Composite bond quality nondestructive evaluation with noncontact Lamb wave system

Adhesively bonded composites have been widely used in aerospace engineering, renewable energies, and automotive industries. However, the potential weak bonding of the composite structures, which exhibits a low cohesion interface between the adhesive and the composite substrate, greatly threatens the reliability of these structures. The occurrence of weak bonding has yet to be well understood and has posed new challenges for the evaluation of weak bonding in composite structures. Ultrasonic Lamb waves have been shown useful for nondestructive evaluation (NDE) due to their ability to propagate a long distance with less energy loss and their sensitivity to small defects. Among various ultrasonic transducers, air-coupled transducers (ACT) eliminate the need for couplant/adhesive and provide a noncontact actuation method. In this study, a hybrid noncontact system was constructed using an ACT for actuation and a scanning laser Doppler vibrometer (SLDV) for sensing. The ACT provided a narrowband wave actuation, while SLDV provided high-quality wavefield signals for damage detection and evaluation. An adhesively bonded composite structure containing good and weak bond quality areas was manufactured using simulated contamination. Then the composite bond quality inspection was conducted using the ACT-SLDV system. To validate the ACT Lamb wave inspection, a second noncontact inspection by a pulsed laser-SLDV method was carried out.