Comparative study on stress acoustic emission changes in damage and failure of raw coal and briquette

When studying the relationship between acoustic emission features and coal samples and fractures, both raw coal and briquette can be used as experimental samples. Most coal seams have soft materials, making it difficult to manufacture standard raw coal samples. Therefore, it is common to use briquettes as research samples in experiments. However, coal briquettes change the original structure of coal, affecting its physical and mechanical properties. The applicability of using briquettes instead of raw coal as experimental samples has always been a focus of academic discussion. And currently, there is relatively limited research on the differences in acoustic emission features between raw coal and briquette in pseudo triaxial compression experiments. In order to solve the above problems, pseudo triaxial compression acoustic emission experiments are conducted on raw coal and briquette, with a focus on discussing and analyzing mechanical properties, fracture modes, spatiotemporal evolution of acoustic emission, frequency band energy distribution, nonlinear features, and other aspects. The results show that the acoustic emission energy released during the loading process and the total peak stress energy are closely related to the strength of the coal sample. The raw coal mainly exhibits a mixed failure mode of shear and tension, while the briquette mainly exhibits a tensile axial crack failure mode. The acoustic emission positions of coal samples correspond to their macroscopic fracture morphology, but their occurrence time and spatial distribution are different. In the pre peak loading stage, the acoustic emission signal of raw coal is relatively small, while the acoustic emission response of briquette is intense and reaches its maximum value at the peak stress moment. Through wavelet packet analysis, it is found that the energy distribution of the acoustic emission frequency band of briquette is smaller than that of raw coal. The acoustic emission signals of raw coal are mainly concentrated in the frequency range of 10-120 kHz, while the acoustic emission signals of briquette only jump in the frequency range of 0-100 kHz, indicating that the micro fracture scale of briquette is larger than that of raw coal. 90% of the waveform energy of raw coal and briquette is active at 0-150 kHz. When the loaded sample approaches instability failure, i.e. around 99% of the peak stress, the Hurst index of the acoustic emission signals of raw coal and briquette are both greater than 0.5. It indicates a long-term correlation between the acoustic emission time series and the loading process.