Some Thoughts about Infrared Radiation Response Characteristics during Loading of Coal and Sandstone Samples

The infrared radiation response characteristics and mechanisms of coal and sandstone samples under uniaxial compression were investigated by using infrared thermal imaging and strain measurement techniques. The stress-strain curves were divided into different damage stages (i.e. crack closure, elastic deformation, stable crack growth, unstable crack growth, and post-failure) by the damage thresholds determined by using transverse and vertical strains. Experimental results show that the infrared radiation response characteristics of coal and sandstone are the superposition of different mechanisms, which lead to either temperature increase or decrease. The response mechanisms of infrared radiation vary depending on the damage stage. During crack closure and elastic deformation stage, temperature variations are governed mainly by thermoelastic effects and nearly adiabatic compression of the air in the pore space. In the stable and unstable crack growth phase, the temperature variations are dominated by two reverse acting processes: heat production by frictional sliding and cooling by the expansion of gases in pore spaces. This leads to complex inhomogeneous radiation and consequently temperature distribution at the sample surface with local hotspots.