Practical issues related to the application of the electromechanical impedance technique in the structural health monitoring of civil structures: I. Experiment

The advent of smart materials such as the piezo-impedance transducer (lead zirconate titanate, PZT) and optical fiber (FBG) has ushered in a new era in the field of structural health monitoring (SHM) based on non-destructive evaluation (NDE). So far, successful research and investigations conducted on the electromechanical impedance (EMI) technique employing a piezo-impedance transducer are often laboratory based and mainly theoretical. Real-life application of the technique, especially under harsh environments, has frequently been questioned. In this research project, investigative studies were conducted to evaluate the problems involved in real-life applications of the EMI technique, attempting to reduce the gap between theory and application. This two-part paper presents a series of experimentation (part I) and numerical verification (part II) on various issues related to real-life application, including the durability of PZT transducers, and the effects of bonding and temperature under conceivable nominal construction site conditions. The repeatability of electrical admittance signatures acquired from the PZT patches surface bonded on aluminum structures was found to be excellent up to a period of one and a half years. Experimental investigations revealed that the bonding thickness should preferably be thinner than one-third of the patch to avoid any adverse effect caused by the PZT patch's resonance on the admittance signatures which reflect the host structural behavior. On the other hand, the effect of temperature on the admittance signatures was found to be closely related to the thickness of bonding, as an increase in temperature would reduce the stiffness of the bonding layer, thus affecting strain transfer. It was concluded that PZT patches with thick bonding thickness and high frequency of excitation are undesirable, especially at elevated temperatures.