Dual PZT sizing for mode decomposition on a composite anisotropic plate

Abstract

Structural Health Monitoring is a multidisciplinary field whose aim is to monitor damages within structures. Damages are detected by means of features - Damage Index (DI) - obtained by comparing measurements obtained from an unknown state with data obtained from a reference state stored in a baseline. Most DIs are calculated using Lamb wave signals generated and received by surface-mounted piezoceramic transducers (PZT) excited around a predefined central frequency. Usually, only the faster mode (S0) is considered on composite structures. However, the symmetric and antisymmetric Lamb wave modes tend to highlight different kinds of damages and are both interesting for SHM purpose. For example, a delamination in a composite plate attenuates the antisymmetric mode and reflects the symmetric one. Using two concentric PZTs (a disc and a ring defining a dual PZT) allows the decomposition of both mode contributions in the received signal. The current work presents a method based on Lamb Wave dispersion curves able to size the dual PZT and to determine the excitation frequencies to use for SHM application. This approach is based on three main observations: (i) Only the first symmetric and antisymmetric modes S0 and A0 must be generated, (ii) the dual PZT used as actuator and the excitation frequencies of the signal used must guarantee the actuation of S0 and/or A0 modes with wave length small enough to interact with the targeted minimal damage size, and (iii) the dual PZT used as sensor must be sensitive to both S0 and A0 modes on the selected range of excitation frequency. This method is applied for the sizing of dual PZT bounded to a highly anisotropic composite plate (CFRP [0,90]16) with a thickness of 2 mm and a minimal targeted damage size of 20 mm. Optimal dimensions of the PZT disk and ring are found and results of mode decomposition method on the optimal range of excitation frequencies obtained on a finite element model are presented. This simple method provides an efficient way for dual-PZT sizing in a SHM context where both S0 and A0 modes are investigated.