Instabilities in diamond under high shear stress

We investigate, through first-principles calculations, lattice instabilities induced in diamond by the application of high shear stresses. For shear stresses as low as 95 GPa a lattice instability will occur, leading to graphitelike layered structures. This effect is highly anisotropic. The reversal of the direction of the applied shear forces may cause a change of 80 GPa in the shear stress value at which the instability develops. The same reversal also causes different bonds to be broken, resulting in a drastic change in the orientation of the resulting graphitelike structures. We also find that an additional compressive stress of 50 GPa along the (111) direction does not eliminate the shear-induced instability.     

The distribution of dislocations in slip bands of Fe-3% Si alloy single crystals deformed by bending

The distribution of dislocations at the ends of slip bands was studied by etching on surfaces parallel to the slip plane. In these places the slip band is formed by groups of asymmetric dislocation loops several hundred microns wide. The long mixed-type parts of these loops running nearly equidistantly and lying in near planes, are the equilibrium arrangement of dislocations of the same sign in the shear stress gradient. From the results we can judge that the dislocation sources are at larger distances from the ends of the slip bands and that the dislocation groups at the ends of the slip bands are sources of large stress fields.     

Plastic deformation transfer through the amorphous intercrystallite phase in nanoceramics

A three-dimensional model is proposed for plastic deformation transfer through the amorphous intercrystallite phase in mechanically loaded nanoceramics. In this model, glide dislocation loops are pressed against amorphous intercrystallite boundaries by the applied local shear stress and initiate in them local longitudinal plastic shears, which causes emission of new glide dislocation loops into neighboring grains. The energy characteristics of these processes and the critical applied stress required for barrierless nucleation of grainboundary and intragrain loops are calculated. As an example, a nanoceramic based on cubic silicon carbide is considered. It is shown that plastic deformation transfer through the amorphous intercrystallite phase in such nanoceramics is energetically favorable and can occur athermically over wide ranges of values of the applied stress and the structural characteristics of the material.     

On the effect of an elastic shear stress upon the magnetization of ferromagnetic sheets

When an elastic shear stress and a cyclical magnetic field, parallel to each other, are applied in the plane of a ferromagnetic sheet, magnetization changes perpendicular to the field are induced in the sample. “Transverse” hysteresis loops, i.e. transverse magnetization plotted as a function of the longitudinal field, were studied in various materials. The characteristic shape of the loop and the differences in sign and magnitude of the “transverse” magnetization have been qualitatively explained in terms of magnetic domain theory. Work supported by G.N.S.M. (CNR).     

冲击加载下铝中氦泡和孔洞的塑性变形特征研究

通过分子动力学模拟研究了在相同冲击加载强度下单晶铝中氦泡和孔洞的塑性变形特征,结果发现氦泡和孔洞的塌缩是由发射剪切型位错环引起的,而没有观测到棱锥型位错环发射. 氦泡和孔洞周围的位错优先成核位置基本一致,但是氦泡周围发射的位错环数目比孔洞多,位错环发射速度明显比孔洞快. 且氦泡和孔洞被冲击波先扫过部分比后扫过部分发射位错困难. 通过滑移面上的分解应力分析发现,氦泡和孔洞周围塑性特征的差别是由于氦泡内压引起最大分解应力分布改变造成的. 氦泡和孔洞被冲击波先后扫过部分塑性不对称是因为冲击波扫过时引起形状变化, 关键词： 分子动力学 冲击波 氦泡 孔洞     

Surface roughness effects on the turbulence statistics in a low Reynolds number channel flow

A high-resolution particle image velocimetry was used to characterize a low Reynolds number turbulent flow in a channel. Experiments were conducted over a sand grain-coated surface of large relative roughness, and the results were compared with measurements over a smooth surface. The roughness perturbation significantly modified the outer layer. Even though the streamwise Reynolds stress shows less sensitivity in the outer layer to the boundary condition, significant enhancements were observed in the wall-normal Reynolds stress and the Reynolds shear stress. These modifications were considered as footprints of the larger-scale eddies transporting intense wall-normal motions away from the rough wall. A quadrant decomposition shows that strong and more frequent ejections are responsible for the larger values of the mean Reynolds shear stress over the rough wall. The results also indicate that spanwise vortex cores with mean vorticity of the same sign as the mean shear are the dominant smaller-scale vortical structures over the smooth and rough walls. A linear stochastic estimation-based analysis shows that the average larger-scale structure associated with these vortices is a shear layer that strongly connects the outer layer flow to the near-wall flow. A proper orthogonal decomposition of the flow suggests that the large-scale eddy is more energetic for the rough wall, and contributes more significantly to the resolved turbulent kinetic energy and the Reynolds shear stress than the smooth wall.     

Instability of interstitial dislocation loops in electron-irradiated dielectrics

The experiments on electron irradiation of yttrium-stabilized zirconium oxide samples show the formation of strong elastic fields near interstitial dislocation loops. The fields increase with an increase in the loop radius and, when the loop radius reaches a certain critical value, the loops became unstable due to the beginning of plastic deformation and the formation of a dislocation network. The mechanism of the occurrence of this instability is suggested. It is based on the accumulation of charges at dislocation loops due to ionization processes in an electron-irradiated dielectric. It is shown that the accumulation of the electric charge at growing dislocation loops in dielectrics may be responsible for an increase in elastic stresses near dislocation loops and for their instability because of the beginning of plastic deformation near the loops when stresses at growing loops become close to the theoretical yield stress of the material.     

Nonaxisymmetric instability of shear-banded Taylor-Couette flow

Recent experiments show that shear-banded flows of semidilute wormlike micelles in Taylor-Couette geometry exhibit a flow instability in the form of Taylor-like vortices. Here we perform the nonaxisymmetric linear stability analysis of the diffusive Johnson-Segalman model of shear banding and show that the nature of this instability depends on the applied shear rate. For the experimentally relevant parameters, we find that at the beginning of the stress plateau the instability is driven by the interface between the bands, while most of the stress plateau is occupied by the bulk instability of the high-shear-rate band. Our work significantly alters the recently proposed stability diagram of shear-banded flows based on axisymmetric analysis.     

Dynamic instability of composite plates subjected to non-uniform in-plane loads

In this paper, the dynamic instability of a shear deformable composite plate subjected to periodic non-uniform in-plane loading is studied for four sets of boundary conditions. The static component and the dynamic component of the applied periodic in-plane loading are assumed to vary according to either parabolic or linear distributions. Initially, the plate membrane problem is solved using the Ritz method to evaluate the plate in-plane stress distributions within the prebuckling range due to the applied non-uniform in-plane edge loading. Subsequently using the evaluated stress distribution within the plate, the equations governing the plate instability boundaries are formulated via Hamilton's variational principle. Employing Galerkin's method, these partial differential equations are reduced into a set of ordinary differential equations (Mathieu type of equations) describing the plate dynamic instability behaviour. Following Bolotin's method, the instability regions are determined from the boundaries of instability, which represents the periodic solution of the differential equations with period T and 2T to the Mathieu equations. The instability regions are determined for uniform, linear and parabolic dynamic in-plane loads using first-order and second-order approximations. Numerical results are also presented to bring out the effects of span to thickness ratio, shear deformation, aspect ratio, boundary conditions and static load factor on the instability regions.     

Effect of korteweg stress in miscible liquid two-layer flow in a microfluidic device

Miscible liquid two-layer flow in a Y-shaped microfluidic device, which consists of microchannels with 120 μm in width and 35 μm in depth, is investigated by particle image velocimetry (PIV) to clarify the flow characteristics at fluid interfaces. The obtained velocities with a spatial resolution of 5.9 x 1.5 μm² around the interface between water and ethanol indicate an imbalance in shear stress at interface. The reason of the imbalance is to be the Korteweg stress generated by interfacial tension gradient due to a concentration gradient by diffusion in a miscible two-layer flow. The stress may cause an interfacial instability and destroy a uniform mixing in two flowing fluids in the case of large concentration gradient.     

Emission of Dislocation Loops from Nanovoids in an FCC Crystal Subjected to Shear Deformation under Post-Cascade Shock Waves

The method of molecular dynamics is applied to the study of the effect of post-cascade shock waves generated in a solid irradiated by high-energy particles on the heterogeneous formation of dislocation loops in a simulated gold crystal containing a spherical nanovoid, which is subjected to shear deformation. The interaction between atoms is described with the use of a potential calculated by the embedded atom method. Shock waves are created by assigning a velocity exceeding the speed of sound in the simulated material to the boundary atoms of the computational cell. It is shown that two regions of increased mechanical stress are formed under shear deformation near the surface of a nanovoid, which are the sources of emerging partial dislocations. The main mechanism for the formation of dislocations is the displacement of a group of atoms towards the inner surface of the void, which does not contradict modern ideas about the heterogeneous formation of dislocations. It is shown that, when the values of shear stress are insufficient for the formation of dislocations, loop emission can be initiated by a post-cascade shock wave generated in the computational cell. As temperature increases, the number of nucleated dislocation loops increases, and, in addition, the formation of Lomer–Cottrell dislocations is observed, which is attributed to the additional tangential stresses created by the unloading wave. In this case, the formation of a stable dislocation loop in which the linear tension is balanced by the Peach–Koehler force due to external stress requires that the shock wave front affect the regions of increased stress near the void surface while propagating through the simulated crystal.     

Age-dependent transient shear banding in soft glasses

We study numerically the formation of long-lived transient shear bands during shear startup within two models of soft glasses (a simple fluidity model and an adapted "soft glassy rheology" model). The degree and duration of banding depends strongly on the applied shear rate, and on sample age before shearing. In both models the ultimate steady flow state is homogeneous at all shear rates, consistent with the underlying constitutive curve being monotonic. However, particularly in the soft glassy rheology case, the transient bands can be extremely long lived. The banding instability is neither "purely viscous" nor "purely elastic" in origin, but is closely associated with stress overshoot in startup flow.     

A composite model of the plastic flow of amorphous covalent materials

A model based on the data available in the literature on the computer simulation of amorphous silicon has been proposed for describing the specific features of the plastic flow of amorphous covalent materials. The mechanism of plastic deformation involves homogeneous nucleation and growth of inclusions of a liquidlike phase under external shear stress. Such inclusions experience plastic shear, which is modeled by glide dislocation loops. The energy changes associated with the nucleation of these inclusions at room and increased temperatures have been calculated. The critical stress has been found, at which the barrierless nucleation of inclusions becomes possible. It has been shown that this stress decreases with an increase in temperature. According to the calculations, the heterogeneous (homogeneous) plastic flow of an amorphous material should be expected at relatively low (high) temperatures. Above the critical stress, the homogeneous flow is gradually replaced by the heterogeneous flow.     

Oscillating viscosity in a lyotropic lamellar phase under shear flow

We report some time-dependent behavior of lyotropic lamellar phase under shear flow. At fixed stress, near a layering instability, the system presents an oscillating shear rate. We build up a new stress versus shear rate diagram that includes temporal behavior. This diagram is made of two distinct branches of stationary states which correspond, respectively, to disordered and ordered multilamellar vesicle phases. When increasing the shear stress, prior to the transition to the ordered structural state, sustained oscillations of the viscosity are recorded. They correspond to periodic structural change of the entire sample between a disordered and a ordered state of multilamellar vesicles.     

低雷诺数下钝体三维尾迹中的涡量符号律

钝体是目前各种工程中广泛应用的一种结构.钝体绕流的尾迹涡动力学也是经典的流体力学研究对象之一.本文通过直接数值模拟,针对低雷诺数下各种钝体结构的不可压缩绕流,当形成三维尾迹时,研究具有特定符号的涡量分布特征.通过分析两类钝体结构,基本的直柱体和受到几何扰动的柱体,总结并得到了更为广泛适用的涡量符号律.通过对比并分析这两类钝体结构,结合理论证明的结果,进一步厘清了对产生涡量符号律的这两类钝体结构之间的内在物理关联,即引起自然失稳的小扰动在惯性力作用下产生的表面涡量只能向下游演化发展,而几何扰动则根据扰动位置,产生的表面涡量可以向扰动上游或下游演化发展.从而可以推测所有钝体结构尾迹中的各种型式的涡脱落模态,从涡量符号律的演化角度来看,实际上是一致的,都是起源于壁面产生特定符号组合规律的∏型涡.     

Spatiotemporal oscillations and rheochaos in a simple model of shear banding

We study a simple model of shear banding in which the flow-induced phase is destabilized by coupling between flow and microstructure (wormlike micellar length). By varying the strength of instability and the applied shear rate, we find a rich variety of oscillatory and chaotic shear banded flows. At low shear and weak instability, the induced phase pulsates next to one wall of the flow cell. For stronger instability, high shear pulses ricochet across the cell. At high shear we see oscillating bands on either side of central defects. We discuss our results in the context of recent experiments.     

Global visualization and quantification of compressible vortex loops

The physics of compressible vortex loops generated due to the rolling up of the shear layer upon the diffraction of a shock wave from a shock tube is far from being understood, especially when shock-vortex interactions are involved. This is mainly due to the lack of global quantitative data available which characterizes the flow. The present study involves the usage of the PIV technique to characterize the velocity and vorticity of compressible vortex loops formed at incident shock Mach numbers ofM=1.54 and1.66. Another perk of the PIV technique over purely qualitative methods, which has been demonstrated in the current study, is that at the same time the results also provide a clear image of the various flow features. Techniques such as schlieren and shadowgraph rely on density gradients present in the flow and fail to capture regions of the flow influenced by the primary flow structure which would have relatively lower pressure and density. Various vortex loops, namely, square, elliptic and circular, were generated using different shape adaptors fitted to the end of the shock tube. The formation of a coaxial vortex loop with opposite circulation along with the generation of a third stronger vortex loop ahead of the primary with same circulation direction are of the interesting findings of the current study.     

Dynamic stress around a cylindrical nano-inhomogeneity with an interface in a half-plane under anti-plane shear waves

Taking into account the size of the nanostructure, the effect of surface/interface stiffness on the dynamic stress around a cylindrical nano-inhomogeneity embedded in an elastic half-plane subjected to anti-plane shear waves is investigated. The boundary condition at the straight edge of the half-plane is traction free, which is satisfied by the image method. The analytical solutions of displacement fields are expressed by employing a wave function expansion method. The addition theorem for a cylindrical wave function is applied to accomplish the superposition of wave fields in the two half-planes. Analyses show that the effect of the interface properties on the dynamic stress is significantly related to the nano-scale distance between the straight edge and the center of the cylindrical nano-inhomogeneity. The frequency and incident angle of incident waves and the shear modulus ratio of the nano-inhomogeneity to matrix also show different effect on the dynamic stress distribution when the inhomogeneity shrinks to nano-scale. Comparison with the existing results is also given.