## 2 Evaluation of the PZT-Metal Transducer Using Double-Sided Tape  
The idea of the PZT-metal transducer proposed in [16] used a commercialized epoxy adhesive for attachment to the host structure. Thus, to make the PZT-metal transducer re-attachable, a simple idea of using a double-sided tape (purchased from www.alpha.co.kr) is evaluated in this section against debonding and crack damage. Figure 1 shows the experimental setup to evaluate the performance of the PZT-metal transducer with double-sided tape attachment. The AD5933 evaluation board is connected to the laptop as shown in the figure where the positive and negative wires from the board are attached to the PZT-metal transducer by soldering. More details on the PZT-metal transducer with its damage detection performance can be found in [16]. For evaluating the performance of the PZT-metal transducer, two glass fiber epoxy laminates of 200 mm \(\times\) 70 mm with 0.4 mm thickness were adhered to each other using a commercialized epoxy (Loctite Quick-set). After fully curing the epoxy for 48 h at room temperature, the impedance signature was measured from 25 to 75 kHz in 100 Hz steps to be used as a reference signature. Then, debonding was achieved by inserting a chisel tip between the two composite plates and seperating them apart 10 mm at a time until 100 mm debondingwas achieved. For each 10 mm of debonding, impedance sigantures were measured. After 100 mm debonding was achieved, the final impedance signature was measured with complete debonding of the bottom composite plate. The reason for this was to observe the difference in the impedance signature subjected to debonding that occurs away from the PZT transducer and which that occurs right below the PZT transducer. To check the reliability of the double-sided tape attachment approach, five identical tests (Test 1, Test 2, Test 3, Test 4, and Test 5) were conducted. Observing Figure 2a, which represents the impedance signatures from Test 1, debonding has caused the amplitude peak (at 60 kHz) to decrease with increase in debonding area. However the amplitude remains virtually the same for the frequency range at 35 kHz where change is very difficult to see. However, when bottom composite plate is removed, the impedance signature peak increases dramatically at the 60 kHz resonance range. This experimentally proves that signature change is significant when debonding occurs below the PZT-metal transducer. Figure 2b shows the limitation of using the double-sided tape as the five impedance signatures seen in this figure were acquired by removing and reattaching the PZT-metal transducer onto the same area of the composite plate. The change is not significant but such change in signatures can cause false alarms when no damage has been experienced by the structure. Figure 1: Experimental setup for the piezoelectric (PZT)–metal transducer experiment. Figure 2: Impedance signature results for: (**a**) Debonding experiment; (**b**) re-attachable performance of the PZT–metal transducer. For evaluating the damage detection performance, the variations between the reference and corresponding signatures (after damage) were quantified using root-mean-square deviation (RMSD) in equations below. In the equations, \(Re(Z_{i})\) and \(Re\!\left(Z_{i}^{o}\right)\) represents the real part of the reference and the corresponding impedance signatures, respectively. \[\text{RMSD}=\sqrt{\sum_{N}\!\left[Re(Z_{i})-Re\!\left(Z_{i}^{o}\right)\right] ^{2}/\sum_{N}\!\left[Re\!\left(Z_{i}^{o}\right)\right]^{2}} \tag{2}\]  
Looking at Figure 3, the RMSD values with the reference signature being the intact case, the RMSD value was 0.6% with 1 cm debonding and increased to 1.37% with 5 cm debonding. However the RMSD values decreased down to 0.97% with 9 cm debonding. This shows the limitation of debonding detection using the PZT-metal transducer, where it seems that the highest value that can be obtained from this test is 1.37%. With full removal of the bottom composite plate, the RMSD resulted in 9.06%.