Crack deflection in shale-liked layered rocks under three-point bend loading

Abstract

The bedding angle has a significant influence on the fracturing of anisotropic geomaterials, such as shale rock, and crack deflection usually takes place during crack propagation. To better understand and estimate the tensile crack deflection in shale, 35 large notched deep beam (NDB) specimens with different bedding angles—0° (Short-Transverse), 15°, 30°, 45°, 60°, 75°, and 90° (Arrester)—were tested under three-point bend (3PB) loading. For mode I dominated loadings, significant expected anisotropies in both fracture toughness and crack deflecting behavior were observed. The nominal mode I fracture toughness showed a progressive decrease from the arrester to the short-transverse orientation, with values from 1.371 MPa·m1/2 to 0.731 MPa·m1/2. When the bedding angle was less than or equal to 60°, the crack deflection induced the difference between the nominal and actual fracture toughness. By introducing the weak plane model to describe the fracture toughness of shale, an anisotropic K-type maximum tangential stress criterion (KMTS criterion) was presented. Using the KMTS criterion, the deflection angle and fracture load for an arbitrary bedding angle could be obtained conveniently with fracture toughness values in both arrester and short-transverse orientations. The comparison between theoretical predictions and experimental data demonstrated that the KMTS criterion could provide a dependable prediction, particularly for the slight deflection when the crack propagates within the matrix.