Influence of specimen geometry on measures of local fracture strain obtained from uniaxial tensile tests of AHSS sheets

Enhanced formability mapping, based on parameters derived from uniaxial tensile tests, has been proposed in order to distinguish between local versus global formability of AHSS sheets. The assessment of fracture surfaces represents the basis of local formability indices, i.e. local fracture strains. Different specimen geometries as well as evaluation methods can be found in uniaxial tensile test standards. It is not yet clear whether the choice of test specimen or evaluation method influences the measures of local fracture strain. This paper aims at contributing to the ongoing discussion on these issues during enhanced formability mapping of AHSS grades. In this study two AHSS grades have been investigated in two thicknesses (t = 1.5mm & 3.0mm). Besides standardized test specimens, the widths w of tensile test specimens were deliberately varied to gain custom tensile test specimens, exhibiting widths of 3.0mm, 6.0mm, 15.0mm and 30.0mm. The resulting measures of local fracture strain were related to the respective width-to-thickness ratio w/t as well as to the respective evaluation method. Distinct trends of all local fracture strains with respect to the w/t ratio could be detected. However, these trends tend to level off for certain w/t ratios, depending on the respective strain hardening behaviour of the investigated AHSS grade. For thicknesses up to 2mm standardized geometries with w larger than 20mm may be used indifferently. For higher thicknesses, e.g. for hot rolled AHSS grades, care has to be taken to choose the tensile specimen accordingly. Local fracture strain measures based on the actual fracture surface tend to show less variability and less dependence on the assessment method as compared to measures representing fracture thickness only. The overall trends as well as outliers depicted in the variability of results were discussed with respect to the fracture surface assessment methods.