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In sonic infrared imaging, the engagement force strongly affects the plate׳s vibration characteristics and frictional heating at the crack, which have been proved by experimental results. We developed a FEM model in which piezoelectric-force analogy was employed to simulate inverse piezoelectric effect. Simulation results revealed that an increase in the engagement force not only enhances the vibration intensity of the test plate but also causes a sharp change in its spatial distribution. However, when the increase exceeds a certain level, the enhanced crack closure and weakened crack differential velocity will lead to a lower heat dissipating at the crack.

Engagement force between ultrasonic horn and target is one of the primary factors which influence defect detection in sonic infrared imaging method. In this paper, an experimental setup which can quantify the engagement force has been designed. A series of experiments under different engagement forces have been performed, and an interesting phenomenon is found that the increase in engagement force enhances superharmonics and diminishes subharmonics in vibration velocity spectrum of the target. A corresponding mechanical model is presented to explain the mechanism of such phenomenon. Frequency-domain analysis on theoretical simulation further confirms the experimental results. Meanwhile, time-domain analysis on theoretical simulation demonstrates the variation of vibration velocity spectrum caused by engagement force. It is clear that the increasing engagement force can shorten the impact interval and prolong associated movement between the horn and target. And the relationship between engagement force and subharmonic-order can be described theoretically using an approximately hyperbolic curve.