## 3 Results  
### Full-Field Strain  
Dynamic deformation of the rock material under blast loading can be obtained using the 3D-DIC technique and analysis of high-speed images. Full-field strains of the AOI for three specimens (BS1, BS3, and BS5) were obtained. The surface strain field of the AOI was calculated using von Mises strain. For BS1, the inter-hole delay was 0 \(\upmu\)s, i.e., simultaneous detonation. The frame rates of the high-speed cameras were set to 20,000 fps, and the image resolution was set to 1064 \(\times\) 762 pixels in the experiment. The obtained surface strain fields of eight frames (from 0 to 500 \(\upmu\)s) for BS1 are shown in Figure 8, which shows the developing strain pattern. As seen in Figure 8, strain concentrations first occurred in the lower part of the AOI at 200 \(\upmu\)s. In the color plots from 250 to 300 \(\upmu\)s, there were two clearly horizontal strain concentration zones in the middle and lower part of the AOI. In addition, a vertical strain concentration zone on the lower right side of the AOI intersects with the two horizontal strain concentration zones. At 300 \(\upmu\)s, two vertical strain concentration zones formed in the  
Figure 7: The location of the AOI on the front surface of the bench specimen. upper part of the AOI. From 350 to 500 \(\upmu\)s, the strain concentration zones became wider and formed macroscopic cracks as time increased. The cracks developed and intersected, breaking the rock into different fragments. From the strain fields analyzed by 3D-DIC, the surface strain development pattern and potential failure zone can be easily estimated. The resolution and frame rate of the high-speed cameras for BS3 was set to 840 \(\times\) 624 pixels and 30,000 fps, respectively. There was approximately 33.3 \(\upmu\)s between adjacent frames. The full-field strain of BS3 is shown in Figure 9, which presents the calculated results obtained by 3D-DIC software from 0 to 566 \(\upmu\)s. At 233 \(\upmu\)s after the detonation of the detonator, strain concentration was observed on the left side of the AOI. From 266 to 333 \(\upmu\)s, the strain concentration zone in the AOI developed upward along the vertical direction. There were secondary strain concentrations near the vertical strain concentration zone at 366 \(\upmu\)s. As the effect of blast loading continued, strain concentration zones developed along the horizontal direction formed in the middle and bottom of the surface strain field. It should be noted that strain concentrations were found on the right side of the AOI at 433 \(\upmu\)s, though these did not form significant strain concentration zones as time increased. At 566 \(\upmu\)s, the middle horizontal strain concentration zone reached the right edge of the surface strain field. Compared with the strain evolution process of BS1, the formation time of the horizontal strain concentration zone for BS3 was relatively late. BS5, with an inter-hole delay of 200 \(\upmu\)s, was selected with the same image resolution and frame rate setting as BS3. Figure 10 shows the surface strain fields for BS5 from 0 to 566 \(\upmu\)s. It can be seen that the strain concentration zone of BS5 from 233 to 333 \(\upmu\)s mainly appeared on the left side of the AOI, which was similar to that of BS3. From 266 to 366 \(\upmu\)s, the left vertical strain concentration zone extended in the middle and branched horizontally. Strain concentration then occurred at 400 \(\upmu\)s in the middle of the AOI and formed a horizontal strain concentration zone as time increased. From 500 to 566 \(\upmu\)s, a downward developing vertical strain concentration zone appeared in the upper right of the AOI. As time increased, the vertical strain concentration zone intersected with the middle horizontal strain concentration zone. Figures 8-10 show the evolution of the strain field on the bench specimens with different delay times. The strain fields obtained by the 3D-DIC technique are favorable for observing the dynamic deformation of the specimen surface before the appearance of macroscopic cracks.