A novel approach combining the extended finite element method and the finite element over-deterministic method to predict mixed-mode fracture of rock by using unstructured coarse mesh

A rapid and appropriate evaluation of the crack asymptote coefficients is essential for estimating the fracture growth in rock materials with inherent cracks and discontinuities. In the present study, the extended finite element method (XFEM), implemented with free coarse mesh, is used in conjunction with the finite element over-deterministic (FEOD) method to determine the stress intensity factors (SIFs) and T-stress for mixed-mode I/II in-plane loading of cracked rock specimens. Then, the generalized strain energy density (GSED) criterion is employed to estimate both fracture load and crack initiation angle for two types of rock pre-cracked specimens. The predictions of the GSED criterion are then benchmarked against the experimental results, here obtained from three-point bend tests on Neyriz marble. It is shown that such an approach to evaluate the fracture load dramatically reduces the computational cost and effort, meanwhile guaranteeing high accuracy and robustness. XFEM is combined with FEOD to obtain stress fields around the crack tip. A new method for extracting test samples from rock cores is introduced. Various mixed-mode fracture tests are conducted on rock samples. The results are satisfying even with unstructured coarse mesh.