Nonlinear Sonic Fatigue Response Prediction from Finite Element Modal Models: A Comparison with Experiments

Aircraft skins can fatigue when exposed to high acoustic levels. Accurate prediction of sonic fatigue response is important in designing aircraft structures for long life. The dynamic response of these stiffened skins is both nonlinear and stochastic. Early prediction methods were based on single-mode, linear models which were not accurate for complex structures or large amplitude response levels. Direct time integration of full, nonlinear, finite element models can provide accurate results but requires a significant computational expense. Recent methods reduce the finite element model to a low order system of nonlinear modal equations. The modal equations can then be integrated in the time domain. The computational burden is greatly reduced and an accurate response prediction can be accomplished. Unfortunately, there have been no thorough comparisons of these new methods to experimental data. In this paper, experimental results from a clamped-clamped beam are compared to response predictions from two nonlinear modal methods. Measured strain and displacement spectra agree well with predictions from nonlinear modal models obtained via the finite element method.