Laser-ultrasonic detection of hidden corrosion in aircraft lap joints: Results from corroded samples

The detection of hidden corrosion has been recognized as a serious problem in the maintenance of aging aircraft structural elements such as lap joints. In the presence of corrosion, the thickness of the metal skin may be significantly reduced and reach a level (generally above 10% of metal loss) that requires repair or replacement. The Industrial Material Institute (IMI) has developed a novel method that uses the spectral analysis of laser-ultrasonic waveforms to determine the residual metal skin thickness of the top skin of a lap joint. Previous work has shown that a characteristic equation can be derived that predicts the resonance frequencies of a paint-metal structure, such as encountered in an aircraft lap joint. Using numerical minimization techniques, this expression is used to process the laser-ultrasonic data and produce thickness maps of both the paint layer and the metal skin of a lap joint. Results from standard samples with flat-bottom holes show that the laser-ultrasonic technique can detect...