岩石试件端面效应的变形局部化数值模拟研究

试件端面效应直接影响试件内的应力分布、试件破坏形式和剪切带图案。采用FLAC - 3D模拟了端面效应对岩石试件剪切带图案、速度场和位移场的影响。试样端部的缚束作用较强时 ,仅在试样中部出现X形的剪切带 ;反之 ,在试样端部出现了倾斜的剪切带。压头刚性越大 ,剪切带倾角越大。块体具有整体平移的特性 ,剪切带边缘位移梯度却较大 ,反映了块体沿剪切带发生滑动的剪切破坏本质。数值模拟结果可在众多的试验结果中找到佐证。     

Cosserat连续体模型中本构参数对应变局部化模拟结果影响的数值分析

基于所发展的压力相关弹塑性Cosserat连续体模型及相应的数值方法,以一维剪切层及二维平板压缩问题为例,数值分析了Cosserat连续体模型中的本构参数Cosserat剪模、软化模量及内部长度参数对应变局部化数值模拟结果的影响.结果表明在一定取值范围内,Cosserat剪模对数值模拟结果几乎没有影响,并给出了具体数值计算时的取值范围;软化模量绝对值越大,后破坏段的荷载-位移曲线越陡,计算得到的剪切带宽度越窄;内部长度参数越大,后破坏段的荷载-位移曲线越平缓,计算得到的剪切带越宽.     

表面粗糙度对TC4钛合金柱壳剪切带形成的影响

剪切带是材料在高应变率加载条件下特有的变形和损伤形式之一,关于影响金属材料中剪切带形成的敏感性因素及其机理的研究,一直是科学研究和工程设计中关注的重点问题. 在柱壳高速坍塌过程中,剪切带优先在内表面形核, 其形核及扩展行为受内表面介观状态的影响显著.本文采用爆轰加载厚壁圆筒坍塌实验技术,结合材料表面处理技术、微结构表征技术和剪切带理论模型分析,研究了内表面粗糙度变化对TC4钛合金柱壳剪切带形成影响的细观动力学规律.结果表明, 在爆炸加载形成的高应变率条件下,表面粗糙度对TC4钛合金柱壳中剪切带形成具有明显影响. 在相同的变形条件下,随着试样内表面粗糙度的增大, 剪切带数量、长度和形核速率均增大;表面粗糙度越大, 部分剪切带扩展速率越快, 剪切带长度差异越大,剪切带的屏蔽效应增强. 分析表明,实验获得的剪切带间距与W-O模型和M模型预测结果基本吻合,具体数值受试样内表面粗糙度影响, 随着表面粗糙度的增大,实验结果逐渐小于预测数值.      

TC4在动态载荷下的剪切行为研究

使用分离式霍布金森压杆(SHPB)对2种TC4(Ti-6Al-4V)试样(单边剪切试样与双边剪切试样)在应变率104 s-1下进行动态剪切加载，利用SIM D8高速照相系统捕捉了绝热剪切带扩展的整个历程，得到了TC4在拍照时刻的应力应变曲线；使用金相显微镜和SEM扫描电镜对TC4绝热剪切带的微观形貌进行观察，发现绝热剪切带宽度为5~12 μm，断口从韧窝断裂演变为解理断裂，可观测到韧窝状与河流花样断口形貌，但是并未看到相变的发生；对2种试样就产生绝热剪切带的形式与敏感性进行了分析，实验表明双边试样更易产生绝热剪切带；通过高速照相系统的标定换算，得到TC4绝热剪切带产生的临界剪切应变在78%~88%之间。在SHPB动态加载条件下，TC4绝热剪切带的扩展速度在460~1 250 m/s之间，且应变率越高，剪切带扩展越快，扩展平均速度与名义应变率近似呈线性关系；另外，在同一加载速率下，剪切带并不是匀速扩展，其扩展速度随载荷的增加而不断增加。     

帽型试样动态绝热剪切破坏演化分析

利用分离式霍普金森压杆加载Ta2钛合金扁平闭合帽形受迫剪切试样,结合数字图像相关法和“冻结”试样的微观金相观察,研究剪切区剪切应变的演化、绝热剪切带形成条件等。结果显示：受迫剪切试样在动态加载过程,剪切区剪切应变不断集中,形成绝热剪切带,裂纹沿绝热剪切带发展;随加载率提高,绝热剪切起始临界应变减小;进一步利用数字图像相关法DIC场应变分析及金相微观观测对比,利用卸载回复特性对绝热剪切带起始临界条件进行了讨论,计算的绝热剪切带起始时温升仅为86℃。材料软化可能不是绝热剪切带起始的控制条件,相反是由于绝热剪切带形成造成的应变高度集中发展导致温度急剧升高。     

白光散斑相关法多尺度分析Portevin-Le Chatelier剪切带变形

在适当的温度、应变率或预变形下,合金材料的拉伸试验中,将会出现伴随雪崩式剪切变形带的锯齿形塑性失稳、即PLC(Portevin-Le Chatelier)效应。本文利用图像相关求位移场的方法,对恒定加载应变率下(10-4/s),拉伸铝合金(A2017)试件时出现的PLC效应从宏观剪切带变形到微观晶粒变形等多尺度进行了观察和定量化的分析。通过对PLC效应发生时采集的试件表面的白光散斑图进行相关运算,得到试件表面剪切变形区域各点的精细位移,并在此基础上计算出剪切带区域的应变分布及剪切带的宽度。实验结果显示,当PLC效应发生时,剪切带区域的应变曲线呈台阶型,带的前后边缘应变梯度较大,中间近似呈平台状,带外区域应变值接近零,塑性拉伸变形主要集中在带内。     

921A 钢纯剪切帽状试件绝热剪切变形的数值模拟

结合相关实验,通过一系列基于921A 钢纯剪切帽状试件的SHPB数值模拟,研究试件的绝热剪 切行为,分析试件内绝热剪切带(ASB)的产生、发展以及相应的试件温度场分布。研究发现:ASB是通过剪 切区两端高温高应变的不稳定区域的扩展而形成;ASB的扩展速率与加载速率相关;在本文加载速率范围 内,ASB带宽无明显变化,均为约70m,基本与所设计的试件剪切区宽度一致;且对应所有加载速率,ASB 均为形变带。     

土的工程力学性质的颗粒流模拟

基于颗粒流理论,引入不同的颗粒接触连接本构模型,分别建立了砂土和粘性土的颗粒流模型．通过颗粒流数值模型试验,对砂土和粘性土的室内平面应变试验及其剪切带形成和发展进行了数值模拟,分别对比了不同围压下颗粒流试样与室内试验的应力应变关系曲线,基本再现了砂土和粘性土试样应力．应变关系．通过砂土和粘性土PFC试样剪切带模拟表明,当围压较小时试样内部颗粒位移量小而且分布范围较广,当围压增大时,试样内部颗粒位移量也增大,而且发生较大位移颗粒的分布范围趋于集中,同时随着围压的增大试样内部形成明显的剪切带．无论砂土还是粘性土的PFC试样,随着围压的增加剪切带的形状趋于集中,而且剪切带宽度在减小．在围压很小时,试样内形成大的破坏区域,在围压较大时出现明显的线破坏区．这些规律基本与室内试验结果相似。     

0Cr17Mn5Ni4Mo3Al不锈钢绝热剪切破坏分析

应用分离式Hopkinson压杆（SHPB）装置对0Cr17Mn5Ni4Mo3Al不锈钢圆柱试件进行动态冲击实验。当用300 mm子弹、以50 m/s的速度加载,真应变达到45%时,不锈钢发生以轴线为对称轴的圆锥型破坏。但是用100 mm的子弹以同样的速度进行多次加载,真应变达到80%时,试件仍保持完好。说明试件的破坏与绝热条件有关。对处于破坏临界状态的试件进行金相观察发现：在横截面上,一个圆环形区域内的晶粒被拉长错位;而通过轴线的纵剖面内则有一个与轴线呈45°、宽度约10μm的狭长矩形区域内的晶粒被拉伸成“Z”字形,形成绝热剪切带,晶粒变形从端面到中心逐渐减弱。当高速变形继续进行时,该区域的温度会继续升高,试件将发生绝热剪切破坏。     

快速加载下TNT材料中绝热剪切带的细观研究

本文对快加载后的国产ＴＮＴ材料，采用光学显微镜和扫描电镜观察，研究了其显微组织，结果表明：在快速加载条件下材料中有绝热剪切带生成。剪切带在细观上的表现形式为：（１）以应变高度集中，晶粒剧烈扭曲为特征的细观组织形貌；（２）在局部区域，晶粒被急剧拉长，形成粗亮而边界清晰的剪切带。在快速加载条件下ＴＮＴ材料中绝热剪切带的产生同时取决于应变和应变切带。在快速的加载条件ＴＮＴ材料中绝热剪切带的产生同时决于应     

A new constitutive model for shear banding instability in metallic glass

Inelastic deformation of metallic glass is through shear banding, characterized by significantly localized deformation and emerged expeditiously under certain stress state. This study establishes a new constitutive model addressing the physical origin of the shear banding. In the modeling, the atomic structural change and the free volume generation are embodied by the plastic shear strain and the associated dilatation. The rugged free energy landscape is adopted to naturally reflect the rate-independent flow stress and flow serrations. Based on this, the conditions for the onset of shear banding instability are established, which enables the explicit calculation of the shear band inclination angle and its extension speed. The study concludes that shear band angle is significantly influenced by the diltancy factor and pressure sensitivity, that a shear band does not increase its thickness once emanated from a deformation unit, that the spreading speed of a shear band is intersonic, and that more shear bands, which lead to higher ductility, can be induced by high strain rates or by the introduction of a second material phase. The analysis also demonstrates that the ductility of metallic glass depends on the sample geometry and/or the stress state.     

试样形状对轴承钢绝热剪切带微观组织的影响

绝热剪切带(ASB)的微观组织受试样几何形状的影响。对圆柱、帽形和剪切压缩型三种不同形状的试样进行分离式霍普金森压杆高速冲击试验,研究试样形状对轴承钢绝热剪切带的形成和微观组织的影响。结果表明,在应变率为1 800～3 100 s^-1的范围内,材料对应变率的敏感性很低。圆柱试样呈现明显的应变硬化,而帽形试样和剪切压缩型试样(SCS)在不同应变率下分别出现应变硬化和无应变硬化的特征,但流变应力并未因应变硬化而提高。试样形状对ASB的微观形貌和组织有很大影响。圆柱试样上产生了窄且细长的ASB,只发生了应变诱发的晶粒细化,属于形变ASB;帽形试样和SCS则形成大片状的ASB,由等轴晶组成,且发生了体心立方体(BCC)马氏体转变为面心立方体(FCC)奥氏体的相变,属于相变ASB。尤其是SCS中ASB的等轴晶,有非常清晰的晶界,是典型的动态再结晶晶粒。温升计算结果显示,圆柱试样ASB的温升远低于奥氏体相变温度,而帽形试样和SCS的温升高于马氏体的熔点,导致局部熔融。     

Adiabatic shear localization in the impact of edge-notched specimens

Impact experiments are performed on edgenotched specimens in the two-dimensional punch geometry. Materials tested include 18Ni(350) maraging steel; S7 tool steel; 4340, 300M, HP 9-4-20 and D-6ac ultra high-strength steels; and Ti6Al4V alloy. These materials have shown a high susceptibility to dynamic shear failure in previous studies. Impact velocity ranged from 25 m/s to 45 m/s, and shear bands were found to form at the notch tip and at the die corner on the back side of the specimen for all materials tested. Metallurgical analysis confirms the existence of adiabatic shear bands followed by a crack propagating through the fully developed shear band. High-speed photography was used to observe the initiation of adiabatic shear bands shortly after impact. Laser-etched lines on the specimen surfaces allowed the determination of the time of impact and the initiation time of shear failure. The elapsed time between the two was used to estimate the stress intensity factor at the time of shear band initiation. Comparisons of shear band initiation stress intensity factors at the notch tip and die corner are made. It is seen that the shear bands initiate at approximately the same stress intensity factor at both the notch tip and die corner. Finite element simulations support the use of a square root singularity for the stress in the plate near the corners of a deformable punch or die.     

三轴试验土样剪切破坏过程的判别

本文在土工三轴试验中应用土样全表面变形数字图像测量系统,通过跟踪包裹土样的橡皮膜表面的标记点,得到加载过程中其全表面的变形和应变分布。根据土样表面的应变分布和实测的应力应变关系曲线,可以得到每一时刻土样表面的应力分布。于是,土样表面剪切破坏开始出现的时刻和剪切破坏带贯穿的时刻可分别依据应变和应力判定。依据应变,将橡皮膜上各点的轴向应变的拐点最晚出现的时刻作为剪切带完全贯穿的时刻;依据应力,将全部点中某一个或几个点最早达到峰值应力的时刻作为剪切带开始出现的时刻;土样各点相继达到应力峰值的过程就是剪切破坏的过程。以剪切破坏的起始点和剪切带贯穿的点为分界,土样剪切破坏过程可以划分成三个阶段:破坏前阶段、破坏阶段和破坏后阶段,在不同阶段土样具有不同的变形特点和变形机理。     

Effects of texture on shear band formation in plane strain tension/compression and bending

In this study, effects of typical texture components observed in rolled aluminum alloy sheets on shear band formation in plane strain tension/compression and bending are systematically studied. The material response is described by a generalized Taylor-type polycrystal model, in which each grain is characterized in terms of an elastic–viscoplastic continuum slip constitutive relation. First, a simple model analysis in which the shear band is assumed to occur in a weaker thin slice of material is performed. From this simple model analysis, two important quantities regarding shear band formation are obtained: i.e. the critical strain at the onset of shear banding and the corresponding orientation of shear band. Second, the shear band development in plane strain tension/compression is analyzed by the finite element method. Predictability of the finite element analysis is compared to that of the simple model analysis. Third, shear band developments in plane strain pure bending of a sheet specimen with the typical textures are studied. Regions near the surfaces in a bent sheet specimen are approximately subjected to plane strain tension or compression. From this viewpoint, the bendability of a sheet specimen may be evaluated, using the knowledge regarding shear band formation in plane strain tension/compression. To confirm this and to encompass overall deformation of a bent sheet specimen, including shear bands, finite element analyses of plane strain pure bending are carried out, and the predicted shear band formation in bent specimens is compared to that in the tension/compression problem. Finally, the present results are compared to previous related studies, and the efficiency of the present method for materials design in future is discussed.     

Axisymmetric bifurcation analysis in soils by the tangential-subloading surface model

This article discusses localized bifurcation modes corresponding to shear band formation and diffuse bifurcation modes corresponding to bulge formation for cylindrical soil specimen subjected to an axisymmetric load under undrained conditions. We employ the tangential-subloading surface model, which exhibits the characteristic regimes of the governing equations: elliptic, hyperbolic and parabolic. Also, conditions for shear band formation, shear band inclination, diffuse bulging formation, and the long and short wavelength limits of diffuse bulging modes are discussed in relation to material properties and their state of stress, i.e. the stress ratio and the normal-yield ratio. Tangential-plastic strain rate term is required for the analyses of shear band and diffuse bulging. The shear band and the diffuse bulging are generated in not only normal-yield but also subyield states and they are severely affected by the normal-yield ratio describing the degree of approach to the normal-yield state.