## 3 Results and Discussion
### Stretch Forming
#### 3.1.3 Force and Strain Evolution of FML  
The stretch forming of FML was carried out at two temperature levels: RT and 235 \({}^{\circ}\)C. Figure 14a illustrates the results of the drawing force for FML01 with varied core thicknesses and two kinds of organosheets, RG and RGUD. At RT, with increased core thickness, the stiffness increased, but the maximum drawing depth, and therefore the maximum drawing force, were reduced. This effect of core thickness in the case of FML01-RGUD was similar to FML01-RG. Additionally, no remarkable difference in the drawing force between FML01-RG/-RGUD was observed. Moreover, the outer cover sheet failed firstly due to the higher tensile stress generated by the stiff organosheet at RT, as expected. Figure 14: _Cont._Regarding the results of the warm stretch forming of FML01, it can be clearly seen in Figure 14b that the formability of FML01 significantly improved at 235 \({}^{\circ}\)C, with molten PA6 matrix in the organosheet core. No remarkable difference in drawing depth and stiffness for the FML01 with varied core thicknesses and organosheets RG and RGUD were stated. However, by stretch forming at 235 \({}^{\circ}\)C, an earlier cracking of the inner steel sheets of FML01 with the 2.0 mm core layer was detected. As can be seen in Figure 14b, points (1) and (2) indicate two force drop locations for both the FML01-RGUD2.0 and FML01-RG2.0. For both at 235 \({}^{\circ}\)C, the first force drop took place without detecting any visual cracking on the outer steel cover sheet (as test end criterion). Therefore, the test was continued until detecting the failure of the outer layer. Upon interpreting the force-displacement progress by DIC unit and the opening size of the crack on the inner and outer layers of FML01 with a core thickness of 2 mm at 235 \({}^{\circ}\)C, it can be stated that cracking took place on the inner cover layer first, as the crack opening size in the inner steel sheet was obviously larger than that in the outer one, as can be seen in Figure 14c,d. This was confirmed repeatedly through the three experimental runs. Inversely for the FML with the thinner core, the cracking pattern of FML01-RG0.5 appeared at the outer cover sheet firstly, and that of FML-RG1.0 appeared in both cover sheets (inner and outer) almost simultaneously, as can be seen in Figure 14e,f. The reason for this may be due to changes in the dynamic friction state between the layers, leading to changes in the tensile stress state on the inner and outer cover sheets, which will be more deeply investigated by means of finite element simulations in a subsequent article. Normally, the outer cover sheets are subjected to higher tensile stress under stretch forming condition, therefore they fail firstly, especially at RT. At 235 \({}^{\circ}\)C, based on the current results, it can be stated that the location of crack initiation and propagation depends on the core thickness; with increasing core thickness, the crack initiation is transferred from the outer cover sheet into the inner one. Furthermore, specimens' pictures for both cold and warm stretch forming are shown in Figure 14g,h, in which the drawing depth was extended up to 33 mm at the warm forming condition compared to only 12 mm at RT, indicating an improvement of almost 300%. Further analysis about the thickness distribution of FML01-RGUD with two core thicknesses (0.5 mm and 2.0 mm) after the stretch forming at 235 \({}^{\circ}\)C is illustrated in Figure 15. As can be seen, the thickness change within FML01-RG0.5 is not remarkable. Conversely, more irregular thickness distribution of the FML01-RG2.0 was found with a thinning of the core in zone (1) and (3) and significant thickening in zone (2), which was due to the flow of molten PA6 matrix from zone (1) and (3) towards the zone (2). In this regard, the total thickness in zone (2) of the FML reaches approximately 5.0 mm, where the dynamic interlayer friction between outer-cover/core layers, as well as inner-cover/core layers, may be reduced accordingly.