Active Thermography for the Detection of Sub-Surface Defects on a Curved and Coated GFRP-Structure

Initial defects, for example, those occurring during the production of a rotor blade, encourage early damages such as rain erosion at the leading edge of wind turbine rotor blades. To investigate the potential that initial defects have for early damage, long-pulse thermography as a non-destructive and contactless measurement technique is applied to a strongly curved and coated test specimen for the first time. This specimen is similar in structural size and design to a rotor blade leading edge and introduced with sub-surface defects whose diameters range between 2mm and 3.5mm at depths between 1.5mm and 2.5mm below the surface. On the curved and coated test specimen, sub-surface defects with a depth-to-diameter ratio of up to 1.04 are successfully detected. In particular, defects are also detectable when being observed from a non-perpendicular viewing angle, where the intensity of the defects decreases with increasing viewing angle due to the strong surface curvature. In conclusion, long-pulse thermography is suitable for the detection of sub-surface defects on coated and curved components and is therefore a promising technique for the on-site application during inspection of rotor blade leading edges.

Automated summary

This study utilized long-pulse thermography for the first time to detect sub-surface defects on a curved and coated test specimen, successfully detecting defects with a depth-to-diameter ratio of up to 1.04. Additionally, defects were detectable from a non-perpendicular viewing angle, showing promising potential for on-site application during the inspection of rotor blade leading edges.