Dynamic Tensile Fracture Of Ductile Metal And Its Critical Damage

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...