Defect detection on the curved surface of a wind turbine blade using piezoelectric flexible line sensors

Blades play a critical role in the wind turbine system. Therefore, their structural health monitoring is very important. Blades are damaged by sudden changes in wind load, cracks due to collision of foreign objects, and disasters, such as lightning strikes, hail, and typhoons. Moreover, blades are expensive to maintain. Defects or damages to wind turbine blades reduce the life span and power generation efficiency of the wind turbine and increase safety risks and maintenance costs. Therefore, it is very important to detect blade damage to prevent problems in the wind turbine. Ultrasonic inspection is suitable for blades made of composite materials. Piezoelectric ceramic, which is a typical piezoelectric element, has relatively high sensitivity compared to other sensors. However, it suffers from brittle fractures and thus difficult to apply to curved structures. To overcome the limitations of piezoelectric ceramics, a piezoelectric flexible line sensor that can be applied to curved surfaces was manufactured using the dice-and-fill method for a [Pb(Li_0.25Nb_0.75)]_0.06 [Pb(Mg_0.33Nb_0.67)]_0.06 [Pb(Zr_0.50Ti_0.50)]_0.88O₃ with 0.7 wt% MnO₂ (PZTNMML) ceramic disc. Instead of a typical ultrasonic inspection method with limited surface contact, a laser capable of producing ultrasonic excitation of ultrasonic waves over a large area from a long distance was used. The possibility of detecting a defect on the wind turbine blade using a piezoelectric flexible line sensor and laser ultrasound was confirmed in this study.