## 4 Conclusions and Outlook  
In this paper, the influences of the material properties (cover sheets: TS275-0.4 mm and TS290-0.3 mm, and core layers: PA6, RG), thicknesses of the core materials (0.5 and 1.0 mm), tool geometries (die radius and gap size), and forming temperatures (RT, 200 \({}^{\circ}\)C and 235 \({}^{\circ}\)C) on the deep-drawing behaviour of several thermoplastic-based FMLs are characterized, using mono-materials and MPM sandwich panels as benchmarks. Based on the obtained results, the following conclusions can be made:  
* The strain evolution of a steel sheet is influenced by its anisotropy. The effect of the core layer PA6 on the MPM strain evolution is mainly due to the tensile stress it exerts on the outer steel sheet at the punch edge, resulting in an increase in strain evolution along the \(\varepsilon_{2}=\frac{1}{2}\varepsilon_{1}\) direction and leading to earlier failure with the increase in the core thickness;
* At RT, the deep-drawability of RG is limited by failure in the flange and die edge regions, attributable to in-plane tangential deformation and fibre compression fracture due to the acting circumferential compressive stresses;
* At RT, the tensile stresses exerted by RG on the outer cover of FML exhibit directional and localised fracture occurrence at the punch edge due to the lack of fibre mobility and anisotropy;
* At RT, the influence of the core layer RG on the strain distribution of the FML in the flange region dominates. This influence decreases with the increasing forming  
Figure 24: Thickness distribution of FML\({}^{01}\)-RG under deep-drawing conditions at different temperatures, with a drawing depth of 40 mm. temperature until it is above the PA6 melting point, where the anisotropy of the cover steel sheet itself dominates;
* At 235 \({}^{\circ}\)C, wrinkling of the inner cover of FML and the waviness of the outer cover can be reduced by increasing the holding force and the cover/core thickness ratio; reducing the die radius and gap size; as well as lowering the forming temperature (here, 200 \({}^{\circ}\)C) to just below the PA6 melting point;
* The minor wrinkling of the inner cover of FML at 200 \({}^{\circ}\)C results from in-plane shear and fibre fractures of the RG in the warp and weft directions due to their limited fibre mobility compared to 235 \({}^{\circ}\)C;
* Warm deep drawings at 200 \({}^{\circ}\)C and 235 \({}^{\circ}\)C can both achieve good improvement of the drawability of the thermoplastic-based FML with an increase in depth by at least 200%; however, the thickness distribution uniformity of FML at 200 \({}^{\circ}\)C is better than that at 235 \({}^{\circ}\)C.  
Moreover, for deep drawing of thermoplastic-based FMLs, varying the process parameters of thermoforming has great potential to improve the product quality of FMLs. Further research regarding the cover/core thickness ratio and the selection of a suitable forming temperature are both possible future research directions. Of greater interest is the selection of fibre-reinforced polymers, especially their weaving style and fibre volume content, both of which have an influence on the mechanical properties and the mobility of the fibres in the molten state of the matrix. Furthermore, for the purpose of forming thermoplastic-based FMLs, reasonable damage models for the FML layers and a cohesive model for interlaminar interaction need to be developed to describe their fracture and delamination behaviours at different temperatures, which will play a decisive role in further numerical research; predicting their forming behaviours; and, thus, improving the thermoformability and final product quality of thermoplastic-based FMLs. Conceptualization, W.H., M.H., G.Z. and H.P. ; methodology, W.H. and M.H. ; software, W.H. ; validation, W.H. ; investigation, W.H. and M.H. ; resources, G.Z. and H.P. ; data curation, W.H. and M.H. ; writing--original draft preparation, W.H. and M.H. ; writing--review and editing, M.H., G.Z., A.C. and H.P. ; visualization, W.H. ; supervision, G.Z., A.C. and H.P.