# Research on Mechanical Properties and Damage Constitutive Model of Water-Bearing Coal
## 3 Experimental Results  
### Uniaxial Compressive Stress-Strain Characteristics of Coal Samples  
The uniaxial compression stress-strain curves of coal samples with varying water contents are shown in Figure 5. When the results of four parallel test groups are compared, the pre-peak stress-strain curves of coal samples are found to be very similar, but the peak stress-strain curves are significantly different, indicating a degree of discreteness. The mechanical properties and deformation characteristics of the four parallel test groups' four groups of coal samples with various water contents show consistent fluctuation trends. The maximum values of the uniaxial compressive strength and Young's modulus of coal samples are reached when the coal sample has a natural moisture content, while the maximum strain of coal samples rises monotonically with increasing moisture content. As an illustration, in test group S1, coal sample moisture content increased from 0% to 3.31%, compressive strength increased from 6.42 MPa to 12.61 MPa, and peak strain increased from 1.60% to 1.63%; in test group S2, coal sample moisture content increased from 3.31% to 13.14%, compressive strength decreased from 12.61 MPa to 6.23 MPa, and peak strain increased from 1.63% to 2.14%. Figure 4: The schematic diagram of the test system. Figure 5: _Cont._  
The stress-strain curves of coal samples with varying moisture content all have a near horizontal step between the fracture compaction stage and the elastic deformation stage, which is referred to in this paper as the pore compaction stage. As a result, the stress-strain curve has five stages: fracture compaction, pore compaction, elastic deformation, plastic deformation, and post-peak failure. From Figure 5a-d, it can be found that the stress-strain curve of the dry coal sample exhibits linear elastic change before the peak, axial stress drops rapidly in linear form after the peak point, and there is almost no falling platform. The fracture and pore compaction stage intervals increase significantly as the moisture content of coal samples increases, the elastic stage interval decreases, and the plastic deformation stage becomes more obvious. In addition, as the water content increases, the axial stress drop velocity after the peak point decreases, revealing a multi-level drop platform. From Figure 5a-d, it can be found that with an increase in water content, the near horizontal step becomes longer, and the interval of the pore compaction stage increases. We believe that this phenomenon may be related to the particularity of the test coal samples. Scholars discovered that coal has a porous structure through CT scans and three-dimensional reconstruction [35], and some researchers discovered that the porosity and permeability of the coal seam sampled in this test are large [36; 37], resulting in the internal micro-pores of coal samples being filled with water after immersion, as shown in Figure 6. The coal sample's micro-cracks closed at the start of loading, the coal sample gradually became more compact as the axial strain increased, its elastic modulus rose as well, and the stress-strain curve was concave upward in this stage. The hydrostatic pressure in the coal sample's micro-pores increased as the external load increases, resulting in micro-pore rupture and a large number of micro-damages in the coal sample. The stress changes are minor at this stage, but the strain develops quickly, and the stress-strain curve appears near the horizontal step. Because the number of micro-damages is positively correlated with water content, the degree of internal damage in coal samples with higher water content is greater than in coal samples with lower water content, and the interval length of the pore compaction stage in coal samples with higher moisture content is also larger than in coal samples with lower water content. Figure 5: Uniaxial compressive stress–strain curves of coal samples of different moisture content.