## 1 Introduction  
The design and application of sandwich materials gain their advantages from its lightweight potential and adapted mechanical properties in comparison to mono-materials. Nowadays, different categories of sandwich materials have been developed. The first category is the one without fibre reinforcement (metal/polymer/metal (MPM)) such as Alucob  
can be found in its core layer properties, where the PA-PE blend combines the formability (due to PE) with the better thermal stability (due to the higher melting pointing of PA). This make Litecor(r) more suitable for the cataphoretic painting while processing at a temperature of approximately 200 \({}^{\circ}\)C. Thus, this MPM has a great possibility to be applied as door and roof structures in body-in-white (BIW), for example. The second kind of the sandwich material is the combination of metal cover and fibre-reinforced polymer core, the so-called fibre metal laminates (FML). A traditional FML is composed normally of metal sheets (cover sheets or even interlayer ones) and fibre-reinforced epoxy resin, providing sandwich structures with excellent fatigue, impact and damage tolerance properties and low density [3]. For instance, ARALL(r) (Al/Aramid-fibre-resin/Al) is developed and is applied as wing panel materials on the Fokker F-27 in the 1980s [4]. Further products of these kinds of FML are GLARE(r) (Al/Glass-fibre-resin/Al) and CARALL(r) (Al/Carbon-fibre-resin/Al), finding increasing application potential in aerospace and automotive industries [5]. For example, GLARE(r) has been further improved with the substitution of aramid fibres with glass fibres to minimise the possibility of failure when used as fuselage material of Airbus A380 [4; 6; 7]. Since the beginning of the 21 st century, further metal sheets, such as steels [8] and magnesium (Mg) alloys, have been used as cover layers to construct FML structures [9]. However, these thermoset-based FML are always accompanied with cost-intensive manufacturing techniques that have too-high cycle times for automotive series applications, as the curing and consolidation of resin in autoclaves due to its nature require elevated processing temperature and pressure for a relatively long period. In contrast, newly-developed thermoplastic-based FML offers equivalent mechanical properties, such as thermoset-based FML, and has an advantage of formability, recyclability and lower process cycle time, leading to cost-reducing production and time-efficiency [10; 11]. For instance, the Leika(r) project, a research project of the \"Research and Technology Centre for Resource Efficient Lightweight Structures for Electromobility\" (FOREL), has developed several components and structures by combining steel/Mg sheets and fibre reinforced thermoplastic, as well as the advanced forming technologies to produce sandwich structures with approximately 24% weight saving compared to the reference floor assembly of e-vehicles, while maintaining the same performance in various crash load cases [12]. Moreover, several research works or concepts by using the thermoplastic-based FML as roof structures, backrest of a backseat and battery package boxes in e-vehicles are carried out by [12; 13; 14; 15]. Other thermoplastic-based FML examples are developed based on thermoplastic matrix like PA6 and PEEK (Polyetheretherketone), such as CAPAAL(r) (Al/Carbon-fibre-PA6/Al) and CAPET(r) (Titanium/Carbon-fibre-PEEK/Titanium) [16; 17; 18]. Conventional lay-up techniques are regularly used to produce the thermoset-based FML with simple geometry [19; 20]. However, depending on the application, the sandwich panel should provide the possibility to be formed into complex-shaped components to be applied in aircraft and automobile sectors. Thus, satisfactory sheet forming potential should be fulfilled under, for instance, deep drawing, draw bending and stretch forming conditions. From those, stretch forming is being widely applied in automotive industries.