Weak bond evaluation of lap bonding structures by SH guided waves using EMATs

In this research, simulations and experiments were conducted for the bonding quality evaluation of an aluminum bonding structure, based on the SH guided wave analysis. In order to simulate the propagation of SH waves in the bonded structure, a 3D finite element model was established based on interfacial spring stiffness modeling. The frequency-wavenumber diagram was obtained by doing Fourier transform along the spatial axis in the time frequency domain simulation results. At the same time, the influence of the bonding interface weakening on the propagation characteristics of the guided wave was analyzed. The simulated dispersion curves of SH-guided waves in different bonding states were in good agreement with the theoretically results. In addition, a periodic permanent magnet electromagnetic acoustic transducer (PPM-EMAT) 3D finite element model was established. An optimized transducer was fabricated to excite pure SH0 mode. Then, a series of aluminum lap bonding structures with different bonding states were prepared. The effect of bonding interfaces on the propagation characteristics of SH waves was investigated. The interfacial spring stiffness can be determined when the STFT of the SH waves matches the theoretical dispersion curves. Thus, this method demonstrated an ultrasonic way to distinguish the weakening of the bonding structures.