Simulative and Empirical Investigation of Test Specimen Geometries for the Determination of Forming Limit States in the Tensile-Compression Range for Austenitic Stainless Steel Foil Material

Abstract

In bipolar plate production, extreme thin foil materials are becoming increasingly important due to the trend towards high-energy dense fuel cells. For a better of the material behavior and component failures, finite element simulations are used. In order to achieve an expressive numerical representation of the forming process, the behavior of material failure in sheet metal forming is described by forming limit curves (FLC). However, especially for thin metal foils, proven testing methods such as the Nakazima test are not applicable because the specimens start wrinkling or fail outside the defect zone specified in the norm. While there are alternative testing methods for the detection of the pure tension area of the FLC, there is no applicable testing method for the evaluation of the forming limit in the tensile-compression zone. Therefore, in this paper simulations as well as physical tests were carried out to define a suitable specimen geometry for the characterization of stainless steel foil (1.4404) with a thickness of 0.1 mm using a scaled Nakazima set up. The simulation results showed that by decreasing the parallel web length as well as the fillet radius the equivalent strain maximum is shifted towards the specimen center. This observation is supported by the physical tests where necking occurred in the specimen center. Additionally to the position of failure, first investigations in physical testing showed maximum strain ratios of ${\upvarepsilon }_{1}$ = 0.23 in major strain and ${\upvarepsilon }_{2}$ = −0.095 in minor strain. The strain ratio therefore represents the uniaxial tension area.