## 4 Analysis of DIC Results at the First Impact  
Compared with that in the static test, the impact loading velocity in the SHPB dynamic test is large, and the specimen failure duration is short. Therefore, a high-speed camera with a resolution of 512 \(\times\) 256 and a shooting rate of 50,000 fps is applied to capture the whole loading process of sandstone. A high-power LED photography lamp is used to illuminate the signal surface of specimens. To obtain the sandstone deformation during the whole loading, special personnel shall operate the loading equipment and LED photography lamp to ensure that the loading, photography, and lighting in a test are strictly synchronized. The DIC is a technique that tracks the strain and displacement changes on the sandstone surface. Therefore, random dots on the surface of the specimen are required as the speckle patterns before an experiment, as presented in Figure 13. During the loading process, the speckle moves with the deformation of the specimen. Then, the strain field in the specimen is given by analyzing the motion of the speckle. Figure 14 gives the vertical strain clouds of sandstone after varying temperature treatments under impact loading. Figure 15 gives the maximum tensile strain at the corresponding loading period of sandstone specimens after varying temperature treatments in Figure 14. The following five conclusions can be obtained. Figure 13: Surface speckle pattern of specimen. (1) In the initial stage of loading, when the strain field in the vertical direction is at 25 \({}^{\circ}\)C and 200 \({}^{\circ}\)C, the distribution of the strain field on specimen surface is relatively uniform, and the average tensile and compressive strains of the specimen are small. In addition, the tensile strain distribution area of sandstone specimens from 400 \({}^{\circ}\)C to 800 \({}^{\circ}\)C gradually increases, and the tensile strain value of the sandstone at 600 \({}^{\circ}\)C is significantly larger than that of the sandstone at other temperature sections. (2) In the middle stage of loading, with a continuous increase in dynamic loading force in the vertical direction of sandstone at 25 \({}^{\circ}\)C, a local hardening zone appears on the edge of specimens. However, the value is small and no significant change occurs in the strain until the later loading stage. It shows that the overall strength of sandstone at 25 \({}^{\circ}\)C is high under impact loading, and the sandstone does not have overall cracks. Figure 14: Vertical strain clouds of sandstone specimens after different temperatures under impact loading. (**a**) Initial loading period. (**b**) Mid-loading period. (**c**) Later loading period. Figure 15: Maximum tensile strain during impact loading. (3) When temperature T = 200 \({}^{\circ}\)C, the tensile strain distribution area of the sandstone increases significantly in the middle of loading. At the initial loading stage, an area of concentrated tensile strain appears. Then, the strain gradually expands until the later loading stage. The lateral strain region parallel to the loading direction evolves into an inclined shear strain region and the tensile strain in this region changes little. The literature [35] has defined two strength values: the initial strength value and the maximum strength value, which correspond to the strength value when the rock begins to fail and the maximum value of the stress time history curve, respectively. It is pointed out that the point corresponding to the crack initiation time is an inflection point of the stress time history curve, but it is not the extreme point. Therefore, compared with that in Figure 10b, the strength of sandstone at 200 \({}^{\circ}\)C changes little from the initiation point to the extreme point. The strain development of sandstone at 400 \({}^{\circ}\)C is almost the same to that of the sandstone at 200 \({}^{\circ}\)C. In short, the DIC technology analysis can explain the dynamic characteristics and failure mode of sandstone under high temperatures.