## 3 Results and Discussion
### 2D and 3D Damage Localization by Utilising Equivalent Circuit  
By employing impedance values, or the electrical properties of a material over a range of excitation frequencies, an equivalent circuit is used in order to simulate the behaviour of the specimen. The information obtained from the single semicircular shape of the curves of the Nyquist plots indicated that only a time constant element should be present in the equivalent circuit. As was explained above, this corresponds to a capacitor element.  
For this reason, a typical circuit consisting of a resistance and a capacitor in parallel was selected. The impedance values were analysed using the Complex Nonlinear Least Squares Fitting Program (LEVM / LEVMW, Version 8.11, Copyright: [PERSON]). The resistance values of the specimen after damage were normalized by the reference values, acquired during the reference scan before impact. The 2D contour topographical image of the smart composite is presented in Figure 14 (left), illustrating the extension of the damage in terms of defective area and degree of structural degradation. The 2D contour topographical image confirmed the results of the impedance plots in Figures 5-7. As expected, the most affected area was near the impact point, in locations (3,2) and (3,3). Figure 14 (left) also illustrates the extent of the damage in more distant areas of the composite, leaving intact only the material between locations (1,1) and (1,2) (Figure 14 (left), deep blue colour). It should be noted that only the loci in the junction points of the carbon fibre strips were measured (experimental) values in the contour images.  
The imaginary part of the impedance also provided valuable information about the damage identification on the laminates. The initial and after impact maximum values of the --Z\" were normalized and the results are presented in a 2D contour topographical image in Figure 14 (right). The values of Z\" offered an identical pattern of the damage extension to the one of resistance values. Comparing the maximum reduced values, the Z\" proved to be a slightly more sensitive element.  
A similar procedure regarding the previous laminate was followed for the representation of the impact damage on the laminate with the 3D self-sensing capabilities. The impedance values of each group of laminas were analysed using an RC in a parallel equivalent electrical circuit. The normalized values of the resistance before and after damage were  
Figure 14: 2D contour topographical image of the composite using resistance values (**left**) and Z” values (**right**) after impact damage.  
used to create the contour topographical damage depiction (Figure 15 (top)). As shown by the Bode and Nyquist plots, in Figure 15 (top), red colour indicates the highest damage level, caused by the impact. The impact affected the centre and the left side of the laminate on the front and the intermediate group of laminas. The blue colour area remained intact after the impact, as the relative resistance before and after impact was negligible. As far as the rear side of the laminate was concerned, the impact inflicted extensive damage. The most affected area was at the centre and the left side of the composite. As should be noted, damage, in this case, occurred on almost the entire back area of the laminates, being distinctly more severe near the impact site (as seen through the thickness of the laminate).  
Exploiting the Z\" values, a contour topographical damage depiction was created (Figure 15 (bottom)). The changes in the Z\" were identical to resistance changes after the impact. Damage (delaminations and microcracks) caused an increase on both real and imaginary parts of the impedance by increasing the resistance and the interfacial polarization due to the disruption of the MWCNTs/CB conductive network and the induction of air inside the composite.  
Figure 15: 3D contour topographical damage depiction of the smart composite with 3D mapping capabilities at three levels using resistance values (**top**) and Z\" values (**bottom**).