Effects of uncertainties on lifetime prediction of aircraft components

Today’s highly competitive environment in the commercial air transportation business instigates airline companies to continue operating their aircrafts beyond the design life, as long as the maintenance ascertaining their safe operation is more cost-effective than the investment into a new aircraft. In the context of damage tolerance of aging aircrafts, the residual strength of the fuselage skin damaged by multiple fatigue cracks received considerable attention. In the stress field of the fuselage skin, the rivet holes of the skin splice joints constitute a number of stress concentrations, where multi-site damage (MSD) may develop: several small cracks may coalesce to form a large crack, after which damage may progress fast to ultimate failure. The small cracks may also lead to widespread fatigue damage (WFD), where unstable crack growth of a lead crack becomes possible due to strength reduction. As experimental work revealed clearly the stochastic nature of MSD and WFD failure, a realistic analysis of the uncertainties is necessary. It is the aim of this paper to develop a probabilistic framework for lifetime predictions of aircraft fuselages with MSD or WFD which incorporates an efficient method frequently employed in deterministic analyses, the finite element alternating method (FEAM). The uncertainties are characterised by random variables and importance sampling is applied in order to obtain robust and efficient estimations of the failure probability as a function of the cycle number. Preliminary results are given in this paper and important aspects of future research addressed.