Characteristics of the damage evolution and the free surface velocity profile with dynamic tensile spallation

The spalling behavior of ductile metals is a process involving void nucleation, growth, and coalescence. Limited by diagnostic techniques, spallation experiments only provide the free surface velocity profiles and the information about recovered targets, but some quantitative damage evolution information about the spalling target is lacking. In this research, the damage nucleation seeds are randomly arranged on the grain boundary in the central region of a target with grain geometry, and a two-dimensional mesoscale numerical model of a plate impact spall experiment is established. By analyzing the free surface velocity profile and the stress history, it is demonstrated that the spall strength obtained with the pull-back velocity essentially corresponds to the maximum tensile stress at the target center. The effects of the impact stress and the stress pulse duration on the dynamic characteristics of the void growth and coalescence are analyzed in-depth by using the damage evolution dissipation energy and the plastic strain contours at different times. The dynamic process of the damage evolution determines the characteristics of the oscillation after the pull-back signal. The stress history controls the damage degree and the kinetic process of the target in the spallation damage process. The impact stress has the most important effect in determining the damage evolution rate, while the stress pulse duration only affects the void coalescence process and irrelevant to the void growth. The damage degree of the void growth and the coalescence process are the result of the joint action of the impact stress and the pulse duration.