Impact of Mechanical Strain Environment on Aircraft Protective Coatings and Corrosion Protection

Abstract

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

The interaction between corrosion management and structural integrity of aging aircraft

“… (Top panel: results of FE analysis of dome‐head fastened joints von Mises stress distribution with sheet ends opening up (true scale) under a maximum allowable load of this joint, and bottom panel: schematic illustration of protective coating at a dome‐head riveted joint; the effective length of the coating covering lap joint sheet ends is length l* 31 …”

“…Any paint surface defects (scratches, dents or even blisters) are undesirable because they act as stress concentrating features which initiate surface cracking and allow moisture penetration. Tiong and Clark 31 , examining published experimental data, demonstrated an applied strain of 14% should have no adverse effect on the mechanical integrity of the fresh polyurethane topcoat. In contrast, aged polyurethane topcoat, i.e.…”

Cerium Compounds Coating as a Single Self-Healing Layer for Corrosion Inhibition on Aluminum 3003

“…This enormous potential continues to stimulate research in corrosion prevention and the improvement of anticorrosion treatments. Various industries have a high interest in developing materials with higher corrosion resistance, such as reinforcements in construction [5][6][7][8], automotive [9][10][11][12][13][14], aeronautics [15][16][17], steel [18][19][20], and biomedical implants [21,22].…”

Thermal Simulation and Testing of Expanded Metal Foils Used for Lightning Protection of Composite Aircraft Structures

“…In the latter case, cracks can propagate into the structure or around fasteners, providing a path for moisture and other environmental species to enter, resulting in corrosion and degradation of the protection measures including expanded metal foils (EMF) required for lightning abatement and safe operation. Consequently, over several decades there have been numerous efforts and investigations [1,2,3] concerning the degradation of surface layer protection schemes.…”