Temporal-frequency distribution and multi-fractal characterization of acoustic emission of rock materials containing two parallel pre-existing flaws

To explore the temporal-frequency distribution and multi-fractal characterization of acoustic emission (AE) signals, a series of uniaxial compressive tests on flawed sandstone containing different flaw geometric arrangements were conducted. The results show that there are primarily low-frequency and low-amplitude signals at relatively low stress levels. With the increase of stress level, the components of high-frequency and high-amplitude signals increase remarkably. Spectrum width ( increment alpha) follows an approximate trend of first decreasing and then increasing with increasing stress levels. When the stress level increases to 0.8 sigma(c), spectrum morphology ( increment alpha(0)) changes from a positive value to a negative value, indicating that the failure mechanism in rock transforms from microcrack damage to large-scale shear rupture. Additionally, spectrum measure subset ( increment f) and increment alpha(0) present an opposite trend. With regard to the flawed sandstone, the fracture mechanism is predominately dominated by the microscopic tensile cracks, whereas the microscopic shear cracks in intact sandstone account for a large proportion.