Post-peak Stress–Strain Curves of Brittle Rocks under Axial- and Lateral-Strain-Controlled Loadings

To systematically study the influence of axial- and lateral-strain-controlled loadings on the strength and post-peak deformation behaviors of brittle rocks, four types of rocks (marble, sandstone, granite, and basalt) are tested under uniaxial and triaxial compressions, using a brittle hard rock testing system named Stiffman with high loading system stiffness. The test results show that the post-peak stress–strain curves of rock specimens under axial-strain-controlled loading are Class I, while those under lateral-strain-controlled loading are mostly Class II when the confining pressure is low. As confining pressure increases, the stress–strain curves change to Class I. Compared with that under lateral-strain-controlled loading, the failure of rock under axial-strain-controlled loading is more intense, the peak strength is higher, and the residual strength is lower. It is demonstrated that Class II post-peak stress–strain curves obtained by lateral-strain-controlled loading are caused by the unloading of the actuator in response to the servo-control system to keep the lateral strain rate constant. In the post-peak deformation stage, large dilation occurs, which leads to a sudden increase of the lateral strain rate; in order to keep the lateral strain rate at the set value under lateral-strain-controlled loading, the servo-control system must force the actuator to unload. When rock dilation occurs in the pre-peak deformation stage, unloading can occur before the peak strength, resulting in a decrease of peak strength compared with the peak strength obtained by axial-strain-controlled loading. It is found that the more brittle and dilatant a rock, the earlier the unloading of the actuator is, and the larger the peak strength decreases and the more obvious the Class II curve is.