Plastic Shear Deformation Research Articles

Temperature effect on ideal shear strength of Al and Cu

According to Frenkel's estimation, at critical shear stress ${\ensuremath{\tau}}_{c}=G/2\ensuremath{\pi}$, where $G$ is the shear modulus, plastic deformation or fracture is initiated even in defect-free materials. In the past few decades it was realized that, if material strength is probed at the nanometer scale, it can be close to the theoretical limit, ${\ensuremath{\tau}}_{c}$. The weakening effect of the free surface and other factors has been discussed in the literature, but the effect of temperature on the ideal strength of metals has not been addressed thus far. In the present study, we perform molecular dynamics simulations to estimate the temperature effect on the ideal shear strength of two fcc metals, Al and Cu. Shear parallel to the close-packed (111) plane along the $[11\overline{2}]$ direction is studied at temperatures up to 800 K using embedded atom method potentials. At room temperature, the ideal shear strength of Al (Cu) is reduced by 25% (22%) compared to its value at 0 K. For both metals, the shear modulus, $G$, and the critical shear stress at which the stacking fault is formed, ${\ensuremath{\tau}}_{c}$, decrease almost linearly with increasing temperature. The ratio $G/{\ensuremath{\tau}}_{c}$ linearly increases with increasing temperature, meaning that ${\ensuremath{\tau}}_{c}$ decreases with temperature faster than $G$. Critical shear strain, ${\ensuremath{\gamma}}_{c}$, also decreases with temperature, but in a nonlinear fashion. The combination of parameters, $G{\ensuremath{\gamma}}_{c}/{\ensuremath{\tau}}_{c}$, introduced by Ogata et al. as a generalization of Frenkel's formula, was found to be almost independent of temperature. We also discuss the simulation cell size effect and compare our results with the results of $\mathit{ab}\phantom{\rule{4pt}{0ex}}\mathit{initio}$ calculations and experimental data.

The Role of Structure and Texture Factors in the Ductility and Deformability Improving of Magnesium Alloys Subjected to ECAP

Equal channel angular pressing (ECAP) in magnesium alloys due to the severe plastic shear deformations provides both grain refinement and the slope of the initial basal texture at 40-50 ° to the pressing direction. Such changes in microstructure and texture contribute to the improvement of low-temperature ductility and deformability of the alloys. On the basis of analysis of data from various researchers, the main structure and texture factors affecting on the ductility increase of the Mg-Al-Zn-Mn alloys were defined.

Elasto-Plastic Properties of Panel Zones and Effects on Story Drift of Steel Frames

Using the plastic shear deformation of panel zones (PZs) to increase the ductility and seismic energy dissipation of structures is a common practice in most countries specifications. This makes PZs generally yielding before columns and beams, and the shear deformation of PZs is relatively large. In order to explore the elasto-plastic properties of PZs, tests of different types of beam-column connections under monotonic loads were conducted firstly. Focus of tests is to explore the influence factors on elasto-plastic properties of PZs. Then based on test results and existing research data, simplified calculation methods of PZs strength and stiffness in the elastic and elasto-plastic stage were derived, and bilinear relationship expression of PZs moment-rotation was also given. Finally, the simplified calculation method for considering the effects of PZs elasto-plastic shear deformation on story drift of steel frames was presented.

Internal stresses induced by plastic shear deformation of Zr–(Cu,Ag)–Al bulk metallic glasses

Effective internal shear stress σi induced by torsional deformation of Zr46(Cu4/5Ag1/5)46Al8 and Zr46Cu46Al8 bulk metallic glasses different by the glass-forming ability of the maternal melts has been determined by measurements of stress relaxation upon stepwise unloading. It has been found that the ratio σi/σ0 (σ0 is the initially applied shear stress) decreases upon increasing the temperature from ≈0.8 at T=450K (T≈0.64×Tg) to ≈0.08 at T=638K (T≈0.91×Tg) independently of σ0 and glass composition. The obtained result is in good agreement with earlier data obtained on ribbon metallic glasses. The origin of deformation-induced internal stresses and their connection with deformation mechanisms of metallic glasses has been briefly discussed.

Interfacial microstructure and mechanical properties of Cu/Al clad sheet fabricated by asymmetrical roll bonding and annealing

Abstract The interfacial microstructure and mechanical properties of Cu/Al clad sheet fabricated by asymmetrical roll bonding and annealing were studied by scanning electron microscope equipped with energy dispersive X-ray detector, micro-hardness, tension and peeling test. The stress state of deformation zone was analyzed qualitatively to express the advantage of asymmetrical roll bonding. The results show that the asymmetrical roll bonding with larger mismatch speed ratio causes severe shear plastic deformation of component metals, and provides a good interface for atoms diffusion during annealing. The interfacial microstructure is improved and interfacial layer thickness increases. The roll bonding with larger speed ratio improves the ultimate tensile strength, elongation and peeling force of clad sheet, and reduces the micro-hardness of interfacial zone.

Soil-Structure Interaction and Failure of Cast-Iron Subway Tunnels Subjected to Medium Internal Blast Loading

The threat of terrorist attacks on subway systems using explosives has intensified considerably in recent years. Explosions inside subway tunnels may lead to the failure of subway structures and result in further socioeconomic losses. Specifically, the century-old, single-track, cast-iron subway tunnels in cities such as New York and London are very vulnerable to this type of attack. In this study, an explicit dynamic finite-element procedure was developed to carry out extensive numerical simulation to investigate the soil-structure interaction and failure of cast-iron tunnels in saturated soils subject to internal explosions using the medium amount of explosives (80 kg TNT) that might be perpetrated by terrorists. The stress path and damage mode of these tunnels, subjected to internal blast loading, were first investigated based on the simulated damage of cast-iron tunnel lining using a hardening elastoplastic model that considered shear damage. The study aims to better understand the soil-structure interaction and its relation to lining damage of cast-iron subway tunnels under internal blast and to investigate the influences of several critical parameters on lining failure, including compressibility of saturated soil, brittleness of lining materials, specific impulse of blast, and strength of the soil-lining interface. The study found that ground-tunnel interaction was one of the governing factors that determined the damage of tunnel lining under medium internal blast loading by providing the necessary confinement to resist internal blast loading and by absorbing blast energy with its plastic shear deformation. Lining damage was mainly triggered by the tensile hoop stress because of large inertia and dynamic forces in the radial direction of the tunnel, and it may exhibit a progressive pattern in the vibration phase. Soil compression significantly influenced the damage of the tunnel under internal blast loading. The damage was more severe with compressible saturated ground.

Plastic shear deformation of a thin strain-hardening disc: variational principles

An analytical solution is derived for the problem of shearing of a thin disc of incompressible rigid, plastic strain-hardening material. The solution is obtained by a methodology introduced by the author in previous publications, relying fundamentally on continuity of the rotation-rate vector field. A variational theorem is derived which shows that the velocity vector field is unique if the boundary values are properly prescribed. The derived solution is compared to experiments, with which it shows good agreement.

Adiabatic shear sensitivity of ductile metal based on gradient-dependent JOHNSON-COOK model

Based on the expression proposed by WANG for the local plastic shear deformation distribution in the adiabatic shear band (ASB) using gradient-dependent plasticity, the effects of 10 parameters on the adiabatic shear sensitivity were studied. The experimental data for a flow line in the ASB obtained by LIAO and DUFFY were fitted by use of the curve-fitting least squares method and the proposed expression. The critical plastic shear strains corresponding to the onset of the ASB for Ti-6Al-4V were assessed at different assigned ASB widths. It is found that the proposed expression describes well the non-linear deformation characteristics of the flow line in the ASB. Some parameters in the JOHNSON-COOK model are back-calculated using different critical plastic shear strains. The adiabatic shear sensitivity decreases as initial static yield stress, work to heat conversion factor and strain-rate parameter decrease, which is opposite to the effects of density, heat capacity, ambient temperature and strain-hardening exponent. The present model can predict the ASB width evolution process. The predicted ASB width decreases with straining until a stable value is reached. The famous model proposed by DODD and BAI only can predict a final stable value.

The Physically Nonlinear Analysis of Circular Plate on Deformable Foundation

A circular reinforced concrete plate on deformable foundation is investigated. The foundation is composed of three layers of soil which are defined as isotropically nonlinear materials with variable physical properties. The reinforced plate is treated as an elastic body. The plate is subjected to dissymmetrical half-circular loading of linearly variable pressure. Generally, the unified system “structure-foundation” is the contact analysis problem, which is modeled by ANSYS software. Plastic shear deformations are evaluated by solving the contact problem by the finite element method. The comparative analysis, including the treatment of physically linear system “structure-foundation”, is performed. The comparison based on linear analysis shows that rigidity of the foundation generates higher extreme stresses in the plate in terms of the Huber-Mises criterion. However, in nonlinear analysis, the intensity of stresses is naturally decreasing, when a more flexible model of the foundation is used.

Comparison of compressive deformation and fracture behaviors of Zr- and Ti-based metallic glasses with notches

Compressive tests on the Zr- and Ti-based metallic glasses with different notches were investigated to compare their shear fracture mechanism and plastic deformation abilities. It is found that the plasticity of the two metallic glasses can be improved by installing two semicircular symmetrical notches even for the Ti-based metallic glass which has nearly zero compressive plasticity. The enhanced plasticity may be ascribed to the easy initiation of shear bands (SBs) around the notches, and the consequent blocking effect of notches on the propagation of shear bands according to the large-scale stress gradient. Additionally, based on a theoretical model originated from the concept of critical steady shear displacement (CSSD), compared with the sizes of smooth regions on the fracture surface, the plasticity difference of the two different metallic glasses was analyzed quantitatively. The current findings might provide an approach to understand and estimate the difference in the plastic deformation abilities on diverse metallic glasses, as well as the ones with large-scale stress gradient.

Atomic-level structural modifications induced by severe plastic shear deformation in bulk metallic glasses

Structural evolution of an Au-based bulk metallic glass (BMG) after severe plastic deformation (SPD) was investigated. The newly formed glass contains high-density shear bands, a reduced ordering and a concomitant excess free volume. Moreover, it exhibits a less-changed local structure even in the supercooled liquid region, but a reduced thermal stability reflected in an accelerated crystal nucleation and growth process. These results suggest that SPD modifies the atomic structure of BMGs by localized shear band formation, thus producing so-called nanoglasses.

A mechanistic approach for determining plane-stress fracture toughness of polyethylene

A new mechanistic approach is used to characterize resistance of polyethylene to deformation and fracture in double-edge-notched tensile test. The new approach considers all three mechanisms involved in the fracture process, i.e. for fracture surface formation, shear plastic deformation, and necking, and can be used to determine values of specific energy consumption for each mechanism. This is different from the conventional approach, known as essential work of fracture (EWF), which does not consider the difference between shear plastic deformation and necking. Results from the new approach for a polyethylene copolymer show that specific energy density for fracture surface formation is about half of that determined from the EWF approach, and specific energy density for necking is very close to that determined from simple tensile test. The latter provides some support for validity of the new approach in characterizing fracture behaviour of polyethylene when accompanied by large deformation and necking. The paper also points out crack growth conditions that have to be met for valid application of the EWF approach and shows that such conditions are not met when deformation and necking occur in polyethylene.

Predicting concrete behaviour from quasi-static loading to hypervelocity impact

ABSTRACT This paper presents the main results obtained during a long and intensive research period on the modelling of the main physical mechanisms related to damage and fracture of concrete. The starting point has been the Mazars one scalar damage model (Mazars, 1984, 1986). On this base Pontiroli & Rouquand have proposed in 1995 a two scalar damage model (PRM model) which takes into account the crack closure mechanism, irreversible strains, strain rate effects and internal friction stresses. This model has been recently completed in order to take into account pore collapse phenomena, plastic shear deformations and the water content effect on the behaviour of concrete for confined loading. All these mechanisms allow the simulation of a large range of loadings from static to high velocity impacts. After a presentation of the most important features of the “PRM coupled model” several applications are given related to quasi static loading, blast or impact effects on concrete structures.

Elasto-plastic materials with lattice defects modeled by second order deformations with non-zero curvature

The paper deals with elasto-plastic materials, whose structural damage is produced by lattice defects, within the constitutive framework of finite elasto-plasticity with second order deformations. For second order deformations, which are represented by distortions and connections, we adopt the decomposition rule into second order elastic and plastic components. The split of the plastic connection into independent components, which involve Bilby’s connection type and a second order tensor, called disclination, plays a fundamental role in defining kinematical variables. The lattice defects are modeled by dislocations and disclinations, which are introduced within the finite deformation formalism and based on the differential geometry arguments. The constitutive framework is compatible with the free energy imbalance, postulated in the configuration with torsion, in such a form to include the presence of the disclinations. The model includes macro forces, which are power conjugated with the appropriate rate of the second order elastic deformations, and micro forces which are related with the plastic behaviour, as well as with the disclinations. The micro balance equations which are associated with the presence of disclinations are derived from a virtual power principle relative to the disclination mechanism. As a consequence of the principle of the free energy imbalance, the free energy function becomes potential for the macro forces. The viscoplastic type equations for micro forces are proposed in such a way to be compatible with the resulting dissipation inequality. Finally, we determine within the proposed constitutive framework, the disclinations as a solution of the micro balance equation associated with the disclination and satisfying the appropriate evolution equation. We suppose that the plastic simple shear deformation is the source for the evolution of disclination. After our knowledge this is a new framework for the mathematical description of disclinations.

Consolidation behavior of Mg–10Gd–2Y–0.5Zr chips during solid-state recycling

Unlike that of sintering of fine powders, chips are consolidated by hot deformation in the solid-state recycling. In this work, conventional extrusion (CE) and cyclic extrusion compression (CEC) are used to investigate single and multi-passes shear deformation on the consolidation of chips during solid-state recycling. The results show that utilization of Mg chips with smaller specific surface area (i.e. coarser powder) contributes to easier solid-state bonding because of the decrease in specific surface area which promotes suppression of oxide contamination in the recycled specimens. Enhanced consolidation of chips is not only ascribed to the physical mechanisms caused by plastic deformation, but also to atom diffusion between chips triggered by shear plastic deformation at elevated temperatures. Multi-passes shear deformation breaks the oxide films easier into small particles which are dispersed within the grains or at grain boundaries. It is postulated that the shear deformation of chips is responsible for the better consolidation of chips by high-temperature deformation.

A TEM Kikuchi pattern study of ECAP AA1200 via routes A, C, BC

Abstract Shear plastic deformation in ECAP occurs through the formation, movement and storage of dislocations. The differences which exist between shearing characteristics lead to important implications concerning the optimum processing route. It is known that the effectiveness of cell evolution into an array of high-angle boundaries (HABs) is in the order BC > C > A, or A > BC > C, depending on the two-channel-angle intersection of the ECAP die. In route A, large portions of HABs are continuously and progressively generated, while routes C and BC cause fully redundant deformation at each 2n passes, and each 4n passes, respectively. This study is focused on dislocation generation, storage and recombination during ECAP for routes A, C and BC. Kikuchi bands identified with TEM were used to quantitatively measure the cell and grain boundary misorientation. ECAP was performed on an AA1200 commercially pure aluminium alloy up to e = 8.64. A different hierarchy for HAB generation efficiency was found. Thermal stability was studied by annealing the alloy at 0.5, 0.6, 0.7 TM (where TM is the alloy melting point) for 2 h after the severe plastic deformation. It appeared that, even if route BC involves the fastest microstructure grain refining, route C is likely to be the most stable upon reheating.

On the failure of low carbon steel resistance spot welds in quasi-static tensile–shear loading

Failure behavior of low carbon steel resistance spot welds in quasi-static tensile–shear test is investigated. Microstructure, hardness profile and mechanical performance of the spot welds were studied. Results showed that spot welds are failed in two distinct failure modes: double-pullout and interfacial failure modes. There is a critical fusion zone size beyond which, pullout failure mode is guaranteed. Metallographic examination showed that failure is a competitive process between shear plastic deformation of weld nugget and necking of the base metal. In pullout failure mode, only the grain pattern of the base metal changes significantly and that of the fusion zone and heat affected zone remains unchanged. Strain localization was occurred in the base metal due to its low hardness. Moreover, the experimental results showed that increasing the holding time which increases the hardness of the fusion zone did not affect the peak load. It was concluded that in the pullout failure mode, the strength of the spot welds is not affected by the fusion zone strength. Fusion zone size proved to be the most important controlling factor for the spot welds’ mechanical performance in terms of peak load and energy absorption.

Deformation-induced crystallization in amorphous Al 85Ni 10La 5 alloy

Helium-atomized amorphous Al 85Ni 10La 5 (at.%) powder was investigated in the as-atomized state and after consolidation at room temperature using high pressure torsion. The samples were investigated by scanning electron microscopy, in situ and ex situ angle-dispersive X-ray diffractometry (XRD), ex situ energy-dispersive XRD, differential scanning calorimetry, and inductively coupled plasma mass spectroscopy. The as-atomized powder shows a glass transition and subsequent crystallization of fcc-Al and intermetallic phases within a narrow temperature range upon continuous heating. Primary crystallization of fcc-Al occurs during isothermal heating below the crystallization temperature. After consolidation, no glass transition occurs upon continuous heating. XRD of consolidated disc-shape samples reveal fcc-Al crystals within the residual amorphous matrix. The amount of fcc-Al crystals increases with plastic shear deformation along the disc radius and with rotation angle. The results indicate that the primary precipitation of fcc-Al is strain-induced and possibly athermal.

Relaxation Behavior of Clamping Force in Bolted Joints of Pultruded GFRP Laminates

The relaxation behavior of clamping force in friction-type FRP bolted joints is one of the significant subjects in application of FRP to structures. The aim of this paper is to obtain the factors of relaxation behaviors in the GFRP bolted joints based on the experimental results. The relaxation behaviors are due to both steel bolt and GFRP laminate. Steel bolt dominates the relaxed force immediately after initial loading but not reloading. The relaxation factors of GFRP laminate except matrix polymer could be both laminate surface geometry and laminate bond, considering the variance of relaxed force and more relaxed force for bonded laminate than single laminate observed in the experimental results. Boltzmann superposition principle can be hardly applicable to the relaxed forces for initial loading and reloading, suggesting that less relaxed force for reloading could be obtained by the strain hardening of bolt threads through the decrease of cross section area due to shear plastic deformation and that surface geometry could be modified by plastic and irreversible viscous deformation considering the viscoplastic property of matrix polymer. More relaxed force could be obtained for the surface geometry farther from the flat surface and the surface geometries could be categorized into the surface with either equal height protuberances or random height ones. Laminate bond dominates the relaxed force and its lower viscosity could give relaxed force more. The viscosity of laminate bond could be more variable than matrix polymer and increase through the modification of structure by plastic and irreversible viscous deformation. The minimum elapsed time could be required to complete the relaxation behavior for the flat surface and the bond viscosity as high as matrix polymer.

A calculation of the redundant deformation factor in the wiredrawing of non-ferrous metals

Axisymmetric wiredrawing in a conical die channel, including localized plastic shear and tension deformations, leads to deformation heterogeneity in both longitudinal and transverse sections of a treated metal. The redundant shear deformation factor is used to analyze it; this factor is determined analytically or on the basis of experimental measurements of the microhardness distribution over the sections of wires or rods of nonferrous metals of large diameters. Based on the Avitzur solution [3] obtained by the upper-bound method using kinematically possible trapezoidal and spherical velocity fields, the expression is derived to calculate the redundant shear deformation factor. The effect of the geometric parameter of the deformation region on the strain heterogeneity in the course of wiredrawing is analyzed. The calculated and experimental data on wiredrawing of copper are compared.