New diagnostic method using instantaneous frequency of ultrasonic pulse (evaluation of adhesive strength of epoxy resin)

Ultrasonic technique is an indispensable diagnostics to evaluate internal conditions. The conventional ultrasonic technique using a wave property, such as a reflected wave, wave speed, attenuation of amplitude and frequency spectrum is, however, not effective in some cases. In this study, we propose to use an instantaneous frequency of the ultrasonic pulse. The instantaneous frequency expresses a transition of the frequency of the ultrasonic pulse, so that it is sensitive to a slight variation of the ultrasonic frequency. The effectiveness is demonstrated through an experiment on adhesive strength of epoxy adhesive, which is controlled by curing process. The conventional ultrasonic technique is difficult for an evaluation of this adhesive strength. On one hand, the instantaneous frequency of an ultrasonic pulse, which passes through the adhesive layer, certainly distinguishes the difference of the adhesive strength. It is also shown that the adhesive characteristics pertaining to their strength are modeled as damping coefficients, which is demonstrated through a simple simulation of one-dimensional ultrasonic wave propagation.