Investigating the failure mechanism of an aircraft longeron fitting and devising the mitigation techniques

Abstract

This study focuses on determining the root cause of crack development in a longeron fitting of a cargo aircraft followed by devising of appropriate mitigation strategy to prevent such failure. This objective is realized through a combination of experimental and computational analyses. The experimental thrust involves compositional, microstructural, and fractographic analyses to determine if there is a variation in composition and microstructure and to observe the failure signs and crack morphology. No deviation from the constituent material (i.e., Aluminum 7075-T6 alloy) in terms of composition and microstructure was noticed. Further fractographic analysis revealed fatigue striation marks and corrosion products near the crack initiation point. The experimental results were validated through computational analysis by calculating stresses and fatigue life of longeron fitting to confirm the cause of failure. The analysis helped in ruling out the design flaw or fatigue loads as the primary causes of crack development. Consequently, the failure is attributed to the corrosion pitting on the open side of the fitting due to exposure to the marine atmosphere which weakened the structure and the subsequent failure occurred due to the fatigue phenomenon. Subsequently, a mitigation technique was developed to prevent corrosion-assisted crack growth by coating the specimens with two distinct coatings, Sulphuric Acid Anodizing (SAA) and Chromic Acid Anodizing (CAA). These specimens were then subjected to salt spray tests and exposed to the environment. SAA coating proved to have better corrosion-resistant properties than the original (uncoated) and CAA-coated samples, confirming it to be a viable mitigation approach. The study will be beneficial in preventing failure in aircraft structures that are exposed to marine environments.