## 1 Introduction
In this regard, several aspects about describing the forming behaviour of sandwich materials are missing the available literature, especially for the thermoplastic-based FML, which requires special forming conditions for reaching defect-free components, e.g., the application of warm forming or one-step thermoforming processes [21]. In contrast, it is possible to form flat MPM semi-products into complex components following the technical guidelines for the monolithic sheets, even at RT. However, its forming potential is less than that of the mono-materials of the same total thickness. Those limits further decreased as core layer thickness increased [22]. Moreover, the thermoset-based FML shows inherently poor formability at RT after the curing/solidification of the resin and could not be reheated to improve its formability due to the nature of the resin [23]. The unique way to enhance its formability is to form the blank sheet before the resin matrix is completely cured. For instance, when forming GLARE(r) with a semi-solidified or uncured matrix, the maximum drawing depth increased, but accompanied with severe wrinkling problems and time cost for final curing. [24; 25]. Thus, the thermoplastic-based FMLgained more attention, as it could be formed at elevated temperatures due to the thermoplastic nature of the matrix. For instance, the stretch forming of the thermoplastic-based FML made of Al and a self-reinforced PP composite is studied at varied temperatures [26], in which the strain experienced by the FML system is affected by holding force, stiffness of the composite and interlaminar adhesion state. Further studies about the thermoforming of thermoplastic-based FML are carried out, in which different punch geometries are considered (e.g., circular cup geometry [17], hat geometry [27] and rectangular geometry) [28]. Some previous studies have also demonstrated the feasibility of thermoforming thermoplastic-based FML into the dome or hat profile in one step, namely joining and forming of the blank sheets at the same time, which is proven to be efficient and highly deformable of the thermoplastic-FML based on steel sheets and organosheets (continuous fibre reinforced thermoplastic) [29; 30; 21]. However, the interlayer is subjected to local flowing in their liquid state during forming, leading to an irregular total blank thickness of the shaped part. Besides the wrinkling and irregular thickness distribution, other defects like cracking or delamination are also found [31]. Additionally, the forming limit diagram (FLD) is an essential method for evaluating the formability of sheet materials, which is developed firstly by [PERSON] and [PERSON] [32]. The forming limit curve (FLC) in FLD depicts the deformations on a sheet as major and minor strains and makes it possible to define limit forming ratios. To keep safe forming conditions, the strains appearing on the deformed part must be located below the FLC level. Thus, it is important to determine the FLC of the sandwich materials as well, offering a chance to evaluate the stamping characteristics of them in comparison to the monolithic metal sheets. It is stated that the FLC level of sandwich materials depends on the material compositions and forming conditions. For instance, FLC level of Al/PE/Al sandwich material is higher than that of its monolithic Al sheet at RT [33]. Moreover, a slight increasing tendency of drawing depth was found to increase the core layer thickness [34]. In comparison, the core thickness has no significant influence on the FLC levels for sandwich materials based on steel covers and polymer core, e.g., steel/PP-PE/steel. With increasing core layer thickness, MPM are subjected to failure at lower strains in the stretching region of the FLC [22; 35]. Investigations about the FLC-determination of FML based on metal sheets and organosheets regarding the influence of temperature changes are rarely found in the literature survey.