Service fatigue cracking in an aircraft bulkhead exposed to a corrosive environment

It has been postulated that for combat aircraft corrosion generally occurs whilst the aircraft is on the ground, while fatigue damage in the form of cracking (if any) grows as result of operational loading which mainly occurs in flight where the conditions are cold and dry and the loading rates are high. This would suggest that (in general) for combat aircraft the effects of the environmental degradation and any fatigue crack growth are decoupled. From a damage prediction viewpoint, this will significantly simplify assessment of such problems (i.e. the growth of fatigue cracks in these aircraft types is not environmentally assisted). This paper presents a supporting case study that examines fatigue cracks that were detected in an F/A-18 Hornet bulkhead during post-service testing and teardown. The in-service phase of the cracking had significant evidence of oxidation on their surfaces which indicated an exposure to a mildly corrosive environment. Both the service and laboratory phases of the cracking were the subject of quantitative fractography and estimates of the crack growth rates were made. A comparison of the in-service and the in-test phases of crack growth indicated that no notable effect on the service part of the fatigue crack could be attributed to its exposure to the service environment.