In order to study the deformation and failure mechanism of hard rocks, true triaxial compression tests were conducted on four type of hard rocks to obtain the complete stress-strain curve and failure modes. Under true triaxial compression condition, the shape of the complete stress-strain curve can be divided into three types: elastic-brittle (EB), elastic-plastic-brittle (EPB), and elastic-plastic-ductile (EPD) types. According to the different post-peak deformation behaviours, the stress-strain curves of elastic-plastic-brittle (EPB) type can be subdivided into three sub-categories: post-peak instantaneous brittle (EPB-I) type, post-peak multi-stage brittle (EPB-M) type, and post-peak delayed brittle (EPB-D) type. The stress-strain curves change from EPD to EPB-D to EPB-M to EPB-I to EB with increasing differential stress (σ2-σ3). The deformation characteristics are dependent on the σ2, σ3, mineral composition and mineral texture to the rock sample. An increase in σ3 leads to an increased ductility, while an increase in σ2 leads to an increased brittleness. Moreover, rocks with regular mineral texture and low mineral hardness are more prone to ductility. When the deformation curve is transformed from EPD to EPB to EB, the tensile crack is gradually dominant, and the macroscopic failure angle is gradually steeper.
Influence of end effect on rock strength in true triaxial compression test