Damage characterization of shale under uniaxial compression by acoustic emission monitoring

Understanding the damage behavior and cracking mechanism of brittle shale is crucial for hydraulic fracturing design.In this research,uniaxial compression tests are conducted on shale samples with different bedding plane orientations,and acoustic emission monitoring is implemented synchronously.The results indicate that the apparent elastic modulus increases with increasing bedding orientation.For the bedding orienta-tions of 45°and 90°,the lateral deformation is anisotropic due to the bedding structure,revealing the anisotropic Poisson effect.A shear failure surface and tensile failure surfaces form parallel to the bedding plane for bedding orientations of 45°and 90°,respectively.For the bedding orientation of 0°,shear failure mainly occurs through the bedding planes.Additionally,the damage mechanism of shale is investigated by crack classification based on AE parameters.It is found that crack initiation is induced by the generation of shear cracks for the bedding orientation of 45°,whereas by the generation of tensile cracks for other bedding orientations.According to damage attributable to different type cracks,shear cracks dominate the damage behavior for bedding orientations of 0°and 45°,whereas tensile cracks dominate the damage behavior for bedding orientation of 90°.Finally,the information entropy is calculated by AE energy.A low value of information entropy,approximately 0.36,predicts failure with a low degree of instability for the bedding orientation of 0°,whereas a high value of information entropy,more than 1.5,predicts failure with a high degree of instability for other bedding orientations.This finding indicates that the failure behavior is gradual progressive damage for bedding orientation of 0°,whereas sudden damage dominates failure behavior for other bedding orientations.