# Study of Crack Growth of Transparent Materials Subjected to Laser Irradiation by Digital Holography
## 4 Experiment and Results  
In the first phase, three groups of light sources to irradiate the tested sample were chosen. In order to increase the irradiation energy of the sample and to obtain a better crack growth effect, each group included two different wavelengths including 430 nm + 440 nm (near-ultraviolet light source, UV), 500 nm + 510 nm (visible light source), and 740 nm + 750 nm (near-infrared light source, IR). The irradiation duration of every group was 5 min. Before the start of the irradiation experiment, digital holographic measurement was performed on the sample yielding an initial state hologram. The initial state measurement is shown in Figure 3a. The experiments on the same sample were performed. Figure 3b shows the new phase after the sample was irradiated with a 740 nm + 750 nm laser; then, the same region was irradiated with a 430 nm + 440 nm laser and the phase is shown in Figure 3c. Furthermore, the same region was irradiated with a 500 nm + 510 nm laser and the new phase is shown in Figure 3d. Figure 3b has an obvious change compared with Figure 3a; however, Figure 3b to Figures 3c and 3d to Figure 3c have almost no change. It is therefore concluded that the largest phase change occurred in the crack head following the irradiation of 740 nm + 750 nm hybrid laser. The results of the phase images indicate that the laser in the near-infrared wavelength has a greater effect on crack growth than the light sources in the visible and near-ultraviolet wavelengths. To further investigate the effect of laser irradiation on cracked samples from near-infrared wavelengths to infrared wavelengths, four different irradiation lasers are selected, namely, 740 + 750 nm hybrid laser, 840 + 850 nm hybrid laser, 940 + 950 nm hybrid laser, and 1040 + 1050 nm hybrid laser. The irradiation duration is also kept at 5 min, and the irradiation frequency is set as 5 MHz. The control sample is shown in Figure 2c as a control variable in this stage. Additional irradiation experiments are performed on the sample with a known crack and obtain the phase difference between the samples after irradiation and the initial state. The unwrapped phase results are shown in Figure 4.  
Figure 3: Hologram phases: (**a**) initial state; (**b**) 740 nm + 750 nm laser irradiation; (**c**) 430 nm + 440 nm laser irradiation; (**d**) 500 nm + 510 nm laser irradiation. Red Circle highlights the changes. Figure 4 shows that the cracked samples have all been expanded under laser irradiation in the near-infrared to infrared wavelength bands. The difference in the degree of extension, however, is not significant. For further detailed analysis, the intercept lines at the head, middle, and tail positions of the cracks were extracted in each phase difference map in Figure 4 for contour analysis. The positions of the intercept lines in Figure 4d are shown in Figure 5. Three intercept lines are located at 457, 1625, and 2525 pixels of the \(x\)-axis on each map, respectively. Taking contour lines for the four-phase difference results yield the results shown in Figures 6-8. Figure 4: Post Irradiation phase difference of the tip crack. (**a**) Post irradiation at 740 nm + 750 nm; (**b**) after irradiation at 840 nm + 850 nm; (**c**) after irradiation at 940 nm + 950 nm; (**d**) after irradiation at 1040 nm + 1050 nm. Figure 5: Position of the contour line of the phase difference. Figure 6: Contour line of phase difference at X = 457 for the position of the intercept line: (**a**) experimental results; (**b**) control experimental results. Figure 7: Contour line of phase difference at the intercept position of X = 1625: (**a**) experimental results; (**b**) control experimental results. Note that during the processing of the experimental results, different numbers of \(y\)-axis pixels are taken for the head, middle, and tail according to the size of the crack and transparent media cavity, with 200 pixels for the head and 950 pixels for both the middle and tail.