*4.1. Experiment*

Figure 7 presents a schematic of the test configuration. The test piece was an aluminum plate with dimensions of 500 × 500 × 5 mm. A crack 4 mm long and less than 0.3 mm wide was introduced at the center of it. A 40 × 40 mm region, including the crack, was scanned by an Nd:YAG pulsed laser (DIVA II, Thales Laser Co., Ltd) at a constant illumination spacing of 0.25 mm (160 × 160 points). The pulse duration was 10 ns with 5 mJ energy per pulse, and the diameter of the beam spot was 2 mm. The ultrasonic wave generated by each illumination was received by a transducer placed 40 mm apart from the crack. Its resonance frequency was 5 MHz, and a 70◦ wedge was used. The received signals were recorded in a computer through an amplifier and a digital oscilloscope.

**Figure 7.** Schematic of an aluminum plate with a crack and the pulsed laser scanning area.

Figure 8a depicts the visualized results of the ultrasonic wave propagation in an aluminum plate with a crack. The traveling waves were reflected at the crack, and the amplitude below the crack was high due to the interference of the traveling and reflected waves, as shown in the maximum amplitude map (Figure 8b). In contrast, the diffracted waves propagated with a low amplitude because of the high directivity of ultrasound, hence the maximum amplitude above the crack was lesser than that in other regions. The approximate location of damage was found from the sudden amplitude change; however, it was difficult to quantitatively evaluate the length because the maximum amplitude distribution was rounded off near the crack tips, as shown in Figure 8b.

**Figure 8.** Inspection results by the visualization method of wave propagation on an aluminum plate with a crack: (**a**) Visualized results of ultrasonic waves; (**b**) Distribution of the maximum amplitude.
