Analysis of crack interactions at adjacent holes and onset of multi-site fatigue damage in aging airframes

Focusing on the geometry of one hot spot in airframes, this paper discusses the onset of the interaction of two collinear cracks at adjacent holes and defines the onset as a criterion for multi-site fatigue damage failure. The finite element method is used to calculate the stress intensity factors at the tips of two collinear cracks at adjacent holes growing towards each other. The stress intensity factor is found to increase rapidly at the onset of interaction. Since a rapid increase in stress intensity factor results in a rapid and unstable growth of the crack, the onset of the interaction is proposed as the point where the multi-site fatigue damage starts. A criterion to avoid multi-site fatigue damage locally is then established based on the separation distance of two crack tips at the onset of the interaction. To speed up the simulation of crack growth under multi-site fatigue damage with the finite element method, a semi-empirical criterion is derived to determine the time at which the stress intensity factors at the tips of the cracks correlate. The numerical examples show that the proposed criterion saves simulation time while incurring negligible relative error in the computation of the final crack length.