## 3 Results
### Cracks Distribution
#### 3.3.2 Interior Cracks  
3D X-ray micro-imaging (3D-XRM) was applied to a selected irradiated rock specimen to obtain the detailed interior structure and crack distributions. CT scanning technology can visually present the microstructure's characteristics (such as cracks, pores and microcracks) of the irradiated rock by using 256 gray scales through the density difference of each imaging unit in the specimen. Then, Dragonfly was applied for post-processing, and data, such as crack sizes inside the irradiated sample, were obtained by means of smooth filtering, threshold segmentation, mesh reconstruction and size measurement. Only images from the case with the laser-power output of 1000 W, beam diameter of 6 mm and beam moving speed of 0.5 m/s were presented because similar damage to irradiated specimens and the interior crack distributions could be expected in the other cases. Figure 11 shows the rendered images of the irradiated granite specimen and its interior cracks, which qualitatively illustrate the crack distributions within and on the surface of the granite specimen after laser irradiation. From the front view of the irradiated specimen shown in Figure 11a, one can see that the cracks are concentrated in the upper part of the specimen where the laser beam was directly irradiated. In addition, it is also found that the laser beam causes thermal damage to both sides of the specimen, and the crack numbers on both sides are similar. Deep cracks can be seen at the front, middle and back of the specimen along the beam scanning direction. From the top view of the rock specimen presented in Figure 11b, the cracks are symmetrically distributed along the central line of the laser moving direction. From the back view of the specimen illustrated in Figure 11c, we can observe that the deepest cracks can reach almost 90% of the specimen's diameter. The distribution of cracks within the irradiated specimen can be illustrated further from the vertical-section view of the specimen at different locations, as shown in Figure 12. From the vertical section through the axis presented in Figure 12a, we can clearly see a groove with a uniform depth at the top of the specimen where the laser beam is irradiated directly on the specimen. Deep, cracked gaps are found at the front, middle and back of the specimen as the laser beam moves from the left to the right end. Large and wide cracks can be seen at the same locations of the irradiated specimen, as illustrated in Figure 12b. It is also indicated from Figure 12 that long and wide cracks are present at the two ends of the specimen, which is consistent with the results shown in Figure 11.  
Figure 11: Rendered images of irradiated specimens from (**a**) front, (**b**) top and (**c**) right views. In order to quantify the crack development within the specimen, the crack length and width at several locations were measured based on the cross- and vertical-section images, as shown in Figure 13. From Figure 13a showing the cross-section located at the beginning of the laser irradiation starting point, the maximum crack width with a value of about 367.48 \(\upmu\)m can be found near the bottom of the U-shaped groove generated by the laser beam, where severe damage was created by the high-temperature gradient in the specimen. The width of the cracks decreased as the cracks developed deeper into the bottom. For instance, the width of the cracks on the left and right sides decreased from 330.08 and 367.48 \(\upmu\)m to 217.76 and 190.24 \(\upmu\)m, respectively. The maximum crack width at the axial section was about 243.98 \(\upmu\)m near the right end of the specimen, and it can be observed that cracks at the middle of the specimen are smaller than those near the end, as shown in Figure 13b. It was also found that the lengths of the two cracks at the central and right sides were about 38.27 and 37.93 mm, respectively, from the bottom of the groove to the end of the cracks.