Fatigue and damage properties of non-consecutive jointed mudstone samples subjected to cyclic triaxial loading

Non-consecutive jointed mudstone samples with dip angles (0° is the horizontal axis) of 15°, 30°, and 45° were subjected to a series of cyclic triaxial loading tests at various frequencies, with the aim of evaluating the fatigue and damage properties of jointed mudstone samples, including the ultimate cycle number, maximum axial strain, damage evolution, and failure behavior. The results from the tests indicate the following: (i) the higher the frequency, the greater the maximum axial strain difference and the higher the value of E_avl (average Young’s modulus during loading) and E_avu (average Young’s modulus during unloading), and (ii) compared with triaxial compressive loading, additional cracks are found in the samples subjected to cyclic loading, and the propagation of cracks initiates from the joint tip toward the two ends of the sample. Meanwhile, under the condition of a lower frequency, additional cracks occur after failure. Based on the experimental data, a new indicator (R_S), considering the effect of the amplitude, peak deviatoric stress, and valley deviatoric stress, was proposed to indicate the stress level. At the same R_S and dip angle, there is a positive correlation between frequency and lgN_f (N_f is the number of cycles before the failure of the rock samples, and lgN_f is the logarithmic value of N_f). Furthermore, a newly proposed damage variable based on shear strain is used to describe the damage evolution of jointed mudstone subjected to cyclic triaxial loading.