Experimental Study on Progressive Damage Evolution in Rocks Subjected to Post-peak Cyclic Loading History

Because of the release of in situ rock stress, rocks surrounding deep underground excavations are mostly driven into a post-peak damage state. Thus, investigations on cyclic loading-induced damage of rocks are of paramount significance in terms of the long-term stability of rock structures. In this study, using a damage-controlled testing method, the effect of post-peak cyclic loading on the failure behavior of different rock types was comprehensively evaluated. This investigation revealed that post-peak cyclic loading imposes no significant influence on the overall failure behavior of rocks. The evolution of damage variables also indicated that the large portion of axial strain damage (D-a) occurs during the first cycle at the peak strength, whereas lateral strain damage (D-I) evolved more rapidly than D-a did in the post-peak regime under further loading and unloading cycles. The cumulative permanent axial strain (Sigma epsilon(p)(a)) evolved nonlinearly with the axial stress ratio (sigma(i)/sigma(a-peak)) in the post-peak regime, which was manifested as sparse-dense-sparse hysteretic loops. A rapid reduction in the crack damage stress ratio was observed for rocks in the initial sparse hysteretic loops, which was followed by a lower decrease rate in the subsequent dense hysteretic loops, and an almost constant value was finally reached in the last sparse hysteretic loops. Furthermore, tangent Young's modulus (E-tan) and Poisson's ratio (upsilon) evolved in three main stages: One, E-tan first increased and then decreased in the initial sparse hysteretic loops, while upsilon continuously increased, representing significant deformation in the lateral direction. Two, E-tan decreased in the dense hysteretic loops, while upsilon increased at a higher rate for stronger rocks because of the decrease in D-a accumulation. Three, the increase rate of upsilon declined in the second set of sparse hysteretic loops, and E-tan decreased until complete failure occurred.