To investigate the direct shear failure mechanism of rock joints, we conducted shear tests on sandstone joints under various normal stresses using the RDS-200 rock joint shear test system. Additionally, we monitored acoustic emissions with the Micro-II Express digital acoustic emission system. By analyzing the evolution characteristics of the ratio of acoustic emission rise time to amplitude (RA) and the ratio of ringing count to duration (AF), we examined the shear failure process of rock joints and discussed an early warning method for shear failure in sandstone joints. The results show that: (1) The evolution characteristics of the cumulative ringing count during the rock joint shear process reveal four distinct stages: low-speed growth, accelerated growth, extreme speed growth, and decelerated growth. For each normal stress level, both AF and RA reach their maximum values as the stage number increases, and the number of signals with higher RA values also increases with the stage number. (2) A specific method, based on JCMS-III B5706, is proposed to determine the threshold k for the tensile-shear fracture type. It is observed that both the threshold k and the tensile fracture ratio alpha decrease as the shear stage number increases under each normal stress condition. (3) In the rock joint shear tests, the coefficient of variation CV(r) of the r value exhibits a decreasing trend before reaching peak shear stress, with a stage identified below the nearest CV(r)value (referred to as the early warning value CV(r)(cr)) before peak shear stress. The CV(r)(cr) initially decreases and then increases with increasing normal stress, showing a quadratic parabolic function relationship. The ratio of early warning time t(d) to instability time t(p) ranges from 5.1% to 24.5%, serving as an effective early warning mechanism. These research findings offer valuable insights for instability warning and disaster prevention in jointed rock masses.
