Strain Uniformity in Biaxial Specimens Is Highly Sensitive to Attachment Details

This study examined how specimen geometry and attachment details influence strain field uniformity during planar biaxial mechanical testing. Finite element modeling and experimental validation demonstrated that the number, spacing, and placement accuracy of attachment points strongly affect the size and magnitude of the uniform-strain region within a specimen. Subtle irregularities in attachment point positioning significantly degraded strain uniformity, particularly under nonequibiaxial loading conditions, while apron geometry had comparatively little effect. Experiments using rubber specimens confirmed model predictions, highlighting that actual specimen strain can differ substantially from nominal strain based on actuator motion. The findings establish critical design guidelines for reliable biaxial testing of small biological and polymeric specimens.