Initiation Of Shear Bands Research Articles

Improving the wear performance of a commercial Vit 1 amorphous alloy by a cryogenic cycling treatment

A commercial Vit 1 amorphous alloy was processed by a cryogenic cycling treatment of different times. The phase structure, thermal, mechanical and wear properties were studied. Although the cryogenic cycling treatment does not change significantly the phase structure, it promotes thermal rejuvenation, i.e., the heterogeneity of the internal structure. The heterogeneous structure promotes initiation of multiple shear bands within the amorphous alloy to bring about dissipative deformation energy, which in turn increases the plasticity. Thanks to the plasticization, the wear properties were significantly improved by the cryogenic cycling treatment. In particular, the wear rate after 30 cycles of treatment (7.55 × 10–5 mm3/N m) decreases to about one third of the as-cast alloy (22.75 × 10–5 mm3/N m). The reason is that with the increase in plasticity, the wear mechanism of Vit 1 BMG changes from three-body abrasive wear to a synergistic mechanism of three-body abrasive wear and oxidative wear. Therefore, improving the plasticity of amorphous alloys by the cryogenic cycling treatment could be an effectively strategy to enhance the wear perfermance.

Free volume evolution dominated by glass forming ability determining mechanical performance in Zr Ti65-Be27.5Cu7.5 metallic glasses

The shear deformation and fracture behaviors of Zr x Ti 65- x Be 27.5 Cu 7.5 metallic glasses (MGs) were systematically investigated in the range x = 20, 25, 30, 35, 40, 45, 50 at% under multi-axial loading mode by small punch test (SPT). Results show that enhanced plasticity can be obtained in MGs with composition ranging from 30 to 45 Zr at%, which is attributed to the large free volume dependent on their better glass forming ability (GFA). It is reasonably concluded that the total free volume consists of two parts: intrinsic free volume ( V 0 ) and excess free volume ( V G ). Intrinsic free volume, involved in an ideal amorphous structure, is defined as the free volume of MGs undergoing fully structural relaxation, which depends on the atomic size of each element. On the basis of atomic dense packing model, intrinsic free volume of the studied MGs is basically same due to a little difference in the atomic size between Zr and Ti. Excess free volume is crucially dominated by the GFA of MGs. In the case of little difference in atomic size of Zr and Ti, the MGs with large GFA possess a much looser atomic stacking structure, leading to the larger content of excess free volume. The large content of free volume not only favor the better atomic mobility, but also facilitate the nucleation of multiple shear bands, which can dramatically improve the plasticity of MGs. Thus, the studied MGs with Zr content ranging from 30 to 45 at% possess the larger content of total free volume than that with 20, 25 and 50 Zr at% due to their better GFA, depicting the enhanced plasticity induced by the initiation and interaction of multiple shear bands. In addition, the MGs with large content of free volume exhibits the lower driving force of shear bands nucleation, showing a soft state.

Numerical study of shear bands formation in a 3D‐printed Zr‐based bulk metallic glass under uniaxial loading

AbstractProcessing metallic materials via selective laser melting (SLM) enables the production of bulk metallic glasses (BMGs) with large size and complex shape, which is difficult or even impossible to achieve using traditional manufacturing techniques. In this study, the initiation and propagation of shear bands as well as the tensile strength in a 3D‐printed BMG with printing defects by means of inherent porosities are studied. A multi‐slips system model based on Mohr‐Coulomb theory is applied.

Effect of Stacking-Fault Energy on the Deformed Structures and Work Hardening of Ag and Ni after Scratching during Early Loading Stage

Friction wear are accompanied by the formation of refined structures and shear bands in the surface layers of metals and alloys. The propensity for refinement and shear band formation is mainly determined by the stacking-fault energy (SFE). Despite the fact that significant studies have been dedicated to plastic deformation during friction, little effort has been directed toward elucidating the early stages of deformation hardening that is accompanied by the effects of SFE. The research results presented herein focused on the initiation of twin and lamellar shear bands, transformation of nano-grained structures during shear banding, and the deformation hardening of surface layers during the scratching of metals with low and high SFE. Flat samples of low (Ag) and high (Ni) SFE were scratched with a diamond indenter for one to ten friction cycles. The friction surfaces were examined with a field-emission scanning electron microscope. It was shown that the scratch track sizes and amounts of pile-up material were lower for Ag than those for Ni. High deformation hardening of Ag relative to Ni was confirmed by the inhibition of cross-slip and the formation of ultra-fine structures. The deformed structure of Ag after ten scratch cycles was characterized by the formation of core shear bands that were bent in the depth of the deformed layers. It was shown that the balance between deformation hardening and softening was crucial in preserving a steady friction state and forming wear particles.

Bending proof strength of Zr61Ti2Cu25Al12 bulk metallic glass and its correlation with shear-banding initiation

Bending proof strength (σp) is a key material property for spring-like flexible components. In this work, initial yielding behavior of Zr61Ti2Cu25Al12 (ZT1) bulk metallic glass (BMG) in bending was investigated by an approach of stepwise cyclic loading-unloading under three-point bending (3 PB). It was shown that to meet the requirement of small-deflection condition in mechanics, a range of span-to-thickness ratio was specified at 10–15 for the BMGs subjected to 3 PB, which is one order of magnitude less than that of conventional polycrystalline metals. With conventional monotonic loading tests, the modulus of elasticity in bending and the proportional limit/flexural yield strength of the ZT1 BMG were determined to be Eb = 84 GPa and 2070 MPa, respectively. Combining stepwise loading-unloading with LOM observations alternately, it was shown that apparent yielding response of ZT1 beam in bending directly correlates with the event of first shear-band initiation. With an approach of stepwise cyclic loading-unloading with a measure in high accuracy, the σp at a permanent strain of 0.005% for the ZT1 BMG was determined to be 1900 ± 10 MPa. It is noticeable that the σp of ZT1 BMG is ~16% higher than its uniaxial tensile strength. In addition, high modulus of resilience (22 MJ/m3) in bending for this BMG manifest significant advantage as a candidate material of flexible components in engineering.

Multi-scale coupling effects on flow localization during micro-compression deformation of Ti–6Al–4V alloy

In this study, the initial microstructure dependency of flow localization behavior was investigated for Ti–6Al–4V alloys with equiaxed, duplex, basket-weave and widmannstatten microstructures. The flow localization behavior was conducted using micro-compression hot deformation tests and the microstructures were characterized by Electron backscatter diffraction (EBSD). The results reveal that initiation of micro shear bands in basket-weave is driven through the absorption of dislocations to colony boundaries, whereas it is implemented via the pinch-off of α lamellae in widmannstatten. A semi-quantitative result based on EBSD observations indicates that the high globularization rate of micro shear bands in basket-weave are found in α lamellae with soft orientation where the c-axis of crystal is tilted of 15–75° from the compression axis or the geometric orientation of lamellae is perpendicular to the shear direction. However, the globularization of widmannstatten is no longer subject to the effects of geometric or crystal orientation, suggesting that there is a noteworthy coupling size effects. In addition, accompanying with the microstructure change from duplex through basket-weave to widmannstatten, the decrease of interlamellar spacing for α lamellae increases the strength of materials through Hall-Petch. At the same time, the decrease of interlamellar spacing promotes slip transmission via the proposed mechanisms of boundary absorption and pinch-off, resulting in easier flow softening, earlier flow localization, more intense shear deformation, narrower and sharper shear bands.

Application of an acoustic emission source-tracing method to visualise shear banding in granular materials

The acoustic emission (AE) technique can be used to locate failure-induced AE sources and provides an alternative to continuously visualise the development of failures inside stressed materials; however, a literature review reveals almost no studies regarding its application for source location in granular materials. Yet the visualisation of ubiquitously observed strain localisation and shear banding is critical to an in-depth understanding of the progressive failure mechanism of granular materials. In this paper, an original AE source three-dimensional (3D) tracing code based on the idea of ‘time difference of arrival’ (TDOA) was developed using Matlab programming, for the first time, to continuously visualise particle-scale interactions involved in saturated granular soils. After validating its feasibility through a series of pencil lead break (PLB) tests, the developed AE source 3D tracing code was applied to saturated granular soils subjected to drained triaxial shearing. Both the results of PLB tests and drained triaxial shearing tests demonstrate a good consistency between the traced AE source and actual PLB/particle interaction-induced sources. This suggests that the developed AE source 3D tracing code could be used to visualise the initiation and evolution of strain localisation and shear banding in saturated granular soils subjected to drained triaxial shearing.

Microstructural characteristics and formation mechanism of adiabatic shear bands in Al–Zn–Mg–Cu alloy under dynamic shear loading

The microstructural characteristics and formation mechanism of adiabatic shear bands in ultra-high strength aluminum alloy are studied under dynamic shear loading. It is found that thermal softening alone is insufficient to trigger the initiation of adiabatic shear bands (ASBs). Dynamic recrystallization induced softening is attributed to be the main cause of ASB initiation, with temperature rise playing a secondary role by accelerating the deformation localization process. It is believed that abrupt temperature rise takes place after ASB initiation, and the temperature rise prior to ASB formation is relatively small. Recrystallized ultrafine grains with a strong texture of {112} <110> are formed in ASBs, based on a progressive subgrain misorientation recrystallization mechanism. It is proposed that the translational–rotational motion of the dynamically recrystallized nanograins in the form of vortexes at the mesoscale can account for the microstructural softening, thus causing shear instability, eventually leading to fracture by the combination of ductile and brittle failure mechanisms.

Особенности низкотемпературной деформации аморфных сплавов системы Al – Ni – Fe – La

X-ray amorphous ribbons of aluminum-based alloys doped with transition (Ni, Fe) and rare-earth (La) metals in various ratios were obtained by melt-spinning. The mechanical characteristics and fracture surfaces of the ribbons under conditions of uniaxial tension with speeds from 0,01 to 10 mm/min were studied. It was shown that with an increase in the rate in all investigated alloys, the strength limit increases and low plasticity remains. Inhomogeneous deformation is realized by the initiation and spread of shear bands. Mixed fracture surface indicates a brittle-viscous fracture with a high ratio of the viscous component. It was confirmed the significant influence of the scale factor on the strength characteristics of the ribbons. The tensile strength of the ribbons increases with iron content rising and nickel content decreasing when the influence of the scale factor is excluded. It is experimentally shown on ribbons with a low-melting tin coating that local heating in shear bands reaches the melting point of tin 231,9 °C.

Mechanical behavior and shear banding of electropulsing-assisted micro-scale shear-compression in Ti-6Al-4V alloy

Various electrically-assisted forming (EAF) technologies have been developed as a promising and new metal forming process, but few studies pay attention to the effect of electropulsing on shear deformation behavior and shear banding evolution. Here the electropulsing-assisted micro-scale shear-compression (EAMSC) experiments were conducted in Ti-6Al-4V alloy with lamellar and equiaxed microstructures under different current densities. Numerical simulation was used to investigate the field distributions during EAMSC. Scanning electron microscope (SEM) and transmission electron microscopy (TEM) were used to comparatively investigate the shear bands evolution and microcracks nucleation of two samples. The results show the electropulsing heating temperature was homogeneously distributed in the shear plane and formed a distinct gradient in the matrix. In lamellar microstructure, shear bands show a distribution of fork-shaped and the localised diffusive rotation occurs at grain boundary α (αGB) layer. The local fluctuations in stress/strain and temperature can contribute to the initiation of shear bands, and the microcracks nucleation will happen in the grain boundaries (GBs). While the formation of elongated dislocation cells, formation of elongated subgrains and break-up of elongated subgrains are the evolutionary process of shear bands for equiaxed microstructure, and the microvoids are nucleated at the breakpoint of subgrains with excessive elongation. In addition, the flow curves of EAMSC display obviously strain softening for lamellar microstructure, while hardening for equiaxed microstructure. It should be mainly attributed to that the equiaxed microstructure EAMSC caused the high dislocation pile-up and deformation twins (DTs). In contrast, the predominance of shear deformation during shear-compression deformation can cause the softening in lamellar microstructure.

Aboundary layer analysis for the initiation of reactive shearbands.

A one-dimensional model for the initiation of shear bands in a reactive material is developed, which accounts for thermal softening, strain hardening and strain-rate effects, and models the chemical reaction using an Arrhenius source term. An inhomogeneity in the heat flux is used as the stimulus for localized plastic deformation, and a solution is sought as a perturbation to the elastic solution. In the analysis, the thin zone of localization is identified as a boundary layer. It is found that the behaviour of the perturbations to the temperature, stress and strain hardening variable in the localization zone are governed by four dimensionless parameters which are known in terms of various material properties including density, the heat of reaction, strain-rate sensitivity, thermal sensitivity and strain sensitivity. The resulting equations are solved numerically and a criterion for the onset of shear banding is discussed. The analysis highlights key physical properties which control the reactive shear banding process and gives a deeper insight into how such a process may be understood as a mechanism for the accidental ignition of reactive materials.

Experimental and computational analysis of initiation and propagation of shear bands in bulk metallic glasses

Shear bands (SBs) play a key role in the mechanical deformation of Bulk Metallic Glasses (BMGs). The key to obtaining an insight into the shear band mechanism is to understand the root cause of shear band initiation and propagation. SB initiation is a difficult event to capture, considering the negligible macroscopic ductility of BMGs. It is the premature shear localisation in the nano scale which is responsible for the noticeable brittle nature in the macro-scale. Plastic deformation of BMGs occurs due to the formation of highly localised SBs of approximately10–20 nm in thickness. These then propagate at a velocity of around 100 ms−1 and results in catastrophic failure in both tension and compression due to the micro-cracks formed through the coalescence of the nano voids. This study develops a technique to capture the early initiation of SBs through a wedge indentation test. The work found that incremental loading (a definite load applying incrementally by 1 kN in stages) did not affect the overall mechanical response of the BMG material, indicating that SB evolution can be examined with an incremental indentation test. A predictive numerical model for SB initiation and propagation in BMGs is presented using two approaches: a strain gradient model and an integral-type non-local approach employing the Vermeer–Brinkgreve modelling strategy(Strömberg, Ristinmaa 1996). Comparisons are made between the outcomes of physical tests and numerical modelling to obtain an insight into the relative merits.

Rejuvenation, embryonic shear bands and improved tensile plasticity of metallic glasses by nanosecond laser shock wave

The structural and mechanical responses of Ti-based and Zr-based metallic glasses (MGs) treated by the nanosecond laser shock wave were investigated. A rejuvenation transition was observed, due to the structural rearrangement of neighboring atoms in the instantaneous shock process. Plenty of embryonic shear bands occurred along the shock-affected zone. A damped model of the shock pressure was established to describe the characteristic of the embryonic shear bands. The results showed that the shear band initiation in the nanosecond scale along the length direction was divided into two stages: the initial low propagation stage and the following fast propagation stage, because of the reduction of the shock pressure. The hardness of the shock-affected zone displayed a maximal decrease of 25% in the critical zone between the two propagation stages, due to the combined effect of the structural rejuvenation and the compressive residual stress. A schematic of the energy landscape was proposed to explain the relationship between the rejuvenation and the formation of the embryonic shear bands. The maximum tensile plasticity increased by 0.46%. The results demonstrated that the MGs possessed different shear band propagation modes in the initiation stages, which deepened the understanding of the deformation mechanism in the MGs.

Transition from shear band propagation to global slab failure in submarine landslides

The risk posed by seabed instability is of increasing significance as offshore activities including oil and gas developments continue to expand in this century. Many studies have considered runout of debris flows resulting from submarine landslides and potential impact on offshore infrastructure. However, initiation of slab failure resulting from shear band propagation (SBP) at the onset of a landslide has received less attention, although it is of key importance for estimating the scale and consequences of landslides. The present paper explores arrest of SBP as well as global slab failure above the shear band. The complete evolution of a submarine landslide from shear band initiation, propagation, slab failure, and arrest of SBP is observed through large-deformation finite element (LDFE) modelling. Governing equations for both dynamic and quasi-static SBP are established and solved, with results showing good agreement with the LDFE results. Lower limits for the final shear band length and for global slab failure are proposed for cases with high strain-rate dependency of shear strength, where shear band propagation is quasi-static; upper limits are proposed for cases without rate effects where the behaviour is dynamic.

Improving the Fatigue Property of Metallic Glass by Tailoring the Microstructure to Suppress Shear Band Formation

Metallic glass (MG) usually exhibits poor fatigue property, limiting its application as structural materials. Shear bands play a key role in fatigue crack initiation and propagation of MG. In this work, to enhance the fatigue property of MG, we proposed and verified a new strategy of suppressing the shear band initiation and then impeding the fatigue crack formation through relaxing the microstructure, in contrast to the previous one of proliferating shear bands through toughening MG. Moreover, simple quasi-static tests including uniaxial tension, compression and notch tension, are employed as feedback to estimate the best microstructure after relaxing and then the optimal fatigue property of MG. It is found that if the relaxed samples exhibit higher compressive elastic limit and tensile strength, but only slightly decreased notch toughness than the as-cast samples, the fatigue property of relaxed samples can be obviously enhanced. The present strategy provides a promising way for enhancing the fatigue endurance property of MGs without the requirement of intrinsically large plasticity, which may thus be beneficial for the structural application of most MGs with high glass forming ability.

Effect of impurities on adiabatic shear bands (ASB) formation and evolution in U-2.5 wt%Nb alloy under impact

Local stress concentration sites and/or microstructural inhomogeneities result in the nucleation and initiation of adiabatic shear bands (ASB). This paper reports the influence of impurities for the initiation and evolution of ASBs in U-2.5 wt%Nb alloy. Both cylindrical specimen and hat-shaped specimen were used to investigate the formation and evolution mechanism of ASBs in U-2.5 wt%Nb alloy using a split Hopkinson pressure bar apparatus. It was observed that brittle inclusions such as carbides in all of the specimens break up to form microcracks at very small strain during impact, then the microcracks coalesce and grow with increasing strain, which revealed that the brittle impurities in specimen not only precedes adiabatic shear failure, but also likely to be a dominant micromechanical factor in the very generation of the band. Adiabatic shear fracture occurs in the top end of hat-shaped specimen, it also appears that the microstructural inhomogeneities play more important role in occurrence of ASBs than the geometrical inhomogeneities. Additionally, it was demonstrated that the ductile inclusion of niobium was susceptible to resist the propagation of shear bands.

Numerical study on toughening mechanism of bulk metallic glass composites from martensite transformation of toughening phase

Numerical simulation was conducted to investigate the toughening mechanisms of bulk metallic glass composites (BMGCs) originated from the martensite transformation of toughening phase (TP). Firstly, a free volume model was chosen to describe the deformation of BMG matrix by addressing the initiation and propagation of shear bands (SBs) in BMG matrix; while a macroscopic phenomenological constitutive model was employed to describe the martensite transformation of TP and its correspondent deformation. Then the two models were implemented into ABAQUS as user material subroutines (UMATs). Finally, the numerically simulated stress-strain curves of BMGCs were compared with the experimental ones to verify the effectiveness of finite element model, and then some numerical analyses were performed to discuss the toughening mechanism of BMGCs caused by the martensite transformation of TP. It is found that the strain hardening and SBs' evolution in BMGCs were significantly influenced by the transformation strain, start stress and hardening modulus of martensite transformation. The excellent toughness of BMGCs mainly results from the phase transition from austenite to martensite phase in TPs.

Prediction of Shear Localisation in Granular Materials based on a Critical State Non-coaxial Model

Experimental evidence indicates that the shear localisation acts as a precursor to the failure in biaxial compression tests of granular materials. Once formed they are persistent and lead to progressive failure of most geotechnical structures. It is generally accepted that the primary mode of deformation within these shear bands is simple shear which is accompanied by rotation of principal axes. Hence, the conventional plasticity theories based on the assumption of coaxility is not sufficient to describe the behaviour within those shear bands. This paper highlights the influence of the non-coaxility on the initiation and orientation of shear bands in both drained and undrained sand. The con-coaxial plasticity theory is integrated into a critical state constitutive model enriched with the state parameter concept. The model is capable of taking account of the variation of lode angle under plane strain condition. Numerical plane strain biaxial compression tests are conducted to observe the effect of non-coaxility on shear localisation. Bifurcation criteria based on the acoustic tensor are checked to predict the onset and inclination of the shear band. Predictions from the non-coaxial model are compared with those of coaxial model. The influence of the initial void ratio for the formation of shear bands is explored. Results are compared qualitatively with experimental observations.

Analysis of mechanisms for chip formation simulation of hardened steel

The machining of hardened steel is characterized by a segmented chip formation. The segmentation is often referred to thermoplastic instabilities or material damage. In this paper the mechanisms for shear band initiation are analyzed and discussed through experimental and simulative investigations. The hardened low alloy steel 51CrV4+Q is machined in an orthogonal cutting process. The chips reveal signs of ductile damage and white layer shear bands. By Finite-Element chip formation simulations the conditions within the primary shear zone are analyzed and based on this an appropriate ductile fracture model for the simulation of shear bands and segmented chips is presented.

XFEM, strong discontinuities and second-order work in shear band modeling of saturated porous media

We investigate shear band initiation and propagation in fully saturated porous media by means of a combination of strong discontinuities (discontinuities in the displacement field) and XFEM. As a constitutive behavior of the solid phase, a Drucker–Prager model is used within a framework of non-associated plasticity to account for dilation of the sample. Strong discontinuities circumvent the difficulties which appear when trying to model shear band formation in the context of classical nonlinear continuum mechanics and when trying to resolve them with classical numerical methods like the finite element method. XFEM, on the other hand, is well suited to deal with problems where a discontinuity propagates, without the need of remeshing. The numerical results are confirmed by the application of Hill’s second-order work criterion which allows to evaluate the material point instability not only locally but also for the whole domain.