Roles of moisture and cyclic loading in microstructures and their effects on mechanical properties for typical Chinese bituminous coals

Abstract

This work aimed at studying the roles of moisture and cyclic loading in microstructures and their effects on mechanical properties for typical Chinese bituminous coals. Different relative moisture contents (100%, 75%, 50%, 25%, and 0%) for Shenmu coal (SM), Hongshaquan coal (HSQ), and Wucaiwan coal (WCW) were chosen to study the effects of moisture. The raw SM was then further selected to investigate the effects of cyclic loading. Images of coals surfaces and mechanical properties during simulated crushing process were recorded and combined to be analyzed. The results showed that the moisture possessed significant effects on coal mechanical properties, which strongly depended on their porosities. As for low porosity coal (SM), the adsorption of moisture can soften and lubricate the microstructures, weakening mechanical properties. While the drying process would destroy the microstructures and decease mechanical properties for high porosity coals (HSQ and WCW). Under the cyclic loading process, the cumulative effects of strain showed a step-up state and the first cyclic loading can typically cause the biggest change of microstructures and produce the largest strain under different stress levels. Finally, a normalized quantitative relationship (σr=-0.67Dr2+1.62Dr+0.07) between the relative fractal dimension and relative stress was built.