## 1 Introduction
Some studies reported to use chopped fibres for a reinforced core in the thermoplastic-based FML under RT, in which the formability of FML is proven to be good due to the poor mechanical properties of GF-Polyurethane (GF-PU) as a core layer in this Al/GF-PU hybrid system [36]. The GF-PU used in the FML has a high elongation at failure of \(\sim\)15% and absolute low stiffness, i.e., elastic modulus of 1.7 GPa [36]. A similar study to determine the FLC of FML based on Al5005/self-reinforced PP/Al5005 at RT is performed in [37], in which the strain evolution of thermoplastic-based FML was found to be larger than that of monolithic Al sheets and therewith a higher FLC level and better formability of thermoplastic-based FML. However, the used self-reinforced PP is the Curv\({}^{\text{TM}}\) from Propex Fabrics, which has an elongation at failure of 20%, tensile strength of 120 MPa and tensile modulus of 4.2 GPa [38]. In short, the above-mentioned studies chose fibre-reinforced cores with good formability for improving the formability of FML due to its nature of chopped fibres or self-reinforcement, but losing its integral strength. Furthermore, there is still a lack of research on the FLC determination of thermoplastic-based FML consisting of metal and organosheets. Since the nature of organosheets dictates the poor formability and good stiffness of such FML at RT, most studies have focused on warm thermoforming processes. However, the results are not abundant for explaining the accompanying defects and mechanism of improving foamability, such as the investigation of thermoforming FML based on steel/GF-PP/steel at two forming temperature levels of 140 \({}^{\circ}\)C and 170 \({}^{\circ}\)C, especially when taking into account that the melting point of PP is 160 \({}^{\circ}\)C [39]. Its result presented significant thickness irregularities of the composite core and flow of the matrix out of the die is detected by stretch forming at higher temperatures, i.e., 170 \({}^{\circ}\)C.  
From the literature survey, rare studies have been carried out to determine the stretch forming-behaviour and FLC levels of thermoplastic-based FML, which is composed of steel covers and composite core. The aim of this study is to investigate the influence of material combinations, forming conditions and thickness ratios on their forming behaviour, especially the FML based on organosheets. The structure of this study is illustrated in Figure 1, describing the study sequence and the utilized investigation methods. Firstly, the material selection, including the mechanical characterization and semi-finished sandwich panel preparation for stretch forming, are described in Section 2.1. A method to determine the FLC levels and the corresponding sample geometries are also introduced. Some interesting technical issues and the development of new warm forming tool to perform the desired tests are introduced in Section 2.2. In Section 2.3, we provide a description of the experimental plan, where the mono-materials and laminates without core reinforcement (MPM) are considered as a benchmark for the study. In addition, the method to determine the FLC levels is explained in Section 2.4. Further, the results are described and analysed in Section 3, which is divided into two parts: Section 3.1 for describing the results of stretch-forming behaviour and Section 3.2 for describing the results of FLC determination. The conclusion is offered in Section 4.