Methods for measuring friction-independent flow stress curve to large strains using hyperbolic shaped compression specimen

Abstract

The accurate measurement of flow stress curve to large strains using cylindrical compression specimen is always a great challenge due to the influence of friction. Recently, the present authors designed a hyperbolic shaped compression (HSC) specimen which can yield an average true stress- strain curve independent of friction and proposed a stress correction function for fast estimation of flow stress curve to large strains. The aim of this paper is threefold. Firstly, to investigate whether the analytical method for stress correction of tensile necking can, or cannot, be extended to HSC specimen for correcting average true stress into flow stress. Secondly, to develop an inverse method based on Kriging surrogate model for identifying the optimal parameters of modified Voce model using HSC specimen. Lastly, the advantages and disadvantages of these three methods were compared and the recommendations for application were also discussed. The results show that the analytical method is more suitable to the stress correction for material with higher n-value but shows worse capability for correcting flow stress related to large strains for material with lower n-value. For Q420 steel, the maximum strain achieved by HSC specimen (0.8) is far higher than that achieved by cylindrical tension specimen (0.55). The analytical method can correct the flow stress in the strain range of 0–0.5 effectively but underestimating the flow stress in the strain range of 0.5–0.8 due to its low n-value. Both inverse method and stress correction function can determine the flow stress in the strain range of 0–0.8 successfully. Thus, for isotropic material with tension–compression yield symmetry, it is recommended to use the HSC specimen instead of conventional tension and compression tests of cylindrical specimens to determine the flow stress curve to large strains.