Feature-guided waves for monitoring adhesive shear modulus in bonded stiffeners

Adhesively bonded stiffeners are employed in aerospace applications to increase structural stiffness. The potential of feature-guided wave modes for the verification of adhesion and curing state in difficult-to-access regions has been investigated. The properties of guided wave modes propagating along a T-shaped stiffener bonded to an aluminium plate were calculated using the Semi-Analytical Finite Element (SAFE) method. Feature-guided modes dominated by shearing motion were identified to be well suited, with energy concentrated at the stiffener and bond line, limiting energy radiation into the plate and thus maximising inspection length. The influences of the bond line stiffness and thickness on the guided wave behaviour were investigated using SAFE and 3D Finite Element calculations, and found to be significant. Experiments were conducted to measure the properties of the guided waves during the curing of an epoxy joint attaching a stiffener to a plate. The feature-guided mode was excited using a piezo-electric shear transducer and measured using a laser interferometer. The measured phase speed changed significantly during curing. The frequency dependency was found to match well with the SAFE calculations for a variation of the shear (Coulomb) modulus of the adhesive. The potential of the feature-guided shear wave mode for bond line inspection and monitoring has been shown and the choice of guided wave mode and frequency range for good sensitivity to the bond line state discussed.