## 5 Discussion  
With an increase in the external load, the internal damage of the rock is continuously enhanced, and the pore structure will change accordingly, as shown in Figure 8. When sandstone specimens were preloaded to 0.6 UCS and 0.7 UCS, the interior of the rock is mainly composed of pores and micro-cracks. Although some large cracks can be seen in Figure 8, they are all less than 5 \(\upmu\)m in length, which is consistent with the NMR results. With the continuous increase in load, the rock structure is destroyed, the pores continue to expand into micro-cracks, and the micro-cracks expand into large cracks or even crack groups. When sandstone specimens are preloaded to 0.8 UCS, it is obvious that the length  
Figure 8: Pore size distribution of rock samples with different axial compression ratios. of multiple fractures is 5 \(\upmu\)m, indicating that the internal expansion of the rock has changed from quantitative to qualitative. The position and type of the atomic nucleus of a specimen can be obtained by detecting the electromagnetic waves emitted from an external gradient magnetic field, and the structural image of the specimen can be drawn accordingly. Based on the MRI data, the NMR imaging of the sandstones with different axial compression ratios is shown in Figure 9 It is noted that the bright color area is the image of the specimen, and black is the background color. The brightness of the image reflects the amount of water content in the rock, with brighter areas corresponding to higher the water contents. 1. NMR images of S-1, S-2, and S-3: The NMR images of the rock specimen are generally dark, with bright spots concentrated at the ends of the sample. Overall, the distribution is a point shape at the ends. This color distribution indicates that there are no apparent cracks throughout the sample before an external load is applied. 2. NMR images of S-4: Compared with the first three samples, the NMR images of S-4 are brighter overall, and the areas that were bright in S-1, S-2, and S-3 are now brighter in this image after an external load has been applied. Bright patches also appear in the center of the rock in a patchy pattern. This indicates the generations of cracks in the rock, and micropores are continuously expanding into macropores and cracks. 3. NMR images of S-5: Compared to the previous images, S-5 is significantly brighter overall, and the brightness covers a much larger area. There are significant bright spots in multiple sections of the sample, especially an obvious bright strip in the diagonal position of the sample, which is regarded as a large crack. This indicates that a large number of cracks interconnected to form even larger cracks. 4. NMR images of S-6: the brightness of the NMR image is significantly brightened, with two bright crack channels clearly seen in the center of the rock. In summary, in the low-damage stage (the low-axial-compression-ratio stage), there are fewer warm or white spots in the NMR images of the rock samples. With an increase in the stress level, the warm spots in the rock samples become more and more bright or colorful, indicating the pore size of the rock sample increases and the pore structure of rock sample is destroyed. The warm spots are gradually connected into sheets from the initial discrete points, which is due to the porosity gradually increasing. The pores connect until the rock is destroyed, and fractures and a water channel in the rock sample are clearly visible. When the stress level reaches the damage threshold, the white spots inside the rock sample suddenly brighten, and the width of the cracks and water channel in the rock sample continue to increase. The NMR images of rock samples with different axial compression ratios are consistent with the T\({}_{2}\) spectrum distribution and porosity analysis. Overall, the dynamic development of fractures in rock samples can be visually shown  
Figure 9: NMR images of sandstone with different axial compression ratios. (**a**) S-1; (**b**) S-2; (**c**) S-3; (**d**) S-4; (**e**) S-5; (**f**) S-6.