Effects of bedding orientation and loading rate on fracture behaviors in coal: Analysis of experiments

To explore the fracture behaviors in coal, laboratory tests are conducted on semi-circular bend specimens with various pre-crack inclinations and bedding orientations subjected to quasi-static and dynamic loadings, respectively. Considering the bedding distribution, the coal is regarded as a transversely isotropic media, its elastic properties are derived from uniaxial compression tests. The critical stress intensity factors are determined using an experimental–numerical method. Findings indicate that the variations of critical stress intensity factors are significantly co-affected by pre-crack inclination, bedding orientation and loading rate. With bedding angle increasing, the planar positional relationship between beddings and loading direction is progressively evolving from orthogonality state to parallelism state, the critical stress intensity factors decline owing to the low cementation strength within beddings. As pre-crack rotates, due to the finite number of angles and strong anisotropy, the variations have no evident regularity. Furthermore, the critical stress intensity factors of coal exhibit sensitivity to loading rate, augmenting with an increase in rate, while anisotropy diminishes. Based on the elastic theory of transversely isotropic body, the conventional fracture criterion, maximum tangential stress, is refined to better describe the fracture onset in coal.