In-Situ Defect Measurement Method for CFRP Plate Based on Air-Coupled Ultrasonic Lamb Wave

The air-coupled ultrasonic technique is capable of in-situ defect measurement for in-service CFRP plates. Traditional air-coupled ultrasonic methods often adopt the XY orthogonal step scanning method. In order to improve the accuracy of defect detection, an in-situ measurement based on the instantaneous energy spectrum of the air-coupled ultrasonic Lamb wave and surround-step scanning method is proposed. 24 air-coupled ultrasonic transducer positions are evenly arranged on a 90mm×90mm CFRP plate to realize 128 defect measurement scanning paths according to the surround-step scanning method. The Morlet wavelet extracts the instantaneous energy spectrum of the Lamb wave of 128 defect measurement scanning paths to obtain the corresponding defect indexes. The ellipse probability damage imaging algorithm and the -6dB method are used to realize the qualitative analysis and quantitative characterization of the delamination defect with a diameter of 10 mm. The absolute error and the relative measurement error of the defect measurement are 4.1 mm
²
and 5.2%, respectively. The proposed method is promising to detect small-size defects of CFRP plates in the aircraft skin in situ.