On AE-Based Classification of the Real-Time Damage Evolution of Granite under Equal-Amplitude Multi-Stage Cyclic Loading–Unloading Test: A New Example to Recognize the Hybrid Tension-Shear Failure

It’s full of challenges to make a real-time observation and determination of three crack stresses for brittle granite with subtle deformation, damage accumulation and a jumping drop of stress under axial compression tests (UCT). The traditional division of pre-deformation dependent on the volumetric-axial strain correlation is effective instead of real-time analysis of AE activities. A new design of equal-amplitude multi-stage cyclic loading–unloading test (EAMSCT) compared with UCT, multi-stage cyclic loading–unloading tests (MSCT) is made to determine three crack stress thresholds of closure stress, initiation stress and damage stress. It can mark the elastic–plastic transition by three suggested descriptors of AE ringing, energy and hit. A further topic about the critical ratio of the Felicity effect is made to successfully distinguish its difference with Kaiser effect. In addition, an AE-based damage model dependent on accumulation AE energy counts is established, which can reveal the non-linear evolution from compaction, and elastic–plastic deformation until failure with a damage jump. This model is effective to describe such subtle damage in the cycling loading–unloading stage under EAMSCT. Further, by a combined observation of AE and SEM, a hybrid tension-shear-induced failure of residual bone-shaped column departing from the outside of cylinder-induced compression-shear behavior is observed.