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In recent years, molecular dynamics (MD) simulation has been used to study the deformation behaviors of glass under nanoindentation, mainly using ideal geometries like a spherical indenter or a 2.5-D sample geometry to simplify post-analysis and save computational costs. To generate stress/strain fields that can be directly compared with experiments, we developed a 3-D nanoindentation protocol in this work to study the deformation behaviors of a model metallic glass under sharp contact loading in MD. Our studies show that the indenter sharpness controls the shear band formation, and the interaction between shear bands dictates the crack initiation in the model metallic glass. Shear bands and residual stress fields in the model metallic glass from our simulated nanoindentation tests are consistent with observations in soda-lime silicate (SLS) glass from the instrumented indentation in experiments, as both of them favor shear deformation under sharp contact loading.

Machining is often the most expensive manufacturing process step when producing titanium alloy components. With a move towards higher metal removal rates to meet aircraft orders, it is important to develop rapid, low-cost methods that assess the machinability of titanium alloys and cutting tool combinations. A cost-effective, small-scale methodology has been developed and validated on the industrial scale using high-speed turning to inform on the machining characteristics of commercial and emerging titanium alloys to aid companies in future developments of new titanium alloys and cutting tool materials. The article demonstrates, using the titanium alloys Ti-6Al-4V (Ti-64) and TIMETAL 407® (Ti-407), that a series of early stage, small-scale methods can identify key machinability characteristics including chip form, tool wear, cutting force and surface damage. It can be concluded using these low-cost machinability assessment methods that Ti-407 exhibits better machining properties to Ti-64 for the aspects of machinability focused on cutting forces and tool wear, whereas the contrary is found for subsurface microstructural features and chip control. [ABSTRACT FROM AUTHOR]

A dispersion of stiff and thin ('rigid line') inclusions (RLIs) in a matrix material may result beneficial for stiffening in the elastic range, but might be detrimental to strength, as material instabilities may be triggered by inclusions when the matrix is brought to a viscoplastic-damaging state. This dual role of RLIs is investigated by means of the embedded reinforcement model. Validated against available analytical predictions, this numerical model is employed to assess the roles of RLIs' orientation, interaction, volume fraction, and distribution, considering up to 1500 inclusions. When the matrix material deforms inelastically, RLIs produce stress concentrations that promote the nucleation of shear bands. These are characterized at collapse for many distributions of RLIs, showing that their effects range from almost negligible to a disrupting alteration of the dominant failure mechanism. In the latter case, it is shown that the dominant shear bands can be fragmented by RLIs into a mosaic of tiny localization bands. These results offer new insights into energy dissipation mechanisms of reinforced materials, as they are promoted or inhibited by the interactions of rigid line inclusions. [ABSTRACT FROM AUTHOR]

Ti-5553 (Ti-5Al-5Mo-5V-3Cr-0.5Fe) is a recently commercialized near-β titanium alloy employed in large-section forging of Boeing 787 landing gear. The machinability of Ti-5553 is low owing to its high strength at elevated temperature, low thermal conductivity and high chemical reactivity. Chip serration occurs during high-speed machining of Ti-5553, which is found to be attributed to the periodic formation of shear bands caused by thermoplastic instability within the primary shear zone. Microscopic observations indicate that the geometry and microstructure of shear bands are significantly related to the shear angle, shear strain and strain rate under the given cutting conditions. In this study, quick-stop turning tests were conducted to understand the chip formation mechanism in machining Ti-5553, and the machined chips were metallographically characterized to investigate their morphology and microstructure evolutions. In the Part I of this study, the shear angle, shear strain and strain rate were quantitatively determined, and the shear band microstructure was examined to reveal the effect of cutting conditions in dry turning of Ti-5553. This study advances the mechanism of shear band formation in machining Ti-5553 and develops a novel experimental approach to directly obtain the actual shear strain and strain rate based on the geometry of the machined chip. [ABSTRACT FROM AUTHOR]

Using molecular dynamics simulations, we provide an atomistic description of the shear band multiplication mechanisms in a heterogeneous metallic glass consisting of two distinct amorphous regions with different amounts of free volume and degrees of short-range order. The structural differences and elastic fluctuations encountered by the developing shear band while crossing the interface between the two regions alter the autocatalytic activation of the shear transformation zones (STZs) and, subsequently, lead to shear band branching. The two-unit STZ-vortex mechanism sheds light on the correlation between the strain distribution during tensile deformation and the directional characteristics of the STZ activation process. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract In this work we discuss a gradient plasticity formulation which relies on the introduction of higher-order gradient contributions as additional arguments of the free energy function. These gradients can be interpreted in terms of the local geometrically necessary dislocation density. This gives rise to physically well-motivated kinematic-type hardening models in contrast to purely phenomenological approaches. At the same time, the framework results into a regularisation of the formulation such that localised plastic deformation processes in softening materials, e.g. the formation of shear bands, can be simulated. The employed theory is based on an extended nonlocal form of the Clausius–Duhem inequality, motivated by a possible energy exchange between particles at the microstructural level. This gives rise to the balance equation of a nonlocal stress tensor that is found to be work-conjugated to the plastic velocity gradient. The solution of the governing system of partial differential equations is approached by means of a multi-field finite element formulation with the solution of the Karush–Kuhn–Tucker conditions being addressed on a global level by means of Fischer–Burmeister complementary functions. We discuss a specific quadratic energy contribution in terms of the incompatibility, respectively dislocation density tensor that results into well-interpretable contributions to the nonlocal stress field and study the formation of shear bands induced by geometric imperfections as well as the constitutive response at a material interface where the yield limit exhibits a jump discontinuity. [ABSTRACT FROM AUTHOR]

Abstract The formability of newly developed steel grades is a key parameter for their successful implementation in the car body. In the present work, the influence of the microstructure on formability during air bending is studied. First- and third-generation advanced high strength steels with tensile strengths above 1000 MPa are investigated, which exhibit a broad range of microstructural constituents (ferritic bainite, bainite, martensite, and retained austenite). Interrupted bending experiments are performed on two grades of dual phase and complex phase steels, respectively, in order to reveal the microscopic deformation and failure processes. The formability in hole expansion tests is determined and used as a measure for the resistance of the materials against crack formation. The macroscopic deformation behavior of the materials is taken into account by measuring the differential strain hardening exponent and performing finite element simulations of the air bending process. Advantages and disadvantages of various microstructural features are worked out, and it is explained why the first-generation dual phase steel exhibits the lowest and the third-generation complex phase steel the best formability during air bending. [ABSTRACT FROM AUTHOR]

Because of the fast dynamics of shear band formation and propagation along with the small size and transient character of the shear transformation zones (STZs), the elementary units of plasticity in metallic glasses, the description of the nanoscale mechanism of shear banding often relies on molecular dynamics (MD) simulations. However, the unrealistic parameters used in the simulations related to time constraints may raise questions about whether quantitative comparison between results from experimental and computational analyses is possible. Here, we have experimentally analyzed the strain field arising across an individual shear band by nanobeam X-ray diffraction and compared the results with the strain characterizing a shear band generated by MD simulations. Despite their largely different spatiotemporal scales, the characteristic features of real and simulated shear bands are strikingly similar: the magnitude of the strain across the shear band is discontinuous in both cases and the direction of the principal strain axes exhibits the same antisymmetric profile. This behavior can be explained by considering the mechanism of STZ activation and percolation at the nanoscale, indicating that the nanoscale effects of shear banding are not limited to the area within the band but they extend well into the surrounding elastic matrix. These findings not only demonstrate the reliability of MD simulations for explaining (also quantitatively) experimental observations of shear banding but also suggest that designed experiments can be used the other way around to verify numerical predictions of the atomic rearrangements occurring within a band. [ABSTRACT FROM AUTHOR]

Ti-6Al-4V powder metallurgy materials with different densities were prepared by hot isostatic pressing technology. Shear band formation and wear mechanisms of the powder metallurgy materials were studied. The wear mechanisms of the powder metallurgy material were revealed that the micro-void torn and accumulated built-up edge in sticking friction and sticking zones, respectively. The wear mechanism of tool rake face was varied when the material density deceased. The shear plane shifted to the residual micro-void, which made a difference on fluctuation of cutting force and caused the appearance of severe micro-fluctuation. The micro-fluctuation of cutting force was weakened with decreasing the material density. Therefore, occasional random shear model of the powder metallurgy material was verified, and the progressions of chip morphology characteristic parameters with material density were studied, the micro-void on improved shear stress concentration. It could be concluded that the shear band mechanism of serrated chip in machining of titanium alloy was a catastrophic shear. [ABSTRACT FROM AUTHOR]

When a granular material such as soil interacts with a hard artificial structure, it commonly involves a considerable degree of material heterogeneity. The shear deformation largely localizes into a narrow band: the shear band. The mechanism of the shear band formation at the soil–structural interface is of significant interest to researchers and engineers, for it explains the process of soil failure under interface shearing as well as the physical essence of the soil behaviors associated with such failure – for example, stress peak, stress softening, and dilatancy. In this paper, we employ the discrete element method (DEM) to analyze the formation of the shear band in a series of 2D interface shear tests in which specimens are initialized with different void ratios. During the interface shearing process, we analyze the shear velocity field v x and its gradient field, both of which are extracted on the basis of kinematic quantities given by grains. In the dense specimen, imperfections – indicated by the discontinuities in the field of v x and – are observed appearing in the stress hardening regime. These imperfections progressively accumulate and converge with each other, threatening to form several potential failure bands. This continues until the stress peak, after which a major failure band appears to be dominant over others. The shear band is then shaped, and the stress softening occurs. In the loose specimen subjected to interface shearing, the domain of the strain localization is so unstable and varied that a steady shear band cannot be steadily formed. In both samples, stress variations are shown to be highly dependent on degree of strain localization. [ABSTRACT FROM AUTHOR]

In this paper, we consider a non-local stochastic parabolic equation that actually serves as a mathematical model describing the adiabatic shear banding formation phenomena in strained metals. We first present the derivation of the mathematical model. Then we investigate under which circumstances a finite-time explosion for this non-local stochastic partial differential equation, corresponding to shear banding formation, occurs. For that purpose, some results related to the maximum principle for this non-local stochastic partial differential equation are derived, and afterwards the Kaplan eigenfunction method is employed. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Numerical simulations with finite element method were carried out and five types of patterns of shear bands and their process of self-organization were obtained. The elasto-plastic theory of constitutive relationship with the non-associated flow rule, vertex-like yielding surfaces and strain softening of strength were adopted to describe the mechanical behavior of geomaterials of rock or soil. The non-symmetrical matrix due to the adoption of the non-associated flow rule was solved with the algorithm of dynamical relaxation of finite element method. The discrete or banded patterns of shear strain in the form of superlattice, parallel strips, super lattice turned parallel strips, rhomboid net and concentric rhomboid loops, were gradually formed as the loading increased. The mechanism of the structural pattern generation and the process of pattern self-organization in geomaterials of rock and soil were a process of mechanical equilibrium of stresses and the allocation of material deformation between the elastic sites and plastic sites. The approach provided a way for researching mechanical origin of shear band pattern. [ABSTRACT FROM AUTHOR]

Abstract: The mechanical behaviors of lithium iron phosphate battery cells are investigated by conducting in-plane and out-of-plane compression tests of representative volume element (RVE) specimens of dry cells. The test results of cell RVE specimens under in-plane constrained compression indicate that the load carrying behavior of the cell RVE specimens is characterized by the buckling, the kink and shear band formation, and the final densification of the cell components. The SEM images of the active materials on electrodes and the test results of the cell RVE specimens under out-of-plane compression suggest that the porosity in the components and the macroscopic gaps between the components is up to 40%. The test results suggest that the lithium-ion battery cells can be modeled as anisotropic foams or cellular materials. The elastic buckling analyses for a beam with lateral constraints indicate that the higher order buckling modes and the critical buckling stresses in general agree with those observed in experiments. The elastic buckling analyses also justify the length selection of the cell RVE specimens. Finally, an idealized kinematic model is presented to explain the physical mechanisms of the kink and shear band formation in the cell RVE specimens under in-plane constrained compression. [Copyright &y& Elsevier]

Abstract: We investigate the influence of various microstructural features on the deformation behavior of binary glasses by molecular dynamics computer simulations and discuss how and why the very same modifications established for enhancing the strengths of crystalline materials, namely the insertion of solutes, precipitates and grain boundaries, can be used for tuning the mechanical properties of metallic glasses. First, by testing bulk samples with and without open surfaces under tensile load, we show that the condensation of shear transformation zones into shear bands can occur as heterogeneous but also as a homogeneous nucleation process. Then, the influence of crystalline nanoprecipitates on shear band nucleation and propagation is investigated. Finally, we study the effect of grain size and composition on the deformation behavior of nanoglasses and nanoglass composites. The results reveal that glass–glass interfaces act as structural heterogeneities ,which promote shear band formation and prevent strain localization. [Copyright &y& Elsevier]

Abstract: In this paper, computational models are developed for simulations of representative volume element (RVE) specimens of lithium-ion battery cells under in-plane constrained compression tests. For cell components in the finite element analyses, the effective compressive moduli are obtained from in-plane constrained compressive tests, the Poisson's ratios are based on the rule of mixture, and the stress–plastic strain curves are obtained from the tensile tests and the rule of mixture. The Gurson's material model is adopted to account for the effect of porosity in separator and electrode sheets. The computational results show that the computational models can be used to examine the micro buckling of the component sheets, the macro buckling of the cell RVE specimens, and the formation of the kinks and shear bands observed in experiments, and to simulate the load–displacement curves of the cell RVE specimens. The initial micro buckling mode of the cover sheets in general agrees with that of an approximate elastic buckling solution. Based on the computational models, the effects of the friction on the deformation pattern and void compaction are identified. Finally, the effects of the initial clearance and biaxial compression on the deformation patterns of the cell RVE specimens are demonstrated. [Copyright &y& Elsevier]

The process simulations based on FEM techniques have been investigated for many years, some fundamental problems are still unsolved, e.g. the element size effect on the computational results. In present contribution, orthogonal cutting simulations of AISI4340 steel are considered. The major concerns are accuracy of computational results, influence of element size and effects of damage model in accommodating modeling of failure phenomenon for cutting process simulations. Numerical simulations are verified with the measured values of cutting force by considering certain case of influencing cutting parameters combination taken from literature. Element size is treated to be the most influencing constituent in the cutting process simulations. The chip morphology is related to the adiabatic assumption considered in the process simulation, the feed value and the element size. The simulation results are presented by neglecting temperature effects to show the influence of failure criterion based on plastic displacement of the numerical results. Though the chip morphology and shear band formation are most sensitive to the element size, the cutting force of process simulations is hardly influenced. The formation of saw-tooth chip in the present simulations is the result of adiabatic shear band at the tool tip and propagating towards the chip's outer surface. The present work confirms that the effect of element size on computational results is reduced significantly if the failure criterion in the process simulation is controlled by a characteristic element length considered from the progressive damage model. [ABSTRACT FROM AUTHOR]

Abstract: Machining of copper-based alloys tends to produce irregularly spaced chip serration. However, turning cold-rolled material could sometimes produce regularly spaced chip serration. The regularly spaced chip serration is indeed layers of shear bands that grow and agglomerate to become chip serration. In this paper, an investigation into the formation of shear bands in the microcutting process of single point diamond turning is presented. The experimental results reveal how shear bands are initiated at the free edge of the chip and then propagated towards the tool tip. A new finite element model is developed to simulate the initiation and formation of these shear bands and to provide evidence for how these shear bands are induced by the elastic strain concentration. In contrast to the popular assumption that materials behave as rigid plastic during machining, the results reveal that elastic strain is one of the dominant factors leading to the formation of shear bands. Finally, an analytical model is proposed to reveal the influence of shear bands on the cutting force and on the formation of serrated chips. [Copyright &y& Elsevier]

The article discusses a study which examined the microstructures and mechanical properties of high-strength TRIPLEX steels with the generic composition Fe-Mn-Al-C. The steels showed low density and superior mechanical properties. Transmission electron microscopy showed homogeneous shear band formation accompanied by dislocation glide in deformed tensile samples. The high flow stresses and tensile strengths were due to effective solid solution hardening and superimposed dispersion strengthening.

Variations in microstructure and chemical compositions of the metallic glasses found in the literature, as well as an overall lack of experimental data on inhomogeneous behavior of metallic glass make the evaluation of the effects of shear band/fracture behavior on mechanical properties of metallic glasses difficult. Investigating the effect of strain localization alone on inhomogeneous flow seems to be a first step in approaching this problem. Mechanical behavior of metallic glasses at room temperature and various strain rates in tension and compression was investigated. Formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension, whereas, the ductility of the material in compression decreases with increasing strain rate. Differences in deformation processes in tension and compression were compared. [Copyright &y& Elsevier]

Despite the utmost importance and decades of experimental studies on fatigue in metallic glasses (MGs), there has been so far little or no atomic-level understanding of the mechanisms involved. Here we perform molecular dynamics simulations of tension-compression fatigue in Cu50Zr50 MGs under strain-controlled cyclic loading. It is shown that the shear band (SB) initiation under cyclic loading is distinctly different from that under monotonic loading. Under cyclic loading, SB initiation takes place when aggregates of shear transformation zones (STZs) accumulating at the MG surface reach a critical size comparable to the SB width, and the accumulation of STZs follows a power law with rate depending on the applied strain. It is further shown that almost the entire fatigue life of nanoscale MGs under low cycle fatigue is spent in the SB-initiation stage, similar to that of crystalline materials. Furthermore, a qualitative investigation of the effect of cycling frequency on the fatigue behavior of MGs suggests that higher cycling frequency leads to more cycles to failure. The present study sheds light on the fundamental fatigue mechanisms of MGs that could be useful in developing strategies for their engineering applications.