Development Of Shear Localization Research Articles

Shear localization in polycrystalline metal at high-strain rates with dynamic recrystallization: Crystal plasticity modeling and texture effect

Shear localization is an important failure mode, or even the dominant mode in metals at high-strain rates. However, it is a great challenge to accurately predict the occurrence and evolution of shear localization in metals at the high-strain rate deformation. Here, a dislocation-based crystal plasticity constitutive model with a crucial mechanism of shear instability, namely dynamic recrystallization, was developed. The evolution equations of dislocation density and grain size in the process of dynamic recrystallization were proposed and incorporated into the new constitutive model. The threshold of the stored energy in crystals was used as the criterion for the occurrence of dynamic recrystallization. Dynamic compression of a nanograin Cu-Al alloy was performed using the crystal plasticity finite element method based on the new constitutive model, and good agreement of the numerical prediction with the existing experimental data validates the new constitutive model. The results show dynamic recrystallization can be a more dominant mechanism for the occurrence of shear instability than thermal softening. In addition, dynamic tension and shear of the Cu-Al alloys with five typical textures were also simulated, showing that both loading mode and texture can significantly affect the formation of shear localization. This work is helpful for us to understand the role of microstructural evolution in the formation of shear localization at high-strain rates and to design the microstructure for artificially controlling or preventing the formation of shear localization.

Evolution of Shear Localization in an Elasto-Plastic Cosserat Material under Shearing

Numerical investigations of shear localization evolution within a layer of granular material under large monotonic shearing are presented. Here, micro-polar (Cosserat) continuum approach is applied within the framework of elasto-plasticity to remove the numerical difficulties of localization modeling encountered in classical continuum. The micro-polar kinematical boundary conditions are used to model the rotation resistance of soil grains along the interface between granular layer and surface of adjoining structure. The finite element results show that shear localization takes place from the beginning of shearing and appears parallel to the direction of shearing, close to the boundary with less restriction of particle rotation. Furthermore, the state variables tend towards asymptotical stationary condition in large shear deformations.

Microstructure and Microtexture Evolution of Shear Localization in Dynamic Deformation with Different Strains in Annealed Copper

Shear localizations with different strains in annealed copper were obtained by a modified split Hopkinson pressure bar. Microstructure and microtexture evolution of the shear localization regions were examined using optical microscopy, electron backscatter diffraction technique, and transmission electron microscopy. The results show that both the mechanical response and deformation behavior are correlated closely to the shear strains. The elongated dislocation cells, stretched subgrains, and the refinement of subgrains are observed within shear localizations during dynamic deformation. Ultrafine grains of 100 to 300 nm with high-angle-boundaries are produced within the shear band with the shear strain of 5.8. Microtexture characterization reveals that a stable orientation, in which 〈110〉 directions of the crystals tend to align with the shear direction, develops both in the deformation and recrystallization areas. The {111} planes of the crystals tend to parallel to the shear plane in the deformation area, whereas the aggregated extent of this orientation becomes weak in the recrystallization area. In addition, some grains exist with the {100} planes parallel to the shear plane in the deformation and recrystallization areas. The rotational dynamic recrystallization is a reasonable mechanism for the microstructure evolution. The effects of cooling stage on the growth of grains and the change of dislocation density are estimated as a complementarity to this mechanism.

Effect of periodic fluctuation of soil particle rotation resistance on interface shear behaviour

The interface behaviour between infinite extended narrow granular layer and bounding structure is numerically investigated using finite element method. The micro-polar (Cosserat) continuum approach within the framework of elasto-plasticity is employed to remove the numerical difficulties caused by strain-softening of materials in classical continuum mechanics. Mechanical properties of cohesionless granular soil are described with Lade's model enhanced with polar terms including Cosserat rotations, curvatures and couple stresses via mean grain diameter as the internal length. The main attention of paper is laid on the influence of spatial periodic fluctuation of rotation resistance of soil particles interlocked with the surface of bounding structure on evolution and location of shear band developed inside granular body. The finite element results demonstrate that the location and evolution of shear localization in granular body is strongly affected by prescribed non-uniform micro-polar kinematic boundary conditions along the interface.

Investigation on Mechanism of Ductile Fracture of AA5754 Sheet

A number of mechanical tests and metallographic techniques have been used to investigate the mechanism of ductile fracture of AA5754 sheet. The sequence of events in the development of shear localization is clarified using in situ strain mapping on both the sample surface and through thickness direction during tensile tests. It is observed that the failure mode changes from cup-cone type to shearing with increasing Fe content in both continuous cast (CC) and direct-chill cast (DC) AA5754 sheets. However, this transition happens in CC with much lower Fe content than DC. As very little damage is found near the fracture surface, this suggests that damage may be a consequence of the shear process rather than a trigger that determines material ductility. For both CC and DC with same Fe content of 0.21%, fracture strain of CC is much lower than DC. It is postulated that this is due to the differences of particle distribution in these two materials, especially the increased fraction of stringer type structures which exist in CC material.

Localized versus distributed shear in transform plate boundary zones: The case of the Dead Sea Transform in the Jericho Valley

Continental transform plate boundaries are typically either localized along a single, usually segmented, major fault or distributed over a broad deformation zone. In the latter, shear is partitioned between major strike slip faults and intervening, often rotating, fault systems. Analog and numerical simulations suggest that such internal fault systems evolve and may be localized or delocalized depending on strain and fault zone strength. This paper examines the degree and evolution of shear localization in the Jericho Valley section of the Dead Sea Transform (DST), which is part of the Africa/Arabia plate boundary. In this section, the current breadth of the transform valley and its penetrative strain field are difficult to reconcile with localized shear. We integrate newly acquired high‐resolution reflection data, reprocessed exploration reflection data (both focused to 2.5–3.5 km depth), relatively relocated earthquake epicenters, and fault plane solutions based on P wave first arrivals and suggest reinterpretations for previously published structural and seismic data in the Jericho Valley and the northern Dead Sea basin. It is found that shear in this section of the DST evolved from an early, probably Miocene‐Pliocene, stage of localized strike slip motion primarily along the Jericho fault to a late stage (Pliocene‐Recent) when shear has been distributed over internal fault sets that apparently lean against a major eastern DST segment. In the early stage the Jericho fault formed the western boundary of a deep, narrow, probably pull‐apart basin filled by fluvial clastics and evaporites. This is overlain by the late stage broad basin, characterized by low‐energy deposition of lacustrine and fluvial sediments. Pliocene rock salt has apparently utilized the Jericho fault zone as a conduit for diapirism, causing the uplift, normal faulting, and surface deformation of Late Pleistocene and Holocene sediments currently observed along the ∼15 km long Jericho Lineament. On the basis of the currently observed dextral‐normal motion across the internal NNE‐NE trending fault system, counterclockwise rotation is expected to have occurred within the transform zone. These tectonic and depositional changes were probably associated with onset of transtension across the DST, related to the previously postulated shift of the Africa/Arabia Euler pole of rotation in the Pliocene.

Prediction of shear localization during large deformation of a continuous cast Al–Mg sheet

In this work, a novel continuum solid mechanics approach has been proposed to predict shear band formation in uniaxial tension, utilizing both plane stress and three-dimensional analyses. The approach postulates a heterogeneous distribution of mechanical properties through the starting material. This may be attributed to the second-phase particles, textural banding or grain size variability of the sheet material. The model is used to study the effect of such heterogeneity on the development of shear localization with increasing strain. We also consider how such a model depends on the constitutive equations that are used. It is found that the Voce equation offers realistic predictions of necking and shear localization whilst the Hollomon equation fails to do so over a realistic range of deformation. The location of necking in each structure is controlled by the largest zone of soft material. The influence of the distribution, size and geometry of soft zones have been extensively studied. The amount of inhomogeneity required to precipitate localization is determined and confirmed using the Marciniak–Kuczynski method. The predicted force–displacement curves, limit strains to failure and the shear band angles all agree well with experimental observations.

Behaviour of interfacial layer along granular soil-structure interfaces

As shear occurs along a soil-structure interface, a localized zone with a thickness of several grain diameters will develop in soil along the interface, forming an interfacial layer. In this paper, the behaviour of a soil-structure interface is studied numerically by modelling the plane shear of a granular layer bounded by rigid plates. The mechanical behaviour of the granular material is described with a micro-polar hypoplastic continuum model. Numerical results are presented to show the development of shear localization along the interface for shearing under conditions of constant normal pressure and constant volume, respectively. Evolution of the resistance on the surface of the bounding plate is considered with respect to the influences of grain rotation.

Numerical investigations of shear localization in a micro‐polar hypoplastic material

AbstractIn this paper a micro‐polar continuum approach is proposed to model the essential properties of cohesionless granular materials like sand. The model takes into account the influence of particle rotations, the mean grain size, the void ratio, the stresses and couple stresses. The constitutive equations for the stresses and couple stresses are incrementally non‐linear and based on the concept of hypoplasticity. For plane strain problems the implementation of the model in a finite element program is described. Numerical studies of the evolution of micro‐polar effects within a granular strip under plane shearing are presented. It is shown that the location and evolution of shear localization is strongly influenced by the initial state and the micro‐polar boundary conditions. For large shearing the state quantities tend towards a stationary state for which a certain coupling between the norm of the stress deviator and the norm of the couple stress tensor can be derived. Copyright © 2003 John Wiley & Sons, Ltd.

Localized failure modes in a compactant porous rock

Since dilatancy is generally observed as a precursor to brittle faulting and the development of shear localization, attention has focused on how localized failure develops in a dilatant rock. However, recent geologic observations and reassessment of bifurcation theory have indicated that strain localization may be pervasive in a compactant porous rock. The localized bands can be in shear or in compaction, and oriented at relatively high angles (up to 90°) to the maximum compression direction. Here we report microstructural characterization of the spatial distribution of damage in failed samples which confirms that compaction bands and high‐angle conjugate shears can develop in sandstones with porosities ranging from 13% to 28%. These failure modes are generally associated with stress states in the transitional regime from brittle faulting to cataclastic ductile flow. The laboratory results suggest that these complex localized features can be pervasive in sandstone formations, not just limited to aeolian sandstone in which they were first documented. They may significantly impact the stress field, strain partitioning and fluid transport in sedimentary formations and accretionary prisms. While bifuraction theory provides an useful framework for analyzing the inception of localization, our data rule out a constitutive model that does not account for the activation of multiple damage mechanisms in the transitional regime.

The catastrophic development of shear localization in thermoviscoplastic materials

Le couplage thermo-mecanique et l'adoucissement thermique sont les proprietes du materiau qui sont a l'origine du developpement des bandes de cisaillement adiabatique. Cependant les defauts geometriques ou metallurgiques fournissent les sites ou le processus de localisation de la deformation plastique est initie. Dans cet article, les effets de la forme et de l'amplitude des defauts sur la localisation de la deformation de cisaillement simple sont analyses a l'aide d'un modele non lineaire: on formule des criteres structurels de localisation asymptotique dans lesquels l'acuite des defauts module l'influence des facteurs rheologiques, et l'on montre que, plus que leur amplitude, la forme des defauts permet d'interpreter la dispersion observee des deformations nominales a la rupture

Development of shear localization in simulated quartz gouge: Effect of cumulative slip and gouge particle size

Frictional sliding experiments were conducted on two types of simulated quartz gouge (with median particle diameters 5 μm and 25 μm, respectively) at confining pressures ranging from 50 MPa to 190 MPa in a conventional triaxial configuration. To investigate the operative micromechanical processes, deformation texture developed in the gouge layer was studied in samples which had accumulated different amounts of frictional slip and undergone different stability modes of sliding. The spatial patterning of shear localization was characterized by a quantitative measurement of the shear band density and orientation. Shear localization in the ultrafine quartz gouge initiated very early before the onset of frictional sliding. Various modes of shear localization were evident, but within the gouge zoneR1-shears were predominant. The density of shear localization increased with cumulative slip, whereas the angle subtended at the rock-gouge interface decreased. Destabilization of the sliding behavior in the ultrafine quartz gouge corresponded to the extension ofR1-shears and formation of boundaryY-shear segments, whereas stabilization with cumulative slip was related to the coalescence ofY-shear segments to form a throughgoing boundary shear. In the coarse quartz gouge, the sliding behavior was relatively stable, probably because shear localization was inhibited by distributed comminution. Two different models were formulated to analyze the stress field within the gouge zone, with fundamentally different predictions on the orientations of the principal stresses. If the rock-gouge interface is assumed to be bonded without any displacement discontinuity, then the maximum principal stress in the gouge zone is predicted to subtend an angle greater than 45° at the interface. If no assumption on displacement or strain continuity is made and if the gouge has yielded as a Coulomb material, then the maximum principal stress in the gouge zone is predicted to subtend an angle less than 45°. If the apparent friction coefficient increases with overall slip (i.e., slip-hardening), then the Riedel shear angle progressively decreases with increasing shear strain within the gouge layer, possibly attaining a zero value which corresponds to a boundaryY-shear. Our quantitative data on shear localization orientation are in reasonable agreement with this second model, which implies the coefficient of internal friction to be about 0.75 for the ultrafine quartz gouge and 0.8 for the coarse gouge. The wide range of orientations for Riedel shear localization observed in natural faults suggests that the orientations of principal stresses vary as much as in an experimental gouge zone.

An Analysis of Shear Localization During Bending of a Polycrystalline Sheet

The development of shear localization in a polycrystalline sheet subject to pure bending is analyzed numerically using a slip-based constitutive model. The material response at each finite element integration point is determined by averaging the stiffness matrices from differently oriented FCC crystals. The effects of texture evolution, hardening, and strain-rate sensitivity are incorporated. The model predicts localized plastic deformation at both the tensile and the compressive surfaces of the sheet during bending. Comparison of the numerical results with a section of the bent sheet indicates that strain localization is predicted at the appropriate strain levels and orientations.