# Study on Magnetic Resonance Characteristics of Coal Sample under Progressive Loads
## 3 Materials and Methods
### Test Scheme  
The coal samples are taken from 15,208 working face in Zhangjiamao Coal Mine. The coal briquettes are processed into standard samples (\(\varphi\) 50 \(\times\) 100 mm) whose non-parallelism between two end faces should be no larger than 0.01 mm and whose diameter deviation of the upper end and lower end should be no larger than 0.02 mm. This is to guarantee the uniform loading on the coal sample. The test scheme is composed of the following steps:  
1. Divide the standard coal samples into two groups and label them as Group S and Group C. Each group contains three samples, which are numbered as S-1, S-2, and S-3 and C-1, C-2, and C-3, respectively. 2. Place all samples into a drying oven to dry for 24 h. The temperature of the drying oven is set at 50 \({}^{\circ}\)C; then, the dried samples are put into water for water-saturation process which lasts for 12 h.
3. Conduct uniaxial loading to the water-saturated samples of Group S with a compression testing machine and obtain the mean peak strength. 4. Conduct low-field NMR test to the water-saturated samples of Group C to obtain the porosity, pore size distribution, and \(T_{2}\) distribution curve; NMR imaging is performed to the coal samples along either transverse or longitudinal direction. 5. Conduct loading on the coal samples of Group C in three loading modes. For the coal sample C-1, uniaxial loading-holding test is conducted under five different loads in turn, namely, 3 MPa, 5 MPa, 7 MPa, 9 MPa, and 11 MPa. For the coal sample C-2, uniaxial loading-holding test is conducted under two different loads in turn, namely, 3 MPa, and 7 MPa. For the coal sample C-3, uniaxial loading-holding test is conducted under two different loads in turn, namely, 3 MPa and 11 MPa. The coal samples should hold the loading for 150 min under each loading condition. After each run of the test, step (2) and (4) should be repeated. As is mentioned above, the vertical stress of coal wall within the range ahead of the working face evolves from 3 MPa to10 MPa gradually. Accordingly, in the loading test, the load also starts from 3 MPa and increases gradually. Meanwhile, given that different recovery rates lead to the difference in stress increase, to better study the evolution characteristics of pores and fissures under different loads, the load exerted on coal samples C-2 and C-3 rises from 3 MPa to 7 MPa, and 11 MPa, respectively. There are two reasons for holding the load on the coal sample for 150 min: one is to better reflect the actual stress situation of coal wall, because the supporting pressure above the coal wall may remain unchanged for some period of time due to the periodicity of recovery; the other is to let the coal sample fully adapt to the current load so that the pores and fissures in the sample could develop fully. Given that the primary fissures of coal samples are relatively developed, once the drying temperature is over 50 \({}^{\circ}\)C, the primary fracture will expand obviously, resulting in the change of the internal structure of the coal sample and the sudden drop of the compressive strength, which makes it impossible to further study the nuclear magnetic resonance characteristics. Therefore, the drying temperature can only be set at 50 \({}^{\circ}\)C.  
Figure 6 shows the stress-strain curve of the coal samples of Group S. As is shown, the stress-strain curve is mainly divided into three stages: compaction stage, elastic stage, and failure stage. The splitting strength between the compaction stage and elastic stage is approximately 4.5 MPa, while that between elastic stage and failure stage is approximately 17.5 MPa. The peak stress of water-saturated coal samples of Group S is 19.14 MPa, 16.60 MPa, and 17.72 MPa, respectively, with the average peak stress being 17.82 MPa. The preset maximum loading (11 MPa) is positioned in the elastic stage of the coal sample.