Ultrasonic Guided Wave Inspection of Bonded Lap Joints: Noncontact Method and Photoelastic Visualization

Abstract The main topic of this paper is the nondestructive inspection of adhesively bonded lap joints by using ultrasonic guided (plate) waves. A noncontact, couplant-free method that employs capacitive air-coupled transducers is demonstrated for the inspection of thin aluminum joints with dimensions typical of aircraft fuselage and wing panels. Two types of bond defects, disbonded regions and regions of poorly cured (low-cohesive-strength) adhesive, are successfully detected by measuring the amplitude decrease of selected plate waves leaking from one adherend to the other one through the bondline. It is shown that proper choice of the vibrating mode structure, in terms of cross-sectional displacement distributions, is needed in order to maximize the sensitivity of the inspection to the presence of the low-cohesive-strength bond. Results from a dynamic photoelasticity study are also presented to visualize fundamental behavior of propagating plate waves and study their interaction with bond defects in glass lap joints. The photoelastic results confirm in a clear, pictorial manner certain assumptions on plate wave leakage through the bond, including the influence of vibrating mode structure on the detection of low-strength bonds.