## 3 Results and Discussion  
### Impedance Analysis, 2D SHM Damage Localization  
Figure 5 depicts the impedance plots of the smart composite for all monitoring sites. Initially, impedance scans were applied to the pristine specimens as a reference, i.e., before the impact test (Figure 5, black curves). The IS scans after high-velocity impact revealed an extensive increase of the magnitude of the impedance at low frequencies for the spectra near the impact spot, that occurred between the areas with coordinates (3,2) and (3,3) (Figure 5, red curves). At low frequencies, the material presented an ohmic behaviour, and therefore the values of the magnitude of the impedance were equal to DC resistance values. This increase can be attributed to the disruption of the MWCNTs/CB conductive network, due to the delaminations and the microcracks [18]. Remarkably, minor damage was identified to a more distant zone of the laminate, stating the high sensitivity of the system.  
Figure 6 presents the phase delay of the impedance scans. At low frequencies the argZ fluctuates at zero degrees indicating the ohmic behaviour of the material, while a drop of the argZ occurred at about 1 kHz, denoting the existence of capacitance effects (non-ohmic behaviour). After the impact test, the transition frequency from ohmic to non-ohmic behaviour shifted to lower values, designating that the conductivity mechanisms need more time to adapt to the external field. This occurred due to the enhancement of the interfacial polarization effect. Interfacial polarization is a polarization mechanism that arises at the interface of two materials or regions with different dielectric properties due to an accumulation of charges. The introduction of air into the cracks and the microcracks after the damage increased the interfacial polarization effect, as the phase of air inducted between the corrupted MWCNT/CB network. The most significant changes were revealed on locus with coordinates (3,2) and (3,3) near the impact area, as these areas were mainly affected by the impact.  
Figure 4: Impacted 2D (**left**) and 3D (**right**) laminate with SHM mapping topography.  
Figure 7 illustrates the imaginary (\(-\)Z\({}^{\prime\prime}\)) versus the real (Z\({}^{\prime}\)) part of the impedance scans in Nyquist plots. The real part of the impedance represents the resistance behaviour of the material, therefore the imaginary part represents the reactance or the energy storage. In this specific case, the reactance of the system occurred due to capacitance, as the phase delay ranged from \(-\)90 to 0 degrees. The values of these plots were calculated from the experimental data according to Equations (4) and (5). After the impact test, the Z\({}^{\prime}\) exhibited an increase caused by the disruption of the MWCNT/CB network, as stated above, while the impedance results for the 2D SHM mapping (Bode plots) are shown in Figure 8.  
Figure 5: Impedance results for the 2D SHM mapping (Bode plots).  
Figure 6: Impedance results for the 2D SHM mapping-phase delay (Bode plots).  
the -Z\" also presented an increased due to the rise of the interfacial polarization. Nyquist plots showed the highest sensitivity for the damage identification, especially in the areas which were mostly affected by the damage. Both the real and the imaginary part values of the impedance were, in some cases, more than doubled in relation to the initial values in the loci with coordinates (3,2) and (3,3).