Numerical verification of a biaxial tensile test method using a cruciform specimen

Abstract

A method of evaluating stress measurement errors in biaxial tensile tests using a cruciform specimen is proposed. The cruciform specimen is assumed to be fabricated from a section of uniformly thick flat sheet metal via laser or water-jet cutting and to have a number of parallel slits cut into each of the four arms. Using finite element analyses with the von Mises yield criterion, the optimum geometry of the cruciform specimen and the optimum strain measurement position necessary to minimize the stress measurement error are determined. Additionally, an experimental validation of the FEA is performed using a sheet material that was experimentally confirmed to be nearly isotropic. The following conclusions are drawn: (i) the thickness of the test material should be less than 0.08B (B: side length of the gauge area of the cruciform specimen); (ii) the geometric parameters for the cruciform specimen should be N≥7, L≥B, ws≤0.01B, and 0.0034≤R/B≤0.1 (N: number of slits, L: length of slits cut into the arms, ws: slit width, and R: corner radius at the junction of the arms to the gauge area); and (iii) the strain components in the gauge area should be measured on the centerline of the specimen parallel to the maximum force direction at a distance of approximately 0.35B from the center of the specimen. The stress measurement error is estimated to be less than 2% when the optimum conditions above are satisfied.