Development of a long-range multi-area scanning ultrasonic propagation imaging system built into a hangar and its application on an actual aircraft

Materials such as aluminum alloys and carbon composites are widely used in aircraft structures. In the case of the use of Al-alloys in aircraft structures, fatigue cracks occur because of excessive and repeated loading and vibrations experienced during frequent flights. Meanwhile, composite materials are also damaged—for example, impact damage and debonding—and have defects, including voids, prepreg gaps, and overlaps created during the manufacturing process. Ultrasonic propagation imaging is a damage visualization technique that is used in structural inspections employing a laser scanning system and ultrasonic sensors. However, conventional ultrasonic propagation imaging or other scanning systems, such as a scanning laser Doppler vibrometer, only permit a single area to be inspected at one time. It is also difficult to inspect inaccessible areas, such as the upper skins of the aircraft wings. In this work, we describe a multi-area scanning ultrasonic propagation imaging system built in a hangar that is able to rapidly scan at a pulse repetition rate of 20 kHz. After acquiring the generated ultrasonic wave signal induced by laser excitation, ultrasonic propagation imaging videos for the in-plate guided wave are displayed. Finally, internal damage can be identified in a damage visualization platform. The developed multi-area scanning ultrasonic propagation imaging system is demonstrated by performing simultaneous inspections on two areas containing manufacturing defects in a large carbon/epoxy laminate. We also performed a demonstration of the hangar-based multi-area scanning ultrasonic propagation imaging system on an actual aircraft containing back surface cracks. The multi-area scanning ultrasonic propagation imaging system with tilting mirror systems installed in the hangar ceiling permitted a clear visualization of the damage. The damage visualization results confirm that the proposed multi-area scanning ultrasonic propagation imaging system and approach have excellent applicability as a built-in ultrasonic propagation imaging system for a Smart Hangar, which is a future structural health monitoring solution that will be used to realize a full-scale structural inspection of an actual aircraft.