Influence of the sampling procedure on the mechanical forming limits in the characterization of sheet metal foils

The aviation industry is committed to reducing its impact on the climate to meet political targets and enhance social acceptance. In particular, the ambitious climate protection targets of Flightpath 2050 require disruptive and revolutionary propulsion concepts. The use of fuel cell systems is one of these propulsion concepts and enables near carbon-neutral operation. A key component of the fuel cell is the bipolar plate, which is manufactured using sheet metal foil with thicknesses ≤ 100 µm. Failure-free forming of the sheet metal foil poses a particular challenge due to the high risk of cracking during forming. To evaluate the failure-free formability of the sheet metal foil, a numerical representation of the forming process is essential. A key parameter is the material-specific failure criterion (forming-limit-diagram (FLD)), which can be determined using Nakajima tests. Due to the low material thickness of the sheet metal foil, it is currently questionable what influence the sampling procedure of the required test specimens has on the resulting mechanical characteristic values. The aim of this paper is therefore to investigate the influence of the sampling procedure on the resulting failure limits in the Nakajima test, considering a corrosion resistant austenitic stainless steel of type 1.4404. The results of this paper show that there is a strong correlation between the selected removal method, the resulting edge zone quality and thus also the determined material properties in the characterisation of foil materials. Machining and wire erosion of the foil using dummy sheets have been identified as promising removal methods.