Shear Band Spacing Research Articles

Growth of thermoplastic shear band

Shear localization is a fundamental and widely observed non-equilibrium phenomenon in metallic materials subjected to impulsive loadings. Despite extensive research over several decades, the evolution of shear bands still remains poorly understood. In this study, we propose a two-stage model to describe the evolution of thermoplastic shear band, where the effects of strain hardening, strain-rate hardening, and thermal softening are involved. Our analysis reveals that the growth of thermoplastic shear band is derived by the competition between momentum diffusion and thermal diffusion processes, which is controlled by the solid-state equivalent of the Prandtl number. By incorporating these factors into our model, we find that the presence of hardening effect retards the evolution of shear bands, resulting in a wider shear band and increased dissipation of energy. Theoretical calculation results of shear bandwidth, shear band strain, propagation velocity, process zone geometry and dimensions, critical dissipation energy, shear band toughness, and shear band spacing are well consistent with available experimental data. Additionally, we explore the potential transition to turbulence controlled by Reynolds number within shear bands and investigate the influence of the Taylor-Quinney coefficient on the evolution of shear bands. These findings are expected to offer valuable insights into the mechanism of shear band evolution.

Dose-dependent strain localization and embrittlement in ferritic materials: A predictive approach based on sub-grain plasticity modelling

This paper presents a theoretical approach addressing plastic-strain spreading in post-irradiated BCC materials accounting for crucial sub-grain scale, dislocation-mediated plasticity mechanisms. The proposed model explicitly provides the number of shear-bands developed in irradiated ( N i r r ) versus non-irradiated ( N 00 d p a ) grain cases, for fixed amounts of plastic deformation. Calculations carried out under various irradiation defect size and number density cases, which helps it appraising important material properties, in particular the dose-dependent, grain-scale uniform elongation threshold. The model ability to handle macro-scale effects is then evaluated using a simple stochastic calculation procedure, taking advantage of actual grain size and orientation maps. The dose-dependent embrittlement amplitude appears to critically depend on the shear band thickness and spacing variations, existing near the fracture surface of failing specimens. That perception allows comparing our predictions with adapted test results, for validation.

Investigation of chip formation mechanism in ultra-precision diamond turning of silk fibroin film

The importance of silk fibroin particles (size from tens of nanometres to hundreds of microns) as effective drug carriers has been well identified due to their superior mechanical strength, biocompatibility and biodegradability. They are usually fabricated by methods that have difficulties in controlling the size and shape to meet the required functions. In addition, using chemical agents in the fabrication process may result in biological risks and degradation of the fibroin structures. In this study, ultra-precision diamond turning (UPDT) technology is researched as an alternative chemical-free manufacturing approach to generate silk particles in the form of cutting chips with controllable size and shape. The machinability in UPDT of silk fibroin film (hereinafter referred to as silk film) was discussed in terms of specific cutting force and chip morphology. The material removal in UPDT of silk film is taken place in the ductile regime. Experimental results reveal that a rise in the cutting speed could significantly reduce the specific cutting force, while decreasing the depth of cut (DOC) could increase the specific cutting force because of the size effect. Moreover, a sharp point tool using a less than 2.5 μm/rev feed rate is preferred to generate helical silk chips. The formation mechanism of shear bands and serrated chips in UPDT of silk film was investigated with the aid of a finite element and smoothed particle hydrodynamics (FE-SPH) hybrid numerical model with Cowper-Symonds (CS) material equation. The material parameters of silk film were determined for the first time at p = 7 and D = 1140 s−1. The simulated specific cutting force with this set of material parameters is 49.0% smaller than the measured one. The simulated chip morphology (e.g. shear band spacing) was also in good agreement with the measured values. The simulation study shows that the shear band formation is done via two joining parts: one part propagates from the cutting edge to the free surface, another part initiates on the free surface and evolves towards the cutting edge. The chip segment is formed by a microcrack propagating from the free surface to the cutting edge. The underlying mechanism of formation of the serrated chip is found to have its roots in the hierarchical structure of silk fibroin.

Analytical Modeling of Shear Localization in Orthogonal Cutting Processes

AbstractThis paper presents an analytical thermo-mechanical model of shear localization and shear band formation in orthogonal cutting of high-strength metallic alloys. The deformation process of the workpiece material includes three stages: homogeneous deformation, shear localization, and chip segmentation. A boundary layer analysis is used to analytically predict the temperature, stress, and strain rate variations in the primary shear zone associated with the shear localization. The predictions of shear band spacing and width from the proposed model are verified by experimental characterization of the chip morphology. The rolling of shear bands on the tool rake face is discussed from the experimental observations. The cutting tool temperature, which is influenced by the heat generated during the shear band formation, is simulated and compared with finite element simulations. The proposed analytical model reveals the fundamental mechanism of the complete shear localization process in orthogonal cutting and predicts the stress and temperature variations with high computational efficiency.

Effect of Cold Rolling on the Evolution of Shear Bands and Nanoindentation Hardness in Zr41.2Ti13.8Cu12.5Ni10Be22.5 Bulk Metallic Glass.

The effect of cold rolling on the evolution of hardness (H) and Young’s modulus (E) on the rolling-width (RW), normal-rolling (NR), and normal-width (NW) planes in Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vitreloy 1) bulk metallic glass (BMG) was investigated systematically using nanoindentation at peak loads in the range of 50 mN–500 mN. The hardness at specimen surface varied with cold rolling percentage (%) and the variation is similar on RW and NR planes at all the different peak loads, whereas the same is insignificant for the core region of the specimen on the NW plane. Three-dimensional (3D) optical surface profilometry studies on the NR plane suggest that the shear band spacing decreases and shear band offset height increases with the increase of cold rolling extent. Meanwhile, the number of the pop-in events during loading for all the planes reduces with the increase of cold rolling extent pointing to more homogeneous deformation upon rolling. Calorimetric studies were performed to correlate the net free volume content and hardness in the differently cold rolled specimens.

Numerical Analysis of Serrated Chip Formation Mechanism with Johnson-Cook Parameters in Micro-Cutting of Ti6Al4V

Previous studies have reported significant differences in the Johnson-Cook (J-C) parameters of Ti6Al4V alloy. Thus, various serrated chip morphologies, cutting forces, and cutting temperatures are obtained when different constitutive parameters are used for numerical and simulation analyses, which decreases the reliability of the simulation model. Therefore, it is necessary to investigate and analyze simulation errors due to differences in the J-C parameters. In this study, the mechanism of the serrated chip formation of Ti6Al4V is thoroughly analyzed using the uniformly proportional J-C parameters. The serrated chip sensitivity, shear band spacing, serrated segmentation frequency, chip serration intensity, temperature field, strain energy, and cutting force is obtained. This study aims to improve the accuracy and reliability of the micro-cutting simulation models, as well as a reference for the selection of J-C constitutive parameters of simulation with Ti6Al4V manufactured with different heat treatments and additive manufacturing.

Intrinsic toughness of the bulk-metallic glass Vitreloy 105 measured using micro-cantilever beams

Bulk-metallic glasses (BMGs) are a class of structural materials with many attractive processing features such as the ability to be processed into parts with fine features, dimensional precision, and repeatability; however, their fracture behavior is complex and size-dependent. Previous work has shown that BMGs can display strong size effects on toughness, where multiple mechanisms on different length-scales, e.g., crack bridging and bifurication, shear band spacing and length, can significantly affect the properies. This length-scale dependence on the fracture toughness has importance not only for advancing the understanding of fracture processes in these materials, but also for the potential future applications of BMGs, such as for microdevices. Here, using in situ scanning electron microscopy (SEM), we report on notched micro-cantilever bending experiments to address the lack of data regarding fracture properties of BMGs at the microscale. Sudden catastrophic propagation of shear bands resulted in failure for these specimens at stress intensities much lower than the bulk material, which may be due to a lack of extrinsic toughening mechanisms at these dimensions. This is explored further with post mortem SEM and transmission electron microscopy (TEM) analysis of the fractured beams while the fracture toughness results are verified using finite element modeling. The excellent agreement between model and micro cantilever beam bending experiments suggests that the intrinsic fracture toughness of Vitreloy 105, 9.03±0.59 MPa.m½, is being reported for the first time.

SEM and AFM analysis of the shear bands in Zr-based BMG after HPT

The surface relief formed by shear bands in bulk metallic glasses (BMG) under high-pressure torsion (HPT) has been investigated by the method of scanning electron microscopy (SEM) and atomic force microscopy (AFM). For this purpose, two halves of disks of the bulk metallic glass were jointed together and processed by HPT. The SEM examination of the internal surfaces of two joint halves of an HPT-processed disk allowed to study the formation and accumulation of shear bands under an increased imposed strain. The maximum density of the shear bands is observed at the edges of the HPT samples and in areas adjacent to the upper anvils. The observed minimum shear band spacing is equal to 0.5 μm after HPT processing for 5 revolutions.

Observation of shear bands in the Vitreloy metallic glass subjected to HPT processing

The parameters of shear band evolution with deformation were examined in the Vit105 bulk metallic glass. For this purpose, two halves of disks of the bulk metallic glass were joined together and processed by high-pressure torsion for various strains: from compression without rotation, to rotation for 5 revolutions. The discrepancy between the experimentally observed and predicted shear strains was detected. The actual strain is significantly smaller than the predicted one. The SEM examination of the internal surfaces of two joint halves of an HPT-processed disk allowed to study the formation and accumulation of shear bands under an increased imposed strain. The maximum density of the shear bands is observed at the edges of the HPT samples and in areas adjacent to the upper anvils. An increase in strain leads to an increase in the shear bands density. The observed minimum shear band spacing is equal to 0.5 μm after HPT processing for 5 revolutions. According to the structural changes recorded by XRD (an increase in the free volume content by about 1.3%) and formation of a high density of shear bands, HPT leads to a significant structural transformation of the amorphous structure of the BMG.

Explosively driven hierarchical particle jetting

When particle rings/shells are subjected to divergent explosive loadings, a dual overlapping particle jetting structure emerges during the shock interaction timescale which consists of a large number of minor jets initiated from the external interface at very early instants and a much reduced number of major jets formed from the internal interface at delayed times but overtaking the minor jets in later times. In the present work, the formation of the hierarchical particle jetting pattern is investigated numerically by discrete element method (DEM) coupled with finite element method (FEM), which execute the mechanical calculations of particles and the explosive/detonation gases, respectively. The numerical results find that the external jetting arises from the spallation of an outer layer pulled away by inward propagating rarefaction waves. Meanwhile an inner compact band re-compressed by a secondary shock remains densely packed while expanding outward. The fragmentation of the inner compact particle band, preceding the internal particle jetting, is caused by the profuse spiral shear failures expanding from the inner radius to the outer radius. The resultant jetting structure depends on the shear-band spacing and the grouping of the clockwise and counterclockwise shear bands as well. The former is a function of the bulk characteristics of the inner compact band, especially the resistance to the shear flows. The latter markedly varies with the microstructure of particle packing, especially the structural order. In the highly ordered extreme, the particle ring with global crystalline structure exhibits six groups of shear bands, probably giving rise to around six fragments. By contrast, the grouping of shear bands in the amorphous packing is far from definite, suggesting an increased number of much smaller fragments to be generated. The dual jetting structure would degenerate into a single jetting pattern if the inner compact band manages to entrain all the spall particles before the shear failure occurs.

Metallurgical Analysis of Chip Forming Process when Machining High Strength Bainitic Steels

In the following work, we propose a metallurgical approach to the chip formation process. We focus on a turning application of high strength steel in which chips are produced by adiabatic shear bands that generate cutting force signals with high frequency components. A spectral analysis of these signals is applied and highlights peaks above 4 kHz depending on the cutting conditions. A microscopic analysis on the chip sections provided data on chip breaking and serration mechanisms. Shear band spacing and excitation frequency of the whole cutting system were calculated and gave a good correlation with cutting forces spectra.

Radiation-induced shear bands enhance softening 316LN austenitic stainless steel

Radiation hardening is usually a major concern for materials used for nuclear reactors. Here, we report that helium (He) radiation could weaken the radiation hardening effect and even soften 316LN austenitic stainless steel by forming shear bands. The experiment results showed that the nanohardness of the steel decreased sharply after radiation by 200 keV He ion to a fluence of 2.3 × 1021 ions/m2. Moreover, it is noteworthy that the number and spacing of shear bands can be controlled indirectly by forming the radiation bubbles with appropriate size and distribution, which is hopeful to extend the application of He bubble engineering.

Chip Morphology and Chip Formation Mechanisms During Machining of ECAE-Processed Titanium

Severe plastic deformation (SPD) processing such as equal channel angular extrusion (ECAE) has been pioneered to produce ultrafine grained (UFG) metals for improved mechanical and physical properties. However, understanding the machining of SPD-processed metals is still limited. This study aims to investigate the differences in chip morphology when machining ECAE-processed UFG and coarse-grained (CG) titanium (Ti) and understand the chip formation mechanism using metallographic analysis, digital imaging correlation (DIC), and nano-indentation. The chip morphology is classified as aperiodic saw-tooth, continuous, or periodic saw-tooth, and changes with the cutting speed. The chip formation mechanism of the ECAE-processed Ti transitions from cyclic shear localization within the low cutting speed regime (such as 0.1 m/s or higher) to uniform shear localization within the moderately high cutting speed regime (such as from 0.5 to 1.0 m/s) and to cyclic shear localization (1.0 m/s). The shear band spacing increases with the cutting speed and is always lower than that of the CG counterpart. If the shear strain rate distribution contains a shift in the chip flow direction, the chip morphology appears saw-tooth, and cyclic shear localization is the chip formation mechanism. If no such shift occurs, the chip formation is considered continuous, and uniform shear localization is the chip formation mechanism. Hardness measurements show that cyclic shear localization is the chip formation mechanism when localized hardness peaks occur, whereas uniform shear localization is operative when the hardness is relatively constant.

Power-law scaling behaviors for shear band intersections in Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass

At the large compressive deformation levels, shear band offsets at intersection sites and the stress drop magnitudes of serrated flow follow power-law distributions for Zr64.13Cu15.75Ni10.12Al10 bulk metallic glass. In addition, it was revealed that shear band interactions evolve into a process of large degree cooperation of many shear bands via investigating the relation between shear band intersections and serrated flow. To clarify the evolution rules of shear band intersections, quantitatively statistical works were performed. With increasing the plastic strain, the distribution of shear band orientations becomes wider and displays a multi-peak distribution. The average shear band spacing varies as a power function of plastic strain and decreases to a very low level in large deformation stages. Small spacing and arbitrary orientations of shear bands greatly enhance the intersecting probability and lead to the density of shear band intersections varying as a power function of plastic strain. Enormous amount of undeformed regions enclosed by shear bands provide many places for shear bands intersecting.

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.

Fabrications and mechanical behaviors of amorphous fibers

Mechanical properties of micro- and nanoscale fibers are superior to their bulk counterparts, and their mechanical behaviors are different from each other. Homogeneous amorphous fibers with smooth surfaces and controllable sizes can be continuously drawn from supercooled liquid. Compared with the preparing of crystalline fibers, the manufacturing of amorphous fibers saves much energy and time. Furthermore, amorphous materials have excellent mechanical properties due to their short-ranged ordered and long-ranged disordered structures. Therefore, amorphous fibers have wide engineering applications and research interest. In this paper we review the fabrication and mechanical behaviors of amorphous fibers with excellent mechanical properties including oxide glass fibers and amorphous alloy fibers.There are continuous and discontinuous oxide glass micro-fibers. Discontinuous oxide glass micro-fibers can be fabricated by techniques in which a thin thread of melt flowing from the bottom of a container is broken into segments. Continuous oxide micro-fibers can be fabricated by techniques in which a filament of supercooled liquid is drawn from melt. However, oxide glass nano-fibers can be fabricated by chemical vapor deposition, laser ablation, sol-gel, and thermal evaporation methods. Fabrication techniques of amorphous alloy fibers are very different from those of oxide glass fibers. These techniques adopt in-rotating-water spinning method, melt-extraction method, Taylor method, nanomoulding method, fast drawing method, melt drawing method, and gas atomization method.Microscale oxide glass fiber has a facture strength as high as 6 GPa. The fracture strength of nanoscale oxide glass fiber can reach 26 GPa which is close to the theoretical strength of 30 GPa. On the other hand, the plasticity of microscale amorphous alloy fibers is mediated by shear banding. The shear band spacing decreases with reducing sample size in bending. However, there is no tensile plasticity in microscale amorphous alloy fibers. When the sample size is smaller than the size of shear band core (500 nm), inhomogeneous plastic deformation transforms into homogeneous plastic deformation. The tensile plasticity of amorphous alloy is significantly improved. The homogeneous plastic deformation is mediated by catalyzed shear transformation. The catalyzed shear transformation may be the origin of hardening behaviors of nanoscale amorphous alloy fibers.Fianlly, we summary the unsolved problems in the fabrications and mechanical behaviors of amorphous fibers, and discuss the prospect of amorphous fibers.

Bulk metallic glasses: “Defects” determines performance

In the current work, the size effect to plasticity of the Zr50Cu40+xAl10−x (x=0, and 2) bulk metallic glasses has been investigated experimentally. It is found that macroscopic plasticity are affected by different of leading factors. The mean stress drop of serrated flow is positively related to the number of weak regions. With greater number of weak regions, more pronounced stress drop occurs in the serrated flow process. The experimental results suggested that fewer number of weak regions lead to larger dimple patterns area. Besides, the average spacing of primary shear bands is associated with the weak regions and the position where shear bands terminate.

Modeling spontaneous shear bands evolution in thick-walled cylinders subjected to external High-strain-rate loading

The evolution of spontaneous shear bands in a 304L stainless steel (304LSS) cylinder subjected to external explosive loading was numerically studied. As an instability process, the initiation of a shear band is strongly dependent on the non-uniformities of the material. A probability factor satisfied the Gaussian distribution was introduced into the macroscopic constitutive relationship to describe the nonuniform distribution of the local yield stress in material. On the basis of that the material inside the shear band undergoes a dynamic recrystallization process (DRX), the traditional J-C model was modified. The item melting temperature, Tm, was replaced by the temperature for DRX, TDRX. Specifically, the TDRX was set to be 0.4Tm at high strain rates (>103s−1) for 304LSS. Using the probability factor and the modified J-C model, we successfully simulated the initiation and propagation of spontaneous shear bands in the collapsing 304LSS cylinder. The pattern of the shear bands agrees well with the experimental observations, at both early stage and late stage. The perturbation amplitude was found to have a slight influence on the shear band spacing and the initiating time of shear bands. Some morphological characteristics of shear bands (such as bifurcation, intersection and countercheck), which are familiar in experiments, were also observed in our simulation. The formation mechanisms of these phenomena were analyzed on the basis of their evolvements.The shear bands in TWC show a single direction spiral pattern, in other words, almost all the shear bands simultaneously propagate along a given direction, clockwise or counterclockwise. The single direction spiral pattern is closely related to the work-hardened layer in the internal surface of the 304LSS cylinder. The microstructures in the work-hardened layer (about 30µm) are significantly different from those in the base material. The grains have been rotated and elongated along the cutting direction during the machining. In the simulation, periodic single direction spiral perturbations were applied to describe the grain orientation in the work-hardened layer, and the single direction spiral pattern of shear bands in 304LSS TWC was successfully replicated. While the spacing of the single direction spiral perturbations exceeds a certain value, the shear bands switch back to the bidirectional spiral pattern. The critical spacing for the pattern transition was found to be close to the spacing of well-developed shear bands.

Numerical analysis of constitutive coefficients effects on FE simulation of the 2D orthogonal cutting process: application to the Ti6Al4V

In this paper, a deep study of constitutive parameters definition effect is done in order to guarantee sufficient reliability of the finite element machining modeling. The case of a particular biphasic titanium alloy Ti6Al4V known by its low machinability is investigated. The Johnson-Cook (JC) elasto-thermo-visco-plastic-damage model combined with the energy-based ductile fracture criteria is used. Segmentation frequency, chip curvature radius, shear band spacing, chip serration sensitivity and intensity, accumulated plastic strain in the formed chip segments, and cutting forces levels are determined where their dependency to every constitutive coefficient is examined and highlighted. It is demonstrated from the separate variation of every plastic and damage parameters that an interesting finite element modeling (FEM) relevance is reached with the adjustment of JC strain hardening coefficients term, thermal softening parameter, exponent fracture factor, and damage evolution energy. Moderate and high cutting speeds are applied to the cutting tool in the aim to test their impact on shear localization, chip segmentation, and numerical forces levels as well as to approve previous highlighted findings related to constitutive parameters definition. In general, this study focuses on a prominent decrease in identification process cost with the previous knowledge of the most affecting constitutive coefficients while keeping an interesting agreement between numerical and experimental results.

Analysis of Shear Banding in Zr-Based Metallic Glass under Laser Shock Peening and Bending

Metallic glasses have been expected to be used as structural materials since their high strength and high hardness. Unfortunately, their catastrophic brittle fracture behavior with poor plasticity becomes the major weakness for structural application. It has been recognized that the mechanism of plastic deformation in metallic glasses is through the formation of shear bands, which provide them with limited ductility. Laser shock peening (LSP) is an innovative surface treatment technique which can introduce deep compressive residual stress layer to materials for improving their mechanical behavior. In this work, a finite-element model has been developed to numerically simulate the pure bending process, LSP and subsequent bending process. An advanced constitutive equation was established based on the large deformation theory of nonlinear mechanics, the free volume model and the Coulomb-Mohr yield criterion. The model is able to capture the following results: (i) for a given bending deflection, the shear band spacing increases with increasing plate thickness; (ii) for a given plate thickness, the free volume increases with the bending deflection; (iii) for a given thickness and a given deflection, the shear bands increase under the effect of LSP.