Acoustic emission features and P-wave first-motion polarity of tensile fractures in the rock

To investigate the acoustic emission (AE) characteristics of tensile fracture in the rock, an AE experiment of granite, marble, and red sandstone using an expanding agent for fracture generation was designed. Characteristic parameters of AE signals and the P-wave first-motion polarity were analyzed in detail. Results show that the cumulative count and energy of the AE signal increase exponentially when a macroscopic failure occurs in all three kinds of rock samples. The centroid frequency of AE signals of granite, marble, and red sandstone samples mainly concentrates in the range of 100–300 kHz, 200–400 kHz, and 200–500 kHz, respectively, and the proportion of high centroid frequency events in the red sandstone test is the highest, followed by marble and granite. As the AE signal’s centroid frequency in all three kinds of rocks changed with the time of expander action, more AE signals with a low centroid frequency appeared in the late loading period, indicating the increase of large-scale fracture in the late loading period. Meanwhile, RA values of AE signals of the three rock samples mainly concentrate between 0 and 1.9. AF values of marble and red sandstone mainly concentrate between 50 kHz and 100 kHz, and AF values of granite mainly concentrate between 200 kHz and 250 kHz. Distribution characteristics of the RA−AF indicate that the tensile failure dominates the cracking process in such an experiment. The P-wave first-motion polarity analysis method was used to obtain the first-motion polarity of AE signals of each rock sample. Results showed that there are 77.82%, 79.5%, and 87.42% T-type crackles in granite, marble, and red sandstone, respectively. Moreover, granite and marble exhibit almost no S-type crackle, while red sandstone samples have 9.93% S-type crackles. Both RA−AF distribution and P-wave first-motion polarity analyses are statistical analysis methods, which can qualitatively analyze the type of rock fracture.