| Although the radial expansion (cold working) of aircraft fastener holes dramatically increases fatigue life, the aircraft industry does not consider most of the cold working benefit in design and sustainment, mainly because fatigue predictions are often nonconservative compared to experiments. The purpose of this research is to identify the key causes of poor correlation between predicted and experimental cold worked hole fatigue crack growth, and to develop new and more accurate prediction methods. Long held assumptions regarding the analysis of fatigue crack growth through residual stress fields were first tested by growing a fatigue crack through a residual stress field created by plastically bending a beam. This experiment minimized or quantified sources of error that had not been considered in previous studies. The correlation between predicted and experimental crack growth rates was shown to be as good as those expected in crack growth experiments in residual stress free material. Next, a method for modeling residual stress relaxation during cyclic loading was developed based on the Chaboche multi kinematic hardening material model with parameters fit to the Ramberg Osgood stress strain curve. The model showed promising agreement with measurements of cold worked hole residual stress relaxation. Finally, the fatigue crack growth of corner cracks from cold worked holes was predicted using 3D finite element models. The analysis included the through thickness variation of the residual stress field, and showed the evolution of the "P shaped" crack front commonly observed in experiments. When based on a conservative estimate of the relaxed residual stress field, the predicted fatigue life was conservative and more accurate than the currently used Department of Defense reduced initial flaw size life prediction approach. |
