Study on the damage and seepage characteristics of water-saturated coal by

Analysis of mechanical parameters of coal samples

The mechanical characteristic curves of axial, radial and body strains of coal samples with the increase of deviatoric stress under different numbers of microwave cycles are shown in Fig. 7a–c.

By analyzing the triaxial mechanical curves, it can be concluded that with the increase in the number of microwave cycles, the deviatoric stress that the coal samples can withstand decreases gradually, and the peak deviatoric stress decreases from 55.29 to 45.51 MPa, which is a decrease of 18.23%; The axial, radial as well as body strains also decreased gradually, with the axial strain decreasing by 0.47%, radial strain by 0.34% and body strain by 1.53%; The modulus of elasticity as well as Poisson’s ratio were analyzed and are shown in Table 2. The modulus of elasticity, as well as Poisson’s ratio, showed a functional relationship, and their fitted curves are shown in Fig. 8.

As shown in Fig. 8, both the modulus of elasticity and Poisson’s ratio fitted curve correlation coefficients have high fitting confidence. The modulus of elasticity and Poisson’s ratio show a negative correlation with the number of microwave cycles, and both values show a decreasing trend; the reason for this is shown in Fig. 9.

The left side is the sample image after 0 times of microwave cycling, and the right is the sample image after 6 times of microwave cycling. It can be clearly seen that the microwave will produce a “softening” effect on the coal samples, coal samples from brittle to plastic transformation, coal samples from the whole large fissures broken, broken after the presentation of the distribution of large blocks, transformed to the local all over the tiny fissures of the “heap” crushing, after crushing deformation The deformation after crushing is not big, not as big as the degree of crushing of coal samples without microwave treatment, so after the microwave cycle, the crushing peak deviatoric stress that the coal samples can withstand is reduced, and the crushing strain is also reduced, so the modulus of elasticity and Poisson’s ratio are also reduced.

Analysis of damage energy evolution in coal samples

The experiment was carried out indoors at room temperature, and the triaxial pressure chamber where the coal samples were located during the experiment is a sealed room. Moreover, each energy change always occurs in the closed cylinder, which is approximated to mean that there is no energy exchange between the coal samples and the outside world, and the total energy absorbed in the loading process satisfies the following relationship^16,17,18,19:

where U is the total energy, MJ/m³; U^e is the elastic energy, MJ/m³; U^d is the dissipated energy, MJ/m³.

Because of the experimental design of the seepage test, we need to consider the change of energy under the condition of gas pressure p. Consulting the relevant literature^20,21 to get the total energy U under the condition of gas pressure p and elastic energy U^e calculation formula:

where Δσ_i is the main stress difference, MPa; ε_1i is the axial strain, %; σ_3i is the radial stress, MPa; p is the pore pressure, 1 MPa; ε_3i is the radial strain, %; and E is the pre-peak modulus of elasticity, MPa.

where E₀ is the initial modulus of elasticity, MPa; D is the damage factor.

The dissipated energy expression is obtained from Eqs. (6), (7), (8) and (9):

The triaxial test data are brought into Eqs. (7), (8) and (10) to calculate the corresponding energy values, and Fig. 10a–d shows the deviatoric stress and energy-strain curves under different microwave cycle times. From the figure, it can be analyzed that the trends of the curves of deviatoric stress, total energy U, elastic energy U^e and dissipation energy U^d have a large similarity, and…