Stress response of advanced high strength steels using tapered tensile specimens and digital image correlation method

The development of constitutive material models for automotive forming and crash simulations requires an accurate description of the flow stress for a wide range of strains and strain rates. Tapered tensile specimens provide new opportunities to obtain this information because deformation rates range an order of magnitude lower and higher than in conventional parallel specimens. Compared with parallel gauge specimens, fewer tests are required to characterize the material response. This study examines the suitability of using tapered tensile specimens, combined with strain measurement via the digital image correlation (DIC) method, to obtain accurate stress-strain-strain rate relations for various advanced high strength steels (AHSS), including TRIP, dual-phase and a new HSLA 780 grade. The results show a good correlation with conventional strain measurements and accurately capture the different types of yielding behaviour, so that the DIC method can be used to accurately measure strain in uniaxial tensile tests for a range of different steels and yielding behaviour, including hot-rolled HSLA 590 steel that exhibits yield point elongation. The use of tapered tensile specimens allows data to be obtained at strain rates ranging approximately three orders of magnitude, both lower and higher, than would be achieved using the same test speed and conventional parallel gauge tensile specimens. The Zerilli-Armstrong equation was used to generate viscoplastic constitutive equation and to fit a flow stress surface (as a function of strain and strain-rate) from two tests for HSLA 590 and HSLA 780 that was in excellent agreement with the experimental data.