In this contribution,the microscopic fracture mechanism and extension criterion for mixed type crack in ductile material under plane mixed mode loading are investigated in details.A universal extension criterion for the mixed type crack,i.e.the crack propagates along the direction of the maximum gradient of equivalent stress,is suggested.This new criterion is used to predict the propagation direction of mixed type crack,showing a good agreement with other theories for different types of mode mixity.Moreover,the numerical verification is also carried out for the case of an edge crack with different mixed mode loadings.Finally,a potential application to three-dimensional fracture in the ductile material induced by holes is also discussed.
