On echelon crack formation in soft solids

The problem of crack propagation under antiplane shear has been long investigated for its mathematical tractability if a crack is assumed to propagate in a self-similar manner. Experiments, however, show a much more complex picture where the crack front breaks up into many daughter cracks, also referred to as echelon cracks. This crack segmentation behavior is pervasive and is observed in soft materials under various mode III and mixed mode loading conditions. The traditional understanding is that echelon crack formation is an instability triggered by roughness in the crack front, micro-cracking, and variation in material properties. However, a complete understanding of why it occurs is still lacking. In this work, we tackle the problem using the complete phase-field approach to brittle fracture. The complete approach can describe propagation according to the Griffith’s criterion, as well as capture nucleation from bulk, pre-existing cracks, or any arbitrary stress concentration or singularity, and has been thoroughly validated with experiments. As such, it provides an opportunity for the first time to thoroughly investigate the reasons behind crack segmentation in soft materials.