Minimisation of Mechanical Cross Talk in Periodic Piezoelectric Composite Arrays

This paper describes an investigation into mechanical cross talk within 1-3 and 2-2 connectivity piezoelectric composite array configurations, comprising a matrix of active piezoelectric elements embedded within a passive, polymeric, material. One way to take full advantage of the reported sensitivity and bandwidth improvements from single crystal materials is to configure them as a piezoelectric composite. For this work, piezoelectric ceramic, lithium niobate and single crystal pmn-pt materials are investigated as the active component in the piezocomposite array designs. Within these piezoelectric configurations, the generation of ultrasonic inter-pillar modes, which arise due to the periodicity of the active piezoelectric elements within the piezocomposite lattice, can be detrimental to the array performance. Consequently, finite element (FE) modelling, using PZFlex, is utilised to provide design techniques for the removal of these inter-pillar modes from the frequency band of interest and the realisation of unimodal piezocomposite transducer structures. Further FE modelling is used to generate dispersion data for 2-2, and doubly periodic 1-3, composite substrates. This dispersion data is used to design the linear arrays, with the objective of minimising mechanical inter-element cross talk. A comparison between the FE predicted mechanical cross coupling between array elements, for each composite material operating in air, is supported by experimentally measured data. Subsequently, the validated FE models are extended to include both operation into a solid load and the introduction of a backing material to simulate the operation of a practical NDE array transducer. The design techniques obtained from PZFlex are shown to produce arrays with low cross talk and the extent of the cross talk in manufactured and modelled ceramic and pmn-pt single crystal arrays is compared. Introduction: This paper investigates the design of 1-3 and 2-2 connectivity piezoelectric composite arrays. Piezoelectric composites comprise a matrix of active piezoelectric elements embedded within a passive, usually polymeric material, as shown in Figure 1. Piezoelectric Material Passive Polymer Piezoelectric Material Passive Polymer Figure 1 (a) and (b). Diagram of a 1-3 composite (left) and a 2-2 composite (right). The work in this paper initially focuses on obtaining uniform surface displacements from a composite substrate and then focuses on obtaining low mechanical coupling between array elements patterned on to the substrate. To achieve these two separate design challenges two different types of waves have to be considered; inter-pillar waves and travelling Lamb waves. Inter-pillar waves exist in any composite configuration due to the microstructure of the substrate. These waves are generated under the electroded areas and if these waves are coupled to the fundamental thickness operating mode as a consequence of their resonant frequencies being similar, the uniformity of the surface displacement can be severely degraded. It is however travelling waves that are responsible for most of the coupling of energy between elements in a composite array. These travelling waves in solid materials are called Lamb waves. Lamb waves are perturbations propagating in a solid plate or layer, for which displacements occur both in the direction of wave propagation and perpendicular to the plane of the plate [1]. It is known that periodic 1-3 and 2-2 piezoelectric composite substrates support Lamb waves that propagate parallel to the plane of the substrate plate. The behaviour of Lamb waves in homogeneous plates of material is well known and understood, however the behaviour of Lamb waves in piezoelectric composite structures is less well documented. In this work, arrays were made from 1-3 and 2-2 composite substrates. Firstly, finite element (FE) modelling, using the PZFlex code [2], was used to determine the frequencies of inter-pillar wave activity, which arise due to the periodicity of the active piezoelectric elements within the piezocomposite lattice. Design techniques to remove these inter-pillar waves from the frequency band of interest are used to produce unimodal piezocomposite transducer substrates. Next, the dispersion data for the composite substrates was taken into consideration to help design the arrays. These arrays were designed with the objective of minimising the mechanical cross coupling between the array elements. The mechanical cross coupling in the manufactured arrays was measured experimentally using a laser vibrometer and this data is compared to the mechanical cross coupling predicted using PZFlex. Further FE modelling was carried out to simulate the cross coupling of arrays operating in to a stainless steel load.