Fracture size effects on crack-strength-acoustic emission coupling in sandstone: experimental and numerical simulations

With the increasing depth of underground engineering, research on the fracture mechanisms of rocks containing nonlinear fissures has attracted significant attention. This study systematically investigates the crack-strength-acoustic emission (AE) coupling effects in nonlinearly fractured sandstone through uniaxial compression-AE tests and particle flow code (PFC) numerical simulations. The experimental design incorporates specimens with filled/unfilled fissures of varying dip angles (0°–90°) and lengths (16–48 mm), combining AE parameters (energy, counts) and mechanical strength (MS) data to reveal the controlling mechanisms of fissure geometry on failure behavior. The results demonstrate that: Increasing fissure dip angle (>45°) and decreasing length enhance peak stress by 4.91–8.32 MPa, while gypsum filling further increases strength by 3.58 %–22.02 % and suppresses crack quantity (W_a) by up to 18.7 %; AE cumulative energy shows a strong correlation with W_a (grey relational grade > 0.83); A multivariate quadratic regression model based on response surface methodology (RSM) and least squares fitting achieves optimal W_a prediction accuracy (MRE = 0.0298) by integrating wave velocity (ξ), MS, and AE parameters; The competition of tribes and cooperation of members (CTCM) further optimizes the exponential model, reducing the mean prediction error by 2.65 %. This study provides a novel quantitative crack prediction method for stability assessment in deep rock mass engineering. However, future work should integrate cross-scale observations and multi-field coupling models to improve applicability in complex environments.