The effect of friction compensation on the slope of flow curves obtained by stack compression tests

Abstract. Knowing the large strain flow curve is essential for many simulation processes in the sheet metal forming technology. A prominent example is the clinching, when the strains are one magnitude higher than in tension, but a simple deep-drawing process could also lead to appropriately large strains. To obtain the flow stress - plastic strain curves beyond the range of tension, several processes are known. In the most multi-axial cases, a conversion factor based on the work equivalence is usually applied on the measurement results to obtain the equivalent quantities. However, at compressive experiments, the friction and the geometrical issues may distort the curve shape even further, hiding the real material behavior. In this study we investigate the effect of the friction determination method and the value of the friction coefficient on the curve shape. Pressure dependent and constant friction coefficients were inversely specified by disk compression tests and related finite element modeling. In possession of the friction values, the conversions of the physically measured compressive stress to equivalent flow stress during disk and stack compression tests with different height-diameter ratios are discussed here.