Comparison between Local and Global Crack Criteria of Crack Initiation Under Mixed Mode

For a cracked plane specimen submitted to mixed load, experimental studies have shown that a transition from Tensile type fracture to Shear type fracture exists. This transition is controlled by the critical value of the mixity parameter, which can be expressed in term of a ratio of the stress intensity factors. The crack will propagate by Tensile type fracture following the direction of the maximum circumferential stresses, and by Shear type fracture along slip bands.Discrete element simulation has been employed within a simulation box in order to investigate the local aspect of the crack initiation. The boundary conditions of the simulation box containing the crack tip were chosen as the displacement field close to the crack tip in a Brazilian crack test. Referring to this approach, a local fracture criterion is carried out. It supposes that cracks begin along the slip bands of grains depending on slip band length, and shear stress range on the slip band.The competition between the tensile and the shear crack propagation modes will be studied in this work. The interaction between the local and global fracture criteria will be analysed as function of the applied mixed mode.