## 4 Results and Discussion  
### Experimental Results and Visual Analysis of Both Temporary Attachment Devices  
Figures 3 and 4 show all impedance signatures described in the previous section. Figure 3a,b represents the impedance signatures using the re-attachable device where the dark orange-colored lines and light orange colored lines represent debonded and bonded area signatures, respectively. Although variations in signatures can be seen, one can clearly identify the difference between the two groups of signatures. The first observation was that the signatures from the debonded area had higher impedance peak amplitudes compared to the bonded area signatures for both figures. For Figure 3a involving the test on Structure_A, the heights of most of the signatures for the debonded area were around 8 kOhms, where  
Figure 2: Composite structure specimens created for this study.  
the heights of the signatures for the bonded area were all less than 7 kOhms. Thus one can visually identify the difference between two different areas. Figure 3b shows a similar outcome using a thicker top plate of 0.4 mm where the heights of the impedance signatures from the bonded area had an amplitude of less than 7 kOhms as well. However, when the measurement was taken from the debonded area, the peaks generally shifted in the left direction (from around 35 kHz to 34 kHz), with most of the signatures resulting in over 7 kOhms. The reason for the increase in peak amplitudes when moving from bonded to debonded area is that with the debonded area, the top plate is free to vibrate in the thickness direction, resulting in higher amplitude. This is why with Figure 3b, the peak amplitudes for the debonded area were lower compared to Figure 3a since the thickness was twice the size, restricting vibration in the thickness direction.  
Figure 4: Impedance signatures measured from bonded and debonded areas using the roller PZT device for (**a**) 0.2 mm top plate; (**b**) 0.4 mm top plate.  
Figure 3: Impedance signatures measured from bonded and debonded areas using the re-attachable device for (**a**) 0.2 mm top plate; (**b**) 0.4 mm top plate.  
Figure 4a,b represents impedance signatures using the roller device where the dark blue colored lines and light blue colored lines represent debonded and bonded area signatures, respectively. For both figures, signature variations were hardly noticeable by visual, and even with the zoom in at 24 kHz, it was very difficult to distinguish the difference between signatures from bonded and debonded areas. With the zoom-in, the variations in signatures existed, but one could say that this variation was due to the replacement of the roller device. Since the EMI technique used a high-frequency range over 20 kHz, replacing the roller device randomly onto another location can cause the signature to change, as shown in the figure. With the resonance peaks located at around 28 kHz, more variations in the signatures can be visually identified for Figure 4a when compared to Figure 4b. This result was quite similar to Figure 3, where the difference (the resonance peak difference between bonded and debonded area) was larger for Structure_A compared to Structure_B due to a thinner top plate of 0.2 mm. Comparing both Figures 3 and 4, it was clear that using the re-attachable device (Figure 3) can clearly distinguish between bonded and debonded areas. However, with the newly proposed idea of roller device, uncertainty remained as signature variations seemed to be random when zoomed in closely at the resonance peak of 24 kHz. Thus, further analysis is required to confirm the possibility of using the roller device for detecting defects in composite structures.