Kissing Bonds Inspection by Measuring and Comparing Natural Frequencies Using Digital Holography

Accurate detection and evaluation of kissing bonds in composite materials is essential to ensure the integrity of the structure, but traditional NDT methods are often ineffective when dealing with zero-volume defects or disbonds. In this study, a vibration analysis method based on digital holography was used to detect kissing bonds by comparing the vibration mode shapes and corresponding natural frequencies of defective and intact samples. The mode shapes observed with the holography were highly consistent with the results of a linear finite element method simulation, validating the capability and accuracy of digital holography when measuring vibrational characteristics. In this study, three composite aluminum plate samples were prepared: a defect-free sample, a sample with a 30 mm-diameter kissing bond, and a sample with a 60 mm-diameter kissing bond, both located in the center of the respective plates. Controlled contamination of the bond area was used to induce kissing bond defects. The experimental results show that as the size of the kissing bond area increases, the natural frequencies of all vibration modes decrease, but the mode shapes remain unchanged. This indicates that the presence and propagation of kissing bonds reduces the structural stiffness, resulting in detectable changes in natural frequencies, thereby providing an effective detection method based on vibrational characteristics. In addition, cyclic loading was used to conduct comparative tests on pristine and defective sample (i.e. with a kissing bond defect). The experimental results show significant changes in the natural frequency of samples with kissing bonds after cyclic loading, due to defect propagation or debonding, while no such change is observed for intact samples. This suggests that in practical engineering applications, the kissing bonds can be detected by comparing the changes in the natural frequency of the same sample before and after cyclic loading, without having to rely on a comparison with an intact sample. This method improves the applicability of this technique, making it more suitable for practical industrial non-destructive testing conditions.