## 3 Results and Discussion
### Deep Drawing of the FML
The reduction in drawing force at 200 \({}^{\circ}\)C or 235 \({}^{\circ}\)C compared to RT was due to the softening of the core material and the consequent increase in fibre mobility in the PA6 matrix. Furthermore, the drawing force increased with the increasing core thickness at 200 \({}^{\circ}\)C and remained almost constant with the increasing core thickness at 235 \({}^{\circ}\)C. The drawing force at 200 \({}^{\circ}\)C (red lines) decreased more rapidly from the drawing depth of 25 mm compared to that at 235 \({}^{\circ}\)C (green and black lines). This was due to fibre fracture in the flange area; this phenomenon is discussed later. It should be mentioned that a sliding of the interlayer could occur at 235 \({}^{\circ}\)C, which is above the melting point of PA6. This is due to the thermoplastic nature of PA6 as well as the adhesion promoter based on PA6, which is degraded at high forming temperatures and forms adhesion strength again after cooling and recrystallization. As observed from the strain evolution analysis, the differences between warm deep drawing and that at RT were significant. The strain in the outer steel sheet developed in the \(\varepsilon_{1}=-\varepsilon_{2}\) direction; however, that in the inner steel sheet developed in the \(\varepsilon_{1}=-2\varepsilon_{2}\) direction, showing a uniaxial tension state as in Nakajima's test [39] (Figure 17a,b). The higher strain evolution in the inner steel sheet was the opposite of that for MPM under deep-drawing conditions. The reason is that under warm deep drawing conditions, the inner steel sheet not only showed wrinkling, but also experienced higher tensile stresses caused by the fibre-reinforced core at the die radius edge (Figure 17c). In addition, the  
Figure 16: (**a**) Wrinkling and waviness in warm deep-drawn FML\({}^{02}\)-RG0.5; and (**b**) drawing force of FML for different cover sheets, core thicknesses, and forming temperatures. Figure 15: Strain evolution in (**a**) FML\({}^{01}\)-RG0.5 and (**b**) FML\({}^{02}\)-RG0.5 under deep-drawing conditions at RT. strain evolution on the outer steel sheet decreased slightly with the increasing temperature and decreasing core layer thickness at 200 \({}^{\circ}\)C (Figure 17d,e). Since the organosheet did not melt completely at 200 \({}^{\circ}\)C, it still stimulated tensile stresses on the steel sheets, leading to an increase in the strain evolution. This difference gradually disappeared as the temperature rose to 235 \({}^{\circ}\)C (Figure 17f). Due to the rapid mobility of the fibrous structure after melting of the PA matrix, their effect on the strain distribution in the outer steel sheet could be neglected. The corresponding FLC levels shown in Figure 17b,d were taken from the previous study, indicating that the typical strain evolution of FML at elevated temperatures was still in a safe forming zone. Figure 17: Strain evolution in the outer and inner steel sheets of FML at (**a**) 200 \({}^{\circ}\)C and (**b**) 235 \({}^{\circ}\)C; (**c**) photogrammetry images; (**d**) strain evolution in the outer steel sheet of FML at 200 and 235 \({}^{\circ}\)C, with different core layer thicknesses at (**e**) 200 \({}^{\circ}\)C and (**f**) 235 \({}^{\circ}\)C.  
In addition, the strain evolution along the cross-sectional lines showed the differences in the various material combinations and forming temperatures. In contrast to the strain evolution at RT, the strain evolution at 235 \({}^{\circ}\)C was significantly extended, as shown in Figure 18a. Although the flange area of FML showed anisotropy at RT, FML\({}^{01}\) did not exhibit different strain values in either direction at 235 \({}^{\circ}\)C, i.e., reduced anisotropy and homogeneous strain distribution. In contrast, FML\({}^{02}\) still showed higher strain values at the die radius edge in the 45\({}^{\circ}\) direction. This was mainly because TS290 itself is anisotropic, whereas TS275 is almost isotropic (see the tensile test results in Figure 1a).