Development Of Shear Bands Research Articles

Simulation of T-bar penetration in soft clay with adhesive contact

To model the deep penetration process of T-bar in soft clay, an adhesive contact algorithm was developed in conjunction with the Coupled Eulerian–Lagrangian approach with consideration of the effect of strain softening. Numerical results show the simulated penetration resistance agrees well with the previous centrifuge experimental data. The failure mechanisms of the clay around the T-bar can be divided into three stages, including shallow penetration stage with global failure mechanism, partially and fully back-flow stages with local failure mechanism. Fluctuations of the penetration resistance can be explained by the formation and evolution of shear bands around the T-bar. Newly formed shear bands would intersect the previously formed shear bands in the partially back-flow stage, which results in the formation of “ear-shaped” areas rotating anticlockwise around the T-bar. The evolution of shear bands would form a similar fabric structure in the fully back-flow stage.

Experimental investigation of shear band and shear strain field evolution during blanking of AA6082 sheet

Interrupted blanking experiments were performed to study the deformation behavior of AA6082 (T6) sheets. An intentional asymmetry was introduced by having the punch and the die of different edge radii. High-resolution microstructural montages of sheet cross-section were used to experimentally measure the shear strain field during each interrupted blanking experiment. This provided a comprehensive experimentally measured spatial–temporal shear strain field for the blanking process. The shear strain was found to be high and localized at the top and bottom surfaces of the sheet which was in contact with the punch and the die. The shear strain was observed to monotonically reduce and get diffused at the interior of the sheet. The strain distribution was also calculated by finite element simulations and found to be in good agreement with the experimentally measured strain distributions. However, the peak strains predicted by the finite element simulations were always marginally lower than those observed by the experimental observations.

The evolution of strain pattern induced by banded structure under uniaxial tension in low-carbon microalloyed steel

The presence of banding inhomogeneities is detrimental to the mechanical behaviors of steels due to the strain anisotropy. In this paper, combined with the in-situ tensile stage, digital image correlation (DIC) method was used to investigate the influence of banding pattern including the size, quantity and morphology on the continuous localized deformation and fracture behavior of low-carbon microalloyed steel under the uniaxial tension load. A zero-deformation test was introduced at the beginning of the experiment, where the accuracy and stability of DIC for this application were validated. The results show that the effect of banded structure on local deformation and damage evolution is significant. Compared to the rolling direction (RD) tensile, the local deformation of banded structure exhibits a higher sensitivity to the transverse direction (TD), and the development of shear bands is clearly affected by the local high strains caused by the deformation of soft banding. The coarse banding (CB) in specimen could decrease the mechanical properties of material. Meanwhile, the mechanical anisotropy is explained by the evolution model of local deformation. This work is helpful to understand the effect of banded structure on the mechanical behavior of steel.

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.

Al‐Based Amorphous Metallic Plastics

A new class of Al‐based metallic glasses is identified, which exhibits polymer‐like thermoplastic formability near the boiling point of water. To satisfy the requirements for thermoplastic behavior, a series of AlSm‐based metallic glasses are examined by Flash DSC to determine the glass transition temperature, Tg. Viscosity measurements confirm that the Tg values of the AlSm‐based glasses are below 100 °C. Bending and imprinting trials in boiling water demonstrate permanent deformation without the development of shear bands as required for thermoplastic behavior. Comparison of the thermoplastic formability index values indicates that the AlSm‐based glasses exhibit behavior similar to that for common Fe‐based metallic glasses. Kinetic measurements are used to evaluate the lifetime before the onset of crystallization. The AlSm‐based glasses have a significant potential in the areas of electromechanical systems, micro‐scale biomedical parts, nanorobots, and micromachines.

Microstructural and textural evolutions in multilayered Ti/Cu composites processed by accumulative roll bonding

Ti/Cu multilayered composites were fabricated via accumulative roll bonding (ARB). During co-deformation of the constituent metals, the hard Ti layers necked preferentially and then fragmented with the development of shear bands. Transmission electron microscopy showed that with increasing ARB cycles, grains in Ti were significantly refined even though dynamic recrystallization has occurred. For Cu the significant grain refinement was only found within the shear banded region when the composite was processed after five ARB cycles. Due to the diffusion of Cu atoms into Ti at the heterophase interfaces, amorphization with a width less than 10 nm was identified even in the composite processed by one cycle. At higher ARB cycles, the width of amorphous region increased and intermetallic compounds CuTi appeared from the region. The lattice defects introduced at the heterophase interfaces under roll bonding was responsible for the formation of the nano-scaled compounds. X-ray diffraction showed that an abnormal {112¯0} fiber texture was developed in Ti layers, while significant brass-type textures were developed in Cu layers. Some orientations along the {112¯0} fiber favored the prismatic < a> slip for Ti. Tensile tests revealed the elevated strength without a substantial sacrifice of ductility in the composites during ARB. The unique mechanical properties were attributed to the significantly refined grains in individual metals, the good bonding between the constituent metals, as well as the development of an abnormal {112¯0} fiber texture in Ti layers.

Soil Particle Movement and Shear Band Development during Plane Strain Compression

Most geotechnical structures failed by formation and development of shear bands in soils. Thus, shear deformation and shear bands development evaluation are necessary to understand shear failure mechanism. During shearing, deformation behaviour analysis for soil particles within entire soil specimen are evaluated to understand the soil behaviour and shear strength characteristics. In this paper, a series of plane strain compression tests using Nevada sand and Ottawa sand were conducted to identify the shear strain and shear failure mechanism. With the results of plane strain compression tests, image analyses using Particle Image Velocimetry (PIV) were carried out in order to measure the change in position of soil particles and shear bands development. Deformation vectors and contours were constructed to see the entire deformation mechanism in the soil specimen. During shearing, shear band was identified after peak stress and most visually distinctive at residual state. However, shear band started to develop invisibly immediately after starting loading and this invisible development was able to be observed by horizontal and vertical movement analyses of PIV. Soil particles moved actively in horizontal and vertical direction to generate shear band in the beginning of shearing. After development of shear band, soil particles moved along the shear band.

Numerical Investigation of Orthogonal Cutting Processes with Tool Vibration of Ti6Al4V Alloy

This article investigates the vibration cutting process of Ti6Al4V alloy numerically and theoretically. The Coupling Eulerian-Lagrangian finite element models with one-tool and double-tool are established, to simulate the cutting processes with tool forced vibration and self-exited vibration respectively. It is shown that low-frequency forced vibration aggravates the periodic shear banding instability and increases the cutting force amplitude, whereas high-frequency forced vibration can improve the machined quality. Furthermore, the self-exited vibration due to fluctuating cutting thickness with low frequency promotes the shear banding evolution in chip. The self-exited vibration stability limit is found dependent on the frictional behavior, penetration resistance and the inherent vibration sources of tool-workpiece system. These simulation results show good agreement with theoretical models, which provide practical guidelines for improving vibration machining.

Investigation on chip formation and surface morphology in orthogonal machining of Zr-based bulk metallic glass

This work presents the preliminary investigation on the orthogonal machining of Zr-based bulk metallic glass. Influence of machining parameters on the chip formation mechanisms has been analyzed. The SEM images of the chip surfaces have been taken for this purpose. It has been observed that at lower speed and uncut chip thickness, the individual serrated sections of chips develop shear bands in the surface. At higher speed and uncut chip thickness, these shear bands aggravate and leads to form fragmented surfaces. Degree of segmentation and serration spacing of chips were analyzed and compared with the morphology of the machined surface.

Simulations of Fine-Meshed Biaxial Tests with Barodesy

Recent experimental studies showed that shear band development starts at the beginning of triaxial tests. In experimental testing, it is impossible to obtain a soil sample with a homogeneous void ratio. Therefore, a homogeneous deformation, i.e., an element test, is questionable well before the peak. In this article we carry out finite element simulations of fine-meshed biaxial tests with the constitutive model barodesy, where the stress rate is formulated as a function of stress, stretching and void ratio. The initial void ratio in the simulations is normally distributed over all elements in a narrow range. In this article, we evaluate the pre-peak shear band development. We further compare stress paths and stress-strain curves of the biaxial test of relevant elements (e.g., in- and outside the shear band) with the results of the average response of all elements. We show how the response in an element test differs from the average response of the fine-meshed test. We present the resulting potential for understanding (early) shear band development and stress-strain behaviour in a biaxial test: The inhomogeneous void ratio distribution in a sample favours early shear band development. This effect is modelled with barodesy. The obtained stress paths and stress-strain curves show that the maximum deviatoric stress is higher in the element test than it is in the average response of the fine-meshed test.

Molecular Dynamics Simulation of Structural Signals of Shear-Band Formation in Zr46Cu46Al₈ Metallic Glasses.

The evolution from initiation to formation of a shear band in Zr46Cu46Al8 metallic glasses is presented via molecular dynamics simulation. The increase in number and the decrease in average size of clusters with the quasi-nearest atoms being 0 correspond to the shear-band evolution from initiation to formation. When the shear band is completely formed, the distribution of the bond orientational order q6 reaches a minimum. The maximum of the number of the polyhedral loss of Cu-centered <0, 0, 12, 0> and the minimum of the number of the polyhedral loss of Zr-centered <0, 2, 8, 5> correspond to the shear-band formation. These findings provide a strong foundation for characterizing the evolution from initiation to formation of shear bands.

Lateral resistance of pipes and strip anchors buried in dense sand

The response of buried pipes and vertical strip anchors in dense sand under lateral loading is compared based on finite-element (FE) modeling. Incorporating strain-softening behaviour of dense sand, the progressive development of shear bands and the mobilization of friction and dilation angles along the shear bands are examined, which can explain the variation of peak and post-peak resistances for anchors and pipes. The normalized peak resistance increases with embedment ratio and remains almost constant at large burial depths. When the height of an anchor is equal to the diameter of the pipe, the anchor gives approximately 10% higher peak resistance than that of the pipe. The transition from the shallow to deep failure mechanisms occurs at a larger embedment ratio for anchors than pipes. A simplified method is proposed to estimate the lateral resistance at the peak and also after softening at large displacements.

Micro Shear Bands: Precursor for Strain Localization in Sheared Granular Materials

AbstractRecent studies have shown that detection of the onset and evolution of micro shear bands (MSBs) in granular materials can be improved using measurements of the kinematic behavior of particl...

A common mechanism for evolution of single shear bands in large-strain deformation of metals

ABSTRACTShear banding, a type of inhomogeneous plastic flow involving very large local strains, occurs in a variety of material systems. We study dynamics of evolution of single shear bands at strain rates of up to per second in three different polycrystalline metal systems, using a special shear deformation framework and a micro-marker technique calibrated to track localised deformation fields at micrometer resolution. Once a band is nucleated as a weak interface, localised plastic flow occurs via Bingham-type viscous sliding between material segments on either side of the interface. As a result, the evolution and magnitude of strains and material displacements in the band vicinity are well-described by a model based on momentum diffusion. The viscosity at the band interface is very small, only a few mPa·sec, and is comparable to those of liquid metals at their melting point. Based on analysis of various contributions to band viscosity at the microscopic level, a plausible explanation based on phonon drag on dislocation motion is presented for the small viscosity. The accuracy of predictions made by the momentum diffusion model for different materials and deformation rates suggests that once nucleated, a shear band evolves by a common mechanism that is relatively insensitive to microstructure details.

Investigations of vibration cutting mechanisms of Ti6Al4V alloy

This work involves systematical study of high-speed vibration cutting process of Ti6Al4V on numerical and theoretical aspects for the first time. In numerical simulations, the one-tool and double-tool cutting models are established based on the coupling Eulerian-Lagrangian (CEL) finite element (FE) method, to simulate forced vibration (FV) and self-excited vibration (SEV) cutting phenomena respectively. In theoretical analysis, linear perturbation method is used to analyze the critical condition of shear localized instability of chip material in the FV cutting process, and stability limits analysis is performed to study the tool vibration stability in the SEV cutting process, which consider coupled effects of wavy cutting thickness and periodic instability of shear bands. Different from vibration assisted machining in low-speed cutting, it is found FV with attainable frequency in industry promotes the evolution of shear bands, increase the cutting force and reduce the machined quality, whereas high-frequency FV can help improve the cutting process. On the other hand, SEV with smooth cutting thickness is found an effective strategy to weaken the evolution of shear bands and decrease cutting force in the high-speed cutting. The stability limit of SEV is related to the friction damping coefficient at the rack face, the penetration damping resistance, the ratio of the oscillation frequency of top wavy surface and the instability frequency of shear bands. These findings would help deepen the understanding towards the vibration effects in metal cutting and provide practical guidance to retrain and utilize vibration in the vibration assisted machining.

Strengthening mechanisms in nanoporous metallic glasses

It has been shown that metallic glasses (MGs) can have enhanced ductility by properly introduced pores. However, the pore effects on the yield strength and the underlying mechanisms have yet to be fully clarified. In this paper, nanoporous MGs with regularly distributed elliptical pores are investigated by large scale molecular dynamics simulations. Obtained results show that, when increasing the semi-axis a in the loading direction and fixing the other semi-axis b of the pores, the strength of the nanoporous MGs increase firstly to reach a maximum value and then decreases. It is found that the porosity and pore shape are the main factors affecting the strength. For a given porosity, the yield strength increases first and then approaches an asymptotic value with increasing a. The underlying mechanism is that the shear bands transit from localization to spreading. For a given pore shape, the yield stress decrease with increasing a. And this is because the increase of porosity leads to reduction in strength. Due to the above mechanisms, there exists a maximum strength at a critical size where the shear band evolution transition occurs. A model for the critical size of acri is proposed to predict the maximum strength.

Ultrahigh-strength titanium gyroid scaffolds manufactured by selective laser melting (SLM) for bone implant applications

Commercially pure titanium (CP–Ti) gyroid scaffolds with interconnected pores and high porosities in the range of 68–73% and three different unit cell sizes of 2, 2.5, and 3 mm were manufactured by selective laser melting (SLM) for bone implant applications. The microstructure and mechanical properties of the scaffolds with different unit cell sizes and sample orientations were evaluated. The microstructure of as-built struts was dominated by massive martensite and the average microhardness of the struts was 2.27 GPa, which is ∼50% higher than that of dense cast CP–Ti. The elastic modulus and yield strength of the as-built scaffolds ranged from 1465 to 2676 MPa and from 44.9 to 56.5 MPa, respectively, values which are close to the elastic modulus of trabecular bone and presumably strong enough to bear the physiological loading of implants. The as-built scaffolds exhibited excellent ductility up to 50% strain and no sign of fracture up to 20–30% strain under compression. The dominant compressive response of the scaffolds was observed by formation of a plastic hinge which led to rotation of the struts about the plastic hinges followed by development of local shear bands in struts in the long plateau region. These SLM-manufactured gyroid CP–Ti scaffolds with significantly enhanced hardness and compressive strength exhibited an elastic modulus close to that of trabecular bone and offer a promising improvement on CP–Ti scaffolds for bone implant applications.

Energy absorption characteristics of metallic triply periodic minimal surface sheet structures under compressive loading

Designing metallic cellular structures with triply periodic minimal surface (TPMS) sheet cores is a novel approach for lightweight and multi-functional structural applications. Different from current honeycombs and lattices, TPMS sheet structures are composed of continuous and smooth shells, allowing for large surface areas and continuous internal channels. In this paper, we investigate the mechanical properties and energy absorption abilities of three types of TPMS sheet structures (Primitive, Diamond, and Gyroid) fabricated by selective laser melting (SLM) with 316 L stainless steel under compression loading and classify their failure mechanisms and printing accuracy with the help of numerical analysis. Experimental results reveal the superior stiffness, plateau stress and energy absorption ability of TPMS sheet structures compared to body-centred cubic lattices, with Diamond-type sheet structures performing best. Nonlinear finite element simulation results also show that Diamond and Gyroid sheet structures display relatively uniform stress distributions across all lattice cells under compression, leading to stable collapse mechanisms and desired energy absorption performance. In contrast, Primitive-type structures display rapid diagonal shear band development followed by localized wall buckling. Lastly, an energy absorption diagram is developed to facilitate a systematic way to select optimal densities of TPMS structures for energy absorbing applications.

Formation mechanism and characterization of shear band in high-speed cutting Inconel718

Nickel-based alloy Inconel718 is one kind of complex alloy with multiple components. There is complex cutting deformation (shear localization of material) in high-speed cutting process. The chip shows a serrated shape. Through analyzing the chip morphology, the change of microstructure and development of shear band is analyzed, and the formation mechanism of the shear band is revealed. Based on the formation mechanism of shear band, the coupled elastoplastic-damage constitutive model is proposed to describe the shear stress during the cutting process. The shear band width has been determined by micrographic observations and theoretical model calculation. The experimental and theoretical results show that the shear band width in chip formation decreases linearly with an increase in the cutting speed.

Grain boundary sliding and rotational mechanisms of intragranular deformation at different creep stages of high-purity aluminum polycrystals at various temperatures and stresses

In the paper we show that grain boundary sliding dominates in the creep of A999 aluminum polycrystals and is accommodated by rotation modes of intragranular deformation. Accommodation mechanisms strongly depend on the applied stress σ and creep temperature T. At low σ and T, accommodation occurs by dislocation glide and mesoscale fragmentation. At high σ and T, there appears lattice curvature, which causes the development of shear bands, multiscale fragmentation, and formation of a quasi-neck with nanosized subgrains and plastic microrotations. Noncrystallographic shears in their subboundaries propagate in local lattice curvature zones under τmax by the plastic distortion mechanism.