Quantifying adhesive thickness and adhesion parameters using higher-order SH guided waves

A method to quantify the interface shear stiffness, adhesive shear modulus and adhesive thickness in an aluminium-epoxy-aluminium joint is presented. Shear horizontal guided waves are considered to infer the properties. A numerical model that employs spring stiffness boundary conditions at the aluminium-epoxy interface was developed to generate dispersion curves. The sensitivity of the first four SH-like modes to epoxy thickness, interface shear stiffness, and adhesive shear modulus are analyzed. The dispersion analysis reveals that higher-order anti-symmetric modes are sensitive to all three parameters, whereas the symmetric modes are sensitive only to adhesive thickness. Hence to prevent false alarms that might arise while assessing the bond conditions, symmetric and anti-symmetric modes should be simultaneously generated. Periodic permanent magnet (PPM) electromagnetic acoustic transducers (EMATs) are used to generate and detect SH-like modes. Utilizing the constant wavelength property of PPM-EMATs, SH₂-like and SH₃-like modes are generated. Short-time Fourier transform (STFT) is used to separate the modes merged in the received time response. By overlaying the dispersion curves of SH₂-like mode on STFT, the thickness of epoxy is quantified. The dispersion curves of SH₃-like mode are generated using the measured thickness and overlaid on STFT to measure the interface shear stiffness and epoxy shear modulus. The proposed method is experimentally demonstrated on aluminium-epoxy-aluminium samples of different surface treatments. The study demonstrates a reliable nondestructive evaluation of adhesive bonds that reduces possible false alarms.