Research on mechanical properties of loaded coal at the micro/nanoscale by coupling nanoindentation and SAXS experiments

Abstract The relationship between the mechanical behaviors at macro and micro scales is of great significance for supplementing and explaining the physical and mechanical properties of loaded coal. An in-situ loading machine suitable for small-angle X-ray scattering (SAXS) experiments was self-designed, and the mechanical properties and damage evolution mechanisms of loaded coal were revealed at the micro/nanoscale. It also looked at the connections between the macro damage evolution rules and the micro mechanical behavior shown by the interior minerals and microstructures of coal. The research results indicate that the microscale deformation of coal undergoes elastic, elastic-plastic, and plastic deformation stage. The linear relationship without intercept between hardness, fracture toughness, and elastic modulus is independent with the indentation depth. As the depth of indentation increases, the mechanical properties at the microscale of coal gradually stabilize, showing the excellent homogeneity. The surface fractal dimensions of coal samples are 2.42, 2.36, and 2.34, respectively. The increase in load does not change the surface roughness of loaded coal, while the complexity of the pore structure increases as the fractal dimension of the pores decreases. The research results also indicate that coal is a composite material composed of micro-pores, fracture structures, and various mineral components. The differences in geometric shapes and mechanical properties among these components are bound to influence the ultimate mechanical behavior of coal. The corresponding elastic modulus determined by the Mori-Tanaka homogenization model is 3.24 MPa, which is a little bit higher than the value determined by macro mechanical testing. An increase in the size or volume of the test sample will lead to an increase in the number of pores, cracks, or other defect structures, which are more likely to expand and evolve under external loads, thereby affecting the performance of macro mechanical behavior.