Abstract
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.
Highlights
The rotor blade leading edge of a wind turbine is heavily loaded by the high rotational speed and the environmental mechanical influences such as rain, wind or dust
The resulting damage due to erosion leads to an increase in surface roughness at the leading edge, which changes the aerodynamic properties of the rotor blade negatively [1]
It is noticeable that the value of the contrast-to-noise ratio (CNR) decreases from defect 1 to defect 3
Summary
The rotor blade leading edge of a wind turbine is heavily loaded by the high rotational speed (over 300 km/h at the blade tip) and the environmental mechanical influences such as rain, wind or dust. The resulting damage due to erosion leads to an increase in surface roughness at the leading edge, which changes the aerodynamic properties of the rotor blade negatively [1]. Such partial surface damages lead to premature transitions of the boundary layer flow from laminar to turbulent [2], which has a significant influence on the degree of efficiency of the wind turbine [3] and leads to losses of the annual energy production [4]. Regular monitoring and early maintenance of the rotor blade is recommended
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