Failure characteristics of obliqued-layers shale specimens from experimental observation

Abstract

Understanding the cracking characteristics of shale is necessary for oil and gas exploitation engineering. The cracking behaviours and failure modes of obliqued-layers shale specimens were studied under uniaxial compression stress. The acoustic emission response was monitored, and the displacement and strain fields were calculated with digital image correlation (DIC). For the flawed shale specimens, the cracks initiated from the flaw tips most of the time and propagated towards the boundaries, and bedding layers led to the branching and diversion of the crack path. The first cracks did not always start from the flaw tips due to microstructure distribution. Tensile cracks dominated the cracking process, and shear cracks mainly contributed to the final failure of the flawed shale specimens. The failure modes remained unchanged when the flaw rotated from 30° to 60°. However, shear cracks along the bedding layers and tensile cracks parallel to the maximum principal stress dominated the failure of the intact shale specimen. The flaw controlled the location of cracking initiation, and the bedding layers affected the cracking path for the flawed specimen. The bedding layers also control the failure mode of the intact specimen. The experimental results contribute to the understanding of cracking properties in layered anisotropic materials.