Cruciform Specimen Design and Verification for Constitutive Identification of Anisotropic Sheets

A novel cruciform specimen design is proposed, which is a composite of existing specimens. The typical shortcomings of many existing designs are that the strains achievable in the test-section before failure are very limited and the stresses in the test-section cannot be easily determined from experimental measurements, or without the use of an inverse finite element analysis. The proposed specimen overcomes both of these difficulties. The proposed geometry features slotted arms; very tight corner radii; a test-section of reduced thickness; and a sharp transition between the arms and the test-section. This new geometry is investigated numerically, at first. It is shown that the stress and strain distributions in the test-section are very uniform, with the exception of a thin outer boundary-layer. This implies that the strains can be measured anywhere in the test-section (except at the boundary-layer) without significant impact on the results. Subsequently, a virtual biaxial experiment is described. Two materials (steel 1018 and aluminum Al-2090-T3) are considered, each with an appropriate material model (Hill 1948 and Yld2004-3D, respectively). In both cases, the proposed specimen identifies the material model very accurately, using only the load cell readings and strains measured within the test-section. Finally, the proposed specimen is used in biaxial experiments to identify an appropriate material model for the dual-phase steel DP 590. It is shown that the Yld2004-3D model captures the material behavior up to almost 10 % strain; however the coefficients of this model should evolve with plastic deformation, for best fitting performance. During the course of the experimental study, the uniformity of the strain fields is assessed using the Digital Image Correlation method.