Inverse characterization of adhesive shear modulus in bonded stiffeners using ultrasonic guided waves

Abstract

Adhesively bonded stiffeners are widely used in aerospace structures. A well-cured bond can increase structural stiffness, and, consequently, enhance the performance of the entire structure. The feasibility of feature-guided wave modes for the inspection of the bond line adhesion in difficult-to-access regions has been already investigated in the literature. However, due to the complexity of the guide wave phenomena in the bond line region, a more comprehensive methodology to identify the curing state remains an open issue. This work introduces a multi-mode and multi-frequency inverse method for the characterization of stiffener bonded line using Semi-Analytical Finite Element (SAFE). Experiments were conducted on a T-shaped stiffener bonded to an aluminum plate. The feature-guided modes were excited using a piezoelectric shear transducer and measured using a laser interferometer at several times along a period of four days. The experimental dispersion curves were computed from the measured data and then systematically compared to the theoretical solutions obtained with the SAFE model. At each measurement, the shear modulus of the adhesive material could be estimated by iteratively minimizing the error between the experimental and numerical data. The results showed an abrupt increase in the shear modulus from the first to the second day, suggesting that the end of the curing processes was achieved. In general, the inverse scheme presented in this work was shown to be very sensitive, being able to distinguish differences of 5% in the shear modulus.