## 3 Test Results and Analysis
### Strain Field Evolution of Sandstone with Different Grain Sizes  
As can be seen from Figure 6, sandstones with different grain sizes exhibit different characteristics in each stage of the stress-strain curve. Compared with fine sandstone, coarse sandstone has a larger particle size, which leads to more voids in coarse sandstone and a lower degree of cementation between rock particles. When entering the elastic stage, the elastic deformation stage of coarse sandstone is the shortest of the three kinds of rock size, and its elastic modulus is also the lowest. Fine sandstone has a compact structure, low deformation characteristics, high strength and a stronger ability to resist elastic deformation. When entering the plastic deformation stage, due to the dense internal structure and good integrity of fine sandstone, the stress-strain curve reaches its peak strength in the form of an approximate straight line, while the coarse sandstone grows in the form of a smooth curve before reaching its peak strength. After reaching peak strength and entering the failure stage, the macroscopic failure of sandstone of any particle size will occur in a very short time. The strain value of coarse sandstone is larger than that of the other two types. That is to say, under the action of external load, coarse sandstone has higher deformation characteristics than fine sandstone, resulting in greater strain. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Rock Sample** & **Loading** & **Peak Stress/MPa** & **Time Required to** & **Stress Growth** & **Elastic** \\
**Number** & **Rate/(mm/min)** & **Reach Peak Stress/s** & **Rate/(MPa/s)** & **Modulus/MPa** \\ \hline X11 & 0.02 & 21.5 & 1755.46 & 0.0122 & 4025.19 \\ X12 & 0.02 & 23.8 & 1874.88 & 0.0126 & 4103.31 \\ X13 & 0.02 & 24.2 & 1924.33 & 0.126 & 4156.48 \\ X21 & 0.1 & 29.12 & 320  
Figure 5: _Cont._  
Figure 5: Evolution cloud map of the strain field under different loading rates: (a) 0.02 mm/min; (b) 0.1 mm/min; (c) 0.5 mm/min. Table 3 shows the comparison of mechanical properties of red sandstone of three kinds of grain size. It can be found that the uniaxial compressive strength of fine-grained sandstone is 1.67 times the uniaxial compressive strength of coarse-grained sandstone, the elastic modulus of fine-grained sandstone is 1.16 times the elastic modulus of coarse-grained sandstone and the density of fine-grained sandstone is 1.35 times the density of coarse-grained sandstone. Fine-grained sandstone has a smaller particle size, tighter internal structure, better compactness and stronger cementation between particles, so it shows stronger mechanical properties on the macro level. Figure 7 shows the evolution cloud diagram of the strain field of red sandstone with different grain sizes under uniaxial compression. For sandstone with a fine particle size, the crack on the surface of the specimen starts from the upper left, approximately parallel to the axial direction, and expands downward with continuous loading. At the same time, secondary cracks start from the end of the main crack. When they extend to the center of the axial direction, an obvious through-crack quickly occurs along the diagonal direction of the specimen surface, which leads to the failure of the specimen. The energy release is completed instantaneously, accompanied by a large noise and a splash of rock. With the increase in sandstone particle size, the failure process of medium and coarse grain size sandstones remains similar. Similarly to fine-grain sandstone, cracks in medium-grain rock samples start from the upper left, forming a strain localization area which is approximately parallel to the axial direction, but different from fine-grain sandstone. The crack growth rate is also slower. The single crack continued to expand downward along the axial direction of the specimen, and the strain localization region gradually expanded.