Polymer coating characterization based on ultrasonic interference and critical attenuation

A characterization technique is proposed for polymer coating structures immersed in water based on the ultrasonic interference and critical attenuation phenomenon. When a coating layer is modeled as a linear viscoelastic material and is subjected to the normal incidence of an ultrasonic wave, the theoretical analysis shows that the reflection spectrum takes local minima at multiple frequencies, which correspond to the resonance frequencies of the coating layer depending on the wave velocity and the thickness under the assumption of relatively low loss factors. Furthermore, the lower envelope of the reflection spectrum yields the critical attenuation condition, which is closely related to the loss factor of the coating. These theoretical findings are applied to the viscoelastic characterization of the coating layers. Two bonded specimens with different substrate thicknesses validate the proposed method. As a result, the experimental data demonstrate that the estimated wave velocities and loss factors of the coatings agree well with the results reported in the previous paper. After the experimental validation, the proposed method is applied to the curing monitoring of a resin coating on a metal substrate. The ultrasonic responses provide the temporal variation of the layer resonance frequencies and the critical attenuation frequency by the resin curing. These results can be used to estimate the viscoelastic properties of the coating, which imply the curing state of the resin.