The first one compares the tensile stress to the tensile strength, while the other uses an energy balance and the material toughness. Within this framework, a coupled criterion was proposed at the beginning of the 2000’s requiring two necessary conditions to be fulfilled simultaneously. In order to overcome this deficiency, the Finite Fracture Mechanics concept assumes the instantaneous formation of cracks of finite size at initiation. The evaluation of the three criteria illustrates that the modified MSED criterion is the least accurate model even in the modified form, while the results predicted by MTS and modified MERR are well matched with the experimental results.Ĭrack initiation in brittle materials is not covered by classical fracture mechanics that deals only with the growth of pre-existing cracks. These modified forms give significantly better predictions of fracture growth paths. We then present modified forms of the ERR and SED functions in which the tensile and shear components are decomposed. It is demonstrated that the energy-based criteria in their classical formulation cannot yield good predictions. The criteria are first assessed in their classical forms employed in the literature.
To evaluate the accuracy of the fracture growth theories, we compare the experiment results for the kink angle and the effective fracture toughness with the predictions of the maximum tangential stress (MTS), the maximum energy release rate (MERR), and the maximum strain energy density (MSED) criteria. Our results show that for certain loading configurations, the anisotropy offsets the loading influence in determining the direction of crack growth, whereas in other configurations, these factors reinforce each other. For each ratio, the influence of the anisotropy orientation on the direction of fracture growth is also analyzed.
The results of 99 fracture toughness tests on the metamorphic Grimsel Granite under four different ratios of mixed-mode I/II loadings are reported.
We evaluate the accuracy of three well-known fracture growth theories to predict crack trajectories in anisotropic rocks through comparison with new experimental data.
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