Research on coal body damage and fractal characteristics under repeated drying and saturated water conditions

In order to study the mechanical properties, damage mechanism, and fractal characteristics of coal under repeated drying saturated conditions, through theoretical analysis and laboratory experiments, repeated non-destructive soaking, ultrasonic velocity measurement, uniaxial compression, and acoustic emission testing were conducted on 50 mm×100 mm cylindrical coal body. The experimental results of different drying and saturated cycles show that: as the number of drying and saturated cycles increases, the moisture content of the coal sample gradually increases, the uniaxial compressive strength gradually decreases, and the dissipated energy increases. When the drying and saturated times are 1, 2, 3, and 4, the moisture content of the coal samples is 6.11%, 8.11%, 10.77%, and 12.38%, respectively; the average strength of coal samples is 21.6 MPa, 18.2 MPa, 16.3 MPa, and 13.7 MPa, respectively (compared with the natural state of coal, the uniaxial compressive strength of dry and water-saturated coal decreases with the increasing frequency), the total energy of the coal sample specimens is 93.2 J, 84.4 J, 61.1 J, and 48.2 J (respectively accounting for 80.1%, 72.5%, 52.5%, and 41.4% of the total energy of the natural coal sample); the damage degree of coal samples varies with different drying and water saturation times, and cracks are dissolved and connected with each other, and the pore size, shape and distribution change significantly. Different drying and water saturation times caused varying degrees of damage to the coal sample, resulting in dissolution and interconnection to form cracks, and significant changes in pore size, shape, and distribution; repeated drying and water saturation reduce the fractal dimension of coal fragmentation blocks, that is, the degree of fragmentation decreases, and the number and quality of large blocks increase. Repeated drying and water saturation reduces the fractal dimension of coal fragmentation blocks, that is, the degree of fragmentation decreases, and the number and quality of large blocks increase. The fractal dimension of the fragmented block ranges from 1.7 to 2.1, with an average fractal dimension of 1.92 for natural coal samples. The coal sample has obvious shear cracks and short tensile cracks after failure, and the tensile cracks are mainly through cracks, indicating that repeated drying and water saturation promote the formation of weak surface, and the weak surface penetrating cracks lead to the instability of coal specimen.