Smart cymbal transducers with Nitinol end-caps for power ultrasonics applications

A Class V cymbal flextensional transducer is composed of a piezoceramic disc or ring sandwiched between two cymbal-shaped shell end-caps, and has primarily been used in sensing and actuation. It has only very recently been studied for high amplitude and high power applications, and consequently implementation in this application space remains undeveloped. Shape memory alloys (SMAs) are a unique class of metals that have been adopted in a wide range of applications as a result of exhibiting two interesting properties. The first is the superelastic effect, which occurs isothermally when the material is stressed at a temperature above its austenitic transformation temperature. The material undergoes a change in which the austenitic microstructure is reoriented to martensite. The second is the shape memory effect (SME), where the material is able to recover to a set configuration under the influence of a phase transformation induced by heating. One type of SMA which has received much attention in recent years is Nitinol, and has been particularly popular due to its excellent strain recovery, durability, corrosion resistance and fatigue strength. However, research aiming to exploit the properties of SMAs in high power ultrasonic applications is limited. The results in this paper demonstrate that the superelastic effect is exhibited as a nonlinearity in the response of the transducer at raised voltage levels, resulting from the local emergence of martensite phase behaviour of the material. It is also shown that the resonance frequency of a Nitinol cymbal transducer can be tuned by utilising the phase transformation of Nitinol.