Foliation Effects on Mechanical and Failure Characteristics of Slate in 3D Space Under Brazilian Test Conditions

Slate frequently encountered in numerous engineering applications basically exhibits significant anisotropies in terms of physico-mechanical properties due to the presence of well-developed foliation structures. The objective of this study is to investigate the mechanisms of foliation effects on the mechanical and failure characteristics of slate in three-dimensional (3D) space under Brazilian test conditions. A series of laboratory tests are conducted on slate with seven different foliation angles ( φ ), which are defined as the angle between the foliation plane and end surfaces of the specimen. On the basis of each foliation angle, five different loading angles ( θ ), which are defined as the projection of the angle between the loading surface and the foliation plane on the front side of the specimen, are selected to ensure that 3D space is involved. The high-speed camera and acoustic emission (AE) system are employed to analyse the failure characteristics of slate during loading process. The testing results indicated that the variations of applied failure force (AFF) with respect to loading angle ( θ ) significantly differ under varied foliation angles. With increase of foliation angle, the anisotropy ratio of the maximum to minimum AFF shows an increasing trend, suggesting that the anisotropy becomes more notable. The high-speed camera visually recorded the initiation and propagation of cracks. The specimen failure can involve two major processes: the local cracks first occurred on the disc flank, and then the fully connected cracks were formed causing the overall failure. The rupture evolution process inside the specimen was characterised by AE energy and AE hits, which was in good agreement with the high-speed camera observations. When the foliation angle is small, i.e. 0° ≤ φ ≤ 15°, or large, i.e. 75° ≤ φ ≤ 90°, the macro-cracks on both sides (i.e., front and back) show similarity to some certain, and the fracture patterns can be considered as two-dimensional (2D). However, when φ is in the range of 30°–60°, the fractured surfaces have 3D spatial distribution characteristics, and the macro-cracks appearing on both sides exhibit an approximately anti-symmetric relationship. It is also revealed that both φ and θ have significant effects on the AFF and the fracture patterns. The results are likely to provide experimental basis for further improving the theory of tensile properties of anisotropic rocks.