Damage detection in an aircraft foam sandwich panel using nonlinear elastic wave spectroscopy

A novel damage detection technique, based on nonlinear elastic wave spectroscopy (NEWS) approach, is presented in this paper. This technique detects the presence of structural changes by monitoring the presence of harmonics and sidebands generated by the interaction between a low frequency and a high frequency harmonic excitation signal, due to the nonlinear material behaviour caused by the presence of damage. The proposed methodology was tested on a sandwich plate after being impacted by a foreign object under low velocity impact conditions. The high frequency signal was modulated in amplitude and the changes of the structural response, in terms of harmonic and sidebands amplitude, were recorded. The spectra and time frequency representation (TFR) were evaluated using wavelet transformations (WT). The experimental harmonic and sidebands amplitude resulted in close agreement with the theoretical behaviour of nonlinear behaviour of damaged materials. More specifically, the 3rd harmonics of the low frequency signal component showed a quadratic dependence with the low frequency response amplitude, as predicted by the theory for a hysteretic nonlinear material. Differently, the experimental second sidebands of the high frequency signal resulted bounded by curves representing the second sideband behaviour for pure hysteretic material (lower bound) and classical nonlinear material (upper bound). In particular, for small low frequency response amplitudes, the experimental second sidebands tended to be closer to the classical nonlinear material curve, while for the largest amplitudes investigated the response resembled a hysteretic material behaviour. The results showed that the proposed methodology was capable of detecting successfully the presence of impact damage and can be used as a first assessment of the presence of damage in aircraft structures where the presence of damage should be quickly estimated.