Loading Rate Effects on Stick–Slip in PMMA: Insights into Fracture Energy and Contact Area Evolution

This study investigates the effects of loading rates on the stick–slip behavior of polymethyl methacrylate material. A series of friction experiments were conducted using a direct shear apparatus to systematically assess how loading rates influences stick–slip behavior. Three loading rates were adopted: 0.1, 1, and 5 mm/min, all under a constant normal stress of 2.5 MPa. The experimental results indicate that loading rates significantly influence the mechanical behavior of stick–slip. The recurrence intervals, shear force drops, and fracture energy decrease as loading rates increase. By monitoring changes in the interface contact area using the total internal reflection method, we observed that the reduction in interface contact area diminishes with increasing loading rates. At lower loading rates, micro-asperities have sufficient time to reform, resulting in stronger interaction forces and fracture dissipated energy; conversely, at higher loading rates, limited recovery of contact area results in reduced fracture dissipated energy. These findings highlight the close relationship between loading rates and interface contact behavior, providing new experimental data and insights for analyzing and understanding fault slip and rupture processes.