Fatigue Fracture of Aluminum Alloy 7178-T6 Aircraft Fuel-Tank Floors

The floors (fabricated from aluminum alloy 7178-T6 sheet, with portions of the sheet chemically milled to reduce thickness) of the fuel tanks in two aircraft failed almost identically after 1076 and 1323 h of service, respectively. Failure in both tanks occurred in the rear chemically milled section of the floor. An alkaline etch-type cleaner was used on the panels before chemical milling and before painting. Various tests and measurements indicated that the aluminum alloy used for the fuel-tank floors conformed to the specifications for 7178-T6. Low power magnification, fractographs taken with a scanning electron, and optical microscopic examination of the milled sections revealed extensive pitting on both sides of the floors. Evidence found supports the conclusions that the floors failed by fatigue cracking that initiated near the center of the fuel-tank floor and ultimately propagated as rapid ductile-overload fractures. The fatigue cracks originated in pits on the fuel-cell side of the tank floors. The pits were attributed to attack caused by the alkaline-etch cleaning process. Recommendations included monitoring of the alkaline-etch cleaning to avoid the formation of pits and careful inspection following alkaline-etch cleaning, to be scheduled before release of the floor panels for painting.