Improving the shear test to determine shear fracture limits for thin stainless steel sheet by shape optimisation

Abstract

Bipolar plates are essential components in proton exchange membrane (PEM) fuel cells for electric vehicles. Micro-formed thin (thickness <0.15 mm) stainless steel bipolar plates have advantages over conventional CNC-machined graphite plates in terms of weight and cost, but the geometry of the final part shape is limited due to the fracture limits of the material. The further optimisation of micro-forming processes, therefore, requires the development of reliable fracture models, with fracture locus covering a wide range of stress states, including shear fracture at zero (or close to zero) stress triaxiality conditions. Few studies have investigated shear tests on thin specimens, as it is challenging to maintain a stable shear condition during the test. There exists a twofold problem that: (a) the sheet buckles prematurely before the failure strain is reached; (b) plane strain fracture initiates prematurely from the edge of the gauge area. In this study, in-planar shear tests are performed on 0.1 mm-thick stainless steel sheet. To achieve fracture initiation in shear, an anti-buckling device was developed and the shear test sample shape was optimised. The shape optimization process is based on finite element analysis (FEA) and verification with experimental shear test trials. The optimised shape has a minimum value of a goal function that encompasses the Lode parameter, stress triaxiality and the ratio of the centre to edge equivalent plastic strain. The experimental trials indicate that the optimised sample shape, in combination with the clamping device, enables testing thin stainless steel for fracture limits in shear. The test routine can be used to calibrate ductile fracture models.