Stability Limit for Mode-I Fracture

In order to study the stability of mode-I fracture, we consider a crack moving along the centerline of a very wide strip and compute its steady-state response to a small, spatially periodic shear stress. We find that, in the presence of this perturbation, the crack remains very nearly straight up to a critical speed $v_c$ of about $0.8\,v_R$, where $v_R$ is the Rayleigh speed. Deviations from straight-line propagation are suppressed by a factor proportional to $W^{-1/2}$, where $W$ is the width of the strip. At $v_c$, however, this suppression disappears and the steady-state crack follows the wavy curve along which the shear stress vanishes in the unbroken strip. We interpret this behavior as a loss of stability and discuss its implications for a more complete dynamical theory of fracture propagation.