Failure mechanisms of coarse-grained sandstone under pure mode I/II loading: insights from energy evolution and acoustic emission

Tensile and shear failure are the main damage modes of rocks in geotechnical engineering. To investigate the energy evolution and failure characteristics of coarse-grained sandstone during pure mode I/II loading, loading-unloading tests were performed on cracked straight-through Brazilian discs (CSTBD) with different unloading levels (i), and its acoustic emission (AE) signals and microstructure were monitored. The results show that the AE signals of the CSTBD are negligible before the peak load (Mode I), or appear when the axial load exceeds a stress threshold and then increases significantly before the peak load (Mode II), and the Kaiser effect was observed in the second loading stage. The specimens' input, elastic and dissipative energies varied quadratically with i. The elastic energy is always greater than the dissipative energy (Mode I), whereas the dissipative energy exceeds the elastic energy at i < 0.5 (Mode II). The crack propagation paths start from the crack tip and extend rapidly and straightly (Mode I) or slowly in the form of wing crack (Mode II) to the loading point. The microstructures of the specimens are mainly intergranular fracture (mode I) and transgranular fracture (mode II). The results provide references for studying energy evolution and failure mechanisms of rocks under mixed modes I+II/I+III loading.