Mixed mode fracture analysis of polycrystalline graphite – A modified MTS criterion

Mixed mode brittle fracture was studied theoretically for two grades of commercial graphite. The results reported previously for the fracture loads were used to calculate the mixed mode fracture resistance of the graphite materials tested by a centrally cracked Brazilian disk specimen. The calculated results show that the experimental data were well above the estimates of well-known mixed mode fracture theories like the maximum tangential stress criterion, the minimum strain energy density criterion and the G-criterion. Then a modified maximum tangential stress criterion was used and hence, significantly improved estimates were achieved for mixed mode fracture resistance of the tested graphite materials. The results obtained also suggest that the empirical equations proposed in other studies are not suitable for modeling the mixed mode test results. This finding was realized due to the fact that these empirical models can be significantly dependent on the geometry and loading configurations used for testing the graphite materials.