Effect of impact velocity on spall behaviors of nanocrystalline iron: Molecular dynamics study

Abstract This study investigates the effect of shock velocity ( u p ) on damage evolution mechanisms in nanocrystalline iron via molecular dynamics simulations. As u p increases, shock wave propagation accelerates, and stress distribution transitions from grain boundary concentration to homogeneity. This causes a transition in fracture mode from cleavage to ductile behavior. When u p exceeds 1.5 km⋅s −1 , micro-spallation emerges as the dominant failure mode. During micro-spallation, localized melting within the material impedes the propagation of the shock wave. As u p increases, the growth rate of the void volume fraction initially rises but then decreases. Higher u p leads to earlier void nucleation. At lower u p , the cavitation of the model is mainly characterized by the growth and penetration of a few voids. With increasing u p , the number of voids grows, and their interactions expand the delamination damage region. The spall strength demonstrates stage-specific dependence on u p . In the classical spallation stage (C_I), temperature softening reduces spall strength. In the plastic strengthening regime (C_II), strain hardening enhances spall strength. In the micro-spallation stage (M_III), further increases in u p cause melting during tensile and compressive phases, reducing spall strength. Finally, in the compression-melting regime (M_IV), local temperatures exceed the melting point, diminishing plastic damage and accelerating spall strength reduction. This study provides new insights into the dynamic response of nanocrystalline iron.