Lamb wave spectral methods are one candidate to characterize invisible damages in composite structures. Unfortunately, multiple reflections resulting from geometric boundaries could distort the Lamb wave spectrum, which may cover the important signatures concerning structural integrity. To eliminate spectral interference, a cepstrum based filtering method is proposed to separate various reflection features. In particular, the fundamental spectrum contributing to structural integrity can be extracted smoothly by removing harmonic fluctuations regarding wave reflections with an optimized filter. Subsequently, to establish the quantitative relationships between fundamental spectrum features and the impact energies, damage indices involving spectrum energy mean and median frequency shift are introduced to quantify the change of spectrum intensity and distribution respectively, which shows good performance on impact damage characterization. Finally, the experiment was carried out on a T300 composite beam, in which strong reflections come after direct waves. Meanwhile, the severity of impact damage was simulated by the free droppings of a steel ball with different heights. The results show the effectiveness of the proposed method for characterizing impact damages with improved sensitivity.
Keywords: Impact damage characterization; Lamb waves; Spectrum filtering; Structural health monitoring.
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