Hygroscopic weakening accelerates the transition to catastrophic failure during brittle creep in clastic rocks

Understanding how water influences slow fracture growth in rocks remains a major gap in our ability to predict time-dependent failure. In particular, it is still unclear how moisture-related weakening push a subcritically stressed rock from stable deformation into sudden collapse.In this study, we investigate how hygroscopic weakening—caused by liquid water entering a notch—affects the creep behavior of a clastic rock loaded below its short-term strength. Using a sandstone beam (400 mm×90 mm×90 mm) in an inverted three-point bending setup, we first load the sample to about 67% of its failure strength more than 5 days, then introduce a controlled water drip directly into the notch.We track the fracture response using digital image correlation, ultrasonic transmission, acoustic emission, and crack-opening measurements. The results show two distinct stages after water arrives:a rapid increase in crack opening and loss of stiffness, consistent with moisture-driven softening; anda slower but sustained rise in microcracking activity, leading to accelerated creep and, in some cases, catastrophic failure.In contrast, identical dry beams remain stable over several days, confirming that water—not load alone—initiates the transition to instability.These findings demonstrate that even small amounts of liquid water can sharply alter the long-term mechanical stability of brittle stressed rocks. This work highlights a potential pathway through which more frequent or intense wetting events could increase the likelihood of sudden rock failure in natural and engineered settings.