Micro And Mesoscopic Characterization And Study On Dynamic Damage Evolution Of Ductile Metal

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.