Figure 5 OCT imaging of sample B using both 1.3 μm and 4 μm OCT. (a) Close-up of the impact area taken with the OCT onboard camera. (b,c) 1.3 μm and 4 μm OCT surface en face projection of the impact area, respectively. (d) XCT verification of subsurface voids. (e,f) 1.3 μm and 4 μm OCT subsurface en face projection of the impact area, respectively. Horizontal dashed lines indicate the scan positions in (g) and (h), which are offset due to different scan orientations. (g,h) Superposition of 10 B-scans using 1.3 μm and 4 μm OCT, respectively.
3.2 | Subsurface bubble detection using 4 μm OCT and X-ray CT
In the previous section, a subsurface void was detected directly below the impact area and the observation was verified by XCT. In this section, the OCT imaging contrast and penetration depth is further compared with images obtained using XCT. Figure 6(a,c) show OCT and XCT en face images, respectively, of the surface of sample B just next to the impact. The area shows signs of strain from the impact in the top-left region, including an exposed surface bubble and a crack. The lower regions appear seemingly unaffected by the impact. Figure 6(b,d) show the corresponding subsurface region, revealing several bubbles of varying diameter. For ease of comparison the observed bubbles are marked (1)-(5). Since the bubbles are different sizes and found at different depths, the OCT projection in Fig. 6(b) could not exclusively capture the dark air region of all bubbles simultaneously. Therefore, bubbles (2) and (4) appear as bright white spots, which marks the strong reflection from the air-coating interface at the top of the bubble. The bubbles are also clearly seen in the cross-sectional images in Figs. 6(e-j). However, compared to XCT it is clear that the penetration depth of 4 μm OCT is still very limited. To measure the penetration depth in physical units using OCT requires knowledge of the refractive index. However, using the known 9 μm voxel size of the XCT images a physical scale bar was generated, and from that the penetration depth could be evaluated. For example, the top and bottom of bubble (4) is located about 247 μm and 425 μm from the surface, respectively. The deepest point observed with OCT is the bottom of bubble (1), which is located 650 μm below the surface, and this was only possible because of the reduced scattering inside the hollow cavity combined with a strong reflection at the air-coating interface. Still, it presents an advantage compared to ultrasonic techniques that cannot image through air. Using the known depth of bubbles (1)-(4), the refractive index of the coating was calculated to be n=1.59±0.03.
Although the XCT images provide a much clearer identification of bubbles, there are other features where OCT provides a better contrast. Because the contrast of OCT is determined by reflectivity and scattering, it is sensitive to small changes in the refractive index and orientation of particles. For this reason, the OCT surface topography shown in Fig. 6(a) is much more detailed than the corresponding XCT image in Fig. 6(c). Similarly, the horizontal cracks seen in the left side of Fig. 6(i) at a similar depth to bubble (5), is not visible in the corresponding XCT image in Fig. 6(j). Only the larger crack located much deeper is seen. OCT could therefore have an advantage in detecting small cracks that have not yet opened sufficiently to be visible by XCT.