Characterization of Crack Propagation in Laminated Shale with Different Dip Angles Based on Ultra-fast Measurements

To investigate the influence of the layered structure of shale on fracture propagation behavior and to establish a quantitative relationship between crack propagation rate and energy release rate, three-point bending experiments were conducted on seven groups of shale semi-circular bending (SCB) specimens with dip angles ranging from 0° to 90°. The entire fracture process was captured using ultra-fast pulsed laser observation technology. Results show that as the dip angle of the bedding layers increases, both the fracture peak load and fracture toughness of the shale specimens decrease significantly. Fracture propagation behavior can be categorized into double-deflection, single-deflection, and non-deflection modes. A good linear relationship exists between the energy release rate and the crack propagation rate, which is highly consistent with the quantitative relationship model established based on the first law of thermodynamics and Griffith's Maximum Energy Release Rate (MERR) criterion. During symmetric loading, no significant angular change occurs during crack initiation, indicating that fracture propagation behavior in shale reflects the interaction between the energy release rate and fracture resistance. Additionally, as the dip angle of the bedding layers increases, the total strain energy, elastic strain energy, and energy release rate decrease, and the fractal dimension of the fracture surface generally decreases, resulting in smoother fractures, which in turn faciliates specimen failure.