## 3 Results and Discussion
### Stretch Forming
#### 3.1.3 Force and Strain Evolution of FML
Therefore, earlier fracture of inner steel sheet occurs. This is because the contact behaviour between the cover sheets and the core layer at 235 \({}^{\circ}\)C can be understood as frictional relative sliding [47], and the friction coefficient between the cover and core layers directly affects the fracture behaviour of the cover sheets [41]. Figure 14: Force evolution of FML under stretch forming at (**a**) RT and (**b**) 235 \({}^{\circ}\)C for different core thicknesses and organosheets; (**c**) DIC images and force evolution of FML01-RGUD2.0 at 235 \({}^{\circ}\)C; (**d**) crack patterns of FML01-RG2.0 and FML01-RGUD2.0, (**e**) of FML01-RG0.5, (**f**) of FML01-RG1.0 at 235 \({}^{\circ}\)C; (**g**) specimen picture at RT and (**h**) at 235 \({}^{\circ}\)C.  
Besides the failure modes appeared on the cover sheets, it is also essential to describe the deformation behaviour of the core material under these warm stretch forming conditions. The cover sheets were removed to be able to detect the distortion of the core layer. Because the cover sheet is tightly bonded to the core layer after the specimen has cooled, in order to remove the cover sheet, it is necessary to reheat the specimen statically to the melting point of PA6 in an external furnace to soften the adhesion layer before separating the outer cover sheet of the FML manually. It can be seen from the specimens' images in Figure 16 that the fibres in RG and RGUD are not cracked, yet the distortion (marked by red arrows) of them occurs in a die edge area near the drawing beads, which is in accordance with the stretch-forming behaviour of the mono-organosheet at 235 \({}^{\circ}\)C, as described in Figure 12. For core material RG2.0 shown in Figure 12b, the shear angle was approximately 90\({}^{\circ}\) at the top. Progressing down from the top side along the weft and warp directions, the shear angle remained unchanged. Inversely, along biaxial direction 45\({}^{\circ}\), the shear angle changed to be larger due to the easy-occurring shear deformation in the 45\({}^{\circ}\) direction [21]. In comparison to RG, the shear deformation of RGUD was different, there was a slight second transverse normal strain for RGUD in the weft direction, which could have been expected [48; 49]. The mobility of fibres in the weft direction was less restricted. Moreover, no wrinkling defects were detected. Unlike the strain evolution of MPM at RT, the strain distribution of FML was located and oriented along or vertical to the RD of steel sheet. The difference in crack orientation for FML01-RGUD and FML01-RG was dependent on the fibre fraction in the weft and warp directions of their cores. As can be seen in Figure 17a,c, crack propagation was vertical to the RD for FML01-RGUD and parallel to RD for FML01-RG, respectively. The reason is that the strengthening effect of organosheet is greater in the weft and warp directions, causing the largest tensile stress on the outer cover sheet. However, the properties of RG in the weft and warp directions are equal. In this case, the cracking behaviour of steel cover sheet was dominant. Those for the RGUD, however, were different, as the strengthening effect of  
Figure 15: Thickness distribution of (**a**) FML01-RGUD0.5 and (**b**) FML01-RGUD2.0 after warm stretch forming at 235 \({}^{\circ}\)C.  
RGUD in the warp direction was more highlighted. Thus, the cracking propagation is more likely vertical to the RD of the steel sheet. In contrast, the influence of the organosheet types on crack orientation was ignorable at higher temperature, i.e., 235 \({}^{\circ}\)C. Moreover, the strain evolutions of both FML01-RG and FML01-RGUD were more extended and homogeneously distributed at 235 \({}^{\circ}\)C compared to those at RT, as can be seen in Figure 17b,d. The cracking propagation is more likely along the RD of steel cover sheet, which resembles the cracking behaviour of the monolithic steel sheets under the stretch-forming condition.