Frequency regulation for thermal excitation effect in the ultrasonic generation system based on pressure-impedance analysis

Ultrasonic thermal excitation enables non-destructive detection of concrete microcracks, but its thermal response depends on the frequency characteristics of the ultrasonic generation system. However, existing studies have mainly focused on the direct mechanical effects of coupling pressure and loading devices. Ultrasonic thermal excitation systems commonly use one single generator to drive multiple transducers, and their impedance matching behaviour differs from that of conventional power ultrasonic systems (such as ultrasonic welding systems). The relationship between series resonant frequency and optimal driving frequency under pressure loading remains unclear. Accordingly, single-transducer impedance measurements and thermal excitation experiments on concrete microcracks were conducted under different initial coupling pressures. From the perspective of the overall electrical operating state of the generator-transducer system, the Butterworth-Van Dyke (BVD) equivalent circuit model was introduced to analyse the relationships among coupling pressure, driving frequency, and thermal response. Results suggest that driving frequency selection should consider the overall operating state of the generator-transducer system in addition to the single-transducer series resonant characteristics. The quantitative relationship between the series resonant frequency and the preferred output frequency under the same coupling pressure requires further verification. These findings provide an experimental basis for frequency regulation under different loading conditions.