Study on acoustic emission characteristics and energy evolution of Brazilian splitting tests of coal samples with different height-diameter ratio

In order to explore the influence of different ratios of height-diameter on the tensile strength and energy evolution of disc coal samples, Brazilian splitting tests were carried out on five groups of coal samples with the same diameter and different heights. The tensile behavior, acoustic emission (AE) characteristics, fracture modes and section morphology characteristics of coal samples with different height-diameter ratio are investigated using a synchronous AE monitoring system and three-dimensional morphology scanning technology. The results show that the tensile strength of coal gradually decreases with the increase of the ratio of height to diameter, and the peak strength of coal reaches the maximum when the height-diameter ratio is 0.4∶1. The failure mode of coal samples gradually changed from the complex fracture mode of multiple cracks dominated by tensile-shear mixed cracks to the tension cracks of single section. The cumulative AE energy increases gradually, and the clustering phenomenon is more significant, showing a stepwise upward trend. The proportion of dissipated energy increases gradually at the lower stress level. When the load increases to the elastic limit, the dissipated energy and its proportion decrease gradually. The ratios of elastic strain energy and dissipated energy tend to be stable when the coal sample approaches the failure stage. When the coal sample enters the post-peak stage, the accumulated elastic strain energy releases rapidly, the proportion of dissipated energy increases sharply, and the macroscopic cracks in the coal sample rapidly expand and coalesce. The width (Δα) of the multifractal spectrum gradually increases, and the spectral measure subset (Δf) is less than zero with the increase in height-diameter ratio, which indicates that the small fracture scale signal dominates in the splitting tests. In addition, the relative frequency of broken section elevation of coal samples under different height-diameter ratios presents a good Gaussian function distribution, and the fractal dimension shows a gradually increasing trend. The smaller the height-diameter ratio, the less complex the fractured section will be, whereas the larger the height-diameter ratio, the more complex the roughness of the broken section will be.