| The PZT-based electro-mechanical impedance (EMI) method has been developed as a promising tool for real-time damage assessment of large and complex structures due to the advantages as follows:being sensitive to local minor damages because of high actuating frequency (greater than 30 kHz); not based on any model and can deal with unpredicted failure patterns; excellent features of PZT under normal working conditions; etc. Unlike the conventional non-destructive evaluation (NDE) method, the EMI technique does not need the location of damage previously or temporarily unusable of structures during detection. The core issues of EMI are the excitation of PZT patches and measurement of coupled impedance (or admittance) which are usually achieved by the bulky and expensive Impedance Analyzer. The actuating voltage is low (smaller than 2V) so that the PZT patches could not be excited well and the signal-to-noise ratio of impedance will be decreased correspondingly. Therefore, it is necessitated to develop an EMI system with high exciting voltage, wild operating frequency and low cost. Moreover, it is difficult to obtain the quantitative results due to the complexity and variety of damages.This paper investigated the experimental system and quantitative evaluation for damages on 1-dimensional beams using EMI method. The new impedance-measuring system is composed by an arbitrary waveform generator, an FFT analyzer and a small current measuring circuit. The actuating voltage is 10V with the working frequency range of 15 MHz, while the frequency range is 2MHz, the system could be excited at 35V. The frequency measurement accuracy is 0.1Hz. A series of drilling holes (diameterφ1mm-φ3mm) and cracks (depth 0.5mm-1.5mm) were made on beam I and beam II respectively to prove the efficiency and sensitivity of the new system. In the detecting frequency range of 120kHz-215kHz, the damage states could be distinguished clearly by the variation of impedance spectra. With the increasing degree of injury, the peaks of impedance spectrum shift to lower frequency and the variation were more significant in the higher frequency range. Experimental results show the efficiency of the detection system.Four types of statistics damage metrics:Root Mean Square Deviation (RMSD). Mean Absolute Percentage Deviation MAPD), Covariance (Cov) and Correlation Coefficient (CC) were introduced for quantitative characterization of damages. The values of RMSD and MAPD increased with the development of damage. The performance of RMSD was more stable and accurate in evaluating the damages. Cov and CC showed opposite tendency to RMSD with the increasing degree of injury. The variation of CC was more accurate in reflecting the correlation of two sets of data because of less influence by the values of data itself. Among the relative-damage metrics, R-cov was the least sensitive one in locating the damages. |
