The objective of this project is to develop and validate a 2nd generation crack-growth prediction methodology for cold-worked fastener holes. The methodology will be based on a three-dimensional weight function, and will handle cracks growing from the edge of the hole, as well as cracks initiating in the residual tension zone away from the hole. A predictive capability, based on probabilistic fatigue analysis, will be used to determine the most probable failure mode for the hole. For cracks initiating in the residual tension zone, the methodology will account for the multi-stage crack growth. It will track the relaxation of the residual stresses as the crack grows through the residual stress in Stage I. In Stage II, the methodology will predict the initiation and the growth of a secondary crack at the hole edge, resulting from the relaxation of the compressive residual stresses. The methodology will track the coalescence of this secondary crack and the primary crack into a single macro-crack in Stage III. To predict the net section stress contour for the probabilistic fatigue analysis, the methodology will be combined with a CAD-based multi-layer joint analysis program to enable the cold-worked hole to be treated as part of a complex structural system, rather than as an isolated hole. the 2nd generation crack-growth prediction methodology will lead to an improved ability to predict the fatigue and crack growth lives for multi-layer joints with cold-worked holes. It will enable the full potential of cold-working(or cold-working plus interference-fit fasteners) to be exploited, leading to a new generation of ultra long life, low maintenance aerostructure.
