## 3 Results and Discussion
### Validation of the Simulation Results and Failure Modes  
After analysing the force and strain evolution of the different materials under deep-drawing conditions, this section focuses on the defects that appear in the FML obtained numerically to be validated with the experiments. After raising the blank holder force to 100 kN and lowering the die radius to 10 mm, the FML\({}^{02}\)-RG0.5-235 \({}^{\circ}\)C (composed of the thin 0.3-mm TS290 steel) still showed severe internal wrinkling and waviness in the outer steel sheet (Figure 19a). However, these conditions showed significant improvements for FML\({}^{01}\)-RG0.5 using the thicker TS275 (0.4 mm) (Figure 19b). This means that with an increase in the cover/core thickness ratio together with the lower gap size between the punch and inner diameter of the die, the waviness of the outer steel sheet and the wrinkles of the inner steel could be reduced, as the thicker cover sheets were able to withstand the circumferential compressive stresses due to their higher geometric stiffness. As the temperature was reduced below the melting point of PA6, i.e., at 200 \({}^{\circ}\)C, the wrinkling of the inner steel sheet was further reduced without a significant reduction in formability (see Figure 19c). Moreover, by increasing the core layer thickness at 200 \({}^{\circ}\)C, the wrinkling of the inner steel sheet was still present and no significant improvement was indicated (Figure 19d). The wrinkling behaviour of the FML's inner cover sheet at 200 \({}^{\circ}\)C can be attributed to the failure of the core material, as illustrated in Figure 20, where fracture of the fibres at 200 \({}^{\circ}\)C at a drawing depth of 25 mm can be observed. Once fibre fracture occurred, the backing support for the inner steel plate was reduced in the fractured area and the inner steel sheet began to wrinkle. As can be seen in the middle image of Figure 20, fractures similar to those seen in monolithic organosheets when deep-drawn at RT appeared in the organosheet in the flange and die edge regions due to high circumferential compressive stresses. Considering the finite element simulation results, it can also be seen that the FML\({}^{01}\)-RG0.5 showed no coincident fibre wrinkles in the flange area in the warp or weft directions with the experimental results in the case of the contact condition of Set 1 in Table 7 (see Figure 21). However, by considering the tangential frictional contact behaviour (Sets 2 and 3) and the cohesive contact behaviour between the FML layers at 200 \({}^{\circ}\)C, all simulation results resulted in wrinkling in the warp and weft directions of the organosheet RG in the flange and die edge areas. Moreover, the degree of the wrinkling of contact Sets 3 and 4 was more severe than that of Set 2 due to the higher stress values indicated in the simulation images. Figure 19: Wrinkling in the flange region and waviness in the sidewall regions of FML after deep drawing at 200 and 235 \({}^{\circ}\)C with a drawing depth of 40 mm: (**a**) FML\({}^{02}\)-RG0.5-235 \({}^{\circ}\)C, (**b**) FML\({}^{01}\)-RG0.5-235 \({}^{\circ}\)C, (**c**) FML\({}^{01}\)-RG0.5-200 \({}^{\circ}\)C, and (**d**) FML\({}^{01}\)-RG1.0-200 \({}^{\circ}\)C.  
Figure 20: Fibre fracture in the core and the wrinkling of the inner steel sheet at a stoke of 25 mm. By comparing the contact of Sets 3 and 4, a significant difference in the stress evolution in the punch edge area was observed, where the stress distribution was more homogeneous in the circular direction, but that of Set 3 showed obvious localization in the warp and weft directions. As the real contact between layers of the semi-finished FML panel could not be simplified as frictional sliding from the beginning of the warm deep-drawing process, the results of Set 4 through mixed-mode delamination of cohesive elements were found to be more reliable. As can be seen in Figure 22a, the cohesive element deletion first occurred at the side wall area at a drawing depth of 10 mm.