## 3 Results and Discussion
### FLC Determination  
The FLC for the monolithic steel sheets and the sandwich panels were determined. Firstly, the FLC for monolithic steel sheets TS275 and TS290 at different temperatures are shown in Figure 19. In Figure 19a, an example of the FLC for TS290 assessed by the time-dependent line fitting method is shown, as described in Figure 9. In Figure 19b, the FLC level of TS290 was kept almost constant for the different test forming temperatures with only a slight tendency to decrease with increasing temperature in the middle and left parts of the FLD, where the plane and tensile strain paths, respectively, dominated. However, there was a slight improvement of the FLC in the right side, i.e., in the biaxial stretching strain. In contrast, as shown in Figure 19c, the FLC level of TS275 decreased significantly in the tensile (left side) and plane strain (at minor strain \(\varepsilon_{2}\) = 0) regions with increasing temperature, which was consistent with the tensile properties of this steel grade, as shown in Figure 3a. Figure 19: _Cont._  
Figure 18: Force (**a**) and strain (**b**) evolution of FML01-RG2.0 with/without prebonding. The results in Figure 19 can be interpreted with the help of the strain distribution over a centreline section, as shown in Figure 20, where the test results in terms of the strain evolution for TS275 and TS290 of hourglass-shaped specimens with a central width of 50 mm at RT and 235 \({}^{\circ}\)C are illustrated. The following characteristic differences can be stated:  
1. At RT, the development of the plastic zone of both TS290 and TS275 revolves around the centre of the specimen, where the strain evolution along the cross-section lines (the red dotted lines in the figure) shows a smooth progression (Figure 20a,b). 2. However, at 235 \({}^{\circ}\)C intermittent slant strain localization bands are detected, indicating the presence of PLC flip-flopped bands for TS275 at elevated temperatures [42] and leading to a reduction in elongation at failure. In contrast, the strain evolution of TS290 at 235 \({}^{\circ}\)C is similar to that at RT, with no detected slant strain localization bands, see Figure 20b. Figure 19: FLC of TS290 (**a**) at RT with the evolution of major and minor strains at different strain paths, (**b**) at different temperatures and (**c**) FLC of TS275 at different temperatures. Furthermore, Figure 21 shows the FLC levels of MPM02 with different core layer thicknesses at RT and 200 \({}^{\circ}\)C. From Figure 21, it can be noted that the FLC level of MPM02 decreased slightly with increasing core thickness, which was in coincidence with the stretch-forming behaviour of MPM01 described in Section 3.1.2. As shown in Figure 21, no significant improvement in the FLC levels of MPM02 was found with increasing temperature, stating that cold forming is satisfactory for processing MPM panels. Finally, Figure 22 shows the results of the FLC determination for FML01 and FML02 to determine the effect of steel sheet properties on the FLC level. Moreover, the core thicknesses of 0.5 mm and 1.0 mm have been chosen, while the 2.0 mm core was neglected due to the result found earlier that the inner steel sheet cracks earlier making the determination of the FLC not reliable. In principle, to determine the FLC, the failure strain can be obtained by the DIC method for the outer layer. In this case, the determined FLC will be not reliable as the outer steel sheet is still within the safe forming zone; however, the inner cover sheet is already cracked. As can be seen from Figure 22, the FLC level of FML02 increased significantly with increasing temperature up to 235 \({}^{\circ}\)C, compared to RT. The effect of core thickness (0.5 and 1.0 mm) on FLC levels at both RT and 235 \({}^{\circ}\)C was negligible, which was similar to the stretch-forming behaviour of FML. Moreover, the FLC levels of FML at RT were found to be very low and the minor strain values positive for all  
Figure 21: FLC levels of MPM02 **(a)** with different core thicknesses and (**b**) at different temperatures.