Depinning dynamics of crack fronts

Nacre, bones or rationally designed artificial materials are all heterogeneous solids with mechanical properties far exceeding those of their constitutive components. Understanding the role of microscale heterogeneities on the macroscale fracture behavior of solids remains a query, especially when toughness gradients are large. In this talk, I will present a combined experimental and theoretical study of the micro-instabilities of a crack front taking place in heterogeneous materials between two successive equilibrium positions. The instabilities are triggered by pinning the front by isolated tough obstacles of controlled strength and size. We show that the depinning dynamics is controlled by a nonlocal line elasticity and the rate dependency of the dissipative mechanisms taking place within the process zone. We model the behavior by an overdamped equation of motion involving a characteristic material speed and provide an analytical solution which captures quantitatively all our experimental observations. The implications of our results on the energy dissipated during fast fracture events and the fracture behavior of materials with randomly distributed obstacles will be discussed.