Local Strain Deflection Angle as a Precursor to Rock Fracture: Insights from Rock-like Model

tudying the correlation between internal deformation and surface misalignment at fracture is of significant importance for structural stability assessment. In this study， four kinds of rock-like models with different compression strengths (ranging from 11.02 MPa to 26.48 MPa)， containing prefabricated fractures， were fabricated using rock-like materials (cement， sand， barite powder， and gypsum). The correlation between the deflection of local strain field directions and surface relative velocity was investigated using embedded strain cubes， digital image correlation， and acoustic emission techniques. The results demonstrated that the deflection angle of the local maximum principal strain ranges within (-45°， 45°)， and its variation correlates strongly with the surface slip velocity during the stable deformation stage， with Spearman correlation coefficients generally above 0.70. When the deflection angle reaches an inflection point， the model often subsequently exhibits a rapid increase in slip velocity， and this inflection point can be regarded as a precursor of model instability. The stress levels corresponding to the peak acoustic emission ringing counts and to the deflection recovery decrease with decreasing strength. Furthermore， the relationship between the absolute change in deflection angle and the relative slip velocity follows a linear function， with the slope decreasing as strength decreases. With decreasing model strength， the damage mechanism of the model transitioned from shear failure to tensile-shear composite damage.