Fatigue Properties and Accumulative Damage Model of Bedded Salt Rock

To investigate the fatigue properties of bedded salt rock and the influences of mudstone interlayer on them, a series of fatigue tests were conducted on bedded salt rock under different cyclic loading conditions. The experimental results showed that the characteristics of stress-strain curve under cyclic loading of bedded salt rock were similar to those of pure salt rock. The mudstone interlayer had an inhibition effect on the residual strain in the salt rock layers, which became more pronounced as the upper-limit stress increased. The macroscopical fatigue failure modes differed significantly between pure and bedded salt rock specimens. Pure salt rock underwent Xshaped conjugate shear failure, whereas bedded salt rock specimens demonstrated interlayer splitting failure, characterized by progressive fracture propagation initiating at the mudstone interlayer and extending into salt rock layers. Moreover, a damage variable was defined by the ratio of accumulative plastic strain to ultimate plastic strain. A novel characterization method for the plastic strain increment was proposed through the integration of the Manson-Coffin equation and Ramberg-Osgood model, with the plastic strain increment utilized to represent the accumulative plastic strain rate. The fatigue life incorporating stress increment and loading frequency was introduced into the fatigue damage rate equation, enabling the derivation of an accumulative damage model describing the entire fatigue deformation process of rocks. The unified function expressions were established between the model parameters and load factors through fitting analysis. Comparative analysis using this model revealed distinct fatigue damage evolution mechanisms between bedded salt rock and pure salt rock.