Post-peak Stress–Strain curves of brittle hard rocks under axial-strain-controlled loading

Abstract

A super stiff rock mechanics testing machine called Stiffman is introduced in this paper. This test facility uses composite loading frames and relay loading by two loading hydraulic actuators and overcomes the technical difficulties of conventional rock mechanics testing machines such as insufficient loading system stiffness, inability to use axial-strain-controlled loading in the post-peak deformation stage, limited straining amount to reach the residual deformation stage. To validate the reliability of the newly developed test machine, a benchmark study was carried out first. Then, systematic uniaxial and triaxial compression tests under axial-strain-controlled loading for three brittle hard rocks were conducted and the results are compared with those obtained using a conventional rock mechanics testing system called Rockman. The test results of Rockman show that under uniaxial compression and triaxial compression under axial-strain-controlled loading at low confining pressures, the rock specimens failed violently in the post-peak deformation stage and it was not possible to obtain complete stress–strain curves of the rocks. On the other hand, it was possible to load the rocks under axial-strain-controlled loading in a stable fashion using Stiffman and complete stress–strain curves of the rocks under uniaxial compression and triaxial compression at low confining pressures could be obtained. Stiffman-tested rock specimens showed signs of spalling but stayed mostly in one piece. Finally, peak and residual strength envelopes of the three brittle rocks are given based on the test results of Stiffman. The experiment results show that Stiffman can be used to capture complete post-peak stress–strain curves of brittle rocks reliably under axial-strain-controlled loading. It provides a new platform for studying post-peak deformation behaviors of brittle hard rocks reliably in a loading control mode that other conventional test machines cannot provide.