## 3 Spider Web Inspired EMI Technique for Damage Detection  
Spiders can pluck and pull their web to send out ripples in every direction to sense the vibrations in each of its eight legs. These vibrations provide the spider with vital information about the state of the web and the presence of prey. Inspired from nature, the authors used three steel wires (where two wires are bent 90 degrees) to be attached to a 35 mm \(\times\) 15 mm PZT transducer, as shown in Figure 3. By bending the wires, it is possible  
Figure 2: Impedance signatures and RMSD values for 0.6 mm composite plate.  
to monitor six different areas, as shown in the figure (A1 to A6), where each end of the wires is attached to the composite plate using epoxy. Since the steel wires are homogenous material, one can expect that the impedance signature change was larger compared to the previous section subjected to debonding damage.  
Here, the test conducted in the previous section was tested again using the proposed idea by introducing debonding damage to all eight areas. Looking at Figure 3, the ends of the wires are attached to the surface of the test specimen. Note that A7 and A8 have no wires attached as the authors wanted to clearly identify the effect of using the wires subjected to debonding damage. One can assume that the impedance signature will dramatically change when debonding occurs directly below the end of wires (from A1 to A6).  
Using the proposed biomimicry idea, the impedance signatures and the calculated RMSD values for each area are shown in Figure 4. At first glance, the impedance signatures have many more peaks and the RMSD values are higher compared to Figure 2. Thus, this experimentally proves that the proposed idea is effective. The highest RMSD value is 3.34 for A1 subjected to debonding where this is more than six times higher compared to A1 in Figure 2 (value of 0.5). However, A7 and A8 show relatively low RMSD values of 0.47 and 0.53 where the reason is due to the fact that the ends of the wires are not attached to this area (Figure 3). Thus, this experimentally shows that the proposed idea performs best when debonding occurs directly below the end of the wires. Furthermore, one explanation of the large differences in the RMSD values between areas can be said to be due to the existence of nodes and antinodes. Debonding damage to certain areas can create larger variations in the signature and this makes the EMI technique difficult to analyze for the existence of damage. Nevertheless, it can be said that the proposed idea for this study is effective when monitoring a composite structure for debonding damage.  
Next, the authors wanted to evaluate the performance of the proposed idea subjected to different thicknesses of the composite structure. Thus, two more tests were conducted with different thicknesses of the top layer, which were 0.4 mm and 0.2 mm, to find out the performance of the EMI technique when subjected to a thinner structure. The same test conducted in the previous test was conducted again with the two different top layers where the impedance signatures and the RMSD values are shown in Figures 5 and 6. By observation, both figures show variations in the impedance signatures subjected to debonding damage where A1 shows the highest RMSD values of 3.13 and 2.71 for 0.4 mm and 0.2 mm thicknesses, respectively. In addition, most of the RMSD values with the  
Figure 3: Biomimicry idea based on a spider.  
0.2 mm thickness structure show higher values compared to Figure 4 which leads to the fact that thinner structures will result in a better outcome when using the EMI technique.