## 2 Materials and Methods
### Composites Manufacturing  
The homogenous dispersion of carbon fillers was performed by high shear mixing under controlled temperature. The mixing took place in a Dispermat AE laboratory dissolver, [PERSON], equipped with a double-wall vacuum container. High shear mixing with an impeller was selected among other dispersion techniques to avoid the degradation both of the fillers and the resin. A 45 mm diameter toothed impeller disk, offering medium shear forces to the dispersion to reduce agglomerates size, was chosen. A fundamental parameter for the manufacturing of the composite was for the nano-filler's concentration at the matrix to exceed the percolation threshold [29; 32; 35]. Therefore, in the case of damage, the conductive network would be interrupted. This would be reflected by an increase in the impedance. The optimum dispersion conditions that exhibited high conductivity, medium viscosity and could enable the SHM mapping, were selected based on previous research [32; 36]. Consequently, 0.5% w/w MWCNTs and 2% w/w CB were dispersed in the epoxy resin (part A) at 3000 revolutions per minute (rpm) at 25 \({}^{\circ}\)C. The dispersion was afterwards mixed with part B. The de-gassing of the resin was skipped as the manufacturing of the laminate was performed under vacuum. For the 2D mapping, an 8-ply [0/90]2s laminate was manufactured using the hand lay-up method under vacuum. Three parallel carbon fibre strips with 2 cm width and 2 cm space between them were positioned on the first layer. Three more carbon fibre strips, with the same width and spacing, were placed after the last layer, at a 90\({}^{\circ}\) angle in relation to those positioned on the top layer. The aim of this specific layout was the formation of sensing elements in the junction points of the top and bottom carbon strips. At the edge of each carbon fabric strip, a copper wire was implanted with conductive silver tape. Figure 1 (left) illustrates the layout of the smart composite with 2D SHM topographic capabilities. The coordinate system of the laminate is presented in Figure 1 (right). A cartesian system was selected to specify the location of the formatted sensing areas. The area of the material under investigation is marked with orange colour. Note that the carbon fibre strips are not in contact. For the manufacturing of the smart composite with 3D SHM capabilities, a more complex structure was selected. A 9-ply laminate with a lamination of (0/90/0/90/0/90/0/90/0) was manufactured by the hand lay-up method using vacuum bagging as well. As previously mentioned, three parallel carbon fibre strips were introduced with 2 cm width and 2 cm space between them, on top of the first layer. Three more carbon fibre strips, with the same width and spacing, were placed between the third and fourth lamina, forming a 90\({}^{\circ}\) angle with the direction of the top strips. Between lamina 6 and 7, three more carbon fibre strips with 0\({}^{\circ}\) direction were added. Finally, three carbon fibre strips were placed after the last lamina ofglass fabric, with a 90\({}^{\circ}\) orientation. As depicted in Figure 2 (left), a copper wire was connected to the edge of each carbon fabric strip using conductive silver tape. This layout led to the formation of 27 sensing areas, providing SHM capabilities at three different depth levels through the thickness of the composite. This through-thickness damage detection property combined with the nine spots in-plane for SHM topography imparted a 3D mapping topography functionality to the smart composite. In this case, a three-dimensional cartesian system was used to describe the locus of the areas were impedance measurements applied. X and Y axes specify the in-plane location of the sensing elements, while the Z-axis specifies the current depth level.