Kissing Bonds Inspection by Comparing Natural Frequencies Using Digital Holography

Accurate detection and evaluation of kissing bonds in composite materials is essential to ensure the integrity of the component structure, but traditional NDT (non-destructive testing) methods struggle to identify imperfect bonds and zero-volume debonds. In this study, a vibration analysis method based on holography was applied to detect kissing bonds by monitoring the changes in natural frequencies of the same sample before and after fatigue loading. Both pristine and kissing bond samples were tested under identical conditions, and their vibration characteristics (natural frequency, amplitude, and mode shape) were measured using holography. The experimental results show for the intact sample exhibited no changes in natural frequency amplitude or mode shape after fatigue loading, confirming that the applied fatigue test did not affect the integrity of its adhesive layer. In contrast, for the sample with a kissing bond, after fatigue loading, the natural frequency decreased by up to 22 Hz due to debonding or delamination, while the vibration amplitude increased, and new localized modes appeared around the debond area. This indicates that in practical engineering applications, kissing bonds can be identified by observing the frequency change of a single sample before and after fatigue testing. This method improves the applicability of this technique, making it more suitable for industrial non-destructive testing conditions.