Radiation efficiency and acoustic efficiency in rock cracking: a new understanding from low and high frequency waveforms

The application of acoustic emission (AE) techniques can capture the formation of discontinuities during crack propagation. Although the received waveforms exhibit significantly different frequency spectrum, the high and low frequency components are usually not separately examined and processed. In addition, the radiation efficiency and acoustic efficiency in acoustic radiation are not well analysed in brittle failure. This study proposed an AE waveform separation based on the frequency response. The acoustic radiation energy showed that P waves account for 0.0137 % and S waves 0.0433 % of the total input energy. However, the allocation of S wave energy between high and low frequency waveforms indicated the occurrence of shear microcracks in tensile crack propagation. The high frequency events were found to more likely be energetic events, featuring a similar profile to foreshocks before earthquakes. This likely results from energy dissipation due to shear microcracking friction The acoustic efficiency was observed to remain nearly independent of frequency and failure mechanisms, following a log-normal distribution. In conclusion, this study revealed a new relationship between acoustic emissions and crack propagation in brittle materials. The results suggest that the ground damage generated by a tensile earthquake could be more destructive than that by a same level shear earthquake owing to a higher percentage of energy radiation. This could be particularly important in the mine earthquake risk assessment. • Acoustic waveforms are separately processed based on frequency in rock fracture test. • Radiation energy allocation confirms shear microcracks in tensile fracture propagation. • Radiation efficiency is lower in shear than in tensile failures, as foreshocks in earthquake. • A calibrated model is proposed to represent radiation efficiency based on failure mode. • Acoustic efficiency follows a log-normal distribution, insensitive to frequency.