## 2 Materials and Methods
### Mechanical Testing and Impact Loading  
All specimens were exposed to a three-point bending (3 PB) test. This type of cyclic flexural test was chosen in order to prevent damage in the area of electrical contacts. The configuration of the loading shown in Figure 2 results in tension/compression loading of the sensor fiber, due to its non-symmetrical integration (see also Figure 8). The cyclic loading was performed by the hydraulic testing system MTS Mini Bionix (MTS, Eden Prairie, MN, USA).  
The specimens were loaded with a loading force of 1.5-15 N (load ratio = 0.1) at a frequency of 0.1 Hz. The maximum loading force was established during the preliminary test. During this test, a strain-gage was installed in the middle part of the outer surface of the specimen. The maximum loading force of 15 N corresponds to a measured longitudinal strain of 3200 \(\mu\)m/m during the preliminary test. This strain level corresponds to a maximal loading level in many applications to avoid intralaminar matrix cracks. The specimens were exposed to 200 load cycles before impact loading and to 200 load cycles after impact loading. Measurements of the changes in electrical resistance were performed using a Keysight 344401A multimeter. The 4-wire resistance measurement method was used in order to eliminate test lead resistances and contact resistances.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Material of CFS** & **Specimen Numbers** & **Placement of CFS between Layers** \\ \hline \multirow{3}{*}{T300 1000-50A} & **30\({}^{1}\)**, 31, 32, 33 & 1 and 2 \\  & **34**, 35, 36, **37** & 4 and 5 \\  & **16**, **17**, **18** & 7 and 8 \\ \hline \multirow{3}{*}{YS-95A-30S} & **1, **2, 4** & 1 and 2 \\  & **5**, 7, **8** & 4 and 5 \\  & **9**, 11, **12** & 7 and 8 \\ \hline \multirow{3}{*}{YSH-70A-30S} & **1, **2, 3** & 1 and 2 \\  & **4**, 5, **6** & 4 and 5 \\ \cline{1-1}  & **7**, 8, 9** & 7 and 8 \\ \hline \hline \end{tabular} \({}^{\dagger}\) The specimens with numbers written in bold underwent computer tomography after the mechanical loading and impact test.  
\end{table}
Table 1: Overview of specimens for mechanical tests.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Label of the Fiber** & **T300** & **95A** & **70A** \\
**Tow** & **T300** **1000-50A** & **YS-95A-30S** & **YSH-70A-30A** \\ \hline Type & PAN & PITCH & PITCH \\ Producer & Toray & Nippon Graphite Fiber Corporation \\ Number of filaments & [–] & 1000 & 3000 & 3000 \\ Tensile modulus & [GPa] & 230 & 893 & 714 \\ Tensile strength & [MPa] & 3530 & 3600 & 3600 \\ Ultimate elongation & [\%] & 1.5 & 0.3 & 0.5 \\ Thermal conductivity & [W/mK] & 10.46 & 600 & 250 \\ Volume resistivity & [\(\mu\Omega\)m] & 17 & 2.2 & 5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: An overview of the carbon fiber tows examined here.  
The possibility of detecting impact damage using CFSs was investigated by a simple drop weight impact test using an impact device made in-house according to ASTM D 7136. The specimens were fixture to the supports according to Figure 3. The groove in the supports was made in order to prevent damage to the sensor during clamping of the specimen. The configuration of the impact test is shown in Figure 3. An impactor 16 mm in diameter and 410 g in weight (m) was used. The height (h) for the drop impact test was 0.5 m, so the impact energy was 2 J. The impact energy was calculated according to Formula (1).  
\[E=m\cdot g\cdot h. \tag{1}\]  
There was no visible damage to the impacted side of the specimen (the mold side of the specimen was impacted) after the 2 J impact. There were damages to the opposite side of the specimens which could be also detected by visual inspection for some specimens.  
Specimens T300-32, T300-33, T300-36, T300-37 were impacted twice because the first impact was outside the area of the CFS. Specimens T300-34 and T300-35 were not subjected to impact loading.