Chapter 31 - C-5A Fuselage Crown Cracking

Abstract

For low temperature applications, analysis and experiments have shown that moderate coefficient of thermal expansion (CTE) composite materials can perform better as fuselage skin repair materials than the more traditional low CTE composite patch materials. The moderate CTE composite material Glare (Glare is a hybrid laminate of 2024-T3 aluminum sheets and unidirectional S-glass/epoxy composite plies) is a viable alternative to traditional composite patch materials such as boron- and carbon-epoxy for pressurized fuselage skin repairs. The moderate CTE patch materials result in a lower stress intensity, Krepaired, near the crack tip and reduced adhesive shear strains due to the better CTE match with the aluminum skin material. The lower Krepaired will result in slower crack growth, or will stop crack growth. The lower adhesive shear strain is an important advantage in the durability of the bonded repair. The finite element model is used to predict K in the unrepaired case and verify the load carrying capabilities of the cracked structure before a repair was applied. The analysis methods are verified using fatigue experiments. The performance of the bonded repairs is tested under C-5A spectrum loading, and easily extends the life of the cracked structure one full lifetime.