Variable Amplitude Fatigue Life Prediction of Rock Samples Under Completely Reversed Loading

Many rock structures may experience cyclic loading with variable amplitude. Therefore, considering the fatigue behavior of rocks subjected to different loading sequences is necessary. In this study, the accumulated fatigue damage for a Green onyx rock sample with two-step high to low sequences of loading levels was investigated under completely reversed loading condition. In order to apply completely reversed loading to the rock specimens, a new rotating fatigue test machine is employed. A comparison between the predicted behavior of Manson damage curve approach and experimental data was conducted. The results showed that the applicability of Manson damage curve approach depends on the fatigue life ratio of specimens subjected to high-low loading conditions. A new damage curve model was proposed based on the experimental observation and verified with a large number of laboratory tests. Also, the results manifested the higher loading level plays a significant role in the fatigue life of samples. Since the amplitude of the dynamic load conducted on the rock structures is not constant, it is necessary that variable amplitude loading to be taken into account. Otherwise, the prediction of fatigue life is overestimating.