## 3 Results and Discussion
### Deep Drawing of the Non-Reinforced MPM  
According to the experimental plan, the effect of the core layer PA6 on the deep drawing of the sandwich panel was investigated subsequently. If only one variable, i.e., the core layer thickness, was varied, it could be observed that the drawing force increased with in  
Figure 11: Deep drawing behaviour of a mono-organosheet RG: (**a**) force–displacement curve; (**b**) failure mode; (**c**) simulation images of deep-drawn RG1.0 with different holding forces and die radii. creasing core thickness. In addition, the MPM demonstrated good drawability in the range of 6 kN to 100 kN, with no remarkable defects such as wrinkling or cracking, indicating the large flexibility of the MPM's working window under deep-drawing conditions, as shown in Figure 12a. However, due to the high tensile stresses exerted by the soft polymer core on the outer steel sheet, the MPM showed differences in the strain evolution of the inner and outer steel sheets compared to that of the monolithic steel sheets. As shown in Figure 12b, the outer steel sheet of MPM\({}^{02}\)-PA0.5 gained further extension in the \(\varepsilon_{2}=\)0 and \(\varepsilon_{2}=\frac{1}{2}\varepsilon_{1}\) directions, which corresponded to the plane strain and biaxial tension states in the Nakajima test, respectively [39]. In the \(\varepsilon_{1}=-\varepsilon_{2}\) direction, on the other hand, the strain evolution was similar to that of the monolithic steel sheet. This phenomenon became more pronounced as the thickness of the core layer increased, as can be seen in Figure 12c. It can be observed that the strain of the outer cover steel sheet in the \(\varepsilon_{2}=\frac{1}{2}\varepsilon_{1}\) direction for MPM\({}^{02}\)-PA1.0 exceeded its corresponding FLC curve, indicating a much higher probability of cracking. The MPM\({}^{02}\)-PA0.5, on the other hand, showed a strain distribution that was still within the safety zone compared to the corresponding FLC curve. Therefore, the formability of the MPM decreases with increasing core thickness. The strain distribution of a cross-sectional line through the centre pointof the drawn cups enabled further analysis of the effect of the core layer PA6 on the strain evolution of the sandwich panel. Firstly, the major strain values were compared for the drawing depths of 20 and 40 mm. It can be seen that at a drawing depth of 20 mm, the strain evolution first  
Figure 12: (**a**) Force–displacement curves of MPM\({}^{02}\) with different core thicknesses; (**b**) strain evolution of the inner and outer steel sheets of MPM\({}^{02}\)-PA0.5; and (**c**) strain evolution of the outer steel sheet of MPM\({}^{02}\) with different core thicknesses and their corresponding FLC curves. appeared in the area of the punch edge (in the range of approx. 50 to 150 mm along the section line). Subsequently, as the drawing depth increased to 40 mm, the strain evolution in the die edge region developed rapidly (see Figure 13a). In contrast, by comparing the strain evolution of TS290 at both drawing depths (25 mm and 50 mm), it can be seen that the maximum major strain values always appeared in the die edge region and doubled as the stroke increased (see Figure 13b). Due to the high tensile stresses exerted by the soft polymer core, the strain evolution of the MPM increased in the punch-edge region and decreased in the die-edge region [13]. Combining Figure 13a,c, it can be concluded that at a 20 mm stroke, the main strain evolution was concentrated at the punch edge, while the corresponding strain distribution was in the \(\varepsilon_{2}=\frac{1}{2}\varepsilon_{1}\) direction. It was the acceleration of the strain evolution around the die radius as the stroke was increased that led to the rapid expansion of the strain distribution along the \(\varepsilon_{1}=-\varepsilon_{2}\) direction in Figure 13c.