Strain field evolution and ultrasonic time-lapse attenuation characteristics of fractured sandstone

Identification of microcrack initiation, propagation and coalescence patterns is fundamental to the study of the development and evolution process of rock mass disasters. In order to explore the development process and mechanism of microcrack in fractured rock, the active ultrasonic measurement and digital image correlation technology (DIC) were used to simultaneously monitor the damage and fracture process of sandstone containing prefabricated fissure under uniaxial compression, and the surface strain field evolution and ultrasonic attenuation characteristics were analyzed. The results show that the local tensile stress concentration at the tip of the prefabricated fissure with a small inclination angle is conducive to earlier crack initiation. As the fissure inclination angle increases, the specimen containing prefabricated fissure changes from a relatively stable progressive rupture to a sudden failure, and its brittleness characteristics become more obvious. The surface strain field can track the initiation and propagation of crack in real time. The attenuation of P-wave velocity, amplitude spectrum and ultrasonic amplitude is closely related to the development of microcracks and the formation of macrocracks. The obvious attenuation of dominant frequency of ultrasonic waves can be regarded as direct evidence for the formation of macrocracks. The differences in P-wave velocity and amplitude attenuation in different ray paths are the results of anisotropy in the accumulation of damage induced by axial stress and prefabricated fissure. In addition, the improved spectral ratio method was used to analyze the time-lapse characteristics of ultrasonic attenuation, and it was found that the ultrasonic attenuation is more sensitive to the development of microcracks in rock media than P-wave velocity does. Further comparison found that the sensitivity of ultrasonic amplitude, surface strain, and P-wave velocity to rock damage identification decreased in order. The results of this study demonstrate that the active ultrasonic attenuation and DIC surface strain simultaneous monitoring are powerful tools for identifying and quantifying precursor information of rock damage and crack propagation.