Unraveling the effect of boundary conditions and strain monitoring on estimation of the constitutive parameters of elastic membranes by biaxial tests

• We analyze the effect of geometry, different gripping methods and strain monitoring.
• For large samples, the gripping method had no effect on the material parameters and the strain distribution at the center of the sample.
• Biaxial strain fields measured using surface markers or DIC methodologies are essential when sutures are used.

The purpose of this study is to study the effects of boundary conditions and strain monitoring on the estimation of the constitutive parameters of elastic membranes by biaxial tests. We analyze the effect of geometry, different gripping methods and strain monitoring. Experiments were conducted on membrane silicon using two samples geometries – cruciform and square – of different sizes, two gripping arrangements – clamped and sutured edges – and two strain monitoring methods – displacement between grips and Digital Image Correlation techniques. The strain maps were compared with that produced by a geometrically matched finite element model.We report two important findings. First, as the St Venant principle pointed for medium or large samples the gripping method had no effect on the material parameters and the strain distribution at the center of the sample. However, due to size the restrictions imposed by soft biological tissues, using sutures is the commonly used technique. Secondly, biaxial strain fields measured using surface markers or DIC methodologies are suitable for clamped method and be essential when sutures are used. As conclusion, the parametric study realized can give guidelines for future researchers on biomechanics on the minimum size of samples and an interesting way to extract data from tests.