Energy evolution analysis and related failure criterion for layered rocks

Energy accumulation and energy release are typical characteristics during the whole deformation process of layered rocks. To reveal the characteristics of the energy evolution and failure mechanism of layered rocks, uniaxial compression tests are carried out on layered shale samples. The anisotropic properties of layered rocks are studied. The energy indexes, namely, the elastic energy, dissipated energy, and total input energy of the specimens, are further investigated, revealing the energy damage evolution mechanism of the layered rocks. The experimental results show that the mechanical properties and failure modes of layered rocks are obviously anisotropic. The failure modes of layered rocks can be divided into three types, and the uniaxial compression strength typically shows “U” shaped with the increasing orientation of bedding planes. The energy reserve of the layered rock mass is also anisotropic due to the influence of bedding planes. Before the peak strength, the energy evolution of the layered rock samples is nearly similar, dominated by energy accumulation. The energy dissipation and energy release predominated after the peak strength. In the postpeak stress stage, the elastic strain energy (U^e) is released suddenly, while the dissipated energy (U^d) increases significantly. After that, the elastic-dissipated energy ratio (U^d/U^e) of the layered rocks is studied. The elastic-dissipated energy ratio slowly decreases with strain in the elastic stage, while it increases significantly in the postpeak fracture stage, which has a mutation point. The mutation point is defined as the critical elastic-dissipated energy ratio (K_c), which also shows a typical “U” shape with the orientation of bedding planes. Therefore, the strength failure criterion according to the energy mutation is proposed, which does not need to consider the tensile or shear failure mode of the layered rocks. The failure criterion is validated by the experimental results of other layered rock samples made of similar materials in the laboratory. The criterion is useful for the strength prediction of layered rocks under uniaxial compression.