As the charge of a thunderstorm cloud accumulates, it discharges to the ground and travels along the air [20,21]. When the tip of the discharge reaches a certain distance from the wind turbine blade, a strong electric field is induced with the metal acceptor of the lightning protection ...
. The attenuation of ultrasonic signals in long-distance transmission is small [125]. Ultrasonic testing needs the help of a liquid couplant, and if the damage is not on the propagation path of ultrasonic waves, the damage will be difficult to detect, or the sensitiv- ity will be reduced....