Transducer arrays for omnidirectional guided wave mode control in plate like structures

For structural health monitoring applications, ultrasonic guided wave mode control is critical for obtaining simple signals that can be easily analyzed as well as special modes and frequencies for improved sensitivity to certain defects. This paper discusses the development of an annular array transducer for omnidirectional guided wave mode control in plate like structures. Using a flexible piezoelectric material like polyvinlydine fluoride (PVDF), annular array transducers that are low cost, low profile and conformable to the structure can be made rapidly. Two different array patterns, namely ones of comb and inter-digital (IDT) type, are studied. The loadings of these transducers on a structure differ from one another and hence so do their source influences. An axi-symmetric finite element modeling is employed to study the surface displacement pattern of these two transducer configurations. The source influence of the transducer configurations is studied experimentally by exciting an anti-symmetric (A₁) and a symmetric (S₁) type guided wave mode in a steel plate. It was observed that IDT type transducers were able to couple well to the guided wave modes at the wavelengths that they are designed for. The comb type transducers have a weak coupling to symmetric guided wave modes at frequencies where the wave structure has high in-plane displacement and negligible out-of-plane displacement on the surface of the structure.