Atomistic understanding of the influences of defects on the spall damage of aluminum under multiple shock loadings

Abstract

The spall damage of solid single crystal Al with initial defects such as voids or He bubbles under multiple decaying shock loadings is studied by molecular dynamics simulations. After the first shock compression and release, plenty of sparse defects in the void collapsed regions and a few disordered atoms surrounding He bubbles are formed in the Al-void and Al–He samples, respectively. The spall mechanism for the Al-void sample is the nucleation–growth–coalescence of voids originated from the defects at the early stage of damage and from the regions without defects at the late stage. Under second shock loading, the first spall region is compressed and the compression extent is dependent on the shock intensity; however, the density after complete compression is lower than the initial value. Meanwhile, a new spall region is formed due to the interaction of the second incident rarefaction wave with the one reflected from the surface of the first spall region into the sample, when the shock intensity is relatively strong. The new spall region exhibits lower spall strength for the samples with initial defects than that for the perfect sample. Additionally, respallation occurs in the first spall region after compression under the strong second shock, its damage evolution is strongly influenced by the defects formed after compression, and the spall strength is much lower than that of first spallation. For the Al–He sample, the spall mechanism is always dominated by the expansion-merging of He bubbles, with the fastest damage development and the lowest spall strength.