Rotational forming of thin SS316L sheets

Abstract

Bipolar half plates (BPPs) are important parts of Proton Exchange Membrane (PEM) hydrogen fuel cells. Metallic BPPs are generally produced by stamping or hydro forming. However, these processes are not continuous and relatively slow compared to rotary forming. Rotary forming is a continuous process and could potentially be a faster BPP production method. The BPPs are plastically deformed in bending and membrane modes. It is necessary to determine the bending and membrane strain because differentiating them can aid in the design of future BPP rotary forming production methods. Bending strain predominantly could lead to fractures on the outside of the bend. Membrane strain causes material thinning and could potentially lead to necking. These deformation modes need to be well understood so that the highest forming levels are achieved with a high level of geometrical consistency without exceeding material failure limits. The goal of this research is to determine the bending and membrane strain of 0.1 mm thick SS316L sheet material formed by rotary forming. This is done by experimental testing and analysis. From the experimentally deformed material, cross-sections are made and analysed. The thickness and bending strain are measured from these cross-sections. The experimental results are compared to FEA simulations. An important assumption is plane strain and it is experimentally validated. Plane strain FEA models are used to further understand the problem. The FEA simulations are validated, using the single channel strain experiments. Simulations with channels aligned with the cylinder axis are validated with a maximum thickness error of 3%. However, channels perpendicular to the axis are validated with an error of 8.9%. The FEA simulations are then used to further analyse equivalent plastic strain, stress, springback and final shape. The maximum equivalent plastic strain is 0.34. The maximum VonMises stress is 1200 MPa and maximum springback is 0.075 mm. The final shapes along the cylinder axis meet the experimental results the best. The bending and membrane strain are experimentally measured and simulated in Abaqus CAE. The bending and membrane strain highly depend on channel orientation and indentation depth. However, the bending strain is greater than membrane strain for both experiments and simulations. This research should be extended to investigate more complex geometries and designs. This is of course a more realistic example for BPPs.