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A high frequency NDE technique has been under investigation at the Center for Intelligent Material Systems and Structures (CJMSS). Physical changes in the structure cause changes in the mechanical impedance. Due to the electromechanical coupling in piezoelectric materials, this change in structural mechanical impedance causes a change in the electrical impedance of the piezoelectric sensor. Hence, by monitoring the electrical impedance and comparing this to a baseline impedance measurement, we can determine when structural damage has either occurred or is imminent. However, there are still basic research issues that need thorough investigation before full-scale development and commercialization can take place. Included in these is the effect of temperature on this impedance based NDE technique. Since piezoelectric materials exhibit strong temperature dependency and change in temperature results in marked changes in the structural dynamic response, any variation that is associated with a change in temperature may be confused as damage. In this paper we analyze temperature effects on the electrical impedance of piezoelectric materials and the structures. We have used an empirical approach due to the complexity of the thermo-electrical-mechanical constitutive models for piezoelectric materials. Through the experimental investigations, it was found that a change in temperature modifies both the magnitude and phase of the electrical impedance of the piezoelectric sensors. A computer algorithm was developed which incorporates temperature compensation into our health monitoring applications. This compensation technique minimizes the effect of temperatures on the electrical impedance of piezoelectric sensors bonded on the structure, in the range from 80 to 1 60 degrees Fahrenheit. In this paper, we show how it is applied successfully to a bolted pipe structure.

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Piezo ceramic patches connected to electronic shunt circuits have formed successful vibration reduction devices. Here a new shunt circuit design with additional capacitors is presented. One drawback of existing electronic shunt circuits is the large inductance required when suppressing low frequency vibration. Also, the large internal resistance associated with this high inductance value exceeds the optimal design resistance needed for vibration suppression. To solve these problems, a modified and improved piezoelectric shunt circuit is designed by connecting additional capacitors in parallel. This design is experimentally demonstrated for a desired tuned mode of an aluminum beam. The results indicate that improved electronic passive damping can significantly reduce the structural vibration response with half of the inductance previously required.