## 3 Results and Discussion  
### Deep Drawing of the Mono-Materials  
According to the experimental plan, the deep-drawing behaviour of the monolithic steel sheets was first analysed in order to further determine the influence of the core layer PA6 and RG on the forming behaviour of the steel sheets in the sandwich panels. Figure 9a shows the force evolution of TS290 for deep-drawing conditions with different forming temperatures and holding forces. It can be seen that the drawing force decreased as the temperature increased and the holding force decreased, as could be expected. In this respect, the drawing behaviour of TS275 was similar to that of TS290.  
In Figure 9b, the strain evolution on both sides of TS290 at RT with major (\(\epsilon_{1}\)) and minor (\(\epsilon_{2}\)) strains converging to \(\epsilon_{1}=-\epsilon_{2}\) was similar to the pure shear deformation behaviour in the Nakajima test [39]. By means of the FLC curve for TS290 at RT (marked with a yellow curve), it can be judged that the strain distribution on the steel sheet remained within the safety zone up to a drawing depth of 50 mm. This highlights the good and deep drawability of the steel sheet TS290, and that the strain evolution on both of its sides was almost identical. The FLC curves in Figure 9b, as well as the ones in the following figures, were all determined at a test speed of 1.5 mm/s in the previous study [31].  
In addition, the anisotropy of TS275 and TS290, obtained earlier by tensile tests, was reflected by their strain evolution under deep-drawing conditions. Figure 10 shows the strain evolution along three cross-sections at \(0^{\circ}\), \(45^{\circ}\), and \(90^{\circ}\) through the sample's centre  
Figure 8: The mesh and boundary conditions of the model.  
Figure 9: (**a**) Force–displacement curves of TS290 at different holding forces and temperatures, deep-drawn up to 40 mm. (**b**) Strain evolution on both sides of TS290 at RT, drawn up to 50 mm.  
point. It can be seen that the strain distribution for TS275 was almost identical in the different directions (Figure 10a), as is similar to the previous tensile tests. In contrast, the strain distribution in the flange region for TS290 was significantly higher in the 45\({}^{\circ}\) direction (see Figure 10b). This was due to its anisotropy in the 45\({}^{\circ}\) direction causing more strain evolution and plastic deformation.  
Figure 11a shows that as the organosheet RG thickness increased, the drawing force was roughly doubled. However, at a drawing depth of about 23 mm, a plateau in the force-displacement curve occurred for both thicknesses of RG, which was due to fibre fracture in the organosheet core, marked by yellow arrows in Figure 11b for the RG1.0 sample. By means of simulation, it can be observed that the RG1.0 was deep-drawn at \(F_{h}=45\) kN and \(r_{d}=15\) mm, and when the stroke reached 25 mm, RG1.0 in the flange and die edge area showed significant stress concentration. By increasing the holding force (100 kN) and reduction in the die radius (10 mm), significant wrinkling occurred due to the large circumferential compressive stress and the restricted plastic deformability. As no fracture criterion was defined for the RG core layer, only areas of stress concentration and wrinkling could be replicated in the simulation image, but fibre fracture could not, as shown in Figure 11c. However experimentally, fracture occurred as the fibres in the flange region were compressed and wrinkled, along with the compression failure of fibres and matrices such as fibre micro-buckling, matrix shear failure, or fibre failure, leading to a decrease in the punch force [40].  
Figure 10: Strain evolution of (**a**) TS275 and (**b**) TS290 in different directions.