Experimental and numerical investigation on the tensile fatigue properties of rocks using the cyclic flattened Brazilian disc method

Rock engineering structures are quite susceptible to cyclic tensile loading. Accurate characterizations of the tensile fatigue properties of rocks are crucial to the long-term stability assessment of rock structures. Using the cyclic flattened Brazilian disc (FBD) testing method, this study experimentally and numerically investigates the tensile fatigue response of brittle rocks to different cyclic loading conditions, involving three loading frequencies, three maximum loads and three amplitudes. Our experimental results systematically reveal the influence of the three cyclic loading parameters on the tensile fatigue properties of FBD specimens, including the fatigue deformation behavior, the tensile fatigue life and the fatigue failure mode. Under higher loading frequency or lower maximum load and amplitude, the FBD specimen is characterized by higher irreversible deformation and higher tensile fatigue life. Nevertheless, the fatigue failure modes of the tested FBD specimens are independent of cyclic loading parameters; all the cyclically failed specimens feature a prominent tensile failure. Furthermore, the progressive fracture behavior of the FBD specimen under representative cyclic tensile loading is numerically assessed via the three-dimensional DEM Code ESyS-Particle. The numerical results reveal that the cyclic FBD testing method indeed guarantees the central crack initiation of the disc specimen, which satisfies the prerequisite for a valid Brazilian-type tensile strength test. After the crack initiation, the central cracks further propagate along the vertical diameter of the FBD specimen, eventually triggering the tensile fatigue failure.