## 2 Materials and Experimental Work
### Evaluation of the Stretch-Forming Behaviour  
In addition to characterizing the stretch-forming behaviour of the sandwich panels (MPM and FML), the stretch-forming behaviour of the monolithic steel sheets, as well as the organosheets, were analysed. For this purpose, the force evolution, major strain and thickness distribution and other failure modes at different adhesion conditions, forming temperatures and core layer thickness, were evaluated following the test plan given in Table 5. Each test condition was repeated 3 times to ensure the repeatability.  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Paint Spray** & **Testing Temperature** & **Product** \\ \hline White paint spray & RT & Dupli color\({}^{\circledR}\), 252570 aqua white 9010 \\ White paint spray & 200 \({}^{\circ}\)C and 235 \({}^{\circ}\)C & Ulfalux\({}^{\circledR}\), Ofenfarben 1190 white-perlmatt \\ Black paint spray & RT, 200 \({}^{\circ}\)C and 235 \({}^{\circ}\)C & Contact Chemistry\({}^{\circledR}\) Graphit 33 76009-AA \\ \hline \hline \end{tabular}
\end{table}
Table 4: Spray pains for DIC analysis at different temperatures.  
Figure 7: **(a) Illustration of heat streaks generated during warm forming and (b) their influence on the strain field measurement obtained by the DIC method.**  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Material/** \\ **Abbreviation** \\ \end{tabular} } & \multicolumn{3}{c}{**Cover Sheet**} & \multicolumn{3}{c}{**Core Layer**} & \multicolumn{3}{c}{**Test**} \\ \cline{2-13}  & **TS275** & **TS290** & & & & & & & & & & & **RGUD (mm)** & \\  & **(mm)** & **(mm)** & & & & & & & & & & & **Stretch Forming** & **FLC** \\ \cline{2-13}  & **0.4** & **0.3** & **0.5** & **1.0** & **1.5** & **0.5** & **1.0** & **2.0** & **0.5** & **1.0** & **2.0** & \\ \hline TS275 & x & & & & & & & & & & & x\({}^{1}\) & x\({}^{1}\) \\ \hline TS290 & & x & & & & & & & & & & & x\({}^{3}\) \\ \hline RG & x & & & & & & x & & & & & x\({}^{1}\) & \\ \hline RGUD & & x & & & & & & & & x & & x\({}^{1}\) & \\ \hline MPM01 & x & & & x & & & & & & & & x\({}^{2}\) & \\  & x & & & & x & & & & & & x\({}^{2}\) & \\ \hline MPM02 & & x & x & & & & & & & & & x\({}^{4}\) & x\({}^{4}\) \\  & x & & & x & & & & & & & & x\({}^{4}\) & x\({}^{4}\) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Test plan for stretch forming and FLC investigation.  
The effect of the cover/core adhesion condition of the stretch-forming behaviour was investigated following 2 approaches:  
1. Semi-products of the sheet-like specimens of MPM and FML are prepared via hot pressing, as is described in Section 2.1 (i.e., the stretch forming is performed on pre-bonded sandwich panels).
2. The stretch forming of FML without pre-bonding is performed at 235 \({}^{\circ}\)C, i.e., the separated cover and core layers are pre-treated, according to Figure 2, and heated up in an external furnace without hot-pressing. The aim of the approach is to investigate the effect of the one-step forming and bonding condition on the stretch forming of the sandwich panels. In this case, at a temperature above the melting point of PA6, free sliding and shearing of the fibre-reinforced polyamide core layer between the two metallic skin sheets is enabled during the forming step. The trial without pre-bonding resembles the one-step thermoforming method that is earlier developed [(43; 44)].