Flexible interdigital PVDF transducers for the generation of Lamb waves in structures

The development of flexible, cheap PVDF transducers for the generation and detection of Lamb waves in plates is described. Lamb waves offer a possible means of inspecting a large area of structure using a small number of sensors, so these transducers may be used in the development of 'smart' structures. In order to obtain a simple signal which can readily be interpreted, it is essential to generate a single Lamb mode over a controlled frequency bandwidth. This has been achieved by using an interdigital design. The process of designing transducers for the generation of a particular mode is discussed and experimental results showing a variety of modes on aluminium, steel, perspex and composite plates are presented. It is shown that the transducers can be operated over the frequency range 0.5-4 MHz. They exert primarily out-of-plane forces on the plate which means that they are more suited to the generation and detection of the lower antisymmetric Lamb modes than the symmetric modes. The antisymmetric modes propagate with minimal attenuation in metal plates so the PVDF interdigital transducers can be used satisfactorily in long range testing applications. However, in perspex and composites. the attenuation in the antisymmetric modes is much higher than in the symmetric modes so ways of increasing the strength of excitation of the symmetric modes must be developed. (C) 1997 Elsevier Science B.V.