Online damage detection systems would reduce costs by minimizing maintenance and inspection cycles. One of the most promising means of developing such systems is through the integration of smart materials such as piezoelectric materials into the structures under monitoring.The piezoelectric ceramic lead zirconate titanate (PZT)-based electromechanical sellekchem impedance (EMI) technique is a very promising technique for SHM. In the EMI method, the electromechanical (EM) admittance signatures of the PZT acquired at different time are used to calculate the damage indicator [8].

The one dimensional EMI model was developed as follows [9]:Y��=(��j)wlh[(?��33T?d312E��)+(ZaZ+Za)d312E��)(tan?��l��l)](1)where w, l and h are the width, length and height of the PZT, respectively; Za is the short Inhibitors,Modulators,Libraries circuit mechanical impedance of PZT and Inhibitors,Modulators,Libraries Z is the mechanical impedance of a structure; �� = E(1+��j) is the complex modulus of elasticity and ?��33T=?33T(1?��j) is the complex electric permittivity at constant stress and j=?1; �� and �� are the mechanical loss factor and dielectric loss factor, respectively; d31 is the piezoelectric constant; �� is the wave number, related to angular frequency Inhibitors,Modulators,Libraries of excitation �� by ��=�ئ�/E��, where �� is density of PZT. The electromechanical admittance signatures consists of a real part (the conductance) and an imaginary part (the susceptance). Conductance has been traditionally used for structural health monitoring due to its better indication of structural changes [10].

The prominent effects of structural damage or material characteristic change on the PZT admittance signatures are the lateral and vertical shifting of the baseline or appearance of new peaks in the signatures, which are the main indicators of damages or material changes [11].Statistical Inhibitors,Modulators,Libraries techniques such as root mean square deviation (RMSD) [6] have been employed to associate the damage or material changes with the changes in the PZT admittance signatures:RMSD(%)=��i=1N(Gi1?Gi0)2��i=1N(Gi0)2��100(2)where Drug_discovery Gi0 is the baseline signature of PZT conductance; and Gi1 is the corresponding conductance for each monitoring time at the ith measurement point. Generally, larger difference between the baseline signature and the subsequent signatures would result in bigger RMSD values.The principle behind this technique is to apply high-frequency structural excitations (typically higher than 30 kHz) through surface-bonded PZT transducers, and measure the impedance of structures. Park et al. [8] recommended a frequency range from 30 kHz to 400 kHz for PZT patches. To achieve selleck kinase inhibitor high sensitivity to damage, high frequency signatures (>200 kHz) have been used to monitor the region close to the PZT location, while low frequency signatures (<100 kHz) have been traditionally ignored.