Impedance domain

The basic concept of impedance method is to use high-frequency structural excitations to monitor the local area of a structure for changes in structural impedance that would indicate imminent damage. The impedance domain technique successfully detects damage that is located near the sensor / actuator. Electro-mechanical impedance is a technique for SHM that uses a collocated piezoelectric actuator / sensor to measure the variations of mechanical impedance of a structural component or assembly. This technique relies on the electro-mechanical coupling between the electrical impedance of the piezoelectric materials and the local mechanical impedance of the structure adjacent to the PZT patches. The PZT patches are used both to actuate the structure and to monitor the response. From the input and output relationship of the piezoelectric materials, the electrical impedance is computed. When the PZT sensor-actuator is bonded to a structure, the electrical impedance is coupled with the local mechanical impedance. Small flaws in the early stages of damage are often undetectable through global vibration signature methods, but these flaws can be detected using the PZT sensor-actuator, provided that they are near the incipient damage. By inspecting the differences between the impedance spectrum of a reference baseline of the structure in an undamaged condition and the impedance spectrum of the same structure with damage, incipient anomalies of the structural integrity can be detected. This health monitoring technique can also be implemented in an on-line fashion to provide a real-time assessment to detect the presence of structural damage. Giurgiutiu and Zagrai [2005] have used high-frequency electro-mechanical impedance spectra for health monitoring of thin plates. Park et al [2003] have summarized the hardware and software issues of impedance-based SHM, where high-frequency structural excitations are used to monitor the local area of a structure from changes in structural impedance. Piezoelectric-wafer active sensor based SHM and damage detection based on elastic wave propagation and electro-mechanical (E/M) impedance technique are reviewed by Giurgiutiu et al [2002].