Microscopic Investigation of Rock Direct Tensile Failure Based on Statistical Analysis of Acoustic Emission Waveforms

Tensile strength is a vital mechanical property of rock and the deformation and failure process of rock under tension is of much significance in rock mechanics and engineering. In this study, a micro analysis on rock direct tensile failure was made based on the newly introduced quantitative analysis of acoustic emission (AE) waveforms. Direct tensile tests of common rocks were carried out, accompanied by real-time AE monitoring. The spatial and temporal distributions of AE signals were studied over the whole loading process. Distribution and evolution laws of dominant frequencies of AE waveforms with normalized applied stress were statistically analyzed. The statistical relationship between the energy ratios of AE waveforms distributed in low and high dominant frequency bands (L-type and H-type waveforms) and peak strengths of rock specimens was built. Microstructural observations with SEM were further conducted. Results show that the dispersion degree of rock tensile strength corresponds to the complexity of mineral composition. Initial release moments of L-type waveforms appeared earlier than H-type waveforms. L-type waveforms of the same proportion carry more energy compared to H-type waveforms in rock subjected to tension. There is an overall downward trend for peak strength of rock specimens with the increasing energy ratios of L-type waveforms. The tensile strength of rock obtained by fitting in this study is smaller than that obtained by averaging. There exist micro shear fractures during the macro tensile failure process of rock according to the microstructural observations with SEM. The peak strength corresponding to the case that L-type waveforms (or micro tensile failures) account for 100% can be considered as the “ideal” tensile strength from the microscopic perspective. The determined tensile strength by fitting can be used as a conservative design parameter for rock engineering.