Failure Analysis of Longeron Fitting via Microstructure Examination and Proposing Viable Mitigation Techniques

Abstract

This study aims to investigate the failure causes in a longeron fitting of transport aircraft and propose a mitigation strategy to prevent such defects. The study involves two phases to accomplish the desired objective. Firstly, an experimental approach to perform a failure analysis of the longeron fitting. This included techniques such as compositional analysis, stereoscopy for dimensional measurement of cracks, and fractographic analysis through XFR, SEM/EDX to determine the cause of the failure. Microscopic analysis revealed a crack originating from the recess area, which was further investigated to determine the cause of failure. The failure analysis revealed the existence of corrosion products along the recess as a major contributor to crack initiation, which was followed by deposition corrosion, as evidenced by the presence of Oxides and undiffused Silicon particles. To confirm the effect of stress concentration within the given design constraints, reverse engineering methods and numerical analysis were implemented to study the mechanical behavior of the fitting under static loading conditions. Furthermore, the mitigation techniques were developed to prevent corrosion-assisted crack growth by selecting a higher strength-to-weight ratio Al-Li (2195 T8) alloy, with a redesigning the longeron fitting with a larger fillet radius for the longeron fitting results in improvement of mechanical properties along with shifting of stress concentration region from recess edge towards the rear support side. Additionally, corrosion resistive coatings of the specimens with Sulphuric Acid Anodizing (SAA) and Chromic Acid Anodizing (CAA) were subjected to salt spray tests and exposed to a saline environment. The findings of the study suggest that SAA perform best under given conditions as most effective corrosion resistive coating, confirming it to be a comprehensive mitigation approach. The results of this study will provide valuable insights into the failure causes of the longeron fitting, which can improve the safety and efficiency of cargo aircraft operations and the risk of downtime and cost of repairs.