内爆加载下热塑性管壳的应力波演化与层裂效应研究

 在引入材料损伤软化、温度软化效应的基础上，对一维柱对称内聚爆加载管壳中应力波的传播、材料的变形和层裂问题开展了研究。成功地通过自编写有限元代码，完成了系列数值模拟计算，得到了管壁中与物理事实一致的应力波传播图像和材料的变形、损伤、层裂的发展规律，成功模拟了内聚爆加载管壳的多次层裂现象。     

不同晶粒度高纯铜层裂损伤演化的有限元模拟

采用Voronoi方法构建了50, 130和200μm三种晶粒度的高纯铜靶板,在晶界处随机预制损伤成核点,建立了平板撞击高纯铜靶板的二维轴对称计算模型,研究了晶粒度和加载应力对高纯铜层裂宏观力学响应和细观损伤演化的影响.基于自由面速度剖面特征分析,揭示了晶粒度和加载应力幅值对Pull-back速度回跳点位置、速度回跳斜率及回跳幅值的影响规律,论证了层裂强度与损伤区拉伸应力峰值相对应本质上表征微损伤早期长大临界应力;基于损伤演化云图特征分析,讨论了长大和聚集过程中微孔洞周围局域化塑性应变场的演变,揭示了晶粒度和加载应力对微孔洞聚集和应力松弛行为的影响.计算结果再现了层裂实验中材料内部的微孔洞长大、聚集的细观物理过程,进一步揭示其与宏观力学响应之间的内禀关系,这对认识层裂损伤演化机制和理论模型构建具有重要的意义.     

高纯铜初始层裂的微损伤特性研究

本文对平面冲击加载下高纯铜初始层裂的微损伤特性进行了研究. 利用准三维的表面轮廓测试技术, 对冲击加载“软回收”的样品截面进行测试. 通过对测试数据的重构、量化和统计分析, 结果表明: 拉伸应力持续时间和加载应力幅值的增加, 都会加剧样品内部损伤局域化程度. 样品内损伤区域宽度是亚微米尺度的损伤演化的结果, 并且亚微米尺度的演化速率随着拉伸应变率的增加而单调递增. 通过统计获得了样品内微损伤的尺寸分布特征, 并分析了其与损伤演化进程的关联.     

非一维应变冲击加载下高纯铜初始层裂行为

提出了一种锥形靶层裂实验新方法,开展非一维应变冲击条件下高纯铜初始层裂行为实验研究,讨论了锥形靶内部损伤分布特征及其与自由面速度典型特征之间的内禀关系.结果显示:1)初始层裂的锥形靶内部出现了连续损伤区,损伤区扩展方向与锥面平行,从锥底到锥顶呈现了不同的损伤状态,从微孔洞独立长大到局部聚集,最后形成宏观裂纹,这种损伤状态分布特征归因于锥形靶内部拉伸应力幅值和持续时间的空间演化;2)通过锥形靶横截面损伤度定量统计分析,揭示损伤演化早期的微孔洞成核与早期长大过程是随机的,而损伤演化后期的微孔洞聚集过程具有显著的局域化特征;3)不同位置处实测的自由面法向粒子速度剖面呈现出典型的层裂Pull-back信号,但是通过与内部损伤分布特征对比,揭示基于Pull-back速度获得高纯铜层裂强度本质是微孔洞成核阈值应力,Pull-back回跳速度斜率反映了损伤演化速率,Pull-back回跳幅值与损伤度引起的应力松弛密切相关.     

单晶铜在动态加载下空洞增长的分子动力学研究

冲击载荷下延性材料的损伤是材料中微空洞的产生和长大演化的结果.利用分子动力学模拟 方法对延性金属单晶铜中单个空洞在动态加载下的演化发展进行了研究，得到了空洞增长过 程中的应力分布及空洞增长演化随冲击强度变化的规律.模拟结果表明，动态加载下的前期 压缩过程对后期拉伸应力场作用下的空洞增长演化特征有不可忽视的影响，微空洞增长的阈 值则与单晶实验中层裂强度随拉伸应力作用时间减少而增加的趋势相一致. 关键词： 层裂 分子动力学 动态加载 空洞     

高加载率条件下LY12铝合金损伤断裂现象的研究

 本文以球形实体中含有一球形空穴为基本单元，通过分析单元的受力状态，推导出了一个韧性介质空穴增长模型。模型中考虑了介质的硬化，应变率效应，同时还考虑了惯性效应。在考虑空穴成核效应时，采用了热激活成核机制。对LY12铝合金进行了二维层裂实验，采用本文提出的动态损伤模型，借助于二维Lagrange弹塑性流动有限差分程序对二维层裂实验进行了数值模拟，计算结果与实验吻合较好。     

冲击载荷下金属材料的微结构-加载特性-层裂响应关系概述

鉴于冲击载荷下金属材料的变形损伤具有微结构和加载特征的依赖性,对国内外有关层裂现象的研究进展进行了简要梳理与总结。概述了晶粒尺寸、织构、晶界类型、相界和元素偏析带等结构对金属材料变形损伤的影响,重点阐述了脉冲宽度、应变率和峰值应力等加载特征对金属材料层裂强度的耦合作用,为了解冲击载荷下金属材料微结构、加载特征与变形损伤行为之间的关系提供参考。     

激光加载锡的动态破碎颗粒尺寸分布

基于Grady能量破坏准则,结合固体动态断裂碎块尺寸分布规律研究结果,提出了材料微层裂破碎颗粒质量分布改进模型,并对强激光加载下锡的熔化破碎实验进行了理论计算。研究显示,理论计算的锡的微层裂破碎颗粒分布数随直径变化规律与实验结果一致,模型进一步改进需考虑应变率变化等更多综合因素影响。     

激光加载锡的动态破碎颗粒尺寸分布

基于Grady能量破坏准则,结合固体动态断裂碎块尺寸分布规律研究结果,提出了材料微层裂破碎颗粒质量分布改进模型,并对强激光加载下锡的熔化破碎实验进行了理论计算。研究显示,理论计算的锡的微层裂破碎颗粒分布数随直径变化规律与实验结果一致,模型进一步改进需考虑应变率变化等更多综合因素影响。     

铝锂合金材料损伤演化方程和层裂准则的研究

 通过平板撞击实验对铝锂合金层裂现象进行实验研究。以细观统计和唯象分析相结合的思想,提出了一种新的微孔洞型损伤演化方程,方程同时计及了孔洞长大、成核以及材料可压缩性的耦合效应对损伤演化的影响。在一维应变波模拟程序中嵌入考虑损伤演化影响的铝锂合金本构方程和状态方程,模拟平板撞击过程靶板自由面质点速度时程曲线,并与实验曲线相对比,得到了铝锂合金材料损伤演化方程及其层裂准则的相关参数。     

Effect of grain size and arrangement on dynamic damage evolution of ductile metal

Plate-impact experiments have been carried out to examine the effect of grain size and grain arrangement on the damage evolution of ultrapure aluminum. Two groups of samples, "cross-cut" and "longitudinal-cut," are obtained from the rolled aluminum rod along different directions. The peak compressive stress is approximately 1.25 GPa-1.61 GPa, which can cause incipient spall damage that is correlated to the material microstructure. The metallographic analyses of all recovered samples show that nearly all damage nucleates at the grain boundaries, especially those with larger curvature. Moreover, under lower shock stress, the spall strength of the "longitudinal-cut" sample is smaller than that of the "crosscut" sample, because the different grain sizes and arrangement of the two samples cause different nucleation, growth, and coalescence processes. In this study, the difference in the damage distribution between "longitudinal-cut" and "cross-cut" samples and the causes for this difference under lower shock-loading conditions are also analyzed by both qualitative and semi-quantitative methods. It is very important for these conclusions to establish a reasonable and perfect equation of damage evolution for ductile metals.     

A study on the effect of stress state on damage evolution in hot deformation of free cutting steels using double notched bars

The effect of stress state on the initiation of damage for leaded free cutting steel has been investigated under hot rolling conditions. Double notched (DN) circumferential tension samples were designed and used to simulate damage development at different stress states and deformation conditions using a Gleeble (3800) thermal-mechanical testing system. Two DN sample geometries with varying notch profiles were used to account for different states of stress. To simulate the conditions of hot rolling the samples were tested at high temperatures (900–1200 °C) and moderate strain rates (0.1–1 s^?1). After testing to failure, which normally occurs at one notch of the specimen, the unfailed notch of each sample was sectioned to examine the sites where damage occurs since the material has been captured in a state very close to failure. Two of the cases examined have shown definitive damage paths occurring from ‘outside–in’ for a sharp notch deformed at T = 900 °C and from ‘inside–out’ for a blunt notch tested at T = 1200 °C for the same strain rate of 0.1 s^?1. The experimental results of the failure initiation sites were compared with computed values of the stress fields around the notch profiles, obtained from FE analysis using a set of viscoplastic constitutive equations calibrated for free cutting steel. The temperature profiles from high temperature mechanical testing were used in the FE calculations of the stress state.     

动态损伤演化的空间不连续性实验研究

对冲击加载下高纯铝的损伤演化进行了实验研究. 利用基于白光轴向色差的表面轮廓测试技术测试冲击加载“软回收”的样品截面, 对测试结果进行三维重构和损伤量化计算. 结果表明: 受到孔洞形核效应、尺寸效应和应力松弛作用, 在损伤演化早期, 损伤度随着空间的分布是不连续的, 除最大损伤度以外还存在一个次高峰. 在损伤演化后期, 受到贯穿作用的影响, 损伤度增量随空间的分布也是不连续的, 贯穿区域损伤度迅速增加, 损伤度曲线的次高峰特征消失. 关键词： 高纯铝 冲击波 损伤演化 空间不连续性     

Study on the negative bias temperature instability effect under dynamic stress

This paper studies negative bias temperature instability (NBTI) under alternant and alternating current (AC) stress.Under alternant stress,the degradation smaller than that of single negative stress is obtained.The smaller degradation is resulted from the recovery of positive stress.There are two reasons for the recovery.One is the passivation of H dangling bonds,and another is the detrapping of charges trapped in the oxide.Under different frequencies of AC stress,the parameters all show regular degradation,and also smaller than that of the direct current stress.The higher the frequency is,the smaller the degradation becomes.As the negative stress time is too small under higher frequency,the deeper defects are hard to be filled in.Therefore,the detrapping of oxide charges is easy to occur under positive bias and the degradation is smaller with higher frequency.     

Modelling of spall damage in ductile materials and its application to the simulation of plate impact on copper

A statistical model of dynamic spall damage due to void nucleation and growth is proposed for ductile materials under intense loading, which takes into account inertia, elastic-plastic effect, and initial void size. To some extent, void interaction could be accounted for in this approach. Based on this model, the simulation of spall experiments for copper is performed with the Lagrangian finite element method. The simulation results are in good agreement with experimental data for the free surface velocity profile, stress record behind copper target, final porosity, and void concentrations across the target. The influence of elastic-plastic effect upon the damage evolution is explored. The correlation between the damage evolution and the history of the stress near the spall plane is also analyzed.     

Modelling of spall damage in ductile materials and its application to the simulation of the plate impact on copper

A statistical model of dynamic spall damage due to void nucleation and growth is proposed for ductile materials under intense loading, which takes into account inertia, the elastic-plastic effect, and initial void size. To some extent, void interaction could be accounted for in this approach. Based on this model, the simulation of spall experiments for copper is performed by using the Lagrangian finite element method. The simulation results are in good agreement with experimental data for the free surface velocity profile, stress record behind copper target, final porosity, and void concentrations across the target. The influence of elastic-plastic effect upon the damage evolution is explored. The correlation between the damage evolution and the history of the stress near the spall plane is also analyzed.     

Influence of boundary structure and near neighbor crystallographic orientation on the dynamic damage evolution during shock loading

The role of crystallographic orientation on damage evolution in ductile metals during shock loading has been investigated. By utilizing large-grained copper specimens, it has been shown that the development of intragranular damage, in the form of void growth and coalescence, is influenced by the grain orientation with respect to the applied load. Additionally, strain incompatibility and the inability to promote transmission or activation of secondary dislocation slip across a grain boundary, are proposed as the likely cause for intergranular failure. Finally, the free surface velocity profiles of each grain, specifically the decay of the oscillations after the pull-back, correlated well with the amount of damage measured within the respective grain.     

延性金属层裂自由面速度曲线物理涵义解读

对延性金属层裂自由面速度曲线上典型特征所蕴含的物理涵义进行了新的解读. 揭示了自由面速度曲线上宏观响应特征和微孔洞成核、长大和聚集的微损伤演化动力学之间的关联: Pullback信号的临界点对应于微孔洞的成核,Pullback幅值表征了损伤成核或起始的条件——成核强度; Pullback信号之后的回跳斜率代表了损伤演化速率, Pullback 信号后速度回跳幅值从一定程度上反映了损伤程度; Pullback信号之后的振荡周期模式反映了损伤的局域化状态.     

Stress transfer around a broken fiber in unidirectional fiber-reinforced composites considering matrix damage evolution and interface slipping

A shear-lag model is applied to study the stress transfer around a broken fiber within unidirectional fiber-reinforced composites(FRC) subjected to uniaxial tensile loading along the fiber direction.The matrix damage and interfacial debonding,which are the main failure modes,are considered in the model.The maximum stress criterion with the linear damage evolution theory is used for the matrix.The slipping friction stress is considered in the interfacial debonding region using Coulomb friction theory,in whic...     

延性材料动态拉伸断裂早期连通过程的逾渗描述

 借鉴逾渗理论,提出了新的应力松弛函数,简称PR函数（Percolation-Release Function）,用以描述断裂发生前由于微孔洞连通而引起的快速应力松弛过程,具有较为明确的物理意义。通过其与损伤度函数模型进行耦合,对无氧铜和45钢平板撞击层裂进行了数值模拟。结果表明：数值计算不仅有效地再现了界面的应力历史和自由面速度历史,而且再现了靶中的损伤分布;新的逾渗松弛函数具有较好的适用性。