Abstract:

When branching is suppressed, rapid cracks undergo a dynamic instability from a straight to an oscillatory path at a critical velocity v c. In a systematic experimental study using a wide range of different brittle materials, we first show how the opening profiles of straight cracks scale with the size nl of the nonlinear zone surrounding a crack's tip. We then show, for all materials tested, that v c is both a fixed fraction of the shear speed and, moreover, that the instability wavelength is proportional to nl. These findings directly verify recent theoretical predictions and suggest that the nonlinear zone is not passive, but rather is closely linked to rapid crack instabilities.