Acoustic fields from PVDF interdigital transducers : Transducer technology for ultrasonic NDT applications

Interdigital transducers (IDTs) made from the piezoelectric polymer PVDF have been built; they transmit ultrasonic Lamb waves into 1-2 mm thick steel and aluminium plates and receive signals reflected from features in the structure. The IDTs are designed to be permanently bonded to the structure under inspection. Such IDTs have considerable potential for use in smart-structure monitoring applications. However, before this potential can be realised, the nature of the acoustic field that they produce must be thoroughly understood. Experimentally measured acoustic fields are presented for two example IDTs, one that produces a collimated beam for line inspection and one that produces a divergent beam for sector inspection. The development of modelling software based on Huygens' principle, which enables the acoustic field from such IDTs to be predicted rapidly, is then described. Example results from this software are presented and compared with experimental measurements. Further predictions made with the model are then used to elucidate certain basic guidelines for IDT design.