Modeling and Experimentation of Thickness Mode E/M Impedance and Rayleigh Wave Propagation for Piezoelectric Wafer Active Sensors on Thick Plates

Abstract

This paper discusses theoretical and experimental analyses of the standing harmonic waves through the electro-mechanical impedance spectroscopy (EMIS) and guided surface acoustic waves (SAW) through the guided wave propagation (GWP) analyses. Both EMIS and GWP analyses have been carried out by utilizing piezoelectric wafer active sensors (PWAS) for in situ structural inspection. PWAS has recently been extensively employed in many applications such as nuclear-structural as well as aero-structural health monitoring and non-destructive evaluations (NDE). EMIS method is utilized for high frequency local modal sensing to determine the dynamic characteristics of PWAS bonded on nuclear-structural component for in-situ ultrasonics. Rayleigh waves a.k.a., SAW, were generated in relatively thick isotropic elastic plates. Rayleigh waves have the property of propagating close to the plate surface, with rapid attenuation with depth. The polarization of Rayleigh waves lies in a plane perpendicular to the surface so that the effective penetration depth is less than a wavelength. Rayleigh waves are a high frequency approximation of the first symmetric (S0) and anti-symmetric (A0) Lamb wave modes. As the frequency becomes very high the S0 and the A0 wave speeds coalesce, and both have the same value. This value is exactly the Rayleigh wave speed and becomes constant along the frequency. In the first part of the study, simplified theoretical constrained PWAS-EMIS model is briefly discussed in relatively high frequency range (in MHz order of magnitude) in terms of thickness mode. Analytical predictive thickness mode impedance simulations of PWAS bonded on plate-like host structures are presented in corresponding with the experiments. For the experimental analyses, PWAS transducers are affixed on isotropic elastic plates such as aluminum plate in relatively high thickness and on a rail I-beam. The extent of the agreement between the experimental and analytical EMIS analyses of PWAS in thickness mode is presented. The study is followed with GWP tests through the pitch-catch method. Rayleigh wave signal packets which are generated in the relatively thick plate and a rail I-beam in high frequency region are assessed along with the experimental thickness mode PWAS-EMIS results. The tuning curve of Rayleigh wave is determined to show the tuning effect of the structure thickness on producing a dominant Rayleigh wave mode. The significant usage of the tuned Rayleigh wave mode is essentially discussed for the applications in the in-situ inspection of relatively thick structures such as nuclear power plant structures. The paper ends with summary, conclusions and suggestions for future work.