Corrosion Pillowing in Aircraft Fuselage Lap Joints

Abstract

*† ‡ This paper presents the results of studies that have been carried out at the National Research Council on the effect that corrosion pillowing has on the structural integrity of fuselage lap joints. Modeling of corrosion pillowing using finite element techniques showed that the stress near the rivet holes increased to the yield strength of the material (Al 2024-T3) when the corrosion present was above 6% thickness loss. In addition, the analysis showed that pillowing resulted in a stress gradient through the skin thickness, which suggested that semi-elliptical cracks with a high aspect ratio could form. During teardowns of service-exposed lap joints, these types of cracks were found to occur at numerous holes and a closer examination of some of the fracture surfaces revealed the presence of fatigue striations. Therefore a new source of multi-site damage other than fatigue was identified. I. Introduction n the mid-1980s, it became apparent that commercial transports would remain in service well beyond their original projected service life. With the continual operation of these aircraft, there was a concern that significant corrosion combined with fatigue damage could lead to catastrophic failure. Since the total elimination of aircraft structural corrosion was impossible, it became necessary to detect corrosion at a level that had little effect on structural integrity. To this end, the Structures Group of the Institute for Aerospace Research of the National Research Council Canada developed the application of D Sight to corrosion detection in transport aircraft fuselage lap joints. D Sight is an enhanced visual inspection technique that was sensitive to out-of-plane displacements as small as 5 µm. As the development of this technique continued, it became apparent that corrosion pillowing could detrimentally affect the structural integrity of corroded lap joints since it was established that the volumetric increase associated with the corrosion products was 6.454 times greater than the volume of pure aluminum. Pillowing was the term used to describe the out-of-plane deformations that occur when corrosion products build-up between two restricted surfaces such as lap joints. Finite element techniques in conjunction with mathematical models were used to simulate the corrosion pillowing present in fuselage lap joints. The results from this study showed that a stress gradient was present through the thickness of the skins resulting in an increase in the stress along the faying surface with a subsequent decrease in stress along the outer surface. This paper reviews the results to-date of studies that are being carried out to determine the effect of corrosion pillowing on the structural integrity of fuselage lap joints. The finite element techniques and mathematical models developed to simulate corrosion pillowing in Al 2024-T3 fuselage lap joints are reviewed. The results indicated that the stress in a joint caused by pillowing is significantly higher than that caused by the material loss alone. The results from a fracture mechanics analysis that was carried out on straight-fronted crack fronts are also presented. These results indicted that corrosion pillowing could cause the formation of semi-elliptical cracks with high aspect ratios. This paper will also present examples of these types of cracks that were found in a number of corroded lap joints. These cracks formed at a number of different rivet holes and thus are a new source of multi-site damage (other than fatigue), which could significantly affect the residual life and strength of a lap joint.