This paper presents theoretical predictive modeling and experimental evaluation of the structural health monitoring capability of piezoelectric wafer active sensors (PWAS) at elevated temperatures. Electromechanical impedance spectroscopy (EMIS) method is first qualified using circular PWAS resonators under traction-free boundary condition and in an ambience with increasing temperature. The theoretical study is conducted regarding temperature dependence of the electrical parameters, the capacitance C<sub>0</sub>, d<sub>31</sub> and g<sub>31</sub>; and the elastic parameters, the in-plane compliance s<sub>11</sub> and Young’s modulus c<sub>11</sub>, of piezoelectric materials. The Curie transition temperature must be well above the operating temperature; otherwise, the piezoelectric material may depolarize under combined temperature and pressure conditions. The material degradation is investigated by introducing the temperature effects on the material parameters that are obtained from experimental observations as well as from related work in literature. The preliminary results from the analytical 2-D circular PWAS-EMIS simulations are presented and validated by the experimental PWAS-EMIS measurements at elevated temperatures. Temperature variation may produce pyro-electric charges, which may interfere with the piezoelectric effect. Therefore, analytical simulations are carried out to simulate the pyro-electric response from the temperature effects on a free circular PWAS-EMIS in in-plane mode. For the experimental validation, PWAS transducers are placed in a fixture that provides the traction-free boundary condition. The fixture is then located in an oven integrated with PID temperature controller. The EMIS measurement is conducted during the temperature increase and the first resonance frequency peak in admittance and impedance spectra was acquired.
This paper discusses theoretical analysis of electro-mechanical impedance spectroscopy (EMIS) of piezoelectric wafer active sensor (PWAS). Both free and constrained PWAS EMIS models are developed for in-plane (lengthwise) and outof plane (thickness wise) mode. The paper starts with the general piezoelectric constitutive equations that express the linear relation between stress, strain, electric field and electric displacement. This is followed by the PWAS EMIS models with two assumptions: 1) constant electric displacement in thickness direction (D3) for out-of-plane mode; 2) constant electric field in thickness direction (E3) for in-plane mode. The effects of these assumptions on the free PWAS in-plane and out-of-plane EMIS models are studied and compared. The effects of internal damping of PWAS are considered in the analytical EMIS models. The analytical EMIS models are verified by Coupled Field Finite Element Method (CF-FEM) simulations and by experimental measurements. The extent of the agreement between the analytical and experimental EMIS results is discussed. The paper ends with summary, conclusions, and suggestions for future work.