Shear Band Zone Research Articles

Rejuvenation of a deformed metallic glass dominated by immature shear band zones during cryogenic thermal cycling

Rejuvenation of a deformed metallic glass dominated by immature shear band zones during cryogenic thermal cycling

Cryogenic thermal cycling effects on serration behavior in wide glass transition temperature Zr-based bulk metallic glasses

Serration behavior serves as a pivotal link connecting micro-deformation mechanism to the mechanical properties of metallic glasses. To elucidate the impact of cryogenic thermal cycling (CTC) and compositional modifications on metallic glasses’ serration behavior, a series of Zr-based metallic glasses, exhibiting a broad range of glass transition temperatures (Tg), were meticulously manufactured. The stress drop magnitude initially decreases before the peak-rejuvenation induced by CTC, then gradually increases, while the number of stress drops exhibits an initial increase and then a decrease. These trends correlate with optimized plasticity of the metallic glasses. Remarkably, metallic glasses with lower Tg predominantly undergo strain hardening during plastic deformation. They exhibit lower viscosity in shear band zones and stress drop compared to others, attributed to the temperature increase around shear band zones approaching Tg. This phenomenon promotes shear band activation in smaller flow units, leading to finer serrations compared to glasses with higher Tg.

Strain localization in the standard triaxial tests of granular materials: Insights into meso‐ and macro‐scale behaviours

AbstractThe standard triaxial tests cease to be valid as material tests since the homogeneity of the granular mass is lost when localized failures such as shear bands occur requiring a different approach to interpreting and analyzing material responses at the meso scale from the macro behaviour. This study sheds light on the above issue by analyzing the standard triaxial tests of the granular specimens undergoing localized failure at different scales using DEM. For the first time, the behaviour of material inside and outside of the shear band as well as the entire sample are quantitatively quantified through DEM simulations. The results enable confirmation of various theoretical hypotheses and experimental observations on the localized failure in granular materials. For example, by quantifying the inter‐particle contact forces of materials inside and outside the shear band zone, it is confirmed that the material outside the localization band undergoes inelastic unloading beyond the bifurcation point, while those inside the localization band experience inelastic shearing to reach the critical state. Moreover, the analysis in this study suggests that if the volume of the localization zone can be precisely measured from the experiment, the mesoscale constitutive responses that truly represent the inelastic behaviour can be quantified. This gives rise to a more appropriate method to obtain inelastic constitutive responses of granular materials from specimens undergoing localized failure in triaxial tests.

Grain refinement mechanism in chip and the machined subsurface during high-speed machining of Inconel 718 alloy

Microstructural defects on the machined surface of Inconel 718 alloy can be a significant source of fatigue cracks, potentially leading to failures of the structural components of aerospace engines. This work focused on the grain refinement mechanism induced by the dynamic recrystallization (DRX) behavior in both the chip and machined subsurface during high-speed machining (HSM) of Inconel 718 alloy. Multiscale metallurgical observations were conducted using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and precession electron diffraction (PED) to characterize the crystal structures, including the grain size, grain boundary distribution, dislocation density, and grain orientation distribution. Finite element (FE) simulations were performed to obtain the strain, strain rate, and cutting temperature to analyze the microstructure evolution mechanism under the effect of severe deformation conditions in HSM. The results indicate that grain refinement in the chip primarily occurs in the shear band and secondary deformation zone. The primary cause of the variation in the final grain size is attributed to the distinct thermomechanical loading conditions experienced by the material. The microstructure observed in the machined subsurface reveals a significant gradient distribution, characterized by the presence of an amorphous structure, nanocrystallites, refined grains, and elongated grains along the depth direction. The equiaxed ultrafine nanocrystals observed in the machined near-surface result from both continuous DRX (cDRX) and discontinuous DRX (dDRX) mechanisms. In the deeper zone, the presence of numerous subgrains and a high geometrically necessary dislocation (GND) density reaching up to 1016/m2 within the grains indicate that the refined grains in this zone result from the cDRX mechanism. The variation in DRX mechanisms observed in the machined subsurface are closely related to the gradient distribution of the thermomechanical load generated in HSM.

Twinning transfer in a near β-Ti alloy under high strain rate dynamic loading

The microstructure evolution of near β-Ti alloys with initially lamellar and equiaxed microstructures subjected to dynamic loading was investigated. Results showed that during high strain rate compression, shear failure is more likely to occur in equiaxed microstructure. Owing to temperature rise and stress concentration, the lamellar α phase in adiabatic shear band (ASB) zone was recrystallized and re-melted, while the equiaxed α phase transformed in strip-shaped following the propagating direction of adiabatic shear band. During high strain rate loading, the twinning transfer was observed between β matrix and α phase, which demonstrates that twinning is the dominant deformation mode to accommodate plastic deformation.

Role of plastic zone porosity and permeability in sand production in weak sandstone reservoirs

Sand production is often characterized as a two-stage process, in which material failure occurs near the cavity, leading to the formation of a plastic zone, from which particles are detached and transported out because of continuous hydrodynamic erosion under the effects of the produced fluid flow. The plastic zone porosity is affected by coupled processes, while the plastic zone permeability has a significant impact on the performance of sand production prediction, especially in weak sandstone reservoirs. Large-scale sand production experiments were conducted using a customized high-pressure consolidation apparatus. The results show that specific stress-fluid pressure conditions may create a plastic zone around a hole, which has lower permeability than the intact zone. The plastic zone comprises two subzones: a high-permeability shear band zone and a low-permeability compaction zone. During sand production, sand migrates from the compaction zone through the shear band zone to the perforation hole. Thus, sand production is associated with the increased permeability and porosity in the compaction zone. Existing sand prediction models were modified according to the new findings, resulting in a modified model with improved performance. The modified model was validated using the sanding data from a weak sandstone reservoir in Kazakhstan.

Shear banding and serrated flow behaviors of high toughness Zr61Ti2Cu25Al12 bulk metallic glass under bending

By interrupted three-point bending experiments, the detailed evolutions of shear-banding and serrated flow as well as their correlation of high toughness Z61Ti2Cu25Al12 bulk metallic glass (BMG) under bending were systematically studied. Three stages can be classified during plastic bending of Z61Ti2Cu25Al12 BMG based on the different characteristics of shear-banding and serrated flow behaviors. Moreover, there has a strong correlation between shear-banding and serrated flow at different stages of plastic bending. It is noteworthy that the driving force for shear bands propagation of BMG beam under bending depends on the bending moment, instead of the applied load. Thus, remarkable positive correlations among bending moment, shear band zone height and average load drop of serrations were found during plastic bending. During the progressive propagation of multiple shear bands, intense interactions and competitions among these shear bands have occurred, which manifested by the self-organized criticality behavior from serrated flow dynamics, as well as the very finer irregular striation patterns and uneven fluctuant morphologies which presented on the shear offset surface. The sizeable plastic zone of Z61Ti2Cu25Al12 BMG is very favorable to achieve the greater macroscopic plasticity and lower the risk of fast catastrophic fracture. Thus, Z61Ti2Cu25Al12 BMG is a promising engineering material for structure elements which are naturally subjected to bending load, such as compliant mechanisms and microelectromechanical systems.

The recrystallization behavior of shear band in room temperature ECAPed Al-Mg-Mn-Sc-Zr alloy

The Al-6 Mg-0.4Mn-0.25Sc-0.1Zr extrusion rod was subjected to ECAP for 1 pass at room temperature to get the nonuniform structure. Then, the samples were annealed under 100 °C, 150 °C, 200 °C, 250 °C, 300 °C, 350 °C, 400 °C, 450 °C, 500 °C and 550 °C for 1 h. It is found that the 1P ECAP induces an extremely concentration of shearing strain. There are two areas in the 1P sample: shear band and non-sheared zone. In the shear band, the hardness is 147 HV and, in the non-sheared zone, the hardness is only 120 HV. Meanwhile, the hardness of the original rod (0P sample) without ECAP is 110 HV. After the annealing, the hardness decreases but the hardness of the 1P sample decreases much faster than that of the 0P sample, leading to the hardness ‘upside down’ phenomenon. Also, in the 1P sample, the hardness of the shear band decreases much faster than that of the non-sheared zone. The tensile test has proved this phenomenon too. After annealed at 500 °C, the Al3(Sc, Zr) particles in the shear band is large and some are incoherent with the matrix, while in the non-sheared zone, the particles are still coherent and only a part of them have grown up. These phenomena are attributed to the drastic recrystallization induced by the high strain in the shear band.

A new geometric model of serrated chip formation in high-speed machining

• The serrated chip geometry can be predicted effectively with average value. • The serrated chip formation can be modeled effectively in time domain without measurement of chip geometry. • The critical shear strain rate for the disappear of crack in machining of cast aluminum alloy are presented. The geometry of serrated chip is of paramount importance to the analysis of physical and thermal phenomena in chip formation process, and it can affect tool fatigue life, surface finish and geometric accuracy of the machining part. This paper presents a novel geometry model of serrated chip segment based on chip formation and cutting force in metal cutting by considering the difference between deformation of shear band and primary shear zone. Where, focusing on the high-speed machining, it key points about the geometry of chip segment, shear band angle, secondary shear angle, third shear angle (curvature angle of second shear band), the strain and its rate, and their evolutions in chip formation process are developed, respectively. Finally, the experimental and simulated instances can verify the proposed method. The results can show that this proposed model having a high accuracy can effectively reflect the evolution of chip segment. The shear angle is silent increased with increase of feed rate, while the difference between shear angles is decreasing with increase of feed rate. While the shear strain is increasing with the increasing feed rate and decreasing with increase of cutting depth, the cutting velocity can enlarge the difference. The critical shear strain rate and cutting velocity for the disappear of crack in chip for feed rate of 0.117mm, 0.14mm and 0.187mm are 1.566e 5 1/s and 32.71m/s, 1.486e 5 1/s and 35.98m/s, 1.279e 5 1/s and 39.25m/s, respectively, which decreased with increased of feed rate.

Shear banding and fracture behaviors of a bulk metallic glass studied via in-situ bending experiments with notched and un-notched specimens

The shear banding and fracture behaviors of Ti32.8Zr30.2Ni5.3Cu9Be22.7 bulk metallic glass (BMG) were studied via in-situ three-point bending experiments using notched and un-notched specimens. A notch strengthening effect was found based on comparing the nominal flexural strength of the notched and un-notched specimens. From in-situ observations of the notched specimens, it was found that the first shear band initiated near the notch tip was followed by a spate of shear band initiations in a wider range of locations, leading to macroscopic yielding. The shear band zone continued to grow until the specimen fractured, with plastic strain being accommodated by repeated branching of shear bands and increase of shear offset in the earlier formed parts of shear bands. Significant differences in the 3-dimensional orientation and shear offset direction of shear bands were observed between the notched and un-notched specimens. These differences were analyzed by relating the elastic stress fields in the general direction of shear band growth, obtained by finite element modeling, to the ellipse criterion.

Grain size influence on chip formation in high-speed machining of pure iron

Same metallic materials with varied grain sizes have different material behaviors and are used in different industrial applications. However, machining parameters are normally determined by the materials, and the grain size effect has not been investigated in detail in a machining process. This paper studies the influence of grain sizes for pure iron on cutting force and chip formation in the high-speed machining with a speed of 24 m/s. A grain size–dependent material constitutive model is proposed and calibrated by the split Hopkinson pressure bar tests and then implemented into a finite element model of the high-speed machining process. The cutting force and chip morphology from simulation results have a good agreement with those from cutting experiments. The results show that a smaller grain size leads to more serrated chips in high-speed machining of pure iron due to the thermal- and mechanical-coupled effect at the shear band zone. Under the high-strain and high-temperature conditions, grain refinement occurs in the serrated chips.

Progressive microforming of pin-shaped plunger parts and the grain size effect on its forming quality

Progressive microforming of bulk microparts by directly using sheet metals has been developed to address the issues of preform/billet positioning, transportation and ejection in microforming, and it has potential to be used for making complex pin-shaped microparts with high productivity and material utilization. In this research, a progressive microforming system with blanking and extrusion operations to make two types of pin-shaped plunger parts by using brass sheets was developed, and the grain size effect was investigated from the aspects of deformation load, dimensional accuracy, microstructural evolution and surface quality. It is revealed that the grain size effect results in the deviation of punch stroke and the variation of part dimension. The formation and characteristics of shear band and dead metal zone are related to velocity gradient and strain accumulation. The forming defects including micro cracks, micro pits, uneven surface and longitudinal surface texture easily occur on the body and tail feature, and micro voids appear with the coarse-grained material. The circular surface, however, shows the desirable quality with each grain size. All these findings provide a basis for mass production of complex pin-shaped microparts by progressive microforming and enrich the knowledges from deformation behavior and product quality assurance aspects.

Mechanics of Grains Size and Orientation on the Machining of AISI S-7 Steel Grade Using Numerical Technique

Two-dimensional models namely homogeneous, uniformly distributed grains and non-uniformly distributed grains were developed for AISI S-7 steel grade metal, to investigate the effect of the grains size and orientation on the chip morphology and temperature rise during the orthogonal machining process. A Johnson-Cook model was used for the simulation study. The machining was done at a medium speed of 60 ms-1 and the depth of cut was maintained at 5 mm in all the above cases. Compared to other models, results showed that nonuniformly distributed grains develop alternate low and high shear bands and generate maximum temperature in the high shear band zone, which was found to be detrimental during the machining process compared to other models. Intergranular chip segmentation was observed in the case of uniformly distributed grains. For the same material, three different chip morphologies and temperature profiles were observed during the orthogonal machining process. Detailed discussion on the mechanics of machining of the above-said models was done that may be advantageous for material and tool design scientist.

In Situ Observation of Adiabatic Shear Band Formation in Aluminum Alloys

We used high-speed X-ray phase contrast imaging and infrared thermal imaging techniques to study the formation processes of adiabatic shear bands in aluminum 7075-T6 and 6061-T6 alloys. A modified compression Kolsky bar setup was used to apply the dynamic loading. A flat hat-shaped specimen design was adopted for generating the shear bands at the designated locations. Experimental results show that 7075-T6 exhibits less ductility and a narrower shear band than 6061-T6. Maximum temperatures of 720 K and 770 K were locally determined within the shear band zones for 7075-T6 and 6061-T6 respectively. This local high temperature zone and the resulting thermal instability were found to relate to the shear band formation in these aluminum alloys.

Effect of deformation routes on the microstructures and mechanical properties of the asymmetrical rolled 7050 Aluminum alloy plates

Asymmetric rolling (ASR) with three different deformation routes and conventional symmetric rolling (CR) were carried out in 7050 Al alloy plates, so as to study the influence of deformation route on the strain distribution, microstructural evolution and mechanical properties. The results show that the ASR process introduces much more effective strain throughout the thickness, resulting in severe deformation or microstructural evolution and improving the alloy's ductility compared to CR deformed samples, irrespective of the deformation routes. Among the three ASR deformation routes, the ASR-R process can introduce highest strains to improve the through-thickness deformation homogeneity, subsequently amount of finer subgrains can be observed in the wider shear band zones due to the intersection of shearing patterns of adjacent rolling passes. Although the strengths of the ASR-processed plates are almost similar, their tensile elongations are obviously affected by the deformation routes, especially the ASR–R route contributes to ~ 23% elongation, which would be mainly responsible by the grain refinement, homogeneous distribution of microstructure and fragmentation of coarse constituent particles.

Numerical investigation of fluid injection into poorly consolidated geomaterial considering shear dilation and compaction

We present a fully coupled poroelastoplastic finite element model to simulate micro-fracture evolution, compaction failure and tensile fracture propagation during fluid injection into poorly consolidated geomaterial. The model includes shear dilation, strain hardening and fracture process zone with permeability evolution. We investigated the effects of geomechanics (stress anisotropy) and fluid injection (fluid rheology) on rock deformation and fracture behavior. Results indicate that fluid rheology dominate the rock failure. Pore pressure increase and water wedge effect can cause two kinds of shear dilation which are prior to tensile failure. The effect of compaction is significant to tensile fracture geometries, which differ from those under elastic and poroelastic conditions. The difference between shear band and compaction zone is the transition of fracture regime from the leak-off dominated to the storage dominated governed by fluid injection.

Experimental investigation of reverse fault rupture propagation through wet granular soil

Surface fault rupture has been previously investigated in dry cohesionless soil; however, wet granular soil has not been investigated in detail; however, apparent cohesion induced by intergranular water causes granular soil to behave differently than dry cohesionless soil. Field evidence shows that all soil that has been wetted will experience some degree of cohesion. In the present study, eight experimental faulting tests were conducted on sand in which the water content varied from 0% (dry) to 15% and the soil layer height ranged from 25 to 35 cm. To determine the behavior of the wet sand, a series of direct shear tests were carried out at low confining stress corresponds to stresses at different depth of the physical model. The results showed that the shear band zone decreased in the wet specimens, creating a sharp scarp at the surface. The required normalized fault displacement for outcropping decreased significantly in these specimens and reached a minimum value at about a 5% water content. The direct shear test results showed that the failure strain of the wet sand can be used for prediction of h0/H. Surface tension cracks at the hanging wall widened in the zone of distortion at the surface in the wet specimens. Other aspects of fault rupture also are discussed.

Plastic deformation behaviours of CuZr amorphous/crystalline nanolaminate: a molecular dynamics study

The plastic deformation behaviours of Cu50Zr50/B2 CuZr amorphous/crystalline nanolaminate were studied at the atomic scale using molecular dynamics simulations. For pure metallic glass, the highly localised shear banding leads to the overall shear failure. And the plastic deformation of B2 CuZr crystal is mainly determined by the martensitic phase transformation. The composite material, nanolaminate, achieves great improvement of plastic deformation compared with the pure metallic glass and the pure crystal. This plasticity enhancement is attributed to two mechanisms: the suppression effect of the nucleation and propagation of the shear band and the regulating effect of the distribution of the shear transformation zones and phase transformation zones. The shear transformation zones can be induced by the interaction between phase transformation zones and interface. The immature shear band or shear transformations zones and phase transformation zones form a network to transmit the strain jointly within the entire sample. Amorphous/crystalline interface connects different layers and transmits the strain within the nanolaminate. Interfaces also plays the roles of source and sink of the shear transformation zones, shear band and phase transformation zones. On the basis of this plastic deformation mechanism, the present study provides a route for controlling the plasticity properties of amorphous/crystalline nanolaminate.

The introduction of highly dense shear bands and their effect on plastic deformation in Zr and Cu-based bulk metallic glasses

The plastic deformation of the Zr64.13Cu15.75Ni10.12Al10 and Cu60Zr30Ti10 bulk metallic glasses (BMGs) with arc-shaped edges were performed by compression. Highly dense shear bands with an average spacing of 520nm are uniformly distributed in a large area in Zr64.13Cu15.75Ni10.12Al10 BMG, and the shear band spacing can be reduced to a minimum value of about 110nm. In the region adjacent to the dense shear band zone in Zr64.13Cu15.75Ni10.12Al10 BMG, the shear band spacing increases linearly. Similarly, highly dense shear bands were also introduced into Cu60Zr30Ti10 BMG. When the width of stress concentration zone is of the order of plastic zone size ahead of crack tip, the internal and external stress limitations can lead to the formation of a large number of uniformly-distributed shear bands. In addition, the pre-existing dense shear bands can lead to a non-localized plastic deformation manner for deformed Zr64.13Cu15.75Ni10.12Al10 BMG.

カップの後方押出し成形における3次元局所分岐後の劈開不安定による破断限界予測

We investigated a method of determining the geometry of the die and punch, which are not allowed to fracture in the forming process of backward extrusion of cup. On the basis of the localized bifurcation theory, it is suggested that final fracture, which is recognized as the separation of a continuum material, may occur in the shear band zone with cleavage. A newly proposed instability condition to the cleaving action in the shear band zone is applied to predict the fracture limit on the forming process. Finite element analysis of the forming process is performed to know the stress state, which is required in the theory. The experimental observation using an optical microscope is also conducted to recognize fracture occurrence during the forming process. Results show that the combination of the punch shoulder radius and the die shoulder radius ensures forming without fracture.