Deformation Rate Effects in the Circumferential Strain-Controlled Post-Peak Loading Tests

To investigate the influence of circumferential deformation rate (CR) on the post-peak failure behavior of brittle rock, standard granite and slate specimens were tested under conventional triaxial compression at different CRs. Acoustic emission (AE) was continuously monitored to compare the internal damage evolution during the test, and thin sections of the post-failure specimens were made to examine the micro-structures. The test results reveal that the granite exhibits mixed Class I and II curves in the circumferential strain-control (CSC) tests at various CRs, whereas the slate is characterized by a complete Class II curve. For granite, the portion of the Class I curve increases while the Class II curve decreases as CR increases. At lower CRs, the Class II curve develops immediately after the peak, whereas at higher CRs, the Class I curve occurs first before the occurrence of the Class II curve. At the highest CR, the stress–strain curve of slate is comparable to that observed under axial strain-control (ASC) loading. The AE analysis indicates that the overall AE hit rate and energy rate increase with increasing CR, indicating that a higher CR leads to a more violent failure process for both rock types. Additionally, the failure modes of the two rock specimens transform from dispersive fractures to pure shear as CR increases. When comparing the peak strength in CSC tests with different CRs, it is found that the CR influences the peak strength in a complex mode. This study also demonstrates that the generation of the Class II curve is influenced not only by CR but also by rock type and confining pressure. For very brittle rocks, more elastic energy is stored within the rock before the peak, which facilitates the generation of a complete Class II curve in the post-peak. The findings of this study will contribute to a deeper understanding of Class II behavior in rocks.