Dynamic crack response to a localized shear pulse perturbation in brittle amorphous materials: on crack surface roughening

Linear Elastic Fracture Mechanics (LEFM) provides a coherent framework to evaluate quantitatively the energy flux released at the tip of a growing crack. However, the way in which the crack chooses its path in response to this energy flux remains far from completely understood: the growing crack creates a structure on its own as conveyed by crack surface roughening even in brittle amorphous materials such as glass. We report here experiments designed to uncover the primary cause of surface roughening in brittle amorphous materials. Therefore, we investigate the response of a growing crack to local perturbation introduced as a shear wave pulse of controlled duration, amplitude, frequency and polarization. This pulse is shown to induce a local mode III perturbation in the loading of the crack front, which makes it twist locally, without fragmenting. This response is linear both in amplitude and frequency with respect to the perturbation, and disappears with it. We also show that shear wave pulses are emitted when the propagating crack encounters the heterogeneity. Implications of these observations for possible sources of roughening are finally discussed.