Mechanical properties of oil shale-coal composite samples

Abstract

In this study, uniaxial compression tests were conducted on the composite samples of coal and oil shale characterised by high strength and low elastic modulus. Their fracturing and failure propagation processes were monitored by an acoustic emission (AE) device and a digital video camera. The strength, macro-failure initiation (MFI) and failure characteristics of composite samples are analysed. Differences in mechanical properties between the coal and oil shale influence the stress states near the interface area, resulting in strength increasing in the coal and strength decreasing in the oil shale. Strength characteristics of composite samples mainly depend on the strength σC of the coal far away from the interface, which is influenced by the coal's original defects and interactions with the oil shale. The large deformation of oil shale reduces the damage accumulation in the coal and the strengths of relatively intact composite samples are greater than σC; in contrast, the strengths of composite samples with more coal's original defects are low and less than σC. MFI occurs within the coal, causing a step-wise stress change accompanying the peak signal of the energy index and representing the beginning of macro-failure in composite samples. MFI models include the tensile fractures of coal near the interface, initiation and propagation of coal's original cracks, and surface spalling between coal's original cracks. The local ejection or surface spalling failure in the coal induces the chain destruction of its peripheral parts to form a large ejection or surface spalling failure zone. Finally, the coal displays the splitting ejection failure or shear failure along the original crack surface. The oil shale is destroyed through the tensile failure or mixed failure by crack propagation in the coal.