The cyclic effects of wetting and drying on rock joints, driven by reservoir-level fluctuations, significantly influence the degradation of their shear mechanical behavior. This study investigates the degradation mechanisms of sandstone joint specimens under controlled wetting–drying cycles (WDCs), with a focus on the evolution of joint morphology and mechanical properties. A systematic experimental approach was adopted to analyze the effects of varying cycle numbers (n), joint wall compressive strengths (JCS), roughness parameters (r), and normal stresses ( \({\sigma }_{n}\) ) on degradation. The key findings reveal that, with increasing WDCs, joint surface roughness decreases, transitioning from rough to smooth, particularly during the initial six cycles. Shear behavior analysis shows a reduction in peak shear strength and an increase in peak shear displacement as the number of cycles increases. The study identifies the progressive weakening of JCS, r, and the basic friction angle ( \({\varphi }_{b}\) ) as the primary drivers of these changes. Two distinct failure mechanisms are observed: shear-off and wear, with wear failures becoming dominant as the number of cycles increases. A predictive shear strength model incorporating the weakening effects of WDCs is developed using regression analysis, achieving a mean relative error of 5.41%, thus validates its reliability. These findings provide new insights into rock joint behavior under fluctuating reservoir conditions and emphasize the critical role of mechanical and morphological degradation in hydraulic stability assessments.
