Strength and failure modes of shale based on Brazilian test

This paper aims to investigate the influence of transversely isotropic property on the splitting strength and failure modes of shale. Brazilian tests and acoustic emission tests were conducted upon the shale specimens with seven bedding orientations. It is revealed that the bedding orientation plays an important role in the splitting strength and the final failure modes. With the increase of inclination angle., the splitting strength decreases gradually. Three failure modes can be identified under different inclination angles from the tests: straight-like crack (theta = 0 degrees, 90 degrees), crescent-like crack (theta = 15 degrees) and arc-like crack (theta = 30 degrees, 45 degrees, 60 degrees, 75 degrees). By analyzing the the normal and shear stress components decomposed along the bedding plane, the failure mode of specimen for theta = 0 degrees is the tensile failure of matrix; the failure mode of specimen for theta = 90 degrees is the tensile failure of bedding plane and the composed failure mode occurres when 15 degrees <= theta <= 75 degrees. The spatial evolution and distribution of the acoustic emission(AE) points we have observed is similar to its macroscopic failure modes. The AE points of straight-like crack is evenly distributed at both sides of the specimen axis; the AE points of arc-like crack is dense on the one side and sparse on the other side of Brazilian disk; the AE points of crescent-like crack are distributed at the middle and one end of the specimen. The fractal dimension can reflect the failure modes of the specimen very well. From the inverted V shaped trend of the fractal dimension, the value D of the straight-like failure mode is close to 2; while D of the crescent-like and arc-like failure modes is between 2.2 and 2.7. It can be drawn from the fact that the larger of the value D is, the larger of the crack's radius curvature is. The smaller of the value D is, the crack is more close to a straight line and failure mode is simpler.