solidified alloys can exhibit the characteristics of a cohesionless granular material, including Reynolds’ dilatancy and strain localization in dilatant shear bands 7–18 mean crystals wide.
Abstract: Compacted granular materials expand in response to shear1, and can exhibit different behaviour from that of the solids, liquids and gases of which they are composed. Application of the physics of granular materials has increased the understanding of avalanches2, geological faults3,4, flow in hoppers and silos5, and soil mechanics6,7. During the equiaxed solidification of metallic alloys, there exists a range of solid fractions where the microstructure consists of a geometrically crowded disordered assembly of crystals saturated with liquid. It is therefore natural to ask if such a microstructure deforms as a granular material and what relevance this might have to solidification processing. Here we show that partially solidified alloys can exhibit the characteristics of a cohesionless granular material, including Reynolds’ dilatancy1 and strain localization in dilatant shear bands 7–18 mean crystals wide. We show that this behaviour is important in defect formation during high pressure die casting of Al and Mg alloys, a global industry that contributes over $7.3 billion to the USA’s economy alone8 and is used in the manufacture of products that include mobile-phone covers and steering wheels. More broadly, these findings highlight the potential to apply the principles and modelling approaches developed in granular mechanics to the field of solidification processing, and also indicate the possible benefits that might be gained from exploring and exploiting further synergies between these fields.
184 citations
TL;DR: A criterion is derived that determines which materials exhibit shear bands based on the initial conditions alone, and it is shown that the shear band width is not set by an inherent diffusion length scale but instead by a dynamical scale that depends on the imposed strain rate.
Abstract: We model a sheared disordered solid using the theory of shear transformation zones (STZs). In this mean-field continuum model the density of zones is governed by an effective temperature that approaches a steady state value as energy is dissipated. We compare the STZ model to simulations by Shi et al. [Phys. Rev. Lett. 98, 185505 (2007)], finding that the model generates solutions that fit the data, exhibit strain localization, and capture important features of the localization process. We show that perturbations to the effective temperature grow due to an instability in the transient dynamics, but unstable systems do not always develop shear bands. Nonlinear energy dissipation processes interact with perturbation growth to determine whether a material exhibits strain localization. By estimating the effects of these interactions, we derive a criterion that determines which materials exhibit shear bands based on the initial conditions alone. We also show that the shear band width is not set by an inherent diffusion length scale but instead by a dynamical scale that depends on the imposed strain rate.
166 citations
TL;DR: In this paper, the authors show that the shear band instability that occurs during plastic deformation of metallic glasses limits the application of these high-strength materials and that this instability can be suppressed in nanometer-scale metallic glasses constrained by ultrafine crystalline layers.
Abstract: The shear band instability that occurs during plastic deformation of metallic glasses limits the application of these high-strength materials. We show that this instability can be suppressed in nanometer-scale metallic glasses constrained by ultrafine crystalline layers. Free-standing Cu/amorphous Pd0.77Si0.23 multilayers consisting either of 10∕90nm glass/Cu or 100∕100nm glass/Cu were deformed to layer thickness reductions greater than 75% by cold rolling or bending, respectively. Transmission electron microscopy showed uniform reduction in the layer thickness with no shear band formation in the amorphous layers. The mechanisms that allow homogeneous codeformation of metallic glasses with nanoscale crystalline layers at high stresses are discussed.
TL;DR: In this article, the influence of particle shape on the global mechanical behavior of dense granular media was studied by means of molecular dynamics simulations of periodic shear cells, and the authors showed that the shear band width is strongly dependent on the particle shape due to the tendency of elongated particles to preferential orientations.
Abstract: We study by means of molecular dynamics simulations of periodic shear cells, the influence of particle shape on the global mechanical behavior of dense granular media At large shear deformation samples with elongated particles, independent of their initial orientation, reach the same stationary value for both shear force and void ratio At the micro-mechanical level the stress, the fabric and the inertia tensors of the particles are used to study the evolution of the media In the case of isotropic particles the direction of the principal axis of the fabric tensor is aligned with the one of the principal stress, while for elongated particles the fabric orientation is strongly dependent on the orientation of the particles The shear band width is shown to depend on the particle shape due to the tendency of elongated particles to preferential orientations and less rotation
TL;DR: In this article, simulated nanoindentation tests on a three-dimensional model of a binary metallic glass-forming alloy reveal how the stress field and material structure interact to control deformation beneath the indenter.
TL;DR: In this article, the effects of typical texture components observed in rolled aluminum alloy sheets on shear band formation in plane strain tension/compression and bending are systematically studied, and the material response is described by a generalized Taylor-type polycrystal model, in which each grain is characterized in terms of an elastic-viscoplastic continuum slip constitutive relation.
25 May 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, the microstructural characterization of induced adiabatic shear bands (ASB) in a hot-rolled interstitial free (IF) steel deformed at high strain rates (>2.8 × 104 s−1) under controlled conditions at −50 and 25°C.
Abstract: We report the microstructural characterization of induced adiabatic shear bands (ASB) in a hot-rolled interstitial free (IF) steel deformed at high strain rates (>2.8 × 104 s−1) in a split Hopkinson bar under controlled conditions at −50 and 25 °C. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used to reveal the degree of subdivision within ASB and neighboring grains. Deformation twins were found in adjacent grains suggesting that twinning occurs before the flow associated to shear banding. Progressive subgrain misorientation (PriSM) recrystallization is a plausible mechanism to explain the development of a new structure consisting of weakly textured ultrafine grains (0.1–0.5 μm) within the ASB. Recrystallization is proposed to occur by the formation and mechanical rotation of subgrains during deformation, coupled with boundary refinement via diffusion during shear band cooling. The presence of elongated subgrains and grains perfectly aligned within regions resembling a former lamellar structure within bands supports the occurrence of such a mechanism.
TL;DR: In this paper, numerical simulation of deep penetration of full-flow penetrometers in strainsoftening, rate-dependent, cohesive soil, and the observed phenomenon of periodic shear bands was conducted using a large deformation finite element approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain-softening.
Abstract: This paper addresses numerical simulation of deep penetration of full-flow penetrometers in strain-softening, rate-dependent, cohesive soil, and the observed phenomenon of periodic shear bands. The analysis was conducted using a large deformation finite element approach, modifying the simple elastic–perfectly plastic Tresca soil model to allow strain-softening, with strain-rate dependency being incorporated in order to avoid spurious mesh dependency. Parametric analyses were carried out varying the strain-softening parameters (hence the relative brittleness of the soil), the rigidity index of the soil, and the strain-rate parameter. Increased brittleness of the soil led to reduction in the penetration resistance, but also to increasingly significant oscillations in the resistance–penetration responses. The oscillation was found to result from periodic shear bands evolving cyclically ahead of the advancing cylindrical and spherical penetrometers. Analyses with different values of rigidity index confirmed f...
TL;DR: In this article, the authors present a historical approach to the development of understanding of the shear localization phenomenon in materials, concentrating particularly on impact, and the most advanced analyses show that shear band widths and spacings are determined by optimizing the diffusion of heat and inertia.
Abstract: This review presents a historical approach to the development of understanding of the shear localization phenomenon in materials, concentrating particularly on impact. Deformation localization under these conditions is widely referred to as adiabatic shear banding as the timescales are such that the distances heat can diffuse are small. Dimensional analysis shows that the phenomenon is ultimately intractable to linear algebraic analysis, as it is a coupled mechanical/thermal problem. However, various linear analyses from the literature are discussed along with their limitations as they shed light on the influence of various material properties. The aim of gaining understanding is to be able to engineer materials with the required localization (and hence fracture) characteristics. The most advanced analyses show that shear band widths and spacings are determined by optimizing the diffusion of heat and inertia. Because inertia is involved, the phenomenon cannot be understood simply as a materials property: geometry and structure must play a role.
TL;DR: In this paper, the authors postulate the physical criterion for dynamic shear band propagation and implement a numerical algorithm and a computation criterion to simulate initiation and propagation of dynamic adiabatic shear bands (ASBs).
Abstract: In this work, we postulate the physical criterion for dynamic shear band propagation, and based on this assumption, we implement a numerical algorithm and a computation criterion to simulate initiation and propagation of dynamic adiabatic shear bands (ASBs). The physical criterion is based on the hypothesis that material inside the shear band region undergoes a dynamic recrystallization process during deformation under high temperature and high strain-rate conditions. In addition to providing a new perspective to the physics of the adiabatic shearbanding process and identifying material properties that play a crucial role in defining the material's susceptibility to ASBs, the proposed criterion is instrumental in numerical simulations of the propagation of ASBs when multi-physics models are adopted to describe and predict the complex constitutive behavior of ASBs in ductile materials. Systematic and large scale meshfree simulations have been conducted to test and validate the proposed criterion by examining the formation, propagation, and post-bifurcation behaviors of ASBs in two materials, 4340 steel and OFHC copper. The effects of heat conduction, in particular the length scale introduced by heat conduction, are also studied. The results of the numerical simulations are compared with experimental observations and a close agreement is found for various characteristic features of ASBs, such as the shear band width, speed of propagation, and maximum temperature.
TL;DR: In this paper, the authors present quasi-static, room temperature compression data for Pd40Ni40P20 metallic glasses for both bulk and microscale specimens, and show a modest increase in the 0.2% offset yield strength in going from bulk specimens to the smallest measured (∼2μm), and attribute this increase to the effect of defects on shear band initiation.
TL;DR: A theoretical study is presented showing that catastrophic failure of viscoelastic materials may occur below Frenkel's ultimate limit as a result of thermal runaway.
Abstract: The first theoretical estimate of the shear strength of a perfect crystal was given by Frenkel [Z. Phys. 37, 572 (1926)]. By assuming that two rigid atomic rows in the crystal would move over each other along a slip plane, he derived the ultimate shear strength to be about one-tenth of the shear modulus. Here we present a theoretical study showing that catastrophic failure of viscoelastic materials may occur below Frenkel's ultimate limit as a result of thermal runaway. The thermal runaway failure mechanism exhibits progressive localization of the strain and temperature profiles in space, thereby producing a narrow region of highly deformed material, i.e., a shear band. We calculate the maximum shear strength ${\ensuremath{\sigma}}_{c}$ of materials and then demonstrate the relevance of this new concept for material failure known to occur at scales ranging from nanometers to kilometers.
TL;DR: In this article, the nanoindentation behavior of Au 49 Ag 5.5 Pd 2.3 Cu 26.9 Si 16.3 bulk metallic glass samples at loading rates ranging from 0.03 to 300mN−s −1.
TL;DR: In this article, the formation of adiabatic shear bands is automatically resolved using very fine meshes and the prediction of following cracks is achieved by means of a recently developed ductile fracture locus.
TL;DR: In this article, a constitutive model which can describe the transformation-induced plasticity (TRIP) accompanying the strain-induced martensitic transformation in TRIP steel under pre-strain is developed.
15 Apr 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a metallurgical analysis on chips obtained by high speed machining of a Ti-6wt.%Al-4wt.V alloy has been performed to provide a better understanding of chip formation mechanisms.
Abstract: A metallurgical analysis on chips obtained by high speed machining of a Ti–6wt.%Al–4wt.%V alloy has been performed to provide a better understanding of chip formation mechanisms. For this purpose, X-ray diffraction, energy dispersive X-ray analysis, scanning electron microscopy and electron backscattering technique were employed. The titanium β phase was observed in all chips for any tested cutting speeds. No evidence of phase transformation was found in the shear bands. Microscopical observations are in agreement with the catastrophic thermoplastic shear model for saw-tooth chip formation, instead of the periodic crack initiation one.
TL;DR: In this paper, the microstructural evolution in a cold-rolled 316L stainless steel during shear localization was comprehensively studied using transmission electron microscopy (TEM), and the TEM results indicate that the main substructure inside a shear band consists of elongated lath, fine rectangular and equiaxed subgrains.
25 Apr 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, cylindrical, flat-nose, 4340 steel projectiles (2.0 cm height) were impacted onto Ti-6Al-4V targets at velocities ranging from 633 to 1027m/s.
Abstract: Cylindrical, flat-nose, 4340 steel projectiles (2.0 cm height) were impacted onto Ti–6Al–4V targets (2.5 thick) at velocities ranging from 633 to 1027 m/s. Plug formation was observable at 633 m/s and exited the target between 1006 and 1027 m/s. Adiabatic shear bands (ASBs) composed of dynamically recrystallized (DRX) grains and cracks formed both vertically and horizontally (parallel or perpendicular) to the impact axis. The ASB characteristically white-band thickness varied from 5 to 40 μm, with the thicker bands occurring at the highest impact velocity. ASBs and cracks increased in frequency with increasing impact velocity, forming a cylindrical flow regime characterizing the plug. TEM analysis showed a greater than order of magnitude decrease in the DRX regime composing the ASBs, while EBSD analysis showed a residual DRX, grain-size regime varying from 50 to 900 nm. There was no evidence for an α → β transformation occurring within the ASB/DRX regime.
TL;DR: In this paper, the effects of non-coaxiality between the principal stress and the principal strain increment direction and the deviation of zero extension direction from horizontal are examined, and a new mesh-free method of evaluating internal strains is proposed.
Abstract: The direct shear test is a widely used method for determining peak and critical state strength parameters for soil and for soil-manufactured material interfaces. The objective of this research was to examine the direct shear test by way of a discrete-continuum approach. Specifically, the effects of non-coaxiality between the principal stress and the principal strain increment direction and the deviation of zero extension direction from horizontal are examined. A new, mesh-free method of evaluating internal strains is proposed, and a new flow rule is also presented. The results from discrete simulations indicate that development of shear bands proceeds from the side boundaries toward the centre with both primary and secondary bands formed. A distinct shear band along the middle plane develops around peak state in a dense sample. Simulation data using the new flow rule and laboratory test data using a conventional flow rule are in good agreement, indicating that the effect of non-coaxiality is small at peak...
TL;DR: In this paper, the authors used two-dimensional discrete element method (DEM) simulations on the strain localization of an idealized interphase system composed of densely packed spherical particles in contact with rough manufactured surfaces.
Abstract: Strain localization is closely associated with the stress–strain behaviour of an interphase system subject to quasi-static direct interface shear, especially after peak stress state is reached. This behaviour is important because it is closely related to deformations experienced by geotechnical composite structures. This paper presents a study using two-dimensional discrete element method (DEM) simulations on the strain localization of an idealized interphase system composed of densely packed spherical particles in contact with rough manufactured surfaces. The manufactured surface is made up of regular or irregular triangular asperities with varying slopes. A new simple method of strain calculation is used in this study to generate strain field inside a simulated direct interface shear box. This method accounts for particle rotation and captures strain localization features at high resolution. Results show that strain localization begins with the onset of non-linear stress–strain behaviour. A distinct but discontinuous shear band emerges above the rough surface just before the peak stress state, which becomes more expansive and coherent with post-peak strain softening. It is found that the shear bands developed by surfaces with smaller roughness are much thinner than those developed by surfaces with greater roughness. The maximum thickness of the intense shear zone is observed to be about 8–10 median particle diameters. The shear band orientations, which are mainly dominated by the rough boundary surface, are parallel with the zero extension direction, which are horizontally oriented. Published in 2007 by John Wiley & Sons, Ltd.
TL;DR: In this paper, the authors conducted indentation studies on two different bulk metallic glasses and for a variety of indenters: spherical, conical and pyramidal, and found that these bands are observed only when we leave the elastoplastic regime of indentation in favour of a fully plastic one.
25 May 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this paper, the effects of particulate reinforcement on the phenomenon of adiabatic heating leading to strain localization in Aluminum 6061-T6 alloy under high velocity impact is investigated.
Abstract: Dynamic deformation behavior of monolithic metallic materials at high strain rates has been extensively studied and reported in the literature Strain localization along shear bands at the final stage of deformation as a result of intensive localized adiabatic heating is a commonly occurring phenomenon that can cause fragmentation in a catastrophic manner In this study, the effects of particulate reinforcement on the phenomenon of adiabatic heating leading to strain localization in Aluminum 6061-T6 alloy under high velocity impact is investigated Results of our investigations show that reinforcing the aluminum alloy with alumina particles increases its strength and stiffness However, the susceptibility of the aluminum alloy to strain localization and adiabatic shear failure increases with particulate reinforcement In addition, the particulate reinforcement leads to systematic cracking of the surface of the impacted test specimens Although the geometry of the surface cracks is similar in all cases, the number of such cracks depends on volume fraction of the particulate reinforcement as well as on impact momentum