Analysis of the accuracy on computing nominal stress in a biaxial test for arteries

Biaxial tests are commonly used to investigate the mechanical behaviour of anisotropic soft biological tissues such as cardiovascular tissues. However, there is still no clear understanding of the influence that the biaxial test set-up conditions may have on the computing material stress of the experimental results. The aim of the present study is to further investigate the accuracy of calculated material stress from measured force during biaxial tests using finite element methods (FEM). The biaxial mechanical response of ascending aorta and pulmonary artery tissue samples was obtained by FEM simulation under two different gripping methods: (a) a system with noodle clamps and (b) a clamped system with needles which leave the specimen's edges free to expand laterally. The results show that the clamped method whose joints allow free movement in the lateral direction produces stresses closer to the universally accepted formulation of biaxial material stress in the central region. However, the system with noodle clamps, commonly used to grip the sample, produces an alteration of the measurement stresses. Our simulations show results giving an inaccurate estimation of the stress at the centre of the sample. In some cases, the stresses are overestimated and in others underestimated depending on the anisotropy of the sample. We can conclude that the clamped system with needles which leave the specimen's edges free to expand laterally should be used as an efficient methodology to other commonly used gripping methods for biological tissues with anisotropic materials