Rate of microcrack nucleation

We propose a phenomenological model for microcrack nucleation in a solid under constant external stress. We argue that the kinetics of the crack nucleation process is qualitatively different from that occurring in simple metastable states such as supercooled liquids or supersaturated solutions. This difference is caused by irreversible processes such as surface diffusion that restructure microcrack surfaces. This restructuring inhibits microcrack healing and thus yields a microcrack nucleation rate that is much faster than that obtained by directly applying nucleation laws appropriate to metastable states in liquids or solutions.