Fracture mechanics description of fracture mirror formation in single crystals

The formation of microbranching and macrobranching in brittle materials has been proposed to occur at a constant stress intensity and strain intensity. The strain intensity and stress intensity criteria are basically the same in their approach and have been shown to be predictive for isotropic materials. A fracture energy criterion can be proposed based on the energy balance approach of Griffith and related to those two criteria. This latter approach is also valid to describe the formation of branching in isotropic materials. The critical test for determining the validity of a criterion for branching is in anisotropic materials. In order to distinguish between the criteria, single crystal Si was fractured and the fracture surfaces were analyzed. In this study, the fracture energy criterion is shown to best describe the formation of microbranching and macrobranching in anisotropic materials. The energy of formation of the mirror-mist boundary is the same in two different orientations: {100} and {110} tensile surface on the {110} fracture plane.