Sub-Rayleigh to supershear transition of dynamic mode-II cracks

•Sub-Rayleigh to supershear transition of mode-II cracks is studied by phase-field fracture modeling.•Crack tip blunting-tearing hypothesis is presented to explain that cracks comply with different mechanisms from sub-Rayleigh to supershear regimes.•A critical prestress level is revealed, below which no supershear transition can be accomplished from the Sub-Rayleigh propagation.•Prediction criteria are proposed for mother-daughter cracks captured for the first time in the phase-field simulations. The Sub-Rayleigh to supershear transition is commonly accepted to be governed by the Burridge-Andrews mechanism. Such a mechanism predicts the direct nucleation of a secondary intersonic crack (daughter crack), which is triggered ahead of the main crack front due to a developing shear stress peak. Yet, some research shows that the supershear transition is not always accompanied by a daughter crack. And to our knowledge, mother-daughter crack pairs are rarely reported with visual observations. By conducting numerical experiments for pre-strained PMMA plates, the mother-daughter crack mechanism is successfully observed in phase-field fracture modeling for the first time. On this basis, both the supershear transition with and without the daughter crack are discussed in terms of the stress field and prestress level. Our results reveal a critical prestress level, below which no supershear transition can be accomplished from the Sub-Rayleigh crack propagation. Furthermore, we also presented the criteria for predicting the occurrence of daughter crack through systematic simulations.