Figure 8 Detection of cracks in cut test blade sample D. (a) Cross-sectional microscope composite image of sample D imaged along the edge of the cut (defocus due to irregular edge). Notations indicate the scale, orientation, coating thickness (double arrow), and crack (arrow). (b) OCT surface topography near the edge of the sample. (c,d) Corresponding OCT volume projections at different depths, revealing cracking along different planes. (e) B-scan near the sample edge, showing the two cracks “c” and “d” at different depths. (f,g) Cross-sections from 10 and 20 B-scans, respectively, in the same orientation as the microscope image in (a). Scan positions are indicated by the blue dashed lines in (c) and (d).
4 | CONCLUSION
This work demonstrates the potential of MIR OCT as a non-destructive method for inspection of coatings on wind turbine blades. It was found that the SC laser is able to penetrate polyurethane leading edge coatings and image subsurface defects, such as bubbles and cracks below 250 μm in depth. In coatings less than 150 μm, OCT can clearly delineate the interface between different coating layers and between the coating and substrate to reveal defects and cracks. OCT is a contactless technology, that unlike ultrasound can image through air gaps, such as cracks and bubbles. However, due to the highly scattering nature of these coatings, and especially the putty layer which has a high content of filler particles, the penetration depth of 4 μm OCT is curently limited to imaging only very close to the surface and requires relatively long integration times on the order of milliseconds per line (seconds per B-scan) to do so. Still, there is currently no other existing non-destructive method that can image just below the surface with sufficient detail to identify coating defects. With further developments in MIR SC lasers and detector technology to increase the wavelength range and reduce the noise, the penetration depth and sensitivity of OCT is expected to improve. OCT therefore has a unique potential to complement existing methods in the quality control of coatings on wind turbine blades and RET specimens, which can contribute to improving the lifetime of turbine blades, reducing waste, and making wind energy production cheaper and more reliable.
ACKNOWLEDGEMENTS
The authors acknowledge the DURALEDGE project partners for providing the test specimens for this work.
ORCID
Christian Rosenberg Petersen (0000-0002-2883-7908), Søren Fæster (0000-0001-5088-2396), Jakob Ilsted Bech (0000-0003-0228-6375), Kristine Munk Jespersen (0000-0002-6796-6200), Niels Møller Israelsen (0000-0001-9632-7902), Ole Bang (0000-0002-8041-9156).
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FIGURE LEGENDS
Figure 1 (a) Photographic overview of the test samples. Arrows indicate impact sites.
Figure 2 XCT data and density segmentation revealing cracks (red) and air bubbles (blue) inside the coated sample B.
Figure 3 (a) Experimental setup for the 4 μm OCT system. (b) OCT imaging depth for different spectrometer integration time. The scale bar corresponds to 200 μm assuming a refractive index of n=1.
Figure 4 OCT imaging of transparent coating sample A at 1.3 μm. (a) Close-up en face optical image of the impact area taken with the OCT onboard camera. (b,c) OCT surface- and subsurface en face projection of the impact area, respectively. (d) Single B-scan across the impact area shown by the vertical dashed line in (b). (e) Superposition of 200 B-scans showing the patterns of cracks in the area between the horizontal dashed lines in (b). The scale bars indicate optical depth assuming of n = 1.
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.
Figure 6 Detection of sub-surface bubbles in sample B using 4 μm OCT with XCT verification. (a,b) OCT and (c,d) XCT surface- and subsurface en face projections of the scan area, respectivly. (e,g,i) OCT and (f,h,j) XCT cross-sections of bubbles (1)-(5). Note that the XCT scale is a physical scale, while the OCT scale is optical path distance (OPD) (i.e. multiplied by n).
Figure 7 (a,b) Microscope- and OCT cross-sections of monolayer sample B, respectively (not the same position). (c,d) Microscope- and OCT cross-sections of multilayer sample C, respectively (not the same position). (e) Line scan average of the two samples showing difference in scattering properties of the coating layers. The two traces represent an averaged over ten adjacent A-scans from ten consecutive B-scans, and are aligned to the strong surface reflection (0 μm OPD). Dashed arrows in (b) and (d) indicate the line scan positions.
Figure 8 Detection of cracks in cut test blade sample D. (a) Cross-sectional microscope composite image of sample D imaged along the edge of the cut (defocus due to irregular edge). Notations indicate the scale, orientation, coating thickness (double arrow), and crack (arrow). (b) OCT surface topography near the edge of the sample. (c,d) Corresponding OCT volume projections at different depths, revealing cracking along different planes. (e) B-scan near the sample edge, showing the two cracks “c” and “d” at different depths. (f,g) Cross-sections from 10 and 20 B-scans, respectively, in the same orientation as the microscope image in (a). Scan positions are indicated by the blue dashed lines in (c) and (d).