Instability of cracks initiation from a mixed-mode crack tip with iso-stress intensity factors KI and KII

For a special case of mixed-mode crack with iso-stress intensity factors K-I and K-II, the instability of cracks initiation from a crack tip has been investigated based on the principle of the energy-based driving force. By using the J(i)-integrals over the crack boundary, the fracture modelling and relevant energy release rate on the multiple-crack initiation have been formulized. It has been found that when stress intensity factors K-I and K-II close to each other, more possible forms of multiple-cracks initiation from a crack tip may occur theoretically including the crack side-branching and kinking, which is remarkable and conducive to better understand the complex fractural phenomena in engineering and fracture tests. By using the present method, which part of the crack tip boundary fracture can be identified and the relevant critical energy release rate can be found.