Study on crack development and damage characteristics of single-cracked red sandstone under uniaxial cyclic loading and unloading based on multi-field monitoring

The progressive failure of cracked rocks under cyclic loading and unloading is a critical issue in rock engineering. It holds significant importance to study the development law and damage characteristics of cracked rocks for the safety and stability of underground structures. In this study, acoustic emission (AE), resistivity, and digital image correlation monitoring (DIC) techniques were employed to conduct dynamic monitoring of the damage process of single-cracked rocks with different prefabricated crack dip angles (PCDAs) and prefabricated crack lengths (PCLs) under uniaxial cyclic loading and unloading. The test results reveal that the peak strength of single-cracked rocks increases with the increase of PCDAs, yet decreases with the increase of PCLs. There are differences in the morphology of new cracks induced by prefabricated cracks with different PCDAs. The underlying cause is that the direction of normal stress generated by the vertical stress at the tip of the prefabricated crack differs at different PCDAs. Differences are also observed in the initiation position of new cracks induced by prefabricated cracks with different PCLs. The reason lies in the variance of the normal stress generated by the vertical stress at the tip of prefabricated cracks with different PCLs. During the cycle, the cumulative AE counts continuously rises, while the resistivity steadily decreases. Among them, the resistivity peak typically emerges near the strain valley, and the resistivity valley appears near the strain peak. The coupling of AE and resistivity parameters characterizes rock damage from the perspective of crack initiation and connectivity. Through comparison with the plastic strain results and digital image correlation monitoring results, the accuracy and rationality of the new coupling damage variable in characterizing rock damage are verified.