The Application of Nonlinear Reverberation Spectroscopy for the Detection of Localized Fatigue Damage

Nonlinear reverberation spectroscopy (NRS) is a non destructive evaluation method exploiting the amplitude dependent change of the resonance frequency in samples made of a nonlinear elastic material. After a sample has been excited near resonance, the immediate vibration frequency of the decaying reverberation signal decays with the decreasing amplitude. The frequency-amplitude dependence can be used to quantify the nonlinearity of the material, typically caused by the damage level. This paper handles the possibilities and difficulties of using NRS to detect a single fatigue crack in specimens made of an linear material like a composite or steel. In this case, the nonlinearity is concentrated in a small zone which is not necessarily affected by the low frequency vibrations used for NRS. First, a proof of concept is given by testing composite beams with increasing levels of fatigue damage. Tests prove that the nonlinear frequency change is more efficient for quantifying early damage than its linear counterparts such as the damping coefficient, at least when the crack is located in a vibration antinode. The method is subsequently used to test steel industrial samples with a complex geometry, showing that efficient damage detection indeed depends on the used vibration mode.