Interacting stress concentration factors and their effect on fatigue of metallic aerostructures.

摘要

During fabrication of certain aerostructure components, a situation can arise where a hole is mislocated so that it interferes with a radius, chem-mill step, machined step, or some other similar detail. The interaction of stress concentration effects between a hole and step are not well understood and the resulting impact on fatigue performance is difficult to predict. There exists a need for fatigue data that can be used to determine the analysis methods for evaluation of the interaction of machined steps and fastener holes. In this dissertation, the effect of stress concentrations placed in close proximity to each other and their impact on fatigue performance is studied. Unique cases of interaction between a hole and radius are analyzed. Physical testing and finite element analysis methods are used to derive thestress concentration factor (SCF, Kt) modification factors (Ktf ) for open hole and joint assembly structures. The most conservative factors derived are recommended for use in fatigue analysis for these instances of holes located at or near radii. The SCF mod factors increase as distance between the hole and radius tangent decreased. For associated geometry, loading and materials similar to those presented within this research, a Ktf value of 1.22 is suggested for use in fatigue analysis of these situations of holes intersecting a radius.Kt and Ktf values have a significant effect on fatigue lifetimes and resulting fatigue margins of safety. Based on a pass/fail experiment, the corrosion specimen passed the test as no fatigue cracking failures associated with the bolt in radius condition were observed.