Cruciform specimen design for large plastic strain during biaxial tensile testing

The cruciform biaxial tensile test can be used to map the hardening evolution of the yield surface over a wide range of loading paths, which is useful for calibrating and validating the advanced material models. However, when cruciform specimens following ISO Standard 16842 are used, equivalent plastic strain in the gauge region is limited to only similar to 0.03 for DP590. In this study, a new method was developed to strengthen the arms of ISO Standard cruciform specimen in order to achieve greater plastic deformation in the gauge region. Arm strength of cruciform specimens was enhanced by laser deposition of thickening layers using materials compatible with the base metal. Furthermore, the slit geometry in the arms was adjusted to improve strain distribution and delay fracture. To verify the proposed method, cruciform specimens of different base materials (Cr4, DP590, DP780 and DP980) were tested in a biaxial tensile testing system with the aid of digital image correlation (DIC) techniques to characterize the strain fields within the gauge region. The laser-deposition-affected zones were negligible for the base materials according to optical microscopy. For DP590, the laser deposition method provided an increase of equivalent plastic strain in the gauge region from similar to 0.03 to similar to 0.11 for various loading paths. Consequently, evolution of an experimental yield locus was obtained at equivalent plastic strains up to similar to 0.11 for DP590. Continuing work with Cr4, DP780 and DP980 materials increased equivalent plastic strains to different degrees under nearly plane strain conditions (biaxial force ratio of 1:2).