Experimental Damage Detection on a Wing Panel Using Vibration Deflection Shapes

This paper discusses the use of a vibration method to detect fatigue cracks in inaccessible internal structures. On aircraft, the lower wing panels are highly stressed causing cracks to initiate from fastener holes inside the wing box. The wing panel internal sections are usually inspected using conventional nondestructive inspection techniques after removing the wing panels from the wing box structure. When a crack is detected during the inspection, it can sometimes be repaired by reinforcing the damaged wing panel integral stiffener with a set of repair doublers. Of concern is whether the repair is intact for extended periods of aircraft service. The integrity of the repair and the condition of the wing interior structure might be determined by measuring the vibration of the outer surface. To investigate this approach, a series of experiments was conducted. Two piezoceramic actuator patches were bonded on the outside of a wing panel above a stiffener to generate vibration up to 80 kHz. A scanning laser doppler vibrometer was used to measure the normal vibration of the outside of the panel over the crack/repair. The measurements were performed for both the healthy and damaged panel. It was found that a crack extending from a fastener hole to the free edge of a stiffener and loosening of a repair doubler could be detected by changes in the vibration response of the outside of the panel. The crack from the fastener hole was not detectable until it reached the free edge of the stiffener.