Size effect of granite in uniaxial compression considering crack propagation mechanism

Rock masses often exhibit significant size effects under uniaxial compression, yet the underlying mesoscopic controlling mechanisms and sensitivity to crack parameters remain poorly understood. In this study, we conducted uniaxial compression and acoustic emission (AE) monitoring experiments on granite specimens of various laboratory scales. By incorporating constraints from X-ray diffraction (XRD) mineral composition and AE-guided micro-crack data, we established a numerical model with a pre-existing micro-crack network using the PFC platform. The results indicate that the uniaxial compressive strength, failure mode, and crack propagation of the specimens demonstrate pronounced size dependence: peak strength decreases with increasing specimen size, and the failure mode transitions from splitting to shearing. In a homogeneous mineral matrix model (without pre-existing micro-cracks), the strength is nearly independent of specimen size, suggesting that pre-existing micro-cracks are the primary factor controlling the size effect. Furthermore, crack length has a significantly greater impact on strength degradation than crack number, with smaller specimens being more sensitive to variations in crack parameters. The established model effectively reproduces the experimental results regarding stress-strain behavior, AE event sequences, AF-RA crack classification, and failure patterns, thereby validating the reliability of the multi-scale numerical approach. These findings provide theoretical support for addressing the strength size effect and enhancing the safety design of engineering rock masses under complex geological conditions.