Characterization and modeling of localized corrosion damage accumulation for aircraft structural integrity

Electrochemical and morphological characterization approaches are being used to quantify corrosion damage accumulation on 7075-T651 in a manner that will support assessments and prognosis of remaining service life of structural airframe components subject to damage by corrosion, and corrosion-assisted fatigue. Electrochemical characterization is based on the use of an electrochemical microcell, which enables the electrochemical behavior to be studied on a phase-byphase basis. Information from these studies will aid in characterizing the role of local dissolution phenomena in a multi-site fatigue crack and corrosion damage model [1]. Morphological characterization is based mainly on optical profiliometry to define the exposure time-dependent pit number density, pit area distributions, and pit depth distributions. Systematic exposure experiments are aimed at characterizing the dependencies of these morphological characteristics on important microstructural, compositional and environmental variables. Data from these experiments are fit to a distribution function, and the dependency of the distribution shape and scale parameters on exposure time, alloy composition, microstructure and environmental variables form the empirical model for corrosion damage accumulation. These models will be used to support a multiscale finite-element based fatigue fracture model (FRANC3D) [2], and a multiscale molecular dynamics-based Internal State Variable (ISV) fatigue cracking model [3]. The approaches selected for characterizing corrosion damage accumulation are experimental and empirical in nature, but characterize damage and damage rate in a numerical form that is acceptable by the various fatigue damage models enabling a more accurate accounting of the damage state.