Characteristics and mechanism of deterioration of tensile properties of coal samples under the action of pressure water immersion duration

Using a self-developed coal rock pressure water immersion test device, Brazilian splitting tests were performed on five sets of coal samples under dry and 3, 6, 9, and 12 months pressure water immersion duration (immersion pressure of 5 MPa) to study the tensile properties of coal rock under pressure water immersion. The results indicated that with an increase in the pressure water immersion duration, the average tensile strength of coal samples in Groups B to E decreased by 42.72%, 50.49%, 55.34%, and 60.19%, respectively, compared with those in Group A. The strain isopotential lines of the coal samples became progressively denser and converged toward the deformation localized zone of the main macro-tensile cracks. The increase in pressure water immersion duration intensified the water–coal rock interaction and disrupted the initial energy storage structure of the coal samples, resulting in a gradual reduction in their pre-peak energy storage capacity. Compared with Group A, the average energy storage capacity (U) of the coal samples in Groups B to E decreased by 50.26%, 68.91%, 76.68%, and 84.46%, respectively. With an increase in the pressure water immersion duration, compared with the A-1 coal samples, the rupture fracture of the B-1, C-1, D-1, and E-1 coal samples exhibited more microcracks, more pores, larger cavities, and looser particle arrangements. Additionally, their surfaces gradually became flocculent. The structure of the coal samples transformed from dense to loose and porous, enhancing the deterioration effect on their tensile properties.