Effects of water on mechanical behavior and acoustic emission characteristics of coal in Brazilian tests

• The variation of tensile strength and acoustic emission in coal samples after water immersion are disclosed. • Quantitative characterization of differences in splitting modes of coal samples induced by water immersion. • The effect of water on pre-existing weaknesses is an important reason for changing the splitting fracture mechanism of coal samples. An in-depth understanding of how water affects mechanical behavior of coal and fracture failure modes is important for evaluating the stability of underground engineering. In this paper, Brazilian splitting tests are carried out on coal samples with different moisture contents (0%, 1.10%, 2.20%, 3.29% and 4.41%), and acoustic emission (AE) signals are collected simultaneously. The fractal dimension of splitting surfaces is calculated by the cubic covering method to quantitatively describe characteristics of specimens. Combined with two microscopic imaging technologies, i.e. computed tomography (CT) technology and scanning electron microscopy (SEM), the mechanism of crack propagation and strength degradation in coal samples under the effect of water is discussed. Experimental results present that moisture contents change the mechanical strength, AE signals and failure mode of coal samples. As moisture contents increase, the tensile strength of coal samples reduces exponentially, the plastic behavior increases, while the wave velocity increases logarithmically. In the perspective of AE signals, the maximum R value (ratio of cumulative AE hits to cumulative AE energy) increases with the increase of moisture contents, indicating that specimens under higher moisture content conditions can induce more low-energy events, and more complicated crack network. After the splitting failure in coal samples, the fractal dimension of splitting surfaces increases logarithmically with the growth of moisture contents. Based on the macroscopic and mesoscopic characterization results such as AE signals, CT and SEM, it is considered that the existence of water reduces the strength of pre-existing weaknesses (beddings and cleats) in coal samples. As a result, the splitting surface develops along pre-existing weaknesses, which increases the fractal dimension of splitting surfaces and reduces the intensity of AE signals.