Origin of the microbranching instability in rapid cracks.

The origin of the microbranching instability is a long-standing unresolved issue in the fracture of brittle amorphous materials. We investigate the onset of this instability by measuring the real-time dynamics and symmetries of the strain fields produced by rapid tensile cracks in brittle gels. We find that once a simple tensile crack is subjected to shear perturbations, cracks undergo the microbranching instability above a finite velocity-dependent threshold. We further reveal a distinct relation between the microbranching and the oscillatory instabilities of rapid cracks.