Abstract: To reveal the failure mode and crack propagation law of rock under different stresses, we carried out loading tests under different loading paths on fine sandstone using uniaxial compression, fixed-angle compression shear, and Brazilian splitting methods. We collected the acoustic emission(AE) full-waveform of the samples in the whole loading process and denoised the original waveform by using the wavelet threshold denoising method. Through fast Fourier transform, we obtained the frequency spectrum characteristics of the signals. By analyzing the temporal and spatial evolution of the full-waveform, dominant frequency (DF), and secondary dominant frequency (SDF), we subdivided the signal frequency into three levels, namely, low frequency (10-70 kHz), intermediate frequency (70-120 kHz), and high frequency (120-180 kHz), and revealed the process of rock failure and instability. The results indicated that the full-waveform of AE varied greatly under different loading methods. The change of amplitude was closely related to the gradual process of rock damage. Under equivalent loading method, the DF and SDF of AE are of self-similarity and their evolution law are relatively consistent. The SDF was more sensitive to the fracture development state. The low frequency signals reflected an inherent property of fine sandstone failure, irrelevant to the loading methods and test methods. Only the intermediate frequency and the high frequency of AE signals corresponded to the shear slip process and shear failure mode respectively.

Key words: fine sandstone, crack propagation, acoustic emission (AE), wavelet threshold denoising method, full-waveform, dominant frequency (DF), secondary dominant frequency (SDF)