Tensile test analysis and strength-fractal model on different pH water infiltrate coal specimens

The strength, deformation and fracture propagation of coal specimens subjected to different pH infiltration tend to complex under stress concentration disturbance. The crack propagation characteristics of different pH water infiltrate coal specimens were analyzed, the solution model of fractal dimension and regression equation of fractal dimension-tensile strength on infiltrate coal specimens was established according to the failure stress ratio. The stress-strain characteristics of different pH water infiltrate coal specimens and the failure instability mechanism were analyzed under cyclic-loading different stress ratio, and the coincidence of the theoretical model, linear regression and binary treatment of fractal dimensional-tensile strength were compared, and the fractal dimension of failure moment was predicted. The results show that fractal dimension can characterize the mechanical behavior. The fractal dimension equation of tensile specimen surface is established on the basis of scale ratio and stress ratio, and the simplified regression model of fractal dimensional-tensile strength, is derived. The average anhydrous tensile strength and peak strain increase 1. 13 and 1. 36 times as much as u-niaxial non-cyclic, respectively. The times of loading cycles (tensile strength) on different pH water infiltrate coal specimens varying from more (high) to less (low) as pH: 7>pH: ll> pH: 3. The more the stress horizontal gradient, the smaller the strain increment, the anhydrous tensile strength is 1. 26, 1. 32 and 1. 85 times of the pH water infiltrate ones in pH: 7, pH:ll and pH: 3, respectively. The tensile strength and peak strain characteristics arc reduced accordance to anhydrous-neutral-alkaline-acid in order. The failure modes of the specimen are as follows: Tensile splitting instability of single main control surface, multifacet-ed disordered tension-shear instability and inclined shear instability near "X" type (or monocline) main control surface. The three curves of fractal dimensional-tensile strength obtained by theoretical calculation, linear regression and binary treatment show slightly concave shape and more consistent, and the correlation reach to 0. 975 8, 0. 958 0 and 0. 992 0, respectively, which verifies the predicting fractal dimension at the failure moment of water-coal specimen is reasonable. © 2024 China University of Mining and Technology. All rights reserved.