Ung Hing Tiong

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

Aircraft paint schemes (paint and sealant) are a key element of corrosion preventive measures, and ensuring that such schemes and other coatings are effective and durable under service conditions is essential if corrosion costs and maintenance costs are to be minimized. Coatings can degrade under the influence of environmental factors such as exposure to moisture, high temperature, ultraviolet radiation, and so forth. Coating failure is often particularly evident at joints, and while failure may be accelerated in some regions by local variations in coating thickness and geometry at features such as edges, or by erosion, it is likely that the displacements which occur at these locations under service loads will be a contributing factor. The impact of in-service mechanical loading on coating degradation has so far received little attention, despite clear evidence that coatings tend to fail first at specific sites such 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 coating degradation, and that development of a thermomechanical history is a more appropriate approach. The likely impact of joint displacements on the protection of aging aircraft is also discussed.

Influence of Mechanical Loading on Failure of Aircraft Protective Coatings

The degradation and failure of protective coatings (paints and sealants) is a key element influencing the service life of aircraft. Such degradation is influenced by the response of coatings to environmental factors such as high temperatures and exposure to ultraviolet radiation, as well as chemical factors. However, the effect of loading and load history on coating durability has received little attention, despite clearly being a factor in determining failure sites (such as joints) and the rate of degradation. This paper describes the key characteristics of coatings at aircraft joints, and the nature of the strains experienced by coatings in locations influenced by in-service loads. It is first step in assessing the complex strain history at joint strain concentration locations as part of developing a prognostic capability for the service life of aircraft coatings. The configuration of coating layers at different joints is important and this research has considered a simplification of a butt strap joint from a RAAF military aircraft and a generic lap joint; predictions of critical movements/displacements have been made using finite element analysis; the predictions will be tested later as part of an experimental program associated with a full-scale fatigue test.

Aircraft Joints and Corrosion Control

Corrosion damage in aircraft structure, if undetected and/or left untreated, can undermine safety. Currently corrosion prevention and management in many civil and military fleets still relies strongly on the use of traditional ‘find and fix’ maintenance practices, although this has been refined by the increasing use of Corrosion Prevention and Control Plans (CPCP) which provide a framework for targeted inspections and treatment to help with corrosion management. Teardowns of high-life service aircraft and parts can also be valuable tools to help identify corrosion-prone areas and relative severity of the corrosion. This paper describes research which supports the development of improved prognostic capability for corrosion, by investigating one particular factor which appears to play a significant role in the development of corrosion. The focus of this research is to better understand and predict the deterioration and breakdown of protective paint coatings at aircraft joints, primarily due to the influence of mechanical displacement. The impact of in-service mechanical loading on coating degradation has so far received little attention, despite clear evidence that coating tend to fail first at specific site such as sheet ends and fastener heads. Potential service/performance implications of the joint displacement on the protection of ageing aircraft are discussed. More importantly, it is argued that appropriate corrective actions are required immediately after the paint cracking detected, even if active corrosion is not fully evident.

Aircraft Joints: the Interaction between Corrosion Protection and Structural Performance

Aircraft joints feature prominently in aircraft structural degradation. Fatigue cracking and corrosion damage can reduce joint strength and degrade service life. Corrosion management can include use of paints and sealants and, increasingly, the application of Corrosion Inhibiting Compounds (CICs) which retard corrosion, 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 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 a reduction in the fatigue life of mechanically-fastened joints after application of CICs (or other lubricants) The lubricating properties of the CICs reduce the friction at the faying surface, which may change the load transfer characteristics of the joint. The paper discusses results from a test program assessing the fatigue life and failure mode of riveted lap joints; the results show a marked reduction in fatigue life for joints containing CICs, and the paper discusses the changes which may 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, since areas such as sheet ends and fastener heads, where displacements are concentrated, will produce concentrated strain in coatings. So far, however, the potential influence of aircraft loading on coating degradation prognostics has received little attention. The paper discusses the role of joint displacement in service as a factor contributing to early degradation of aircraft coatings, 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 initial analyses of displacements in aircraft joints, to identify the levels of coating strain and the roles and relative contributions of the various deflections in the joints. The results indicate the potential for very large strains in coatings.

Properties of PH 13-8 Mo steel for fatigue application in helicopters

While many fixed-wing aircraft have adopted damage-tolerant design in recent years, helicopter design is still based predominantly on a safe life approach, in which relatively simple Stress Life (S-N) data underpins the tools used for life prediction. Due to their unique loading, helicopter structures experience a high number of loading cycles as compared to fixed-wing aircraft, and this presents a more challenging fatigue life management problem. To minimise the fatigue damage, the helicopter community tends to design components such that most of the loading experienced falls below the fatigue limit of the selected material. These materials are usually of high strength and have good fatigue properties, although the large number of cycles experienced by some components raises the possibility of fatigue in the “gigacycle” regime where the fatigue limit drops to a new, lower level. This paper discusses the suitability of a high-quality PH 13-8 Mo steel for critical helicopter usage, using a simulated application in Australian service to evaluate its fatigue performance particularly at high R ratio and other properties such as density, corrosion properties and cost in terms of the operational environment experienced in Australian helicopter operations.

Surface Damage in Riveted Aircraft Aluminium Lap Joints, in the Presence of Lubricants

This paper discusses experiments investigating the effect of lubrication on surface damage in riveted lap joints which experience fatigue loading in aircraft structure. As part of a larger investigation into the effect of lubricants (specifically Corrosion Inhibiting Compounds, CICs) on fatigue performance, the fracture surfaces were examined to determine the crack initiation sites, and the failure modes involved. Scanning Electron Microscopy was also utilized to assess the nature of the fracture surfaces. The results showed that under the loading used, all the specimens which had not been treated with the CICs, and some treated specimens, failed by tensile failure of the sheet. Some treated specimens failed by rivet shearing. The results suggested that the presence of lubrication at the contacting surface might have reduced frictional load transfer, contributing to the change in failure mode. For specimens that failed in the sheet material, fatigue cracking and micro-void coalescence were the fracture modes, with the potential influence of fretting as a fatigue source.