## 2 Materials and Experimental Work  
### Materials and Sandwich Production  
In this study, two thicknesses (0.3 mm and 0.4 mm) of electrolytic-galvanised steel sheets were used as cover sheets for the production of sandwich panels, namely, TS290 and TS275, which were supplied by Thyssenkrupp Steel Europe (Duisburg, Germany). These unalloyed low carbon steel grades are typical of those used in the food packaging industry and, more recently, also in the automotive industry. For the non-reinforced sandwich panels, PA6 was used as the core material (supported by Infiana Germany GmbH & Co. KG, Forchheim, Germany), the thickness of which could be varied through stacking and thermal fusing. The organosheet (GF-PA6 from Lanzess Deutschland GmbH, Koln, Germany) was used for the preparation of a semi-finished thermoplastic-based FML, Tepx (abbreviated as RG), containing a fibre volume fraction of 47 vol-% (the fibres were equally distributed in the weft and wrap directions, i.e., 50 vol-% fibres in each weft and warp direction and weaving style of twill 2/2). This material was supplied by Lanzess. A summary of the materials used is given in Table 1.  
The mechanical properties of the mono-materials were firstly determined at a strain rate of 0.008 s\({}^{-1}\) (gauge length of 80 mm) via the video extensometer, as this affects their later forming behaviour. For instance, the anisotropy of the steel sheet influences the strain evolution pattern of the sandwich panel and failure location under deep drawing conditions, e.g., the earing effect. In order to predict and explain this forming behaviour, it is necessary to determine the mechanical anisotropy of the monolithic steel sheets. For this purpose, tensile tests were carried out on TS275 and TS290 at RT in three directions (0\({}^{\circ}\), 45\({}^{\circ}\) and 90\({}^{\circ}\)) to the rolling direction (RD), and their results are shown in Figure 1a. It can be observed that the strength levels of TS275 were similar in all three directions, indicating a higher degree of isotropic behaviour. In contrast, the strength levels of TS290 showed clear differences in the three directions, i.e., the strengths measured in the 0\({}^{\circ}\) and 90\({}^{\circ}\) directions were slightly higher than that in the 45\({}^{\circ}\) direction. Moreover, the mechanical properties along RD of the monolithic steel sheets decreased with the increasing temperature (see Figure 1b), as expected. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials/** & & & & \\
**Abbreviation** & **Thickness (mm)** & **Coating** & **Fibre Content (vol-\%)** & **Supplier** \\ \hline TS275 & 0.4 & Zinc & – & Thyssenkrupp Steel Europe AG \\ TS290 & 0.3 & Zinc & – & Infiana Germany GmbH \& Co. KG \\ PA6 & 0.5 & – & 47 & Lanzess Deutschland GmbH \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of the used materials. As for the core material, more attention was paid to the organosheet RG, which showed a linear elastic behaviour. The ultra-tensile strength was about 340 MPa, and the elastic modulus was 18 GPa in the warp (0\({}^{\circ}\)) and weft (90\({}^{\circ}\)) directions, respectively. Moreover, the tensile behaviour of the RG core layer in the 45\({}^{\circ}\) direction showed higher elongation at failure as well as reduced strength, mainly due to the fact that the fibres were more susceptible to shear deformation in the 45\({}^{\circ}\) direction. This phenomenon has been described previously [28]. The soft core PA6 has a 200% elongation at failure and a tensile strength of 73 MPa, showing rather good formability. This study focuses on the deep drawing of sandwich panels, which were developed arbitrarily for a systematic understanding of the different material combinations and process parameters on their deep drawability. Therefore, firstly, sandwich panels were produced.