Prediction of Fatigue Crack Growth in Preloaded Metallic Nested-Angle Specimens

This paper presents numerical fatigue crack growth predictions from a rotorcraft structural component subjected to a random load spectrum. The structural component is a riveted joint used on cabin frame cap splices of several civilian and military helicopters modeled here as a lap-joined nested angle sub-assembly. The fatigue life of this component depends on the local stress gradient induced by fastener preload, contact friction, assembly loads and effects of machined fastener holes. Coefficient of friction plays a significant role affecting the local stress and fastener load distributions. In this paper, a systematic approach that combines 3D finite element modeling (FEA) and 2D damage tolerance analysis is used to predict the fatigue life of the nested-angle specimen. In the numerical analysis phase, fatigue crack growth for through-the-thickness crack initiating from the critical fastener hole is estimated using 2D standard and weight function NASGRO models. The effect of friction is parametrically included in the damage tolerance analysis and results are compared with a frictionless analysis. It is concluded that NASGROS's TC03 and TC13 standard and weight function models when used with stress gradient inputs generated by half symmetry 3D FEA model shows a good agreement with the observed experimental crack growth data.