A materials selection framework for fastener-in-panel geometries using FEM and LEFM, to mitigate coating degradation and environmentally assisted cracking

Abstract

A finite element method (FEM) model was used to determine the potential, current, and current density distributions for a multi-material fastener/hole-in-plate configuration in an atmospheric environment. The plate consisted of bare Pyrowear 675 with or without a ZnNi coating, and the fastener consisted of Ti-6Al-4V with or without an anodization surface treatment. The potential distribution within the fastener hole was the focus of this work, as this region would represent the location of highest stress within the fastener/panel geometry. Separately, existing literature data were augmented by fracture mechanics-based evaluations of the environment-assisted cracking behavior of Pyrowear 675 as a function of electrode potential Pyrowear 675 in 3.0 M NaCl. Combining the fracture mechanics and FEM results allowed for a unified framework which was able to narrow a material selection matrix of 13 total scenarios to three scenarios, in an atmospheric environment of 0.6 M NaCl, specifically. The delayed drying out of occluded regions after the surface electrolyte has been evaporated was accounted for, with only one of the studied material selection combinations able to reduce the crack susceptibility under those conditions. A single pit within the fastener hole was also considered, and it was determined that the size of the pit had a larger impact on the potential than the location of the pit, which will aid in future computational assumptions.