## 4 Discussion  
While the results of the isolated experiments shown in the previous section did not always provide an unambiguous result about the influence of the testing temperature, the particular tendencies sum up to a clearer picture. At quasi-static testing the most obvious difference is the change in damage behaviour of the hybrid that can be regarded in the force displacement diagram and in the thermograms as well. While the specimens at room temperature break in short, abrupt damage events, the heated ones fail without those events and in a smoother curve after lasting a higher displacement. Looking at two subsequent thermograms, clear and concrete areas in which abrupt damage events take place can be identified, whereas at 60 \({}^{\circ}\)C no suddenly emerging damage can be observed. On the other hand, the specimens of the single materials (CFRP and PPA-GF30) break at similar displacements with the same damage behaviour  
Figure 14: Two-dimensional projections of the hybrid specimens; (**a**) reference, (**b**) after dynamical testing @ 23 \({}^{\circ}\)C, and (**c**) @ 60 \({}^{\circ}\)C, respectively. respectively for both temperatures. The heated specimens endure a slightly lower force and maintain a lower Young's modulus. This leads to the assumption that the elevated temperature primarily effects the interfaces in the hybrid specimen. The reason could be a premature debonding of CFRP like it is described in [21]. A possible reason for this debonding could be residual stresses caused by different thermal expansion coefficients of the three single materials. Such effects are described in [23]. This would not necessarily affect the overall performance of the hybrid too much due to the geometrical interlocking of the inlay and it would explain the absence of the abrupt damage events. Another assumption under these observations is a softening of the single materials in the hybrid at elevated temperatures leading to a reduction of locally concentrated interfacial tension during experiments due to higher polymer mobility. Higher local tensions can lead to a localized damage event, even if the global tension to initiate the component failure is not reached yet. A local damage like this would weaken the specimen as a whole and lead to tensions on other places and thus to a damage cascade as observed in Figure 4c. This is also evidenced by the local, sudden temperature peaks which only occur at 23 \({}^{\circ}\)C corresponding to an abrupt unloading of these localized tensions. The CT-projections after quasi-static testing cannot deliver further information to support or contradict this hypothesis. Looking at the dynamical experiments with regard to the observations made in quasi-static testing a few more things stand out. Firstly, there is a reduction in life cycles as well as in the initial stiffness for the 60 \({}^{\circ}\)C hybrid specimens. The single-material specimens, in contrast, only show a small difference in their mechanical behaviour at the two tested temperatures. This fact points to the influence of the temperature on the interfaces. Thermography does not show differences in the damage mechanisms in dynamical testing between the two temperatures. However, the CT-projections after dynamical testing support the local tension hypothesis from the quasi-static experiments. The cracks in aluminium occurring in the hybrid specimens at 23 \({}^{\circ}\)C can also be explained due to high local tensions that lead to fatigue crack growth in the metal. [PERSON] [14] suggests that this happens due to dislocation gliding which is the predominant deforming mechanism in aluminium. The absence of these cracks at 60 \({}^{\circ}\)C on the other hand shows that a softening of the polymers may reduce those tensions. Summarizing the effects, the mechanical behaviour of a metal-CFRP hybrid structure under quasi-static and dynamic load changes at elevated temperature. The switch of damage mechanisms occurs at temperature differences, which a component out of these materials could be exposed to in daily life use.