## 3 Damage Detection Problems and Possibilities  
In the previous section, the reattachment of the PZT device caused the impedance signatures to vary, which can result in difficulty when identifying damage. The highest RMSD values produced for the two different reattachment methods were approximately 7% when using the drone and 3.5% for manual placement. Thus, it would be ideal for a structure experiencing damage to have higher values as these two values can be defined as the threshold values, in order to differentiate between an intact case and a damage case. Keeping this in mind, the purpose of this section is to conduct two experiments involving a progressive damage case and a thickness loss damage case. The setup of the test specimens is shown in Figure 5. Figure 5a shows the PZT device attached to the metal plate, where damage is introduced by cutting 1 mm each time (up to 20 mm), with the impedance signatures being measured at every step. In Figure 5b, five plates are stacked with the PZT device attached at one end of the stack. The stack is held together by the magnetic force (without any additional magnet) of the PZT device, where it is detached and reattached 10 times for acquiring impedance signatures. After measuring the impedance signature, a single layer of the metal plate is removed from the stack and the PZT device is detached and reattached another 10 times, with the impedance signatures being measured at each step. This process is repeated until the last plate of the stack is remaining, totaling 50 impedance signatures. Figure 4: Reattachment of the PZT device: (**a**) by the drone; (**b**) manually. Figure 3: (**a**) Piezoelectric (PZT) device reattachment test; (**b**) root mean square deviation (RMSD) results for the reattachment test. Figure 6a shows the impedance signatures for the experiment in Figure 5a, where 21 impedance signatures (including the reference signature) can be observed. Although the signatures vary subject to progressive damage, the intensity of signature variations are less severe compared to Figure 4. Thus, this causes difficulty when identifying damage as damaged structure results in smaller signature variations than variations caused from reattachment of the PZT device. Figure 6b shows RMSD values by selecting the intact case as the reference signature, with all other signatures being compared to this reference. The first RMSD value which is calculated from the initial 1 mm cut damage results in 1.01% (red square dot), and the remaining RMSD values range between 1.4% and 1.8% (purple circle dots). One possible explanation for the relatively low RMSD result (1.01%) for the first RMSD value is that the damage was conducted near a node (a point that vibrates with minimum amplitude), causing small variations in the impedance signature. Using a line of best fit (excluding the first RMSD value), shows a rising trend with an increase in damage intensity. For this progressive damage case, the RMSD values are smaller than the values calculated from manually reattaching the PZT device (2.5%-3.5%). Thus, a different approach other than using the RMSD equation, needs to be researched in order to differentiate between an intact case and a progressive damage case using the presented PZT device. Figure 7a shows 50 impedance signatures measured for the experiment conducted in Figure 5b. When observing this, a trend exists for the resonance peaks located at 35 kHz and 63 kHz, where both peaks show a decrease in the amplitudes and a shift to the left direction as the thickness of the  
Figure 5: Experiment setup for: (**a**) the progressive damage case; (**b**) thickness reduction case. Figure 6: Progressive damage case: (**a**) Impedance signatures; (**b**) RMSD results. metal stack layer is reduced. Figure 7b shows the RMSD values calculated using the first impedance signature, with the remaining signatures being compared to this value.