Comparison of high-frequency components in acoustic emissions from rock fracture under Mode Ⅰ and Mode Ⅱ dominated loading

High-frequency components in acoustic emission (AE) or seismic radiation attract much attention as they are directly related to the fracture process. One of the main issues in our knowledge of high-frequency radiation is a lack of definition on the relationship between high-frequency content and source mechanisms. In this study, high-frequency emission waves due to rock fracture in tension and shear are studied experimentally and theoretically. An improved AE-signal processing method, focusing on direct-arrived waves, is proposed to pinpoint the exact high-frequency radiation in a rupture event. Current data support that for each individual AE signal the later arriving waves contain lots of low-frequency components, while the high-frequency components are mostly carried by the direct arrivals. In comparison with tensile cracks, shear cracking generates more high-frequency components and tends to radiate energy in the form of waves with larger RA values (designated as rise time/amplitude). Combining the theories from dynamic fracture and acoustic radiation, the difference in high-frequency energy release from shear and tensile cracking can be attributed to the rate of stress drop at the tips. The present findings provide a framework for a new way to identify the tensile failure in rock. It will be helpful in solving the difficulty tracking seismic or micro-seismic events from tensile sources associated with karst collapsing, hydraulic fracturing and roof falling in mines.