Autowave mechanics of plastic flow in solids

The regularities of the development of localized plastic deformation of solids have been analyzed. A correlation of the products of linear and velocity parameters of elastic and plastic deformations is found for some metals and nonmetals. A hypothesis about relationship between the characteristics of elastic and plastic deformation is formulated on this basis. An empirical relationship between the regularities of plastic flow and quantum-mechanical characteristics is found.     

Depinning Dynamics of Fluid Monolayer on a Quenched Substrate

Langevin simulations are preformed on the depinning dynamics of fluidmonolayer on a quenched substrate. With increase in the strength of thesubstrate, we find for the first time a crossover from elastic crystal tosmectic flows as well as a crossover from smectic to plastic flows above thedepinning. A power-law scaling relationship can be derived between the driftvelocity and the driving force for both the elastic crystal and smecticflows, but fails to be obtained for the plastic flow. The power-lawexponents are found to be no larger than 1 for the elastic crystal flow andlarger than 1 for the smectic flow. The critical driving force and theaveraged intensity of Bragg peaks remain invariant basically in the regimeof smectic flow. A sudden increase in the critical driving force is observedwithin the crossover from the smectic to plastic flows, and the averagedintensity of Bragg peaks shows sudden decreases within the crossovers bothfrom the elastic crystal to smectic flows and from the smectic to plastic flows.The results are helpful for understanding the slip dynamics of fluids on a molecular level.     

Plastic flow macrolocalization in aluminum and the Hall-Petch relation

The parameters of plastic deformation macrolocalization are compared to the parameters of the Hall-Petch relation for the flow stress in polycrystalline aluminum samples with a grain size of 0.008–5.000 mm. Two types of the dependence of the localized plastic deformation autowave length on the grain size and two versions of hardening according to the Hall-Petch relation are found in the grain size range under study. The boundary between these versions is shown to correspond to d ≈ 0.1 mm for both cases. A relation between localized plastic flow patterns and the Hall-Petch relation is revealed.     

Dispersion of autowaves in a localized plastic flow

The dispersion relation for localized plastic deformation autowaves emerging at the easy glide and strain-hardening stages in metals and alloys is established experimentally and analyzed. The quadratic form of this relation and the form of the wavenumber dependence of the phase and group velocities of autowaves are explained, as well as the quantitative interrelation between the characteristics of autowave processes of plastic flow localization and crystal lattice parameters of strained solids.     

Significant correlation between macroscopic and microscopic parameters for the description of localized plastic flow auto-waves in deforming alloys

Understanding of mechanical properties of materials and a possibility to predicting them from ab initio calculations have fundamental importance for solid state theory. In this work we establish a significant correlation between the product of the macroscopic parameters of localized plastic flow auto-waves in deforming alloys, their length and propagation rate and the product of the microscopic (lattice) parameters of these materials, the spacing between close-packed planes of the lattice and the rate of transverse elastic waves. Thus, these products can be regard as invariants of plastic and elastic deformation processes, respectively. Moreover, the established regularity suggests that the elastic and the plastic processes simultaneously involved in the deformation are closely related. Our work also demonstrates that ab initio simulations can be used for the prediction of parameters of localized plastic flow auto-waves in deforming alloys.     

非晶力学流变的自组织临界行为

非晶材料是由液体快冷冻结而成的结构无序的亚稳态固体.在受力条件下,非晶材料表现出独特和复杂的流变行为,具有跨尺度的高度时空不均匀特征,并在一定条件下表现出自组织临界行为,和自然界以及物理系统中许多复杂体系的动力学行为相似.本文结合作者近年来在非晶合金流变行为方面的研究结果,对非晶材料流变的研究进展和物理机制的认识进行介绍,包括非晶材料流变的跨尺度特征、表征和微观结构机制,以及近年来发现的非晶力学流变的自组织临界行为、物理机制等.最后,对非晶材料流变行为研究中亟需解决的问题进行了总结和展望.     

气液两相流中的声速研究

气液两相流在工业各领域中广泛存在,而声速是描述其声学性质的一个重要参数。本文从流体的体积弹性模量的定义出发,推导了气液两相流中的声速随含气率的变化关系式,即混合流体的Wood声速公式,将其声速的部分计算结果和其他作者的实验数据进行了比较,吻合良好。并通过COMSOL有限元模拟软件得到不同气体分布下圆管谐振腔最低阶模式的共振频率,间接数值模拟研究了含气率对声速的影响。模拟结果与理论计算结果一致,当气液两相流中含气率较低时,声速随含气率的增大急剧减小。本研究结果为确定声速与气液两相流中的含气率间的关系提供了参考依据。     

Origin of Elastic–Plastic Deformation Invariant

We consider the autowave mechanism of evolution of a localized plastic deformation of crystalline solids of different origins. It is found that localization of the plastic flow is determined by the relation between elastic and plastic phenomena in deforming materials. It is shown that the main parameter of deformation processes is the elastic–plastic deformation invariant, viz., a dimensionless quantity connecting quantitatively the parameters of elastic waves and self-sustained waves (autowaves) of localized plastic deformation. The correctness of this statement is verified for metals, alkali-halide crystals, and rocks. The physical origin of the invariant is explained on the basis of thermodynamic considerations.     

Characteristics of plastic flow localization autowaves

The dispersion relations for localized plastic strain autowaves are considered for some metals at the stages of easy slip and linear strain hardening. The quadratic form of this relation, the type of the dependences of the phase and group velocities of such autowaves on the wavenumber, and the quantitative relation between the characteristics of plastic flow localization wave processes and elastic wave parameters in solids being deformed are established and explained. An invariant for strain effects at micro- and macroscopic levels is introduced.     

Plastic Strain Localization and Its Stages in Al-Mg Alloys

The paper describes an experimental technique based on the use of a Vic-3D contactless digital optical system and digital image correlation for research in the mechanical behavior of a solid and its plastic deformation with space-time inhomogeneities. Using this technique, we analyze the evolution of inhomogeneous strain and local strain rate fields in AMg2m alloy at constant uniaxial tension rates. The analysis reveals quasi-periodic strain field homogenization in jerky flow: alternating phases of active local plastic flow (shear banding) and macroscale strain levelling. Also analyzed are the parameters of localized microscale plastic flow such as the height and width of shear bands, their velocity, and coefficient of plastic strain inhomogeneity. From a series of mechanical tests, the influence of the specimen geometry and loading rate on these parameters is estimated.     

An analytical-based method for studying the nonlinear evolution of localized vortices in planar homogenous shear flows

Recent experimental and numerical studies have shown that the interaction between a localized vortical disturbance and the shear of an external base flow can lead to the formation of counter-rotating vortex pairs and hairpin vortices that are frequently observed in wall bounded and free turbulent shear flows as well as in subcritical shear flows. In this paper an analytical-based solution method is developed. The method is capable of following (numerically) the evolution of finite-amplitude localized vortical disturbances embedded in shear flows. Due to their localization in space, the surrounding base flow is assumed to have homogeneous shear to leading order. The method can solve in a novel way the interaction between a general family of unbounded planar homogeneous shear flows and any localized disturbance. The solution is carried out using Lagrangian variables in Fourier space which is convenient and enables fast computations. The potential of the method is demonstrated by following the evolved structures of large amplitude disturbances in three canonical base flows, including simple shear, plane stagnation (extensional) and pure rotation flows, and a general case. The results obtained by the current method for plane stagnation and simple shear flows are compared with the published results. The proposed method could be extended to other flows (e.g. geophysical and rotating flows) and to include periodic disturbances as well.     

Reconstruction of the autowave structure upon deformation of polycrystalline aluminum

The evolution of zones of localized plastic deformation in polycrystalline aluminum was investigated. At the stage of the linear strain hardening, such zones were established to move synchronously, whereas at the stage of parabolic strengthening they are stationary. The quantitative characteristics (wavelength, propagation velocity) of deformation waves that are formed at the stage of linear strengthening were determined. A relation between the quantitative characteristics of the process of deformation localization and the grain size was found. The distribution of local deformations upon transition from one stage of plastic flow to another was investigated. A model that explains the generation of coarse-scale structures of localized plastic deformation is suggested.     

Model of Plastic Deformation and Failure of Solids

Russian Physics Journal - A model of localized plastic flow development based on the idea about interaction between plasticity carriers and acoustic emission pulses generated by developing...     

Mathematical modeling of the thermal and optical parameters of the zone of laser influence for determining the feedback parameters for a system of adaptive precision laser material processing

We develop a mathematical model of channel formation in the ablation regime at the initial non-stationary stage of growth that establishes the relationship between channel parameters such as temperature, optical parameters of the irradiated surface and erosion flow, and the parameters of laser action. The solution to a two-dimensional heat equation, coordinated with the solution to the problem of propagation of an ablation front and the formation of flows of evaporated material and plasma, is found in a grid with variable boundaries.     

Acoustic emission during heterogeneous and homogeneous plastic flow of a metallic glass

Measurements have been made of the acoustic emission that occurs during the plastic deformation of metallic glass subjected to various heat treatments. It is shown that for the same temperature and deformation rate it is possible to have either homogeneous flow with no acoustic emission or localized flow accompanied by intense discrete acoustic emission, depending on the preliminary annealing conditions. From an analysis of the experimental results, it is deduced that the nature of the plastic flow of metallic glasses is determined by the structural relaxation rate at the experimental temperature and the conditions of intense structural relaxation, that plastic flow is homogeneous viscoelastic, and, under the conditions of kinetically inhibited structural relaxation, that it is localized and occurs by a dislocation-like mechanism. Fiz. Tverd. Tela (St. Petersburg) 39, 885–888 (May 1997)     

Phenomenology of wave processes in a localized plastic flow

The motion of localized deformation sites in mono-and polycrystallites of metals and alloys is investigated experimentally at the stages of easy slip and linear strain hardening. The wave nature of localization of deformation at these stages of plastic flow is established, and the velocity of waves and their dispersion relation are determined. The scale effect and the dependence of the localized deformation wavelength on the grain size are investigated. A qualitative interpretation of the results obtained is proposed.     

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.     

On the localization of plastic flow under compression of NaCl and KCl crystals

The plastic flow localization patterns for alkali halide crystals under compression are investigated. The main spatiotemporal regularities of the strain localization at the stages of deformation hardening in these single crystals are established. The relation is traced between the orientation of localized strain zones and the crystallography of slip systems of the test specimens at the initial stages of plastic deformation. The velocity of motion of localized strain zones under compression is determined.     

Plastic response of a 2D Lennard-Jones amorphous solid: Detailed analysis of the local rearrangements at very slow strain rate

We analyze in detail the atomistic response of a model amorphous material submitted to plastic shear in the athermal, quasi-static limit. After a linear stress-strain behavior, the system undergoes a noisy plastic flow. We show that the plastic flow is spatially heterogeneous. Two kinds of plastic events occur in the system: quadrupolar localized rearrangements, and shear bands. The analysis of the individual motion of a particle shows also two regimes: a hyper-diffusive regime followed by a diffusive regime, even at zero temperature.