Use of relative baseline features of guided waves for in situ structural health monitoring

This article presents a new signal-processing technique, which utilizes "relative baselines" instead of "pre-stored baselines," for Lamb wave based SHM. Several successful SHM methods utilizing wave propagations usually involve recording baseline measurements and comparing them to a newly measured response for structural damage identification. However, maintaining an accurate database of baselines remains challenging because of the effects of varying environmental conditions. Therefore, in this study, the relative baseline concept is proposed, in which measured Lamb waves are correlated between different sensor-actuator sets, as opposed to being correlated to pre-stored baseline data. This study focuses on determining the feature best used for this relative baseline concept, and cross-correlation and power spectral density analysis techniques are performed on data sets recorded from composite and aluminum plates. Experiments are performed with these plates under the presence of temperature variations in order to demonstrate the capability of the relative baseline concept. Our experimental results clearly indicate that the proposed technique reduces the complications associated with using pre-stored baselines for SHM under varying environmental conditions, and provides a quantitative means of identifying structural damage.