冲击加载下Zr51Ti5Ni10Cu25Al9金属玻璃的塑性行为

进行了10—27 GPa应力范围内Zr₅₁Ti₅Ni₁₀Cu₂₅Al₉金属玻璃的平面冲击实验以研究其高压-高应变率加载下的塑性行为.由样品自由面粒子速度剖面的分析获得了冲击加载过程的轴向应力,并通过轴向应力与静水压线的比较获得剪应力.实验结果表明,尽管存在明显的松弛效应,但Zr基金属玻璃的Hugoniot弹性极限随着冲击应力的增加而增加.然而,塑性波阵面上的剪应力则显示先硬化而后软化现象,而且软化的幅度随冲击应力的增加而增加.冲击加载下Zr基金属玻璃的上述剪应力变化特征与分子动力学模拟结果比较一致,但与压剪实验结果和一维应力冲击实验结果明显不同.     

Shear strength of metals behind shock fronts

Results are presented from a numerical modeling of experiments, conducted by J. Lipkin, R. Asay, L. Chabildas, and J. Wise, in which shock-compressed aluminum and polycrystalline beryllium were loaded again by shock waves. Reproduction of the experimental structures of the impulses in the calculations made it possible to determine the stresses acting in the materials during the primary and secondary shock loading, as well as to study the dynamics of the secondary shock load. It is shown that resistance to plastic strain is maintained in aluminum and beryllium behind the front of weak shock waves with intensities up to 10 GPa. The shear strength of the material behind the front does not correspond to the shear stresses at the Hugoniot elastic limit. For aluminum alloy 6061-T6 and beryllium, shear strength behind the fronts of shock waves with amplitudes up to 10 GPa is lower than in the elastic precursor but is greater than the static yield point of the material in the initial state.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 62–66, October, 1995.     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.     

Change of the kinetics of shock-wave deformation and fracture of VT1-0 titanium as a result of annealing

The paper presents the results of measurements of shock-wave compression profiles of VT1-0 titanium samples after rolling and in the annealed state. In the experiments, the pressure of shock compression and distance passed by the wave before emerging to the sample surface were varied. From measurements of the elastic precursor decay and compression rate in a plastic shock wave of different amplitudes, the plastic strain and the corresponding shear stresses in the initial and subsequent stages of high-rate deformation in an elastoplastic shock wave are determined. It is found that the reduction in the dislocation density as a result of annealing reduces the hardness of the material but significantly increases its dynamic yield strengh, corresponding to the strain rate above 10⁴ s^–1. With a reduction in the strain rate, this anomalous difference in the flow stresses is leveled off.     

含微孔洞脆性材料的冲击响应特性与介观演化机制

微孔洞显著地影响着脆性材料的冲击响应,理解其介观演化机制和宏观响应规律将使微孔洞有利于而无害于脆性材料的工程应用.通过建立能够准确表现材料弹性性质和断裂演化的格点-弹簧模型,本文揭示了孔洞的演化对于脆性材料的影响.冲击下孔洞导致的塌缩变形和从孔洞发射的剪切裂纹所导致的滑移变形产生了显著的应力松弛,并调制了冲击波的传播.在多孔脆性材料中,冲击波逐渐展宽为弹性波和变形波.变形波在宏观上类似于延性金属材料的塑性波,在介观上对应于塌缩变形和滑移变形过程.样品中的气孔率决定了脆性材料的弹性极限,气孔率和冲击应力共同影响着变形波的传播速度和冲击终态的应力幅值.含微孔洞脆性材料在冲击波复杂加载实验、功能材料失效的预防、建筑物防护等方面具有潜在的应用价值.所获得的冲击响应规律有助于针对特定应用优化设计脆性材料的冲击响应和动态力学性能.     

Mesoscale plastic flow instability in a solid under high-rate deformation

Here we consider high-rate deformation in solids in the context of a nonlocal transport theory, present a dynamic stress-strain diagram with elastic and plastic portions defined from a single standpoint, determine the conditions for pulse stress accumulation, and propose a mathematical model of momentum and energy exchange between scales and an instability criterion for transient plastic flow under shock loading. Phe instability criterion for high-rate deformation is verified by the example of shock loading of high-strength 30CrNi4Mo steel.     

Shock wave chemistry

Abstract

Shock wave chemistry, a new scientific trend, deals with investigations of chemical aspects of the substance state under this new type of effect. Indeed, shock wave effect is not a greater imposition than pressure and temperature actions. Characteristic features of the effect are the tremendous rates of substance loading and subsequent unloading. The effects result in a substance in a strongly non- equilibrium state. The lifetime of the state is governed by the relaxation process of those phenomena which are provoked by shock waves in the substance. For instance, in the case of substance consisting of complex molecules with a large number of internal degrees of freedom, differing strongly in excitation times, all kinetic parts of the shock energy are at first absorbed by the translational degrees of freedom inside the shock wave front. Then, the energy is redistributed to the vibrational degrees of freedom. The non-equilibrium state time is not longer than the excitation time of the most slowly excited vibrational degrees of freedom (10¹⁰-10^?9 s). The same order of magnitude is the relaxation time of liquid substance polarization caused by dipolar molecules mechanically turning under the shock discontinuity zone effect. In polymers the zone turns some separate groups of polymer molecule atoms. In such a case the relaxation period, on the contrary, may last as long as it can. As far as “hot are concerned, their lifetime is determined by thermal relaxation regularities and it depends on their size. The hot spots in solids appear during the shock compression process at the sites of an imperfect substance structure. In liquids the hot spots can orighate when a shock wave front passes through negative density fluctuations. It transforms the fluctuations of very small size and of high probability into some positive temperature regions of large size and extremely low probability at equilibrium state behind the wave front. The hot spots in perfect solids (possibly in liquids too) appear due to the effect of shear stresses in shock front. Pointed and lengthy defects of solid structure occur under the effect. The lengthy defects appear in the shock wave front due to the transition from one-dimensional to volume compression. The transition takes place if the wave intensity is larger than the dynamic elastic limit of the solid under investigation. In brittle materials the transition results in their grinding into fragments and in the relative displacement of the fragments. Some liquid melted layers of substance appear between the fragments in the process of displacement. Their lifetime is also determined by the thermal relaxation regularities and probably is small. Nevertheless, the layers obviously govern the spall strength of brittle solids and promote solid-phase shock reactions. The defects created in solids by the shock effect can exist for a very long time if the solid substance residual temperature is lower than its recrystallization temperature. Therefore, solid substance treatment by shocks of proper intensity can increase their chemical reactivity.     

Evolution of substructures under microlevel and mesolevel plastic deformation

The existence of a hierarchy of structural levels of plastic deformation can be considered to be an experimentally and theoretically proven fact [1–3]. Mescheryakov [1] showed that a noncrytallographic level of deformation arises in elastoplastic waves, manifesting itself as macrofluxes of particles of the medium; the velocity of the particles relative to each other at velocity has dispersion and the particles move in the direction of the wave propagation. Displacement of macrofragments of the crystal, which is also a manifestation of noncrystallographic structural levels of deformation, has been detected in highly excited systems [2]. The relaxation approach used increasingly to describe plastic deformation assumes that defects are created, move, and are restructured during deformation in a way so that the level of stresses inside the material drops. The nonuniformity of the stress field gives rise to nonuniform plastic deformation and local shears and rotations at points of stress concentration. These concepts make it possible to use the principles of synergetics to build specific theoretical models and to consider loaded material as a nonequilibrum dissipative structure [3]. To date, however, the construction of the theory describing multilevel plastic deformation processes has not been completed. In particular, it is not yet known what levels are added, depending on the rate and duration of the loading and on how the levels are linked.St. Petersburg Branch of the A. A. Blagonravov Institute of Mechanical Engineering. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 7–12, October, 1992.     

Nonequilibrium processes in condensed media: Part 1. Experimental studies in light of nonlocal transport theory

Based on experimental research in shock loading of solid-state materials it is shown that among the important dynamic characteristics of the process, like spatial-temporal mass velocity profiles of shock waves, are the mass velocity variation, velocity defect, and structural instability threshold recorded in real time. Analysis of these characteristics depending on the strain rate, target thickness, and structural state of material demonstrates that conventional approaches of continuum mechanics fail to provide their adequate interpretation and simulation of shock wave processes. A new concept of shock wave processes in condensed media is proposed. The concept, being based on nonlocal nonequilibrium transport theory, allows describing the transition from elastic to hydrodynamic response of a medium depending on the loading rate and time. A nonstationary elastoplastic wave model is proposed for describing the relaxation of an elastic precursor and formation of a retarded plastic front during the wave propagation in a medium with regard to structural evolution. Analysis of the experimental data shows that the division of stresses and strains into elastic and plastic components is incorrect for shock loading.     

Resistance to dynamic deformation and fracture of tantalum with different grain and defect structures

This paper presents the results of measurements of the strength properties of technically pure tantalum under shock wave loading. It has been found that a decrease in the grain size under severe plastic deformation leads to an increase in the hardness of the material by approximately 25%, but the experimentally measured values of the dynamic yield stress for the fine-grained material prove to be less than those of the initial coarse-grained specimens. This effect has been explained by a higher rate of stress relaxation in the fine-grained material. The hardening of tantalum under shock wave loading at a pressure in the range 40–100 GPa leads to a further increase in the rate of stress relaxation, a decrease in the dynamic yield stress, and the disappearance of the difference between its values for the coarse-grained and fine-grained materials. The spall strength of tantalum increases by approximately 5% with a decrease in the grain size and remains unchanged after the shock wave loading. The maximum fracture stresses are observed in tantalum single crystals.     

纳米多晶铜中冲击波阵面的分子动力学研究

冲击波阵面反映材料在冲击压缩下的弹塑性变形行为以及屈服强度、应变率条件等宏观量, 还与冲击压缩后的强度变化联系. 本文使用分子动力学方法, 模拟研究了冲击压缩下纳米多晶铜中的动态塑性变形过程, 考察了冲击波阵面和弹塑性机理对晶界存在的依赖, 并与纳米多晶铝的冲击压缩进行了比较. 研究发现: 相比晶界对纳米多晶铝的贡献而言, 纳米多晶铜中晶界对冲击波阵面宽度的影响较小; 并且其塑性变形机理主要以不全位错的发射和传播为主, 很少观察到全位错和形变孪晶的出现. 模拟还发现纳米多晶铜的冲击波阵面宽度随着冲击应力的增加而减小, 并得到了冲击波阵面宽度与冲击应力之间的定量反比关系, 该定量关系与他人纳米多晶铜模拟结果相近, 而与粗晶铜的冲击压缩实验结果相差较大.     

Relaxation waves in plastic deformation

Conclusion Experimental study of distortion fields of plastically deformed solids performed on a wide range of materials including fine- and coarse-grain body- and face-centered polycrystals, as well as amorphous alloys reveals that in these materials plastic deformation develops in the form of waves having translational and rotational components. This fact is in accordance with the currently developed theory of a turbulent mechanical field, which also has translational and rotational components.The plastic deformation waves are observable at a macroscopic structural level, and their spatial period (wavelength) is determined by the dimensions of the deformed object and dimensions of the basic structural elements (for a polycrystal, the grain size). The propagation rate of these waves is significantly less than the characteristic propagation rate of an elastic excitation and the velocity of previously described plastic waves which are produced by shock deformation, which latter speed is determined by the hardening coefficient.The character of plasticity waves depends on the form of the material's deformation curve, and on the stage of the hardening curve. The distribution of plastic distortion components changes especially significantly in prefailure regions, which allows detection of the latter long before formation of a macroscopic crack. The role of rotations in forming the failure process has been established.A synergetic interpretation of plasticity wave formation has been proposed, based on synchronization of relaxation acts occurring at stress concentrators during the deformation process.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 19–35, February, 1990.     

Supersonic dislocation stability and nano-twin formation at high strain rate

Improved understanding of the plastic deformation of metals during high-strain-rate shock loading is key to predicting their resulting material properties. This paper presents the results of molecular-dynamics simulations which address two fundamental questions related to materials deformation: the stability of supersonic dislocations and the mechanism of nano-twin formation. The results show that aluminium plastically deforms by the subsonic motion of edge dislocations when subjected to applied shear stresses of up to 600?MPa. Although higher applied stresses initially drive transonic dislocations, this motion is transient, and the dislocations decelerate to a sustained subsonic saturation velocity. Slowing of the transonic dislocation is controlled by the interaction with excited Rayleigh waves. 800?MPa marks a critical shear stress at which dislocation glide gives way to nano-twin formation via the homogeneous nucleation of Shockley partial dislocation dipoles. At still higher applied stresses, additional dislocation dipole nucleation produces a mid-stacking fault transformation of the twinned material.     

Plastic deformation under high-rate loading: The multiscale approach

A two-level approach has been proposed for describing the plastic deformation under high-rate loading of metals. The characteristics of the motion of dislocations under shear stresses have been investigated at the atomistic level by using the molecular dynamics simulation. The macroscopic motion of a material has been described at the continuum level with the use of the model of continuum mechanics with dislocations, which uses information obtained at the atomistic level on the dislocation dynamics. The proposed approach has been used to study the evolution of the dislocation subsystem under shock-wave loading of an aluminum target. The behavior of the dynamic yield stress with an increase in the temperature has been analyzed. The results of the calculations are in good agreement with experimental data.     

Plastic deformation of filamentary silicon crystals. Relation to formation of dislocations at the surface and evolution of ensemble of these in volume

Direct methods, as well as high-sensitivity indirect methods, have shown that in initially dislocation-free filamentary crystals with high Peierls barriers dislocations originate at the surface and become abruptly localized at sites of stress concentration. Formation of the first dislocations does not lead to large-scale multiplication of these and to irreversible plastic deformation; rather, a gradient of the dislocation density appears, reaching 10¹⁷ m^–3. After penetration of the shear front to about half of the radius of the filamentary crystals, a change in the mechanism of plastic deformation occurs.Voronezh Polytechnical Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 60–68, May, 1993.     

多孔脆性材料对高能量密度脉冲的吸收和抵抗能力

作用在脆性结构材料表面的高能量密度脉冲会以冲击波的形式传播进入材料内部, 导致压缩破坏和功能失效. 通过设计并引入微孔洞, 显著增强了脆性材料冲击下的塑性变形能力, 从而使脆性结构材料可以有效地吸收耗散冲击波能量, 并抑制冲击诱导裂纹的扩展贯通. 建立格点-弹簧模型并用于模拟研究致密和多孔脆性材料在高能量密度脉冲加载下的冲击塑性机理、能量吸收耗散过程和裂纹扩展过程. 冲击波压缩下孔洞塌缩, 导致体积收缩变形和滑移以及转动变形, 使得多孔脆性材料表现出显著的冲击塑性. 对致密样品、气孔率5%和10%的多孔样品吸能能力的计算表明, 多孔脆性材料吸收耗散高能量密度脉冲的能力远优于致密脆性材料. 在短脉冲加载下, 相较于遭受整体破坏的致密脆性材料, 多孔脆性材料以增加局部区域的损伤程度为代价, 阻止了严重的冲击破坏扩展贯通整个样品, 避免了材料的整体功能失效.     

Finely striped structure at the edges of localized deformation bands

Under conditions of high-rate loading, plastic strain localization is a result of tension in the zone of interference of unloading waves rather than of thermal softening. At stresses close to the dynamic strength of the material, the microstructure of localized strain bands consists of strongly deformed material, with a large number of incipient microdiscontinuities. At stresses below the Hugoniot elastic limit, the microstructure looks as a set of barely visible stripes. The finely striped structure at the edges of the bands of a spall damage arises as a result of the stretching of initially rounded damage centers attached to the matrix material during dynamic deformation.     

Collective properties of defects,multiscale plasticity,and shock induced phenomena in solids

A statistically based approach is developed for the construction of constitutive equations that provides linkages between defect-induced mechanisms of structural relaxation, thermally activated plastic flow, and material response to extreme loading conditions. The collective properties of defects have been studied to establish the interaction of multiscale defect dynamics and plastic flow, and to explain the mechanisms leading to the universal self-similar structure of shock wave fronts. Pn explanation for structural universality of the steady-state plastic shock front (the four power law) and the self-similarity of shock wave profiles under reloading (unloading) is proposed. Structural characterization under transition from thermally activated dislocation glide to nonlinear dislocation drag effects is developed in terms of scaling invariants (effective temperatures) related to mesodefect induced morphology formed during the different stages of plastic deformation.     

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.     

Arch Failure in Sandstone in the Case of Nonlinear Flow

A mathematical model of elastic--plastic deformations and stability of sand in a compacted zone around a perforation with a nonlinear flow is developed for a gas well. Laser processing of porous materials is accompanied by similar problems too. The stresses in porous material and the effect of flowing fluids have been analyzed theoretically. The criteria describing stability of sand arches are given. Critical rates of production are found that do not result in destruction of the reservoir near the perforation opening by tensile and shear stresses. The influence of variable permeability on the stability of the arches around the perforations has been studied. The significant influence of water on elastic–plastic deformation and destruction of sand arches is shown within the approach of constant saturation. The results are obtained for both fluid production from the well and gas injection into the reservoir. The conditions of arch stability near the perforated opening are always fulfilled with fluid injection.