Fractal characteristics and acoustic emission of anisotropic shale in Brazilian tests

The strata of shale contain structural weak planes such as laminations and joints, which do not have the same mechanical properties as intact rock masses. Tensile strength is a critical parameter that determines the capacity of a rock and its resistance to deformation and failure. Focusing on tensile strength and acoustic emission (AE), the characteristics of shale were investigated at various orientations of the laminations with respect to the loading direction. By coupling Brazilian test and the AE technique, the mechanical properties and damage patterns of shale can be explored. The shale exhibits clear laminations and contains a high proportion of brittle minerals by XRD and SEM analysis. The stress-time curve of the Brazilian test can be divided into three stages with distinct brittleness characteristics, and the tensile strength exhibited undulatory trend as bedding angles increases. At low bedding angles, the compaction of fissures and pores within the shale is not significant, and the cumulative AE count-time curves exhibited a fiat-to-sharply rising trend. By contrast, the curves showed a gradually increasing "stepped" tendency at high bedding angles. Analysis of the AE time sequence based on fractal theory reveals that fractal dimension values fluctuate with increase of the stress, signifying the initiation of complex microcracks within the shale. The fractal dimension values sharply dropped when approaching the limit of tensile strength, signifying the occurrence of major cracks. The sudden drop of AE time sequence correlation dimension values can serve as an early warning for the coming failures. The research findings could be instrumental in the monitoring of rock mass instability, microcrack mechanisms, and earthquake sequences.