Abstract Corrosion fatigue is a major damage mechanism responsible for the premature failure of the aircrafts and turbine parts, especially in the marine environment. The process starts with the surface degradation caused by the corrosion pits, which become fatigue initiation sites for the initial damage when the part is loaded. Microstructure and morphology of the surface pits is critical for the crack initiation. Cracks are initiated at sharp corners and at the bottoms of the narrow “micropits.” The paper provides a statistical characterization of the crack initiation process based on the pit density and microstructure distribution. Based on the analysis of surface damage initiation, new morphological characteristics combining pit size and highest curvature are introduced. The distribution of these introduced morphological characteristics is estimated from the measured data and have exhibited to be an efficient metric for pitting fatigue. The effects of pitting morphology are evaluated for various heat-treated 2024 aluminum alloy specimens with varying distribution of pit shapes and curvatures. The statistical distribution of specimen life is estimated using the “weakest link” approach; that is, by computing the probability that at least one crack is initiated somewhere on the surface. The paper contains a detailed description of crack initiation’s statistical model, the methodology for corrosion parameter estimation and representative numerical examples of statistical modeling. Surface pit characterization has been nondestructively measured on a Zygo ZeGage 3D surface profiler. The ZeGage uses a coherence scanning interferometry (CSI) technique based on the wavelength of light. This profiler can measure a wide range of surface qualities with repeatable results in sub-nanometer precision. Large-area scanning was done using a segmentation approach. A large field of view of the selected lens and automated rapid scanning allowed for reasonably smaller data files after stitching these individual segments.
Starr et al. (Thu,) studied this question.