Characterization of Early Stages of Corrosion Fatigue in Aircraft Skin. Phase II: Interpretation of Corrosion-Fatigue Fracture Surfaces in Alclad 2024-T3 Aluminum Alloy Sheet
Abstract
New methods were developed and applied to extract and interpret information from the fracture surfaces of corrosion-fatigue specimens of clad and bare 2024-T3 aluminum sheet. We developed two fracture surface topography analysis methods that characterize the effects of environment and a fracture surface analysis and finite element modeling approach to detect and quantify overloads that occurred during fatigue crack propagation. We used these methods to perform a detailed analysis of the fracture surface topography produced in the early stages of corrosion fatigue, and developed an understanding of the effect of environment on the crack nucleation mechanism based on fracture surface features. In addition, cyclic load experiments were conducted on bare and clad 2024-T3 sheet under vacuum and in air, salt water, and arsenated salt water environments to assess the effects of environment, stress, and material condition on nucleation and early propagation kinetics of corrosion-fatigue cracks. For bare 2024-T3 sheet, substantial reductions in fatigue life were observed when the environment produced elongated pits associated with constituent particle corrosion. For clad 2024-T3 sheet, reductions in fatigue life were observed in environments that did not produce pitting (e.g., laboratory air and salt water). A likely mechanism for enhanced crack nucleation in 2024-T3 sheet with pure aluminum clad is the weakening effect of hydrogen on pure aluminum and the concomitant enhancement of cracking kinetics. The analytical methods developed and the analyses performed bear directly on the understanding of corrosion-fatigue in commercial aircraft through the relevancy of the materials, loading conditions, and test environments.
Document Details
- Document Type
- Technical Report
- Publication Date
- May 01, 1999
- Accession Number
- ADA365106
Entities
People
- C. G. Schmidt
- D. A. Shockey
- T. Kobayashi
Organizations
- SRI International