Interface waves for SHM of adhesively bonded joints

Adhesive bonds are being increasingly used in many load bearing and safety critical members or structures. The strength and reliability of adhesive bonds need to be monitored for structural integrity. Ultrasonic guided waves offer potentially infinite sets of inspection possibilities in terms of the frequency and phase velocity combinations. In this paper, a Lamb wave ultrasonic nondestructive testing (NDT) and structural health monitoring (SHM) technique for adhesively bonded metal plates is considered. SHM of such structures for detection of defects in the adhesive joint region can be more efficient if the wave structure of the Lamb wave mode is such that the energy is concentrated in the adhesive region instead of in the adherends. In this regard, the following method for determining such modes was attempted. The adhesively bonded joint was modeled in two different ways. The first model considers the bonded assembly i.e. the adherends (aluminum) and the adhesive (epoxy) to be three layers having finite thickness in perfect contact. In the second model the adhesive is considered to be a thin layer embedded between two half spaces of adherend material. The analytical dispersion curves from both the models were studied and it was observed that the regions where the dispersion curves from the models coincide have predominant displacement in the adhesive region. Parameters were extracted from analytical results with an aim to generate single mode interface waves in lap joints. Finite element simulations were performed to verify generation of single mode interface waves.