United States Department of Transportation
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Characteristics of Early Stages of Corrosion Fatigue in Aircraft Skin
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1996-02-01
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[PDF-613.07 KB]
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NTL Classification:NTL-AVIATION-Aviation Safety/Airworthiness
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Abstract:SRI International is conducting research to characterize and quantitatively describe the early stages of corrosion fatigue in the fuselage skin of commercial aircraft. Specific objectives are to gain an improved deterministic understanding of the transition from corrosion pit to short crack to long crack and to delineate the effects of environment, skin surface condition, and loading conditions on crack nucleation and propagation rates. This Phase 1 report summarizes the effort in the first two years of this five-year program.
The results suggest that corrosion fatigue in Alclad 2024-T3 involves two competing crack nucleation mechanisms--hydrogen effects in the cladding and pitting at constituent particles in the core alloy. In a givenn situation, the mechanism which dominates depends on environment (particularly pH) and (weakly) on specimen orientation. Cracks do not necessarily nucleate at the largest corrosion pit, suggesting that the main effect of a pit is not to raise the local stress. Rather, a high local hydrogen concentration associated with accelerated corrosion at a pit could cause cracking in a nearby favorably oriented grain.
Propagation rates of short cracks were slightly higher in acidic environments and in specimens with painted surfaces, but were unaffected by material orientation and surface roughness. Corrosion fatigue observations and data from the laboratory experiments are being compared with those from field components to check whether research results represent in-service experience.
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