延性金属动态拉伸断裂及其临界损伤度研究

【作者】 王永刚；

【导师】 经福谦； 贺红亮； 王礼立；

【作者基本信息】 中国工程物理研究院 ， 凝聚态物理， 2006， 博士

【摘要】 延性金属动态拉伸断裂过程是一个多尺度的科学问题，大体上涉及三个物理层次——原子层次、细观层次和宏观层次。早期学者们通常把实验测量的宏观物理量——层裂强度（最大拉伸强度）作为描述延性金属动态拉伸断裂的特征物理量，但是大量的实验结果表明，层裂强度与加载应力、拉伸应变率相关，因而它不是一个材料物性常数。后来，Curran等人（Physics Reports，147（5&6），253-388，1987）通过对回收样品的金相分析，在细观（介观）层次上认识到延性金属动态拉伸断裂是一个微损伤随时间不断累积而诱发灾变断裂的过程。最近，Strachan等人（Phys．Rev．B，63，060103，2001）和Sepp（?）l（?）等人（Phys．Rev．Lett．，93，245503，2004 and Phys．Rev．B，71，064112，2005）在原子层次上的分子动力学模拟结果表明了延性金属动态拉伸断裂具有某种临界行为。本文在Curran等人（Physics Reports，147（5&6），253-388，1987）的实验研究和封加波等人提出的损伤度函数模型（J．Appl．Phys．，81（6），2575-8，1997）基础上，探讨采用特征物理量——临界损伤度来表征延性金属动态拉伸断裂特性；并基于Strachan等人（Phys．Rev．B，63，060103，2001）的分子动力学研究成果和逾渗理论，提出了一个新的逾渗软化函数，用于描述损伤演化后期到断裂灾变之前由于微孔洞聚集而导致的材料快速软化过程。本文以20^#钢和纯铝为主要模型材料，通过实验和数值模拟方法，对爆炸与冲击作用下延性金属动态拉伸断裂的一般特性进行了比较系统的研究。研究结果表明：在一维应变平面冲击加载条件下，断裂临界损伤度对加载应力和拉伸应变率变化不敏感，并在预测金属柱壳动态拉伸断裂中也具有适用性，从而初步证实了断裂临界损伤度是一个材料物性常数，可以用于复杂应力环境下表征延性金属动态拉伸断裂性质。本文的主要工作和创新点简要归纳如下： 1．采用一级轻气炮加载，通过激光速度干涉测速系统（VISAR）测量平板样品自由面速度剖面，对20^#钢的层裂特性进行了比较系统的实验研究。通过改变飞片和样品的厚度及飞片速度调整拉伸应变率与加载应力，拉伸应变率变化范围为10⁴～10⁶s^-1，加载应力5～10 GPa。实验结果显示：20^#钢的层裂强度随拉伸应变率的增高而增大，10⁶s^-1条件下层裂强度比10⁴s^-1时提高近60％；但在5～10 GPa加载应力范围内和拉伸应变率基本不变的条件下，层裂强度基本不变。文中还对强激光辐照加载条件下纯铝的损伤演化行为和层裂特性开展了类似的实验研究，发现纯铝的层裂强度也随着拉伸应变率增加而增大，特别在拉伸应变率超过10⁶s^-1以后，层裂强度随着拉伸应变率的提高更加显著。 2．对微孔洞演化过程中的聚集行为进行了分析与讨论。文中借鉴Strachan等人（Phys．Rev．B，63，060103，2001）的分子动力学研究成果和逾渗理论，提出了逾渗软更多还原

【Abstract】 The dynamic tensile fracture process in the ductile metals is of multiple scales, including microscopie, mesoscopic and macroscopic scales. Usually, spall strength proposed by early investigators is regarded as a characteristic physical parameter for describing the dynamic tensile fracture. However, extensive experiments indicate that spall strength depends on both of the impact stress and the tensile strain rate, which is not a material constant. Later, mesoscopic studies from metallographic observations of the shocked samples by Curran et al. （Physics Reports, 147（5&6）, 253-388, 1987） showed clearly that the dynamic tensile fracture is resulted from that an accumulation of microdamages which triggers the catastrophic fracture. Recently, by means of molecular dynamic simulation, it was revealed by Strachan et al. （Phys. Rev. B, 63, 060103, 2001） and Seppala et al. （Phys. Rev. Lett., 93,245503, 2004 and Phys. Rev. B, 71, 064112, 2005） that there is a critical behavior in the dynamic tensile fracture of ductile metals at the microscopic atomic level. In this paper, based on experimental studies performed by Curran et al. （Physics Reports, 147（5&6）, 253-388, 1987） and the damage function model proposed by Feng Jiapo et al. （J. Appl. Phys., 81（6）, 2575-8, 1997）, a critical damage parameter is introduced to describe the intrinsic characteristic of the dynamic tensile fracture in the ductile metals. On the basis of the molecular dynamic simulation performed by Strachan et al. （Phys. Rev. B, 63, 060103, 2001） and the percolation theory, a Percolation-Softening （P-S） function is proposed to describe the material’s rapid softening during the void-coalescence process. The universal characteristic for dynamic tensile fracture of ductile metals under the explosion and shock wave loading has been evaluated by experiments and numerical simulation. Results indicate that the critical damage parameter is independent on the impact stress and the tensile strain rate, and it is applicable to predicting the dynamic tensile fracture behavior in metal cylinders, therefore may be regarded as a material constant to identify the intrinsic characteristic of the dynamic tensile fracture in explosion and shock wave events. The main and / or innovative points of the thesis are summarized as follows:1. Firstly, using a gas gun, a set of plate impact experiments were performed for 20 steel by measuring the rear free-surface velocity profiles with a Velocity Interferometer System for Any Reflector （VISAR）. Experiments were arranged in two variations: i) by adjusting the flyer velocity to change impact stress, and ii) by adjusting the thicknesses of flyer and sample to change the tensile strain rate, in order to investigate the effects of loading stress and tensile strain rate on the spall strength. The measured results show that the impact stress has less influence on the spall strength in the range of 5-10 GPa, but an apparent increase of spall strength with tensile strain rate is evidenced, and 60% increase of spall strength is determined in the present tensile strain rate range of 10⁴¹0⁶s^-1. Secondly, the dynamic tensile spallation of pure aluminium subjected to intense laser shock has been studied experimentally. Spall strength calculated from the measured free surface velocity is characterized as a function of更多还原