The mechanisms of plastic strain accommodation during the high strain rate collapse of corrugated Ni–Al laminate cylinders

The Thick-Walled Cylinder method was used on corrugated Ni–Al reactive laminates to examine how their mesostructures accommodate large strain, high strain rate plastic deformation and to examine the potential for intermetallic reaction initiation due to mechanical stimuli. Three main mesoscale mechanisms of large plastic strain accommodation were observed in addition to the bulk distributed uniform plastic flow: (a) the extrusion of wedge-shaped regions into the interior of the cylinder along planes of easy slip provided by angled layers, (b) the development of trans-layer shear bands in the layers with orientation close to radial and (c) the cooperative buckling of neighbouring layers perpendicular to the radius. These mesoscale mechanisms acted to block the development of periodic patterns of multiple, uniformly distributed, shear bands that have been observed in all previously examined solid homogeneous materials and granular materials. The high-strain plastic flow within the shear bands resulted in the dramatic elongation and fragmentation of Ni and Al layers. The quenched reaction between Al and Ni was observed inside these trans-layer shear bands and in a number of the interfacial extruded wedge-shaped regions. The reaction initiated in these spots did not ignite the bulk of the material, demonstrating that these mesostructured Ni-Al laminates are able to withstand high-strain, high-strain rate deformation without reaction. Numerical simulations of the explosively collapsed samples were performed using the digitized geometry of corrugated laminates and predictions of the final, deformed mesostructures agree with the observed deformation patterns.     

An elastic,plastic, viscous model for slow shear of a liquid foam

We suggest a scalar model for deformation and flow of an amorphous material such as a foam or an emulsion. To describe elastic, plastic and viscous behaviours, we use three scalar variables: elastic deformation, plastic deformation rate and total deformation rate; and three material-specific parameters: shear modulus, yield deformation and viscosity. We obtain equations valid for different types of deformations and flows slower than the relaxation rate towards mechanical equilibrium. In particular, they are valid both in transient or steady flow regimes, even at large elastic deformation. We discuss why viscosity can be relevant even in this slow shear (often called “quasi-static”) limit. Predictions of the storage and loss moduli agree with the experimental literature, and explain with simple arguments the non-linear large amplitude trends.     

微扩张管道内幂律流体非定常电渗流动

研究了电渗驱动下幂律流体在有限长微扩张管道内非稳态流动特性.基于Ostwald-de Wael幂律模型,采用高精度紧致差分离散二维Poisson-Nernst-Planck方程及修正的Cauchy动量方程,数值模拟了初始及稳态时刻微扩张管道内幂律流体电渗流流场分布情况,研究了管道截面改变对幂律流体无量纲剪切应变率及无量纲表观黏度的影响,以及无量纲表观黏度对拟塑性流体与胀流型流体流速分布的影响.数值模拟结果显示,当扩张角和无量纲电动宽度一定时,电场驱动下的幂律流体在近壁区域速度响应都很快;初始时刻,近壁处表观黏度的变化受到剪切应变率变化的影响,从而影响了三种幂律流体速度峰值的分布,出现拟塑性流体流速在扩张段上游及扩张段近壁处速度峰值均为幂律流体中最大、而在扩张段下游三种幂律流体速度峰值相近的现象;稳态时刻,幂律流体速度剖面呈现塞型分布,且满足连续性条件下,幂律流体流速随扩张管半径增大而减小,牛顿流体流动规律与宏观尺度下流动规律相同;初始时刻,在相同电动宽度、不同壁面电势作用下,幂律流体在扩张管近壁处剪切应变率分布的差异导致表观黏度分布的差异,并最终导致拟塑性流体与胀流型流体流速分布的差异.     

Activation energy of shear transformation zones: a key for understanding rheology of glasses and liquids

Key manifestations of the glassy and liquid states, such as viscous flow and structural relaxation, occur spatial and temporal heterogeneously, within highly localized rare events, termed shear transformation zones. Characterization of these basic entities with respect to thermal activation and mechanical response is vital for understanding the rheology of glasses across length scales. This is achieved in classical molecular dynamics computer simulations on the model glass, CuTi, by determining the activation energy barrier and plastic yield strain of individual shear transformation zones as a function of size and external stress loading. Sizes of approximately equal to 140 atoms are identified to be especially energetically favorable with an activation energy barrier of approximately equal to 0.35 eV. Using these parameters, a rheology model is proposed to quantitatively explain viscosity.     

高压、高应变率与低压、高应变率实验的本构关联性

 指出Johnson-Cook（J-C）、Zerilli-Armstrong（Z-A）、Bodner-Parton（B-P）本构方程在一定条件下的适用性,表明对于低压、高应变率实验,单一曲线假定似乎可以采用。通过等效应力、等效应变,可以将不同应力状态下的流动应力函数采用统一的方程描述。然而,这些本构方程的确立,并不包括平面冲击波实验。对适合于平面冲击波实验的Steinberg-Cochran-Guinan（SCG）本构方程,讨论了其方程中所包含的高压与高应变率耦合效应。指出,以剪切模量度量的流动应力具有应变率相关性。基于温度效应的新发现以及直接测量平面冲击波流动应力的新进展,分别用J-C本构及SCG本构方程估算了钨材料在高压、高应变率加载下的流动应力。结果表明,采用J-C本构估算的流动应力仅在压力为10 GPa以下才能与实验数据相近,当压力高于10 GPa时,流动应力只能采用SCG本构估算。也指出了高压、高应变率本构方程与低压、高应变率本构方程所对应的不同物理背景。     

Instability of plastic flow in the microstructure of alkali halide crystals

Spatial localization of deformation bands in LiF and KCl single crystals caused by instability of plastic flow in the strain rate range from 5 × 10^?6 to 2 × 10^?4 s^?1 was studied experimentally. The geometrical parameters of localized shift bands (LSB) were studied as a function of strain rate and temperature. To study the LSB relief, a surface profilometry technique was used for the first time, which made it possible to determine the LSB parameters at the early stages of plastic flow (for strains in the range from 0.5 to 2%). The formation and branching of LSB steps on the surface of a deformed crystal due to the generation and motion of dislocations were found to be scaled. It was shown experimentally that the LSB formation is a thermally activated process that occurs through dislocation glide and is limited by dislocation creep.     

Effect of Hydrogen Charging on Mechanical Twinning,Strain Hardening,and Fracture of ‹111› and ‹144› Hadfield Steel Single Crystals

This paper studies the effect of electrolytic hydrogen charging on the plastic deformation and fracture of Hadfield steel single crystals oriented for tension along the ?111? and ?144? directions, which the major deformation mechanism is mechanical twinning. Electrolytic hydrogen charging for five hours at a current density of 100 A/m² slightly affects the stages of plastic flow, deformation mechanism, and the value of uniform elongation of ?111? and ?144? single clreystals. Hydrogen saturation causes shear microlocalization and a decrease of the strain hardening coefficient in twinning in one system, but slightly affects the strain hardening characteristics in multiple twinning. Hydrogen charging increases the fraction of the brittle component on fracture surfaces and leads to microand macrocracking near the fracture zone on the lateral surface of deformed specimens. It has been found experimentally that the stress relaxation rate in loaded ?111? single clreystals after hydrogen saturation decreases. Mechanisms of describing this phenomenon have been proposed.     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

Plasticity and dynamical heterogeneity in driven glassy materials

Many amorphous glassy materials exhibit complex spatio-temporal mechanical response and rheology, characterized by an intermittent stress strain response and a fluctuating velocity profile. Under quasistatic and athermal deformation protocols this heterogeneous plastic flow was shown to be composed of plastic events of various sizes, ranging from local quadrupolar plastic rearrangements to system spanning shear bands. In this paper, through numerical study of a 2D Lennard-Jones amorphous solid, we generalize the study of the heterogeneous dynamics of glassy materials to the finite shear rate ( [(g)\dot] \dot{{\gamma}} 1 \neq 0 and temperature case (T 1 \neq 0 . In practice, we choose an effectively athermal limit (T ∼ 0 and focus on the influence of shear rate on the rheology of the glass. In line with previous works we find that the model Lennard-Jones glass follows the rheological behavior of a yield stress fluid with a Herschel-Bulkley response of the form, s \sigma = s_Y \sigma_{{Y}}^{} + c ₁ [(g)\dot]^b \dot{{\gamma}}^{{\beta}}_{} . The global mechanical response obtained through the use of Molecular Dynamics is shown to converge in the limit [(g)\dot] \dot{{\gamma}} ? \rightarrow 0 to the quasistatic limit obtained with an energy minimization protocol. The detailed analysis of the plastic deformation at different shear rates shows that the glass follows different flow regimes. At sufficiently low shear rates the mechanical response reaches a shear-rate-independent regime that exhibits all the characteristics of the quasistatic response (finite-size effects, cascades of plastic rearrangements, yield stress, ...). At intermediate shear rates the rheological properties are determined by the externally applied shear rate and the response deviates from the quasistatic limit. Finally at higher shear the system reaches a shear-rate-independent homogeneous regime. The existence of these three regimes is also confirmed by the detailed analysis of the atomic motion. The computation of the four-point correlation function shows that the transition from the shear-rate-dominated to the quasistatic regime is accompanied by the growth of a dynamical cooperativity length scale x \xi that is shown to diverge with shear rate as x \xi μ \propto [(g)\dot]^-n \dot{{\gamma}}^{{-\nu}}_{} , with n \nu ∼ 0.2 -0.3. This scaling is compared with the prediction of a simple model that assumes the diffusive propagation of plastic events.     

Localization of plastic strain under shock-wave loading of titanium alloy with a crack

The effect of shock-wave loading with an amplitude of 4 GPa and a duration of 10 μs on the localization of plastic flow and the dynamic fracture of VT-1 cast titanium alloy containing a disk-shaped crack is considered. It is found that the crack size decreases in the direction of shock wave propagation, the dislocation density grows and adiabatic shear bands appear near the crack, and secondary cracks originate in the areas of localized flow near the crack edges. The strain, strain rate, dislocation density, rate of dislocation generation, and dislocation velocity near and away from the healing crack are estimated.     

Prediction of flow stress and textures of AZ31 magnesium alloy at elevated temperature

The viscoplastic behaviour of magnesium alloys at high temperatures leads to highly temperature-dependent mechanical properties. While at high strain rates a notable strain hardening response is observed, at low strain rates the material shows a smooth plastic response with negligible amount of hardening. This complicated behaviour is due to different deformation mechanisms that are active at different strain rate regimes, resulting in different strain rate sensitivity parameters. In this study we show, by utilizing both numerical simulations and experiments, that this behaviour can be predicted by a model that combines two deformation mechanisms, grain boundary sliding mechanism and dislocation glide mechanism. We discuss the importance of each deformation mechanism at different strain rate regimes based on the findings of modelling and experimental results for AZ3 magnesium alloy. By developing a model that includes the above-mentioned two deformation mechanism, the prediction of flow properties is expanded to a wide range of strain rate regimes compared to previous study. The obtained numerical findings for the stress–strain behaviour as well as texture evolution show good agreement with the experimental results.     

Computational mesomechanics of titanium surface-hardened by ultrasonic treatment

This paper studies plastic strain localization and stress-strain evolution in commercial titanium specimens with an ultrasonically treated surface. A dynamic plane strain boundary-value problem is numerically solved by the finite difference method. The microstructure and mechanical properties of the composition are specified in the calculations based on microhardness measurements, mechanical tensile tests, and metallographic studies. The dependences of the plastic flow localization characteristics on the geometry and mechanical properties of ultrasonically treated surface layers have been established. Plastic strain localization is found to depend on the geometry and mechanical properties of ultrasonically treated surface layers.     

Modeling microstructure evolution of binary systems subjected to irradiation and mechanical loading

We study a change in mechanical properties of binary systems subjected to irradiation influence described by ballistic flux of atomic mixing having regular and stochastic contributions. By using numerical modeling based on the phase field approach we study dynamics of deformation fields in a previously irradiated system and in the binary system deformed during irradiation. An influence of both deterministic and stochastic components of ballistic flux onto both yield strength and ultimate strength is studied. We have found that degradation of mechanical properties relates to the formation of percolating clusters of shear bands. Considering a hardening coefficient we analyze stages of plastic deformation of both initially irradiated alloy and alloy subjected to sustained irradiation. Stability of binary alloy under mechanical loading in the form of shear strain with a constant rate and cyclic deformation is discussed.     

Molecular dynamics study of coupled layer thickness and strain rate effect on tensile behaviors of Ti/Ni multilayered nanowires

Novel properties and applications of multilayered nanowires(MNWs) urge researchers to understand their mechanical behaviors comprehensively.Using the molecular dynamic simulation,tensile behaviors of Ti/Ni MNWs are investigated under a series of layer thickness values(1.31,2.34,and 7.17 nm) and strain rates(1.0 × 10~8 s~(-1) ≤ε≤5.0 × 10~(10) s~(-1)).The results demonstrate that deformation mechanisms of isopachous Ti/Ni MNWs are determined by the layer thickness and strain rate.Four distinct strain rate regions in the tensile process can be discovered,which are small,intermediate,critical,and large strain rate regions.As the strain rate increases,the initial plastic behaviors transform from interface shear(the shortest sample) and grain reorientation(the longest sample) in small strain rate region to amorphization of crystalline structures(all samples) in large strain rate region.Microstructure evolutions reveal that the disparate tensile behaviors are ascribed to the atomic fractions of different structures in small strain rate region,and only related to collapse of crystalline atoms in high strain rate region.A layer thickness-strain rate-dependent mechanism diagram is given to illustrate the couple effect on the plastic deformation mechanisms of the isopachous nanowires.The results also indicate that the modulation ratio significantly affects the tensile properties of unequal Ti/Ni MNWs,but barely affect the plastic deformation mechanisms of the materials.The observations from this work will promote theoretical researches and practical applications of Ti/Ni MNWs.     

Strain Localization in Titanium with a Modified Surface Layer

Plastic flow localization in commercially pure titanium (VT1-0 according to the Russian classification) with the surface modified by low-energy high current electron beams has been numerically studied. The structure and mechanical properties of the modified surface layer and titanium substrate correspond to the experimentally observed ones and are taken into account explicitly as initial data of a dynamic boundary value problem. The tension of titanium structures with a modified surface layer is simulated by the finite difference method in a plane strain formulation. The dependence of the plastic strain localization parameters on the mechanical properties of structural elements in the titanium substrate has been determined.     

The influence of a static magnetic field up to 15 T on the manifestation of the Portevin-Le Chatelier effect in NaCl: Eu crystals

The influence of a static magnetic field on the instability of plastic flow (the Portevin-Le Chatelier effect) is revealed in NaCl: Eu quenched crystals. It is found that, in an external magnetic field, the yield stress of the crystals is reduced, the probability of plastic strain jumps and their amplitude decrease, and the amplitude distribution of the plastic strain jumps becomes random. The number of shear bands formed on the surface of crystals strained in the magnetic field is halved as compared to that observed without a magnetic field.     

Localization of plastic deformation in calcium fluoride crystals at elevated temperatures

The strain distribution was experimentally studied in CaF₂ crystals subjected to compression tests along [110] and [112] at a constant strain rate at temperatures T = 373–1253 K. At T > 845 K, the plastic deformation in deformed samples is found to be strongly localized in narrow bands, where the shear strain reaches several hundred percent. The physical deformation conditions are determined under which the plastic flow loses its stability and, as a result, the deformation is localized. The temperature dependence of the critical stress of the transition to a localized flow is found. A scenario is proposed for the nucleation and development of large localized shears during high-temperature deformation of single crystals.     

Study of the Acoustoelastic Effect in an Anisotropic Plastically Deformed Material

Acoustical Physics - The paper discusses the dependence of shear elastic waves birefringence on applied mechanical stress and preliminary plastic deformation of a material. A technique involving...     

Modeling the strain rate-dependent constitutive behavior in nanotwinned polycrystalline metals

Experimental studies have demonstrated that both strain rate and temperature influence the mechanical behavior of nanostructured metals significantly. In this work, a theoretical model is developed to describe the strain-rate-dependent constitutive behavior of nanotwinned polycrystalline metals. The athermal flow stress and thermal-activated flow stress are both considered in modeling the plastic deformation of a nanotwinned metal. Numerical results are consistent with the experimental results, showing that the present model can well describe the strain rate-dependent deformation behavior of nanotwinned polycrystalline copper. Henceforth, the constitutive behaviors of nanotwinned copper at different strain rates and temperatures can be predicted, which will be useful for optimizing the dynamic mechanical properties at various temperatures for nanotwinned metals.