Mechanism of Frictional Strength Degradation of Natural Rock Fractures under Cyclic Loading and Unloading

Laboratory and field experiments have demonstrated that the breakdown pressure of rock containing natural fractures decreases under cyclic loading and unloading, yet the underlying mechanism remains insufficiently understood. This paper studies the weakening mechanism of fracture surface frictional strength under cyclic loading through microstructural mechanical analysis, aiming to elucidate the failure process of naturally fractured rock. A microstructure model is developed based on the shear deformation behavior. of granular materials on fracture surfaces, in which particle-formed force chains are idealized as elastic rods capable of bearing combined compressive and shearing loads. The model further assumes that when the shear direction reverses, the force chains reorganize into new elastic rods. By analyzing the deformation and failure mechanics of these elastic rods, the study systematically explores the mechanical behavior. during slip initiation, arrest, and reversal, and derives corresponding frictional constitutive equations for each stage. The results indicate that slip-induced elastic deformation leads to a reduction in the apparent frictional strength of fracture surfaces, which is governed by the elastic properties of the granular materials. Reversal of slip direction results in additional energy dissipation and causes sudden fluctuations in the friction coefficient. By aligning with the phenomenological slip-weakening friction law, the model parameters are constrained through calibration against existing experimental data on fault gouge friction. This microstructural approach provides a novel framework for understanding the weakening mechanisms of rocks subjected to cyclic loading and unloading