【作者】 范端；

【导师】 经福谦； 祁美兰；

【作者基本信息】 武汉理工大学 ， 一般力学与力学基础， 2011， 博士

【摘要】 延性金属材料在强冲击载荷作用下的拉伸型损伤与力学失效是许多工程技术领域中的重要科学问题。层裂(spallation)是这类问题相关的一种典型破坏现象,它是在冲击波作用下,由于相向稀疏波相互作用产生的拉伸应力引起材料内部微损伤成核、长大以及贯通,最后导致材料发生灾变式断裂的一种物理、力学现象。国内宁波大学的王永刚等人(中国工程物理研究院博士学位论文,2006年)在Curran等人(Physics Reports,147(5 & 6),253-388,1987)的实验研究和封加坡等人(J.Appl.Phys.,81(6),2575-8,1997)的模型基础上,提出一个逾渗软化函数,用于描述损伤演化后期到灾变断裂之前由于微孔洞聚集导致的材料快速软化过程,并据此引入两个损伤特征物理量：孔洞聚集临界损伤度和断裂临界损伤度,用于表征微孔洞之间发生聚集的临界点和微孔洞聚集后期诱发灾变式断裂的临界点。但是要想进一步建立能够真实反映材料损伤演化过程外在表现和内在规律的物理模型,必须关注材料内部损伤演化的微观特征,从材料学的角度来对这些演化特征进行系统的描述。祁美兰等人(武汉理工大学博士学位论文,2006年)通过建立损伤分布统计方法,对同一发实验的自由面速度剖面信号和“软回收”样品损伤分布的实验和计算结果分别做了比对,验证了模型的合理性。但是这些学者的大部分工作都是建立在完全层裂或是接近断裂临界损伤度的情形下,对于在较低的聚集临界损伤度附近,损伤演化模型及参数的适用性还没有的到很好的证明。本文以高纯铝(99.999%)作为延性金属的模拟材料,在一级气体炮上开展了一维应变平面冲击波加载实验,采用了不同成型的铝型材加工成实验样品(包括棒材和退火处理的板材),分析了高纯铝在较低损伤度下的损伤演化规律,定量统计了微孔洞的尺寸和损伤分布特征,进一步验证了损伤演化模型的普适性,同时,使用高分辨透射电镜对样品进行了分析与表征,获得孔洞周围亚微米及纳米尺度的结构特征信息。论文的主要工作和创新点归纳如下：1.通过改变飞片厚度(2.0mm-3mm),控制不同的加载条件,获得了未完全层裂的低损伤度下的实验样品。基于层裂损伤样品的定量金相分析方法,对历经了冲击加载的高纯铝受损样品进行了细观统计分析,并与损伤演化模型计算的结果进行了比较,证明该模型及模型参数在高纯铝材料动态损伤演化初期行为预测上的普适性。2.使用金相分析的方法,分析了高纯铝材料在动态损伤演化发生的初期,材料中孔洞成核与生长行为。结果表明,在高纯铝中,在拉伸载荷作用下,孔洞的成核,生长主要在晶界上发生,沿晶界断裂是最主要的断裂模式。3.完成了高纯铝动态拉伸损伤试样的透射电镜分析,基于高分辨透射电镜的微观观察,揭示出微孔洞成核可能存在一种新的机制,即熔融成核机制。在冲击加载过程中,首先高纯铝试样在压缩波的作用下,发生塑性变形,在较高的应变率下,能量局域于试样中的某些区域,在高压作用下,局部升温熔化。而在应力波拉伸作用时,熔融区域首先出现微孔洞,产生了新的自由面,使熔融区域物质快速卸压、淬火凝固析出纳米晶在损伤演化中,成核区微孔洞内侧析出纳米晶,纳米晶粒尺寸约数纳米到数十纳米,且可能受到拉伸应力的作用而存在一定的晶体取向。基于对冲击波压缩和拉伸两个作用阶段的物理分析,熔融成核机制将为延性金属材料层裂的物理过程提供新的认识,为进一步建立能够真实反映材料损伤演化过程外在表现和内在规律的物理模型提供了依据。更多还原

【Abstract】 The dynamic tensile failure and fracture in ductile metals is of scientific importance for many engineering projects, about which spallation is one of the typical fracture phenomena concerned. Under dynamic loading, the nucleation, growth and coalescence of microscopic voids inside the specimen will be induced due to the interaction of rarefaction waves from both free-surfaces of the impactor and the specimen, and ultimately the catastrophic fracture occurs.A critical damage parameter is introduced by Wang Yonggang(Ph.D thesis, China Academy of Engineering Physics,2006) to describe the intrinsic characteristic of the dynamic tensile fracture in the ductile metals, 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 Percolation-Softening (P-S) function is proposed to describe the material’s rapid softening during the void-coalescence processand and two physical parameters, named as the critical linking damage Dl and the critical fracturing damage Df, are proposed. Dl indicates the critical value of damage for the onset of void coalescence, and Df the critical value for the occurrence of catastrophic fracture. But to establish a physical model that really reflects the contact the characteristics and microstructure, the microstructure of the material damage evolution must be studied and described In the field of materials science.Qi Meilan(Ph.D thesis, Wuhan University of Technology,2006) investigate the critical behavior in dynamic tensile fracture. Using a method for accounting the micro-voids of the shock damaged HPA samples, Qi validate the reasonableness and feasibility of the model constructed by way of comparing the free surface velocity profile, sample damage distribution of the "soft-recovery" of the shocked specimen and the calculated results. But almost all scholars’ works are based on complete spallation or close to the critical fracturing damage Dfo The damage evolution model has not be validated under the conditions of very low damage.In this thesis, one-dimensional strain impact experiments were performed for the High Purity Aluminum (99.999%, Different molding of aluminium bar,including aluminium bar and aluminium plate after annealing process). A quantitative analysis method for accounting the micro-voids of the shock damaged HPA samples has been used. The universality of damage evolution model was be verified. And the microstructure of the HPA samples were characterized by a transmission electron microscope (TEM) and a high-resolution TEM. The main and /or innovative points of the thesis are summarized as follows:1. With an thickness of the flying from 2.0mm to 3mm the shock compressed HPA samples have been prepared. A quantitative analysis method for accounting the micro-voids of the shock damaged HPA samples has been used. The size distribution of micro-voids and the damage evolution of the spalled samples have been analyzed. Comparing the results of experiment and calculation, the Damage evolution model and the critical damage parameters for describing the tensile fracture has been validated under the conditions of very low damage, and they are independent on the dynamic loading conditions.2. Based on metallographic method, a series of shocked samples were analyzed for understanding the evolution law of ductile metal under dynamic shock. Result indicates that under tensile loading, nucleation, growth and coalescence of voids occur on the grain boundary primarily. The fracture of along grain boundary is the uppermost fracture mode. These analysis results are helpful to understand the evolution process of ductile metal dynamic fracture and establish the damage evolution model.3. The microstructures of microvoid, which result from dynamic tensile loading in high pure aluminum (99.999%), were characterized by a transmission electron microscope (TEM) and a high-resolution TEM. It was found that there may be a new nucleation mechanism of damage evolution in a ductile metal, which was called melt nucleation. During shock compression, shock energy gives rise to local melting in high pure aluminum, and then a new free surface is generated under the tensile stress in the melting areas. Nanocrystalline amorphous metal is produced by rapid quenching a molten aluminum. In our experimental observations, the grain size of Nanocrystalline amorphous aluminum is 5-20 nm. This will increase understanding of the physical processes of dynamic tensile fracture of materials under high strain rate deformation.更多还原