Characteristics and mechanisms for a new damage region near the loading side of polycrystalline aluminum with helium bubbles under strongly decaying shock waves

The damage evolution of polycrystalline Al with helium (He) bubbles under strongly decaying shock waves is studied by molecular dynamics simulations. A new damage region is observed near the loading side of the sample, and the evolution characteristics and underlying mechanisms are elucidated. The development of damage in the new damage region begins after complete unloading of the incident shock wave and is further enhanced when the tensile stress arrives later. The damage evolution is completely controlled by the expansion–merging of He bubbles, without nucleation–growth of voids. This new damage region can be divided into two sections, each of which exhibits a unique dominant mechanism. The damage in the section closer to the loading side is due to the reverse velocity gradient formed after complete unloading of the incident shock wave, depending on the rate of decrease and the amplitude of the initial peak pressure. A high initial peak pressure that can lead to melting of material near the loading side is a necessary condition for the formation of the new damage region, since a significant reverse velocity gradient can only be established if melting occurs. The dominant mechanism in the section distant from the loading side is the action of tensile stress, associated with the profile of the incident shock wave upon reaching the free surface, which determines the material phase near the free surface. Moreover, the presence of He bubbles is another critical factor for formation of the new damage region, which does not occur in pure Al samples.