Effect of Loading Rate on the Progressive Damage and Crack Classification of Granite Based on Acoustic Emission Technique

In this study, we investigated the strength, deformation, failure, and acoustic emission (AE) characteristics of granite during the five uniaxial incremental rates of (0.5, 1.0, 1.5, 2.0, and 2.5 kN/s) for cyclic loading and unloading. It is found that the phase difference between the stress–strain and loading/unloading rate is dependent on the stress–strain hysteresis. A steady rise in the elastic moduli during unloading, a characteristic of strain hardening behavior, is seen. The AE signal can be divided into four phases: quiet, transition, active, and rapid AE development phases. It is observed that the Kaiser effect is more pronounced for the first and second cycles when the cyclic loading is below 1.5 kN/s. This directly reflects that the crack damage threshold rises with cycle number, and the loading rate decreases and remains constant. On the other hand, the Felicity effect is more significant for the second and third cycles during the above 2.0 kN/s cyclic loading. The Felicity ratio (FR) drops with increasing cycle count and loading rate. Hence, FR variation can also be classified into four phases during rock deformation and failure: Phase I, FR ≥ 1; Phase II, 0.85