Shear instability identification method and its damage characteristics based on automatic recognition of three-dimensional curvature of limestone joint surfaces

• Shear damage area proportion on limestone joint surfaces increases with effective normal stress, while high-curvature proportion decreases. • The critical threshold range for inducing shear fractures in limestone is identified as effective normal stress-to-shear stress ratio between 1.4 and 1.6. • A novel method for automatic recognition of three-dimensional curvature of rock joint surfaces is proposed, enabling quantitative analysis of joint surface morphology. • Pre-peak acoustic emission cumulative energy and damage increase sharply near the ratio of 1.6, validating the critical threshold for shear instability.