Corrosion treatments and the fatigue of aerospace structural joints

Abstract

Corrosion Inhibiting Compounds (CICs) retard or prevent degradation by corrosion, and are used widely in aviation, particularly in large aircraft with well-developed Corrosion Prevention and Control Programs. The diverse General Aviation sector, including small airliners, also benefit, but are poorly supported in terms of guidelines for CIC selection and application. One risk is that lubricating chemicals (like CICs) might affect the fatigue performance of structural joints, and potentially affect the overall economic or safe life of the aircraft.This paper discusses the effect of the application of Corrosion Inhibiting Compounds (CICs) on the fatigue life of riveted joints, and on the failure mechanisms involved. Single lap joints treated and untreated with CICs were tested under constant amplitude fatigue loading, with multiple load levels. Two different CICs, one oil-based, and one a soft-waxy CIC, were applied. The results showed that the presence of CICs had a significant influence on the fatigue life, and also on the failure mode of the joints. At high load levels, the application of CICs caused a reduction in the fatigue life of the joint by more than a factor of two. In this load range, the CICs appeared to cause the failure mode to change from tensile failure of the sheet (the prevalent mode at medium load levels) to shear failure of the rivets (observed at the highest load levels). Specimens that failed by rivet shearing showed some fatigue cracks propagating along the critical rivet row. In treated specimens tested at medium load levels, a reduction in the fatigue life still occurred, with all specimens failing in the sheet. At low load levels, there was little difference in fatigue life for the three conditions, although specimen test run-outs meant that further testing will be needed.The results are believed to have significance for managing the small aircraft in which these joints are common.