# Study on Magnetic Resonance Characteristics of Coal Sample under Progressive Loads
## 4 Testing Results and Analysis
### Magnetic Resonance Image (MRI)  
Figure 11 shows the NMR images of the cross section and longitudinal section of the same position before and after uniaxial loading on the coal samples. These images are generated by the imaging module of the NMR, which mainly uses the proton density to create contrast. The sequence used in MRI is NiuSE, includes the following specific parameters: the TE is 500 ms, the TR is 5.89 ms, the averages are 4, the read size is 256, the phase size is 192, the echo position is 10%, the RF center frequency is 12 MHz, the RF offset frequency is 696643 Hz, the GxOffset is 0, the GyOffset is 15, the GzOffset is 20, the RFA90 is 1.8%, and the RFA180 is 3.8%. It is worth noting that the generated image is originally a gray image; the black area in the image is the background color, the white bright spot represents the area where the water molecules are located, and the brightness of the image reflects the water content in the sample. The higher the brightness of an area in the image, the higher the water content in that area, indicating that the pores and fissures in the area are larger. In order to improve the aesthetics of the image and facilitate the reader's reading, image processing software is used to convert the gray image into the color image, as shown in Figure 11.  
Figure 12a displays the NMR images of the cross section of the same position in C-1 under different loads. As is shown, before uniaxial loading, the pores are distributed evenly throughout the whole sample, intermingled with few fissures. When the load increases to 3 MPa, 5 MPa, and 7 MPa, respectively, the pore and fissure distribution changes greatly. The brightness of the image decreases gradually, indicating that most micro fissures close up after loading; meanwhile, the brightness congregate somewhere in the sample, which implies that some pores and fissures begin to expand, converge, and fuse with each other gradually. When the load reaches 11 MPa, the fissures feed through completely and form roughly Y-shaped macro-cracks. This verifies the characteristics of pore  
Figure 11: Conversion between gray image and color image. size distribution: as the load increases, the micro pores and fissures tend to grow into macropores and cracks. Figure 12b displays the NMR images of the cross section and longitudinal section of the same position in C-2 under different loads. As is shown, one cross section and five longitudinal sections are  
Figure 12: NMR images of the coal samples in Group C: (**a**) Sample C-1, (**b**) Sample C-2, (**c**) Sample C-3. selected to analyze the evolution law of pore and fissures. The five longitudinal sections are spaced at the interval of 15 mm; Sections 1 and 5 are 20 mm away to the two end faces, respectively; the cross section is vertical to the longitudinal sections. From the cross section image it can be known that before loading, pores and fissures are widely distributed in C-2 and there are some clear vertical micro-cracks. When the load reaches 3 MPa, part of pores and fissures in the sample close up; however, the distribution is relatively even and the macropores transform to micropores; when the load reaches 7 MPa, the distribution of pores and fissures is relatively concentrated, and some fissures close up. As can be seen from the five longitudinal sections, there are some clear striped fractures in the coal sample. This demonstrates that the pores and fissures are featured with centralized development in the elastic stage. With the closing of primary fissures and the opening of secondary fissures, the fissures form macro-cracks in the end. Figure 12c shows the NMR images of the cross section of the same position in C-3 under different loads. To analyze more efficiently the evolution law of pores and fissures before and after uniaxial loading, the brightness observed are divided into two types.