Macro- and microdamage characteristics and multiscale damage constitutive model of gas-bearing coal under loading

Abstract

To obtain the correlation between the macro- and microdamage of coal under the coupling action of gas adsorption and stress, the pore structure and mechanical properties of gas-bearing coal under axial loading and the correlations between these factors were studied. The results show that the pore size distribution of coal hardly changed with the change in axial stress, while 45.26% of the micropores and small pores were transformed into mesopores and macropores with the increase in adsorbed gas content within the tested range. Due to different microdeterioration mechanisms, the strength and elastic modulus of gas-bearing coal do not obey a single damage evolution criterion, and their deterioration rates with increasing adsorbed gas content were 27.96% and 11.20%, respectively, within the tested range. The trends of the variations in strength deterioration rate, fracture fractal dimension and pore volume fractal dimension of gas-bearing coal were consistent, and their correlations were greater than 99.2%, but the degrees of the changes in these factors were quite different, indicating that the damage factor at the macroscale cannot be extended to the microscale. To overcome this limitation, a multiscale damage constitutive model considering macro- and microdamage was constructed by using micro-based phenomenological damage theory. The verification indicated that the model accuracy could reach 98.97%, indicating a high reliability.