Subcritical crack growth and fracture behavior of rocks and long-term strength estimation

• The strain evolution and fracture process in the tensile surface of the specimen were successfully observed by optical photography. • The fracture characteristics and subcritical crack propagation behavior of five kinds of rocks are compared. • The long-term strengths of the five rocks were compared based on subcritical crack growth parameters, fracture toughness and tensile strength. Understanding the potential subcritical fracture behavior is very essential to assess the perennial stability of rock engineering. The purpose of this study is to reveal the fracture characteristics and subcritical crack propagation of rocks with different lithologies, emphasizing that the influence of time-dependent fractures on the long-term strength of rocks cannot be ignored. The fracture toughness, K I - V curves and subcritical growth index of five types of rocks were obtained by the double-torsion (DT) technique. Subcritical fracture propagation exists in the rock as the stress intensity level is between K 0 and K IC , and the ratio of the two values is between 0.73 and 0.83. Using a three-dimensional digital image correlation (3D-DIC) technique and a plane mirror, the evolution of tensile strain and crack propagation process on the bottom surface of specimens were observed successfully. The granite and basalt showed a faster growth rate of subcritical crack propagation, with fracture rate ranging from 10 -2.5 to 10 -6.9 m/s. The long-term strength of rocks was estimated based on the tensile strength, fracture toughness, SCI and A values. In a short term, basalts with the highest tensile strength have the highest long-term strength, while granites show the highest long-term strength when the time-to-failure exceeds 408.2 years. Therefore, the time-dependent cracking behavior of rock engineering must be considered when assessing the long-term stability.