A Nitinol Langevin transducer with resonance tuneability for adaptive ultrasonic applications

Abstract

In power ultrasonics, the Langevin ultrasonic transducer has been widely utilised across medical and industrial applications, including for bone surgery, food cutting, and cavitation generation. Transducers for these applications are typically tuned to a fundamental operating mode, often the first longitudinal, for optimal interaction with a target material or structure. Currently, there is a growing interest in ultrasonic devices with tuneable dynamic properties, including resonance frequency, for optimising performance in these applications. To overcome limited frequency tuning capabilities of current configurations, this study demonstrates a Langevin transducer which is designed and fabricated incorporating the shape memory alloy Nitinol as its end masses. The rationale is that the change in elastic properties of these end masses with temperature will induce a change in the fundamental resonance frequency of the transducer, thereby demonstrating a viable and novel approach to controlling resonance frequency. Laser Doppler Vibrometry was used to characterise the first and third longitudinal modes at room temperature, correlating closely with finite element analysis results. Harmonic analysis was then conducted at various environmental temperatures to show changes in the resonance frequencies and vibration amplitudes of both modes as functions of temperature. The tuneable resonance of the Nitinol Langevin transducer (NLT) has a dependency on changes in the thermomechanical properties of Nitinol from its martensitic phase transformation, demonstrated through structural design factors. The transducer exhibits maximum resonance frequency increases of above 15 % and 10 % for the L1 and L3 modes respectively, between 30 °C and 100 °C. This research enables a new generation of Langevin ultrasonic transducers fabricated using advanced materials for multifrequency and tuneable resonance applications.