Effect of loading direction on the critical characteristic strength and energy evolution of quartz mica schist and microscale mechanisms

In this study, micro and uniaxial compression tests were performed on two types of quartz mica schist. The effect of schistosity orientation on their mechanical properties was discussed in terms of the critical characteristic strength (CCS) and energy. Furthermore, the microscale mechanism of the mechanical properties of schistose rocks was revealed. In view of the good correspondence between the energy evolution curves and stages of rock damage, the damage evolution process of rock before failure can be divided into five stages. Energy evolution curves can be used to quickly and accurately determine the CCS of specimens, including the initiation, closure, and damage strength of cracks. This suggests that the energy and CCS of schist depend closely on the schistose angle α. The specimen with α = 30° has the minimum crack initiation strength and the highest level of crack initiation stress, while the specimen with α = 90° has the lowest level of crack damage stress and higher elastic and dissipation energy. The mechanical anisotropy, closely related to the orientation dependence of crack initiation and propagation, is essentially controlled by the typical interbedded quasi-microstructure composed of clustered mica layers and granular mineral layers. The CCS and energy are significantly different for the two tested schist samples, and they are determined by microscale factors, such as the proportions of soft and hard minerals, aggregation degree of flaky minerals, and continuity of oriented mica clusters. The CCS and energy of schistose rocks are more sensitive to the various microscale factors at α = 30° than at α = 90°.