Size effect on uniaxial compressive strength of single coal particle under different failure conditions

Strength of coal particles is an important mechanical property that is of great significance in the prevention of coal and gas outburst. The force-displacement curve for each small particle was obtained by the uniaxial compression of 1200 coal particles of sizes in the range 0.2-7 mm and different degrees of deterioration ranging from low-rank bituminous to high-rank anthracite. The experimental data were used to analyze various theories describing the effect of size on material strength; these include the traditional statistical theory, random distribution of structural strength based on the Weibull statistical theory, fracture energy release theory, and crack fractal theory. The results show that one of the traditional statistical models that receives little attention is more suitable than the others to describe the size effect on particle crushing strength. The intact coal strength, i.e the strength when particle size d -> 0, is in the range 10-88.79 MPa, and the coal mass strength, i.e., the strength when d is in the range 0.92-8.03 MPa. The tensile strength of the matrix is 0.7 times the intact coal strength. Based on different failure conditions, the uniaxial compressive strength is subdivided into yield strength, crushing strength, and ultimate strength. When the particle size tends to infinitesimal, the ratios of yield strength to ultimate strength and crushing strength to ultimate strength are 0.73 and 1, respectively. Both ratios decrease with increase in particle size and tend towards constant values thereafter. Various factors affecting the breakage of coal particles were analyzed. The firmness coefficient, maximum vitrinite reflectance, and apparent density have a significant positive correlation with particle strength whereas the proximate analysis parameters have little effect on strength. (C) 2019 Elsevier B.V. All rights reserved.