Shear localization in dynamic deformation of copper single crystals

Dynamic deformation of copper single crystals, especially of fatigued copper single crystals with different orientations, was conducted on a split-Hopkinson pressure bar apparatus. The strain rates were in the range 2???9?×?10³?s^?1. After dynamic deformation, the adiabatic shear bands (ASBs) were examined in a light microscope and SEM. The width and spacing of ASBs formed under different strain rates in a fatigued copper single crystal were measured and the spacing of ASBs is one-order of magnitude smaller than the theoretical predictions. The possible reasons for the discrepancy were discussed. The critical strains for the ASB formation in four different orientated single crystals at stain rate of about 4?×?10³?s^?1 were determined by examining the post-deformation specimens and dynamic stress–strain curves. It is clearly indicated that the critical strains for the ASB formation are orientation-dependent in copper single crystals. A simple microscopic mechanism for ASB formation in fatigued single crystals was proposed.     

不同应变率下两种岩石的压缩破碎特征试验研究

为了研究不同岩石在不同应变率下压缩时裂纹的产生规律及破坏模式,将石灰岩和红砂岩制成试件,研究其在不同应变率和受力模式下裂纹的形成模式。开展了两种岩石的准静态压缩和动态压缩试验,采用高速摄影机记录了裂纹的产生和破坏模式。对两种岩石试件的裂纹形态进行对比,基于岩石的物理性质、受力状态、能量演化分析,得到了在不同应变率下压缩时产生差异性的原因。结果表明:准静态压缩下岩石试件受压的破坏模式也会因应变率的不同而存在差异,并且破坏模式的差异对岩石试件的抗压强度将产生显著的影响;从能量演化的角度分析,入射能量的大小将会决定岩石试样动态抗压强度曲线是否出现起伏;动态压缩时,裂纹的周向扩展速度与岩石抗压强度呈正相关。     

结构钢中绝热剪切带形成与扩展的光学观测与数值模拟

绝热剪切带(Adiabatic Shear Band,ASB)是许多金属材料在冲击载荷作用下发生破坏的主要原因之一,它是近年来冲击动力学和损伤力学研究的前沿和热点。相关的理论研究主要针对一维剪切条件,分析应力、应变、剪切速度、材料热物理和力学性能、初始缺陷大小之间的关系,得到一个由多个物理量组合而成的量来判别材料出现剪切带的难易。ASB的实验主要利用Hopkinson压杆、扭杆、压剪炮等加载技术,研究钛合金、钨合金、高强结构钢等材料的剪切带特征,包括局部温度和变形分布、剪切带出现的阈值等。但是,对剪切带演化过程的在位观察及其动态实时演化的研究还较少见,妨碍了人们对由于剪切局部化而导致的材料破坏机理的深入认识。针对45钢,在Hopkinson压杆上,开展了不同冲击加载条件下剪切带演化过程的在位观察及可视化研究。利用自行设计的高分辨力的光学观测系统和基于数字相关理论的图像处理软件,捕捉了单一试样在冲击加载条件下ASB逐渐形成和扩展的过程。同时,利用LS-DYNA商业程序对试样的冲击压缩过程进行了数值模拟,所得主要结果与实验观测基本一致。     

Microstructural evolution of AZ31 magnesium alloy subjected to sliding friction treatment

Microstructural evolution and grain refinement mechanism in AZ31 magnesium alloy subjected to sliding friction treatment were investigated by means of transmission electron microscopy. The process of grain refinement was found to involve the following stages: (I) coarse grains were divided into fine twin plates through mechanical twinning; then the twin plates were transformed to lamellae with the accumulation of residual dislocations at the twin boundaries; (II) the lamellae were separated into subgrains with increasing grain boundary misorientation and evolution of high angle boundaries into random boundaries by continuous dynamic recrystallisation (cDRX); (III) the formation of nanograins. The mechanisms for the final stage, the formation of nanograins, can be classified into three types: (i) cDRX; (ii) discontinuous dynamic recrystallisation (dDRX); (iii) a combined mechanism of prior shear-band and subsequent dDRX. Stored strain energy plays an important role in determining deformation mechanisms during plastic deformation.     

Temperature evolution associated with dynamic phase transformation in shape-memory TiNi alloys

In this paper, temperature evolution in shape-memory TiNi alloys was in situ measured in the process of phase transition under shock loading tests. It was shown that the temperature increased during loading, while unloading, it would keep the maximum value constant or decreased, depending on the strain rate. The corresponding dynamic response indicated that the hardening property is related to phase transition strain and the transformation path has remarkable strain-rate effect. The reason for this phenomenon is essentially due to the temperature dependence of the phase transformation stress. Moreover, the evolution of temperature was consistent with the change of stress, suggesting that this dynamic phase transition deformation is uniform. The theoretical calculation shows that the mechanism of temperature evolution results from the release of latent heat and the hysteresis dissipated energy. The results of this investigation provide insight into intriguing strain-rate-dependent phenomena intrinsic of TiNi alloys.     

Al-Mg合金中锯齿形屈服现象的热分析

研究了在室温、定加载应变率拉伸的情况下Al-Mg合金中的锯齿形屈服现象.伴随着锯齿形屈服现象的发生,试件表面温度场会发生变化.而红外相机能以较高的时间、空间分辨率记录下随时间变化的试件表面温度场图像.通过分析这些热图像,探讨了A,B两种类型带的传播规律,得到了局域变形带的带宽、倾角、传播速度等特征参数.在此基础上,引入热传导方程,求得了带内的应变率.实验和计算都发现B类型带产生时试件表面带外区域存在弹性收缩现象,由此提出以是否存在带外收缩变形作为划分A,B类型带的新标准.     

Deformation and stress-induced transformation of polybutene-1

The deformation at room temperature of thick (ca. 2 mm) and thin (ca. 1000 Å) films of polybutene was characterized by electron microscopy and electron x-ray diffraction. The thin films, as studied, draw by micronecking whereas the thick films draw uniformly. In both types of samples the details of the deformation process varied with the orientation of the lamellae in the original spherulitic structure; in all cases, however, the lamellae broke up into “mosaic” blocks prior to uniform fibril formation. The tetragonal-hexagonal phase transformation accompanying the deformation of fresh polybutene films generally appeared to occur within the blocks except in those situations in which the lamellae are being drawn perpendicular to the molecular axes, that is, parallel to the lamella surface; in the latter case the tetragonal form can be at least partially retained until unfolding and fibril formation occurs.     

Microscopical study of the formation of adiabatic shear bands in 4340 steel during dynamic loading

In this study, optical microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction and electron probe microanalyser were used to analyse the changes in microstructure of AISI 4340 steel specimens caused by impact at high strain rates and large strains. The structures of the steel prior to dynamic deformation and after dynamic deformation were examined to understand on a microscale level, the mechanism of formation of adiabatic shear bands (ASBs). The study also includes the structural changes that occur during post-deformation annealing processes which may relate to understanding of the mechanism of formation of ASBs. Prior to deformation, the tempered steel specimens consisted of lenticular laths of α-ferrite with precipitated platelet and spherical M₃C carbides. After impact, the structure inside the shear band was characterized by refined and recrystallized grains immersed in dense dislocation structures. In addition, residual carbide particles were observed inside the shear bands due to deformation induced carbide dissolution. Regions away from the shear bands developed ‘knitted’ dislocation walls, evolving gradually into sub-boundaries and highly misoriented grain boundaries at increasing strains, leading to grain refinement of the ferrite. After impact, annealing the shear bands at 350?°C resulted in an increase in hardness regardless of the heat treatment before impact, amount of deformation and the time of annealing. This is because of the occurrence of extensive reprecipitation of dissolved carbides that existed in the steel structure prior to deformation. It is concluded that dynamic recovery/recrystallization, development of dislocation structures and carbide dissolution all contribute simultaneously to the formation of ASBs in quench-hardened steels.     

Three-dimensional strain localization of water-saturated clay and numerical simulation using an elasto-viscoplastic model

Since strain localization is a precursor of failure, it is an important subject to address in the field of geomechanics. Strain localization has been analysed for geomaterials by several researchers. Many of the studies, however, treated the problems brought about by strain localization as two-dimensional problems, although the phenomena are generally three-dimensional. In the present study, undrained triaxial compression tests using rectangular specimens and their numerical simulation are conducted in order to investigate the strain localization behaviour of geomaterials under three-dimensional conditions. In the experiments, both normally consolidated and over-consolidated clay samples are tested with different strain rates. Using the distribution of shear strain obtained by an image analysis of digital photographs taken during deformation, the effects of the strain rates, the dilation, and the over-consolidation on strain localization are studied in detail. The analysis method used in the numerical simulation is a coupled fluid-structure finite element method. The method is based on the finite deformation theory, in which an elasto-viscoplastic model for water-saturated clay, which can consider structural changes, is adopted. The results of the simulation include not only the distribution of shear strain on the surfaces of the specimens, but also the distributions of strain, stress, and pore water pressure inside the specimens. Through a comparison of the experimental results and the simulation results, the mechanisms of strain localization are studied under three-dimensional conditions.     

一维应变压缩下TC4钛合金绝热剪切研究

 利用内径为57 mm的压缩气炮，在撞击速度为0.2~1.2 km/s（相应的靶中压力为3~15 GPa）范围内进行对称碰撞实验，以研究TC4（Ti-Al6-V4）钛合金在一维应变冲击压缩条件下的绝热剪切现象。对回收得到的受冲击样品，在扫描电镜（SEM）下进行细观金相分析。结果指出，一维应变冲击压缩条件下，TC4钛合金中绝热剪切带产生的对称碰撞速度阈值为500 m/s（相当于样品中的压力为5.87 GPa）；主剪切带与冲击方向约为45°角，带上有圆形和椭圆形两种孔洞且随碰撞速度的增大而增多和长大，这是典型的韧性损伤特征。随碰撞速度增大，产生与主剪切带成15°角的支剪切带。这些与理论预言相符。X射线能谱分析结果指出，剪切带内材料发生了(α+β)→β相的转变，是典型的相变带。剪切带的温度估算与实验提供的信息吻合。     

Distinctive features of plastic strain localization under severe plastic deformation of metals

Mechanical testing is performed and the structure of zirconium and aluminum predeformed by ∼450% using multiaxial forging (MAF) and equal channel angular pressing (ECAP) is investigated. Tensile loading tests of the severely deformed specimens exhibited their tendency to necking, with the ductility of the material in the neck, however, being superior to that in the neck of initial coarse-grained specimens. The results of the experiments imply that a fundamental stage of plastic flow of solids under severe plastic deformation (SPD) is the formation of cellular-banded structure and strain localization in the fine-grain bands. This considerably retards further deformation-induced refinement of the structure by SPD, and also results in the rapid formation of a fracture neck in the materials with this structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 43–49, November, 2007.     

某PBX炸药的动态力学性能研究

 PBX炸药作为现代武器的主装药,它的力学行为决定着武器的生存能力。为了研究PBX炸药的动态力学特性,采用分离式霍普金森压杆（Split Hopkinson Pressure Bar,SHPB）作为加载手段,结合半导体应变片测试技术和压电晶体监测技术,保证了实验数据的有效性。利用SHPB加载波整形技术,实现了材料两端应力平衡和常应变率加载,得到了不同应变率（90~410 s^-1）下材料的应力-应变曲线。根据材料的模量、破坏强度和破坏应变随应变率的变化规律,采用粘性修正的Sargin模型,得到了该PBX炸药在单轴压缩下的唯象本构模型,模拟结果与实验曲线符合较好。     

Superplasticity of a microcrystalline aluminum-lithium alloy under torsion

The deformation of an aluminum-lithium alloy under torsion in the temperature range 523–673 K at angular velocities of 0.322 and 0.0322 rad/s is studied. The shear strain γ prior to failure is found to increase with decreasing strain rate, and its temperature dependence has a maximum at 553 K (γ≈30). The initial loading-induced jump in the dependence of the torque on the angle of rotation (stress-strain curve) is followed by a softening stage, which changes into a hardening stage or a stage with a constant torque at all temperatures except those near 673 K. The stress distribution over a cross section is analyzed, and the dependence of the shear-strain rate \(\dot \gamma \) on the stress τ and temperature T is found to be \(\dot \gamma \sim \tau ^n \exp ( - {U \mathord{\left/ {\vphantom {U {kT}}} \right. \kern-\nulldelimiterspace} {kT}})\). The results are compared with those obtained earlier from tensile tests of this alloy.     

Relaxation and Creep of Dry Bulk Solids

In former investigations it has been shown that creep (constant stress, altering strain) and relaxation (constant strain, decreasing stress) can be observed with dry bulk solids. Both effects are covered when investigating the time dependent behaviour of bulk solids where time dependence can also mean an increase of the deformation resistance with increasing deformation rate. In this paper the investigated time dependent effects do not include time consolidation. The effects of creep and relaxation are often neglected for bulk solids because in many applications the influence of these time dependent behaviours is of minor importance. A deeper insight into the bulk solids flow characteristics and mechanisms can only be obtained when time dependence is taken into consideration.     

Experimental study of different modes of block sliding along interface. Part 1. Laboratory experiments

This paper is the first part of an experimental work on studying the formation of different deformation modes of rock discontinuities under laboratory and field conditions. The formation conditions of different sliding modes were studied under laboratory conditions for several types of discontinuities, such as rigid surface contact and cracks filled with quartz sand, talc, and clay. A wide range of shear deformation modes were experimentally reproduced—from dynamic slip with a maximum velocity of tens of mm/s to stable sliding with a velocity of 1 µm/s. The behavior of a crack with a clay-containing gouge drastically changes after its wetting. The larger is the content of clay, the longer is the slip duration. The motion of a block consists of a long phase (~100 s) in which displacement velocity smoothly increases, and a retardation phase of almost the same duration in which displacement velocity decreases down to a few tens of µm/s. The used sensors detected no acoustic emission prior to the beginning of block sliding as well as on all stages of block motion until its full stop. It is shown that slow slip events have all stages typical for stick-slip motion: acceleration, long sliding, retardation, arrest, and quiescence. The conducted laboratory experiments substantiate the earlier statement that all types of deformation processes in the Earth’s crust produce a common range of phenomena.     

Dynamic versus quasi-static loading of X65 offshore steel pipes

Anchors or trawl gear occasionally impact offshore pipelines, resulting in large local and global deformations. Impact velocities are typically less than 5?m/s, but local strain rates may be very high. In this study strain rate effects in an X65 offshore material was characterised by split Hopkinson bar tests, while the cross-section homogeneity and possible anisotropic behaviour were determined by quasi-static material tests. Further, dynamic impact tests at prescribed velocities were carried out on simply supported full scale X65 steel pipes. Next, deformation-controlled quasi-static tests with the same boundary conditions were conducted. The level of deformation in the quasi-static tests was set to be equal to that attained in the dynamic tests. Finally, an assessment of the differences between the dynamically and quasi-statically loaded pipes was made in terms of force-displacement response, energy absorbed, and fracture. An optical light microscope and a scanning electron microscope were used to investigate fracture surfaces arising from the various tests.     

Role of packing-defect energy in the localization of plastic strain during shock-wave loading of copper-based solid solutions

The formation of deformation structure accompanying a change in the packing-defect energy (PDE) and the conditions of deformation of copper-based solid solutions under shock-wave loading were investigated by the method of optical microscopy. It is shown that in the case of solid solutions with high PDE plastic strain is localized. This localization increases with the amplitude and duration of the shock pulse, and this in turn gives rise to the generation and growth of microcracks. V. D. Kuznetsov Siberian Physicotechnical Institute at Tomsk State University. Translated from Izvestiya Vysshykh Uchebnykh Zavedenii, Fizika, No. 2, pp. 30–34, February, 1993.     

入射波整形技术的实验和理论研究

 在硅橡胶材料的分离式霍普金森压杆实验中,实验研究了如何实现常应变率加载,并且得到了整形器尺寸与加载应变率之间,以及加载应变率与试样厚度之间的定量关系。根据预估入射波形的理论模型,给出了采用H62黄铜整形器整形后入射波形的计算结果和实验结果,二者基本上是一致的。     

Point defect generation,nano-void formation and growth. I. Validation

The volume fraction of point defects generated as a function of plastic shear strain squared, γ², was derived from crystal plasticity concepts. The evolution was determined from the stress–strain values using a new constitutive relation which replicates the measured behavior with at least two fitted loci. Assuming that nano-voids form by clustering of vacancies, the nano-void diameter was found to be proportional to their spacing and shear strain with the constant being characteristic of point defect production during deformation. The predicted amount of point defect generated was validated using the previously determined resistivity of [100] copper single crystals deformed at 4.2?K and annealed at 296?K. Similar analysis of super-pure polycrystalline copper data affirmed that the dynamic annihilation parameter extrinsically incorporated in the new derivation is larger due to formation of slip clusters. Moreover, the temperature dependence of the mean slip-distance to inter-forest spacing ratio at Stage II to III transition indicates that the thermally activated drag of vacancy-creating jogs occurs above 150?K. For polycrystalline aluminum deformed at 296?K, it was concluded that the nuclei of the nano-voids were not part of the evolving dislocation array but were embedded in the grown-in microstructure. This hypothesis is pursued in the accompanying paper, Part II, and its prediction results in a criterion for ductile failure.     

熔合反应中动力学势垒的研究

用改进的量子分子动力学模型研究了与入射能量相关的重离子熔合势垒. 随着 入射能的降低可以观察到动力学势垒的最低值, 这个最低动力学势垒与绝热势垒 非常接近；动力学势垒随着入射能的增加而升高, 最终接近于静态势垒(非绝热势垒). 基于动力学势垒的研究, 对于重离子熔合反应的额外推动(extra-push)给出了微观理解,对势垒贯穿给出了一种新的解释. 为进一步理解动力学势垒, 还研究了颈部的形成和体系的动力学形变, 分析了动力学势垒降低的原因.