Measurement of Adhesive Bonding Strength With an EMAT in the Resonant Mode

The electromagnetic acoustic resonance (EMAR) method with shear wave is sensitive to boundary conditions and plate thickness. In this paper, a new noncontact ultrasonic testing method based on the electromagnetic acoustic transducer (EMAT) in the resonant mode is proposed for the bonding strength evaluation in metal-based adhesive structures. Different from the conventional pulse-echo method using short-burst excitation for bonding inspection, the attenuation coefficient feature of the resonant ultrasonic signal with long-burst excitation is applied to increase the signal-to-noise ratio (SNR) and detecting sensitivity of the EMAT for adhesive bonding strength evaluation. A theoretical model for adhesive bonding testing with EMAT signals in the resonant mode is established. To extract the signal feature representing the reflection coefficient, the time-domain signal was processed by Hilbert transformation and exponential curve fitting. Through the simulation on the received signal, the correlation between the attenuation coefficient of the exponent fitted curve and the strength on the adhesive imperfect interface were confirmed. Finally, the proposed correlation is verified by an experiment on stainless steel plates bonded with polymethyl methacrylate plates by epoxy adhesion via a permanent magnetic EMAT.