# Effect of Methane Adsorption on Mechanical Performance of Coal
## 3 Results  
Figures 2 and 3 show the stress-strain curves of coal samples under different combinations of confining and equilibrium pressure. As observed, each coal sample experienced four typical stages before reaching peak strength: (a) microfracture compression and closure; (b) elastic deformation; (c) stable fracture expansion; and (d) rapid fracture expansion. At the initial stage, the external force compressed and closed the microfractures inside the sample, causing an increase in axial deformation. After the fracture completely closed, the increase in stress resulted in a linear variation of the axial and radial strain of the sample. That is, elastic deformation occurred. With a further increase in stress, internal fractures gradually appeared and steadily developed. When the stress was about to reach peak strength, the internal fracture expansion rate increased, the axial stress-strain curve did not show a yield point, the radial strain rate gradually increased, and finally the sample reached peak failure and entered the post-peak stage. In the elastic stage, the slopes of the stress-strain curves in each subgraph were similar, the curve shapes were also relatively similar, and the peak stress variation of rock samples was less than 5%. These data indicates that the experimental method, process, and results obtained in the present research are reliable. Figure 1: Structure diagram of test system. At a constant confining pressure, the shapes of the stress-strain curves of coal before and after methane adsorption were very similar. However, after methane adsorption, as the compressive strength of coal decreased, the equilibrium pressure for methane adsorption became higher and the compressive strength of coal became lower. When the adsorption equilibrium pressure was constant, the compressive strength, elastic modulus, and maximum strain of coal samples increased with the increase in confining pressure. In addition, when the confining pressure was constant, the compressive strength, elastic modulus, and maximum strain of coal samples decreased with the increase in confining pressure. As shown in Table 1, in meager coal, when the adsorption equilibrium pressure was 1 MPa and as the confining pressure increased from 0 to 3, 6, and 9 MPa, the compressive strength increased from 9.17 to 14.08, 22.38, and 26.21 MPa, respectively. Moreover, the elastic modulus increased from 1.98 to 2.59, 3.87, and 4.86 Gpa, respectively. When the confining pressure was 3 MPa and the adsorption equilibrium pressure was 1 MPa, the amount of methane adsorbed was less than that observed at 2 and 3 MPa. Additionally, the compressive strength decreased from 14.08 to 13.87 and 13.61 MPa, respectively, and the elastic modulus from 2.59 to 2.41 and 2.29 GPa, correspondingly. Figure 2: Meager coal triaxial compression stress-strain curve. (**a**) Stress-strain curve of Meager coal when confining pressure is 0 MPa. (**b**) Stress-strain curve of Meager coal when confining pressure is 3 MPa. (**c**) Stress-strain curve of Meager coal when confining pressure is 6 MPa. (**d**) Stress-strain curve of Meager coal when confining pressure is 9 MPa. Table 1 shows the compressive strength \(\sigma_{c}\) and elastic modulus E of coal samples under different equilibrium pressures for methane adsorption. As shown in Table 1, under the same adsorption equilibrium pressure, the compressive strength and elastic modulus of meager coal were much larger than that of gas coal. In addition, the percentage of strength reduction for the meager coal after methane adsorption was higher than that observed in gas coal. The data in Table 1 shows that the elastic modulus of coal decreased with the increase in methane adsorption equilibrium pressure. This probably occurred because of the plastic effect of coal caused by methane adsorption. The plastic effect of coal occurs because the segmental mobility in the polymeric coal structure increases after methane adsorption. This produces coal softening, which weakens the structure of coal.