High Frequency Guided Waves for Disbond Detection in Multi-Layered Structures

Aerospace structures often contain multi-layered metallic components where hidden defects such as localized disbonds can develop, necessitating non-destructive testing. Model structures consisting of two adhesively bonded aluminium plates and artificial defects in the bond layer were manufactured. Immersion ultrasonic C-scans were used to check the uniformity of the bond layer and the manufactured defects. Employing standard wedge transducers, high frequency guided ultrasonic waves that penetrate through the complete specimen thickness were generated. Interference occurs between the wave modes during propagation along the structure, resulting in a frequency dependent variation of the energy through the thickness with distance. Significant propagation distance with a strong, non-dispersive main wave pulse was achieved. The interaction of the high frequency guided ultrasonic waves with small disbonds in the sealant layer and lack of sealant in the multilayer structure was investigated. Guided wave pulse-echo measurements were conducted to verify the detection sensitivity and the influence of the stand-off distance predicted from the finite element simulations. The results demonstrated the potential of high frequency guided waves for hidden defect detection at critical and difficult to access locations in aerospace structures from a stand-off distance.