## 3 Results and Discussion
### Mechanical Testing  
Most composite structures for which impact damage detection is an intending aspect are also exposed to cyclic loading. The influence of cyclic mechanical loading on the measured signal of impact sensors is therefore of great interest. The purpose of the mechanical testing was to describe the response of the measured signal of the investigated sensors with regard to:  
1. Number of cycles;
2. The positioning of the CFS in the composites lay-up;
3. Influence of different materials on the CFS signal. Specimens with integrated sensors were exposed to cyclic flexural loading before and after impact loading. During mechanical loading, the room temperature was measured in order to exclude this influence. The influence of a change in temperature on the measured signal of CFSs is described in Section 3.1. In practical applications, it is not always possible to make temperature measurements simultaneously at the position where impact detection is needed. It is possible to use temperature compensation of the half-bridge circuit as used, for instance for strain-gauges [28]. Another option is to evaluate the peak-to-peak values of the cyclic loading [29] (see Figure 6). The schematic representation shows the evaluation procedure applied for the two signals. In the upper part of the figure is depicted an example of a signal from CFS during measurement under constant temperature. The first three cycles show signal before impact loading and the next three cycles represent signal after impact loading under cyclic loading and constant temperature. In the lower part of the figure depicts a graph of the measured signal from CFS under rising temperature before and after impact loading. We assume, that the rising of the temperature is much slower than the change of mechanical strain applied \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen Nos.** & **m [\(\Omega\)/\({}^{\circ}\)C]** & **R\({}_{0}\) \({}^{\circ}\)C [\(\Omega\)]** & **R\({}^{2}\)** & **RMSE [\({}^{\circ}\)C]** \\ \hline T300-12 & \(-9.10\times 10^{-3}\) & 27.79 & \(1-3368\times 10^{-5}\) & 0.206 \\ T300-13 & \(-8.93\times 10^{-3}\) & 27.76 & \(1-5498\times 10^{-6}\) & 0.083 \\ T300-14 & \(-9.02\times 10^{-3}\) & 27.91 & \(1-1286\times 10^{-6}\) & 0.128 \\ T300-15 & \(-9.04\times 10^{-3}\) & 27.90 & \(1-1123\times 10^{-6}\) & 0.119 \\ \hline T300 mean & \(-9.02\times 10^{-3}\) & 27.84 & & \\ \hline
70A-10 & \(-3.12\times 10^{-3}\) & 3.49 & 0.9985 & 1.365 \\
70A-11 & \(-3.17\times 10^{-3}\) & 3.54 & 0.9978 & 1.682 \\
70A-12 & \(-3.19\times 10^{-3}\) & 3.53 & 0.9986 & 1.309 \\
70A-13 & \(-3.32\times 10^{-3}\) & 3.48 & 0.9983 & 1.462 \\ \hline
70A mean & \(-3.20\times 10^{-3}\) & 3.51 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The calculated values of the linear fit, and also the R2 and RMSE values. during cyclic loading. This is often the case in practical applications. The graph on the right side show measured peak to peak values of electrical resistance for constant and rising temperature. When using this procedure, temperature measurements are not needed. Figure 6: Evaluating the peak-to peak data. Figure 7: Peak-to-peak relative values of the measured signal from CFSs integrated into the specimens before and after impact loading: (**a**) results for specimens T300; (**b**) results for specimens T300–detail; (**c**) results for specimens 95 A; (**d**) results for specimens 70 A. The relative values were determined to allow an easier comparison of sensors made of different carbon fiber tows. The peak-to-peak relative values were determined for the first and last cycle before impact loading (cycle 1 and cycle 200) and for the first and last cycle after impact loading (cycle 201 and cycle 400). The peak-to-peak values were related to the measured value of the electrical resistance of the sensor after implementation.