Strain-based criteria for mixed-mode fracture of polycrystalline graphite

The mixed-mode brittle fracture of two types of commercial graphite is investigated focusing on strain-based fracture criteria. The previously published experiments using centrally-cracked Brazilian disk specimens subjected to mixed-mode loadings are simulated by two strain-based fracture criteria: the traditional maximum tangential strain (MTSN) criterion only considering the singular terms, and the extended maximum tangential strain (EMTSN) criterion, which considers the first nonsingular strain term as well as the singular terms. Numerical simulations on the centrally-cracked Brazilian disk specimen show that the first nonsingular tangential strain term significantly influences the tangential strain distribution around the crack tip. The comparison of the evaluations by the MTSN and EMTSN criteria with the experimental data shows that the EMTSN criterion is more capable of successfully estimating the fracture resistance of graphite materials rather than the traditional MTSN criterion. In addition, when the first nonsingular term is considered, the strain-based fracture criterion provides better predictions for near mode II loadings than the stress-based fracture criterion.