An intrinsic nonlinear scale governs oscillations in rapid fracture

TL;DR

This paper demonstrates that a nonlinear zone size governs the oscillatory instability in rapid cracks across various brittle materials, confirming theoretical predictions and highlighting the zone's active role in crack dynamics.

Contribution

It provides experimental evidence linking nonlinear zone size to crack oscillations, validating recent theories and showing universality across materials.

Findings

Oscillation onset velocity is a fixed fraction of shear speed.

Instability wavelength scales with nonlinear zone size.

Experimental results confirm theoretical predictions.

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 $\ell_{nl}$ 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 $\ell_{nl}$. 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.