## 3 Results
### Mechanical Testing of Welded joints  
The verification of the selected welding parameters determined by numeric simulation was carried out based on etched microsections for both weld insert types. In addition, chisel tests were conducted, in order to verify a sufficient strength of the weld nugget. In Figure 10, the mentioned microsections and photographs of samples after the chisel tests are shown. For the weld nuggets, diameters of 3.9 mm (type A) and 4.4 mm (type B) as well as heights of 1.2 mm (type A) and 1.3 mm (type B) were determined. The diameters of the weld nuggets as well as the shape of the heat affected zone are within the range predicted by numeric simulation. With regard to the chisel test, the element heads were peeled off (type A) or deformed (type B). Subsequently, the TPC was unbuttoned, with the weld nugget withstanding. Accordingly, a sufficient strength of the welded joints can be assumed, as the failure mode matches the aspired failure for REW, which is  
Figure 10: Analysis of the joining zone after resistance element welding (REW) (top: micrographs of welded joints using weld insert type A (**a**), and type B (**b**), bottom: pictures of welded specimen using weld insert type A (**c**) and type B (**d**) after chisel tests). Figure 9: Simulative generated weldability lobes for weld insert type A (**a**) and type B (**b**). unbuttoning of the weld nugget from the base sheet or a head failure, respectively an unbuttoning of the TPC. As the pictures of the chiselled specimen show, no residues of molten plastic or smoke can be seen on the metallic joining partner. Furthermore, as can be seen from the pictures, the plastic matrix of the TPC does not show any thermal damage. For further investigation of the load-bearing capacity of the joints, shear tests on single-lap joints were carried out under quasi-static load application on five specimens each for both types of weld inserts. The corresponding load-displacement curves, evaluated arithmetic means regarding the ultimate shear loads, the corresponding standard deviations (SD) and exemplary failure pictures of the specimens are shown in Figure 11.  
It can be seen that the ultimate shear load of the welded joints using weld insert type A is 3.7 kN on average with a maximum displacement of 13 mm to15 mm. The average ultimate shear load of joints using type B weld inserts is significantly lower (3.0 kN) with a maximum displacement of approximately 5 mm to 6 mm. This phenomenon can be explained by the corresponding failure characteristics. For type A insert, the failure behaviour of the specimens is characterised by the deformation with subsequent peeling off of the element head. Due to the fact that the specimen bend during the shear tests, the joint is loaded with an additional bending moment, which causes the flat head of the weld insert to be deformed before breaking off. Thus a bearing failure in the TPC is induced before the remaining element shank is unbuttoned, which explains the higher load-bearing capacity and in particular the increased displacement at failure. In contrast to that the smaller element head of the type B insert merely deforms during the shear test. Due to the smaller head diameter, the weld insert unbuttons from the TPC at lower load and lower displacement. Table 3 summarises the mechanical characteristics of embedded weld inserts and welded joints as well as geometric sizes of the weld nuggets for both weld insert types. Figure 11: Results of single-lap shear tests under quasi-static load for both types of weld inserts: (**a**) load-displacement curves, (**b**) ultimate shear load, (**c**) characteristic failure behaviour. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Type A** & **Type B** \\ \hline push-out load before welding (mean) & 1.7 kN & 0.8 kN \\ nugget height (mean) & 1.2 mm & 1.3 mm \\ nugget diameter (mean) & 3.9 mm & 4.4 mm \\ ultimate shear load (mean) & 3.7 kN & 3.0 kN \\ \hline \hline \end{tabular}
\end{table}