# Piezoelectric Electro-Mechanical Impedance (EMI) Based Structural Crack Monitoring
## 4 Experiments and Discussion  
To verify the above theoretical model and finite element analysis, an experimental device was set up, as shown in Figure 7, and experiments were conducted. The dimension of the test specimen was consistent with the simulation model and the piezoelectric patch was bonded on the center of the one surface of the beam, as shown in Figure 8. When the piezoelectric patch was bonded to the host structure with epoxy resin, two optical fibers of 0.125 mm diameter were placed parallel on the surface of the beam with the epoxy resin to ensure the consistent adhesive layer thickness for all test pieces. First, the PZT patch, bonded on the without crack beam, was performed the piezoelectric impedance test by the WK6530B impedance analyzer ([PERSON], West Sussex, UK). Then, a crack with length of 6 mm was machined and the test was performed again. At last, the piezoelectric impedance test was carried out with crack length of 12 mm machined on the beam. The piezoelectric admittance spectra of the beam with different crack lengths (0, 6 and 12 mm) were measured and the peak frequencies in the admittance spectra are extracted, as shown in Figures 9-11. As the figures indicate, as the crack grows, the peak frequency, i.e., the resonant frequency of the coupled system shifts to the left. That means the frequency decreases as the crack grows. There may be a slight difference between the simulation and experimental results, which may be resulted from an error between the actual and the designed positions of the damage and PZT. The experimental results have the same trend with the theoretical analysis and finite element simulation results. The tested resonant frequencies are also consistent with the simulation results; therefore, the finite element simulation results can be used to guide the experiment to select an appropriate scanning frequency band of the piezoelectric impedance measurement. Figure 7: Testing device. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **19.2–19.3 kHz** & **62.6–62.8 kHz** & **174.5–175 kHz** \\ \hline \(\Delta f_{1}\) (Hz) & \(-10\) & \(-25\) & \(-55\) \\ \(\Delta f_{2}\) (Hz) & \(-18\) & \(-40\) & \(-105\) \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Peak frequency shifts at different frequency bands. Figure 8: The aluminum plate sample with a crack. Figure 10: Experimental results (**a**) piezoelectric admittance spectrum and (**b**) peak frequency tendencies in 62.2 kHz–62.6 Hz with different crack sizes. Figure 9: Experimental results (**a**) piezoelectric admittance spectrum and (**b**) peak frequency tendencies in 18.9 kHz–19.2 kHz with different crack sizes. The peak frequency shifts extracted from the piezoelectric admittance spectra at different frequency bands are shown in Table 4. The peak frequencies in the piezoelectric admittance spectra decrease with the increase of crack length in each frequency band. Moreover, at the same crack length, the peak frequency shifts increase with the increase of the scanning frequency band, which means increasing the scanning frequency band can increase the sensitivity of the proposed method. Therefore, the crack occurrence and propagation can be interrogated by the peak frequency shift in the imaginary part of the piezoelectric admittance spectrum. With the increase of the scanning frequency, the higher the frequency band is, and the larger the shift of peak frequency is, i.e., the higher the frequency band is, the more sensitive is the method to damage. The experimental studies reveal that the peak frequency in the piezoelectric admittance spectrum, i.e., the resonant frequency of the coupled system of the beam and piezoelectric patch, decreases with the increase of crack length, which indicates that the peak frequency shift can well reflect the size of cracks. Therefore, the peak frequency can be used as a feature parameter to characterize the crack length. With the increase of the scanning frequency band, the shift of peak frequency tends to increase.