Laser-ultrasonic detection of hidden corrosion in aircraft structure

Corrosion has been recognized as a serious problem in the maintenance of aging aircraft. The Industrial Materials Institute (IMI) has explored the use of laser-ultrasonics for the detection of hidden corrosion in metallic lap joint structures. For inspection with painted surfaces, IMI has shown that a resonance spectroscopy approach using a simple two-layer model can be used to determine the thickness of the paint layer and of the top metal skin. Validation of the model has been made using a test sample with a broad range of paint thickness. Once combined with a numerical inversion method, the model is used to produce a thickness map of the top metal skin from measured resonance frequencies. Results from standard samples with flat-bottom holes showed that the laser-ultrasonic technique could detect metal loss below 1%. The reliability of the method was also demonstrated on accelerated corrosion samples. Comparison to X-ray images showed that the laser-ultrasonic method presented a thickness map that had the same accuracy as the X-ray system without the need for dismantling the sample. These results indicated that laser-ultrasonics could be a useful tool not only to inspect aircraft during routine maintenance but also to provide valuable data in the study of corrosion inception and growth in lap joint structures.Peer reviewed: YesNRC publication: Ye