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.
