Comparative study on damage characterization and damage evolution of rock under uniaxial compression

The evolution of stress-strain data, acoustic emission (AE) and ultrasonic characteristics of granite under uniaxial compression are synchronously tested by using the ultrasonic wave and AE synchronous monitoring devices. The macro- and meso-characteristics of the stress thresholds are analyzed. Also, the quantitative damage evolution of granite is inferred by the crack volumetric strain, AE parameters and ultrasonic characteristics. The results show that the crack initiation stress inferred by the crack volumetric strain and ultrasonic velocity coincides well, and the stress thresholds inferred by the AE events, AE amplitude distribution and b-value are basically identical, but the damage accumulation inferred by the AE events initially begins earlier than that by the macroscopic deformation and ultrasonic testing. At the initial loading stage, the ultrasonic velocity and velocity anisotropy coefficient K increase, and the increase rate slows down gradually. After the crack initiation stress, the lateral velocity begins to decrease, while K gradually increases. With the rapid coalescence of cracks, the amplitudes of AE signals increase significantly prior to the rock failure, accompanied by the rapid decline of b-value and dramatic increase of AE energy. Assuming that the damage accumulation begins only after the crack initiation stress, the damage evolution of granite is characterized by various parameters, such as the crack volumetric strain and AE event. It’s revealed that the majority of damage occurs after the damage stress. The damage characterized by the crack volumetric strain has a clear physical meaning, but the selection of Poisson's ratio for calculating the crack volumetric strain is somewhat subjective. The damages haracterized by the crack volumetric strain, AE energy and modulus all increase significantly prior to the peak stress, which coincides with the rapid decline of b-value. It’s suggested that the damage estimation using the AE energy method should be preferred from the perspective of the reliability of the obtained damage values.