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.     

Strain localization under dynamic loading

A spallation model of strain localization is suggested according to which localization bands under pulsed loading result from unloading wave interference such that negative stresses in the extension zone are lower than the ultimate strength of the material. The temperature in the localization band is estimated to be close to the environmental temperature.     

Non-linear oscillatory rheological properties of a generic continuum foam model: Comparison with experiments and shear-banding predictions

The occurrence of shear bands in a complex fluid is generally understood as resulting from a structural evolution of the material under shear, which leads (from a theoretical perspective) to a non-monotonic stationary flow curve related to the coexistence of different states of the material under shear. In this paper we present a scenario for shear-banding in a particular class of complex fluids, namely foams and concentrated emulsions, which differs from other scenarios in two important ways. First, the appearance of shear bands is shown to be possible both without any intrinsic physical evolution of the material (e.g. via a parameter coupled to the flow such as concentration or entanglements) and without any finite critical shear rate below which the flow does not remain stationary and homogeneous. Secondly, the appearance of shear bands depends on the initial conditions, i.e. the preparation of the material. In other words, it is history dependent. This behaviour relies on the tensorial character of the underlying model (2D or 3D) and is triggered by an initially inhomogeneous strain distribution in the material. The shear rate displays a discontinuity at the band boundary whose amplitude is history dependent and thus depends on the sample preparation.     

On the conditions of strain localization and microstructure fragmentation under high-rate loading

The paper reports on research in the deformation and fragmentation mechanisms of coarse- and fine-grained materials under high-rate loading. The study was performed by an experimental procedure based on collapse of thick-walled hollow cylinders and by molecular dynamics simulation. The key issue was to study the formation of plastic strain localization bands. It is found that the pattern of plastic deformation is governed by loading conditions and characteristic grain sizes. For a coarse-grained material, the governing mechanism is dislocation deformation resulting in localization bands. For a fine-grained material, the governing mechanism is grain boundary sliding with attendant fragmentation of the material. A dependence of the strain rate and degree on the critical grain size was disclosed. The computer simulation revealed mechanisms of grain boundary sliding on the scales studied.     

Boundary-driven colloidal crystallization in simple shear flow

Using confocal microscopy, we directly observe that simple shear flow induces transient crystallization of colloids by wall-normal propagation of crystallization fronts from each shearing surface. The initial rate of the front propagation was 1.75±0.07 colloidal layers per unit of applied strain. The rate slowed to 0.29±0.04 colloidal layers per unit of applied strain as the two fronts approached each other at the midplane. The retardation of the front propagation is caused by self-concentration of shear strain in the growing bands of the lower-viscosity crystal, an effect that leads to a progressive reduction of the shear rate in the remaining amorphous material. These findings differ significantly from previous hypotheses for flow-induced colloidal crystallization by homogeneous mechanisms.     

孪晶对Be材料冲击加-卸载动力学影响的数值模拟研究

在高压、高应变率加载条件下,孪晶变形对材料的塑性变形具有重要的贡献,而目前孪晶对金属材料的动态屈服强度、冲击响应等的影响还没有被充分揭示.为此,本文考虑孪晶变形和晶粒碎化,针对铍(Be)材料在高应变率加载下的动态力学响应发展了含孪晶的热弹-黏塑性晶体塑性模型.经过和实验结果的对比,发现该模型可以更准确地预测Be材料在动态加载下,尤其是高压动态加载下的屈服强度.进一步,基于该塑性模型研究了Be材料在冲击加载下的准弹性卸载行为,结果表明剪切波速随着压力和剪应变的变化而发生变化是材料产生准弹性卸载现象的主要原因.此外,研究了冲击波卸载过程中Be材料孪晶的演化过程,发现Be材料卸载过程中也伴随着孪晶的产生.     

Portevin-Le Chatelier effect triggered by complex loading paths in an Al–Cu aluminium alloy

Serrated flow has been observed during non monotonic tensile tests of an Al–Cu aluminium alloy in the naturally aged state. The associated propagative localisation bands were observed by digital image correlation (DIC). In particular, the Portevin-Le Chatelier (PLC) effect and also Lüders bands were observed in interrupted tests during which the specimen was held for a length of time and also in tests with partial unloading followed by a holding time. Increasing strain rate jumps also triggered the PLC effect. These observations indicate the existence of the PLC effect in this material which was formerly considered insensitive to it at room temperature under monotonic loading conditions. There is no evidence of PLC serrations during constant strain rate tests. A strain ageing finite element model is used that captures the experimentally found PLC triggering effects.     

Extraordinary plasticity of ductile bulk metallic glasses

Shear bands generally initiate strain softening and result in low ductility of metallic glasses. In this Letter, we report high-resolution electron microscope observations of shear bands in a ductile metallic glass. Strain softening caused by localized shearing was found to be effectively prevented by nanocrystallization that is in situ produced by plastic flow within the shear bands, leading to large plasticity and strain hardening. These atomic-scale observations not only well explain the extraordinary plasticity that was recently observed in some bulk metallic glasses, but also reveal a novel deformation mechanism that can effectively improve the ductility of monolithic metallic glasses.     

Dynamic recrystallization as a potential cause for adiabatic shear failure

Dynamic recrystallization (DRX) is almost universally observed in the microstructure of adiabatic shear bands. It is usually admitted that DRX results from the large temperatures that develop in the band along with very high local strains. This paper reports the observation of dynamically recrystallized nanograins in Ti6Al4V alloy specimens that were impact loaded to only half the failure strain at which the adiabatic shear band develops. This observation shows that DRX not only precedes adiabatic shear failure but it is also likely to be a dominant micromechanical factor in the very generation of the band. This result means that adiabatic shear failure is not only a mechanical instability but also the outcome of strong microstructural evolutions leading to localized material softening prior to any thermal softening.     

Modeling of surface cleaning by cavitation bubble dynamics and collapse

Surface cleaning using cavitation bubble dynamics is investigated numerically through modeling of bubble dynamics, dirt particle motion, and fluid material interaction. Three fluid dynamics models; a potential flow model, a viscous model, and a compressible model, are used to describe the flow field generated by the bubble all showing the strong effects bubble explosive growth and collapse have on a dirt particle and on a layer of material to remove. Bubble deformation and reentrant jet formation are seen to be responsible for generating concentrated pressures, shear, and lift forces on the dirt particle and high impulsive loads on a layer of material to remove. Bubble explosive growth is also an important mechanism for removal of dirt particles, since strong suction forces in addition to shear are generated around the explosively growing bubble and can exert strong forces lifting the particles from the surface to clean and sucking them toward the bubble. To model material failure and removal, a finite element structure code is used and enables simulation of full fluid–structure interaction and investigation of the effects of various parameters. High impulsive pressures are generated during bubble collapse due to the impact of the bubble reentrant jet on the material surface and the subsequent collapse of the resulting toroidal bubble. Pits and material removal develop on the material surface when the impulsive pressure is large enough to result in high equivalent stresses exceeding the material yield stress or its ultimate strain. Cleaning depends on parameters such as the relative size between the bubble at its maximum volume and the particle size, the bubble standoff distance from the particle and from the material wall, and the excitation pressure field driving the bubble dynamics. These effects are discussed in this contribution.     

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.     

Chemical transformations in the zone of spall damageability

The results of experiments on studying the perlite–ferrite structure in steels under short-term negative pressures are described. It is shown that in the localized deformation bands formed in the zone of interference of unloading waves, where the tension stress is lower than the dynamic strength of the material, the cementite bands in perlite are crushed, their fragments are in part dissolved and enriched with carbon, and the cementite can pass into a steady spherical form on the boundary with ferrite. At relatively high shock-wave amplitudes, the perlite in its entirety acquires a spheroidal shape.     

Deformation behavior of amorphous Co-Fe-Cr-Si-B alloys in the initial stages of severe plastic deformation

The mechanical behavior and morphological features of the formation of strain relief in amorphous Co-Fe-Cr-Si-B alloys are studied in the initial stage of severe plastic deformation. Patterns of propagation in shear bands upon pressure torsion are revealed. The effects of surface etching and the decoration of shear bands are studied in the alloys. The causes behind these phenomena are discussed using a set of methods for structural investigation.     

Acceleration of Mass Transfer under Dynamic Loading

Under dynamic loading, the interaction of unloading waves with each other and with the faces causes the deformation of the sample due to the formation of standing waves, at the nodes of which the localized- deformation bands arise (if the high-speed tension does not exceed the spall strength, so the material in the zone of interference of unloading waves preserves its continuity). A consequence of deformation localization is the mass transfer of atoms and the doping phase particles to nascent localized-deformation bands. The duration of stress oscillations in standing waves in the ultrasonic frequency range exceeds the duration of the initial impulse by at least two orders of magnitude, which ensures an increase in the distance traveled by atoms and dispersed particles during deformation in standing waves.     

Microalloying and the mechanical properties of amorphous solids

The mechanical properties of amorphous solids like metallic glasses can be dramatically changed by adding small concentrations (as low as 0.1%) of foreign elements. The glass-forming-ability, the ductility, the yield stress and the elastic moduli can all be greatly effected. This paper presents theoretical considerations with the aim of explaining the magnitude of these changes in light of the small concentrations involved. The theory is built around the experimental evidence that the microalloying elements organise around them a neighbourhood that differs from both the crystalline and the glassy phases of the material in the absence of the additional elements. These regions act as isotropic defects that in unstressed systems modify the shear moduli. When strained, these defects interact with the incipient plastic responses which are quadrupolar in nature. It will be shown that this interaction interferes with the creation of system-spanning shear bands and increases the yield strain. We offer experimentally testable estimates of the lengths of nano-shear bands in the presence of the additional elements.     

多孔脆性介质冲击波压缩破坏的细观机理和图像

本文采用一种具有良好定量性质的离散元模型研究了带孔洞的各向同性脆性介质在细观尺度上的压缩破坏特征. 通过对孤立孔洞、三种简单的孔洞排布方式和大量孔洞随机排布等几种情况的模拟, 认识到了剪切破坏和局域拉伸破坏是冲击波压缩下多孔介质的基本破坏模式; 孔洞之间的损伤贯通会促进孔洞在较低应力下发生塌缩, 但损伤区的应力松弛过程却会对一定范围内的介质起到损伤屏蔽作用; 不同区域中损伤促进和损伤屏蔽的综合效果是在多孔脆性介质中形成一种高损伤区与低损伤区间错排布的奇特损伤分布. 本文的研究结果为深入理解脆性材料冲击波压缩破坏的演化过程和机理提供了细观尺度上的初步物理图像.     

设计脆性材料的冲击塑性

通过微结构设计提升脆性功能材料的冲击塑性, 将有助于避免或延缓失效的发生. 提出在脆性材料中植入特定的微小孔洞以改善其冲击塑性的设计方法. 采用一种能够定量表现脆性材料力学性质的格点-弹簧模型, 研究了孔洞排布方式对脆性材料冲击响应的影响. 孔洞随机排布的多孔脆性材料具有明显高于致密脆性材料的冲击塑性, 而设计规则的孔洞排布方式将有助于进一步提升脆性材料的冲击塑性. 对150 m/s活塞冲击下气孔率5%的多孔样品的介观变形特征分析表明, 孔洞规则排布的样品中孔洞贯通和体积收缩变形占主导, 而孔洞随机排布的样品中剪切裂纹长距离扩展和滑移与转动变形占主导. 尽管在宏观的Hugoniot应力-应变曲线上, 两种孔洞排布方式的样品都表现出三段式响应特征(线弹性阶段、塌缩变形阶段和滑移与转动变形阶段), 但孔洞规则排布时孔洞塌缩变形阶段对整体冲击塑性的贡献更大. 研究揭示的规则排布孔洞增强脆性材料冲击塑性的原理, 将有助于脆性材料冲击诱导功能失效的预防.     

Surface Morphology of Deformed Amorphous-Nanocrystalline Materials and the Formation of Nanocrystals

The formation of nanocrystals under deformation in Al-, Fe-, and Co-based amorphous alloys are studied using X-ray diffraction and transmission and scanning microscopy methods. It is shown that the presence of shear bands in deformed amorphous alloys is an insufficient condition for nanocrystal formation. The presence of a large number of intersecting shear bands and an increase in the material temperature in the shear-band region in the case of intense plastic deformation contributes to nanocrystal formation.     

动高压下拉格朗日声速的测定及其应用

 通过实时记录冲击加载—再加载和冲击加载—卸载波的粒子速度剖面,得到了再加载和卸载波在93W合金中传播的拉格朗日声速。通过拉格朗日声速从准弹性到纯塑性的转化,获得了材料在一次冲击终态时的剪应力、剪切模量以及考虑了剪应力修正后的93W合金的冲击绝热关系。     

高应变率压缩加载下LY12铝圆管的剪切断裂研究

 通过回收破片的断口观察和金相分析研究了LY12铝圆管在高应变率压缩过程中的剪切断裂现象；回收破片具有典型的剪切断裂特征，断口与径向近似成45°夹角，断口观察发现断口面经历过径向摩擦；破片金相分析表明，绝热剪切带和裂纹首先在圆管内壁附近产生，然后沿最大剪应力线向外壁扩展。利用数值计算给出了圆管在内爆压缩加载下管壁内微元的应力、应变历史，为分析LY12铝圆管的剪切断裂提供了参考。