## 3 Results
### Dynamical Testing
\begin{table}
\begin{tabular}{c c c c c} \hline
**Specimen Type** & **Initial Stiffness [kh/mm]** & \multicolumn{2}{c}{**Cycles**} \\ \hline  & 23 \({}^{\circ}\)C & 60 \({}^{\circ}\)C & 23 \({}^{\circ}\)C & 60 \({}^{\circ}\)C \\ PPA-GF30 & 3.38 \(\pm\) 0.02 & 3.08 \(\pm\) 0.2 & 1607 \(\pm\) 392 & 956 \(\pm\) 728 \\ CFRP & 6.79 \(\pm\) 0.04 & 6.57 \(\pm\) 0.09 & 1,000,000 & 1,000,000 \\ Hybrid & 24.00 \(\pm\) 1.2 & 21.34 \(\pm\) 0.32 & 103,000 \(\pm\) 43,778 & 50,000 \(\pm\) 12,468 \\ \hline \end{tabular}
\end{table} 
Table 3: Comparative analysis at the two different testing temperatures. The stop criterium is 1,000,000 cycles. Figure 10: \(\Delta\)T-thermograms of the **(a)** PPA-GF30 specimens at 23 \({}^{\circ}\)C **(left)** and 60 \({}^{\circ}\)C **(right)** after failure and **(b)** CFRP specimens 23 \({}^{\circ}\)C **(left)** and 60 \({}^{\circ}\)C **(right)** after one million cycles in dynamical testing. fast failure occurring at those specimens. Significant differences in the heat up between the specimens tested at 23 \({}^{\circ}\)C and 60 \({}^{\circ}\)C cannot be observed. The CFRP specimens show no significant temperature increase during fatigue testing. The marginal difference of \(\pm 1\)\({}^{\circ}\)C is in the region of day and night fluctuations. This negligible difference can be explained through the fact that the biggest fraction of the load is taken up by the linear elastic carbon fibres and not by the viscoelastic polymer itself. That leads to a lower damping and to a smaller heating. While no failure occurs, no damage events can be identified. Regarding the thermograms of the hybrid specimens in Figure 11, a heating around the inlay can be observed for all number of cycles at both testing temperatures. The heated-up area grows with rising number of cycles. The temperature of the surrounding CFRP remains unchanged. For both specimens, there is a little stronger heating between the arms of the inlay at first. Subsequently, a hot area emerges below the inlay and shortly before failure there is a strong heating up to 5 \({}^{\circ}\)C around the inlay. Significant differences in the fundamental damage mechanisms cannot be detected with this method. Using Lock-In thermography displaying the phase contrast of thermal waves provides further information about the ongoing damage mechanisms. Delamination growth can be observed in dependence of the number of cycles, and it can be differentiated between delaminated area and delamination front (Figure 12). For both testing temperatures, the delamination growth starts below the inlay and between the arms of the inlay (see Figure 13). From these areas, the delaminations enlarge until they grow together to one global delamination, and it comes to failure. A fundamental difference in the damage mechanisms between those two temperatures cannot be detected either. Figure 12: Differentiation between delaminated area and delamination front in Lock-In thermography. In contrast to thermography, a significant difference in damage progression can be observed in the X-ray projections (Figure 14) recorded after mechanical testing. While the specimen tested at 23 \({}^{\circ}\)C shows a crack in the aluminium inlay, the specimen tested at 60 \({}^{\circ}\)C does not show any damage in the aluminium inlay. Both specimens have slightly bent \"arms\". Hence, the mechanical data as well as the NDT-data imply an influence of the temperature on the fatigue behaviour of the hybrid. After regarding all the different mechanical, thermographic, and CT data, it has to be analysed why some of them show more temperature effect than others. To conclude this paper, a discussion about the underlying reasons considering the behaviour of the hybrid as well as the influence of the specific single materials is necessary.