Research on cross-interface behaviors of tension fractures at coal-rock interface

In order to deeply understand the complex fracture propagation mechanisms and control mechanism in the interface of coal-rock under tension, Brazilian tests were performed on coal-rock specimens in various approximation angle with respect to the loading direction to study the fracture propagation patterns on the temporal and spatial evolution and then to explore the fracture cross interface propagation mechanisms and control mechanism under tension, assist by Digital Image Correlation (DIC), Acoustic Emission (AE) and high-speed camera. For Brazilian condition, it is shown that tension fractures initiation in coal, then propagates nearly alone the loading direction to coal-rock interface, and non-planar distortion occurs near the interface, producing a complex fracture pattern with fracture branching, deflection and penetration. When the fracture cross the interface, the fracture usually turn to the loading direction due to the local stress, producing arc-shaped fracture path. When the fracture propagate alone the coal-rock interface, the fracture propagate mainly in the lower strength side of the coal-rock combination, producing simple fracture pattern. The fracture primarily turns to interface and propagate alone the interface for a length, then towards to the principal stress, producing complex fracture path, when it propagates to the interface at an acute angle. Meanwhile, when the fracture is perpendicular to the interface, it will penetrate the interface, producing simple fracture pattern. Brazilian tests of coal-rock combined specimens show that: The main fracture behavior is tension fracture, even the fracture occurs deflection or inflection which does not change the main behavior, obtained by the finally fracture geometry, the propagation path, the surface displacement field acquire by DIC data and the RA value by AE. The research results provide important theoretical guidance for deeply understanding of the cross-interface propagation behavior and control mechanism of tensile fractures at the coal-rock interface.