Mechanical properties and acoustic emission characteristics of granite under discontinuous multilevel fatigue loading

The study investigates the effects of discontinuous multilevel fatigue (DMLF) loading on the mechanical properties, acoustic emission characteristics, and energy evolution patterns of granite. The research results show that after DMLF loading, the post-peak deformation ratio of granite under uniaxial compression increases with creep time, exhibiting a trend of brittle-ductile transition, and the extent of cracking intensifies during failure. The energy analysis results indicate that during DMLF loading, both the elastic and dissipated energies of granite increase with the stress level. The elastic energy is stored in a linear form, while the energy dissipation follows a combination of linear and quadratic functions. During the creep loading phase, the external input energy is mainly converted into dissipated energy. In the later stages of DMLF loading, the average intensity of acoustic emission ringing counts increases, and the introduction of creep loading promotes the development and expansion of internal cracks in the rock. The acoustic emission b value overall exhibits a two-stage characteristic, with the b value of granite decreasing from 1.6 to 0.4 during failure.