A B S T R A C T Aircraft joints feature prominently in aircraft structural degradation; fatigue cracking and corrosion damage are major issues, which can reduce joint strength and degrade service life. Protecting the structure against corrosion usually involves use of highly developed protective coatings -paints and sealants -and, increasingly, the application of corrosion inhibiting compounds (CICs) which retard corrosion principally by penetrating into crevices and cracks, and displacing water. A combination of coatings and CIC use can provide effective corrosion protection, but both interact -in different ways -with joint structural performance and overall system durability. This paper discusses the interaction between these two corrosion protection measures and fatigue performance of joints. The first issue relates to the extent to which application of CICs (or other lubricants) can cause a reduction in the fatigue life of mechanically fastened joints. The CICs are lubricants which will reduce the friction at the faying surface of the joint, and change the load transfer characteristics of the joint. This paper discusses results from a test program assessing the fatigue life and failure mode of simple riveted lap joints; the results show a distinct reduction in fatigue life for joints containing CICs, and the paper discusses the changes thought to be responsible for the reduction. The second issue discussed is the degradation of protective coatings in service. Joints are key locations for coating cracking and failure, because areas such as sheet ends and fastener heads, where displacements are concentrated, may produce concentrated strain in coatings. So far, however, the potential influence of aircraft loading on coating degradation prognostics has received little attention. This paper discusses the role of joint displacement in service as a factor contributing to degradation in aircraft coatings at joints, and argues that this local strain effect, and indeed structural loading history, needs to be considered in predicting and assessing rates of coating degradation. It describes analyses of displacements in aircraft joints, to identify the levels of strain and to identify the roles and relative contributions of the various deflections in the joints. The results indicate the potential for very large strains in coatings.
Aircraft paint schemes (paint and sealant) are a key element of corrosion preventive measures, and ensuring that sueh schemes and other coatings are effective and durable under service conditions is essential If corrosion eosts and maintenance costs are to be minimized. Coatings can degrade under the influence (if environmental factors sueh as exposure to moisture, high temperature, ultraviolet radiation, and so forth. Coaling failure is often particularly evident at joints, and while failure may be accelerated in some regions hy local variations in coating thickness and geometry at features sueh as edges, or by erosion, it is likely that the displacements which oeeur at these locations under service loads will he a eontrihuting factor. The impact of in-serviee mechanical loading on coating degradation has so far received little attention, despite clear evidence that coatings tend to fail Hrst at specific sites sueh as sheet ends and around fastener heads. This paper argues that the magnitude of the applied service loads and the nature of the load history should be considered in predicting and assessing rates of eoating degradation, and that development of a thermomeehanieal history is a more appropriate approaeh. The likely impaet of joint displacements on the protection of aging aircraft is also discussed. Nomenclature it "( D E, e (', I. r k t '• It) ß Al* e* £/, e", fí «ihCo (Ta = crack length/depth, tntn = critical intertacial delamination length, tnm = diameter of fastener shank, mm = elastic tnodulus of coating, MPa = edge margin, mm = critical energy release of coating, i/nr = spacing between two perpendicular cracks in the coating, mm = effective length of coating, mm = critical stress intensity factor, MPa (m)''' = critical buckling stress constant = thickness of sheet, tnm = thickness of coating, mm = width of the sheet = initial postbuckling slope = geometry factor = change of effective length, mm = strain at the bottom surface of the plate = hygroscopic strain = mechanically-induced strain = strain at the top surface of the plate = thermally-induced strain = compressive residual stress, MPa = critical tensile stress, MPa = Pois.son's ratio of coating
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