Design of an Octo-Strain Specimen for Biaxial Tension Testing

A custom biaxial testing fixture was designed to evaluate a new specimen geometry for complex loading paths. Biaxial testing is commonly used to evaluate work-hardening behavior of sheet metal in biaxial tension to study the accumulation of plastic strains to determine the anisotropic yield loci. The current state-of-the-art specimen geometry that is used for biaxial testing is the cruciform specimen. Cruciform specimens are machined into a geometry that resembles a cross with four arms arranged at 90 degrees. However, this geometry is prone to premature failure and non-homogenous strain distribution within the gauge region. These problems persist even with the addition of complex features (e.g., slits and multi-step pockets). Therefore, the primary goal of the new specimen geometry is to achieve a large and uniform strain field within the gauge region. One of the main problems of the cruciform specimen is the formation of stress concentration within the gauge region. Therefore, the proposed specimen geometry is comprised of four additional arms between the existing cruciform arms. This geometry is termed an ‘Octo-Strain’ specimen after the eight arms that are arranged in a 45-degree planar pattern. It is hypothesized that the additional arms will stabilize the stress concentrations and, thus, achieve increased failure strain and uniformity as compared to the cruciform geometry. This work focuses on the comparison of the cruciform specimen to the Octo-Strain specimen during balanced biaxial deformation of mild steel. It is found that the Octo-Strain specimen achieved twice the failure strain and increased strain uniformity within the gauge region as compared to the cruciform specimen.