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

Abstract

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 setup 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: (i) a system with noodle clamps and (ii) 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.

Combining Unique Planar Biaxial Testing with Full‐Field Thickness and Displacement Measurement for Spatial Characterization of Soft Tissues

“…Sutures allow rotation at the sample boundaries, ensuring force application perpendicular to the sample's boundary. Unfortunately, they also introduce damage to the sample and create stress concentrations around the puncture sites (Cilla, Corral, Peña, & Peña, 2020; Eilaghi, Flanagan, Brodland, & Ethier, 2009; Sun et al., 2005). A few biomechanists have created custom systems with larger numbers of actuators and load cells to actually measure in‐plane shear forces (Khalsa, Hoffman, & Grigg, 1996; Malcolm, Nielsen, Hunter, & Charette, 2002).…”

The Use of Contravariant Tensors to Model Anisotropic Soft Tissues

“…A homogeneous square plate of dimensions L = H = 5mm was meshed using 256, 4 noded, quadrilateral elements.The parameters for the proposed model were chosen from table 1. Seven nodes (symmetrically chosen) along each edge were stretched in the horizontal and vertical directions upto 12 percent.This was attempted to recreate the clamping procedure associated with equibiaxial tests [50]. Figure ??…”

An improved parameter fitting approach of a planar biaxial test including the experimental prestretch

“…clamps, sutures or rakes, to the sample shape, i.e. squared or cruciform [10,11,12,13]. The lack of a standardized protocol makes a comparison between different studies unreliable [14].…”

“…To overcome this, studies have been conducted to gain more insight in various aspects of the experimental protocol and data analysis. The influence of the gripping mechanism and placement on different sample shapes has been studied [12,13,15,16,17,18], as well as other aspects of the protocol such as the effect of preconditioning [19,20], the effect of flattening an excised sample prior to sample mounting [21], the difference between a force-and displacement-based protocol, the sample side on which the deformation is tracked [14] and the variations of the experimental stress calculation and parameter fitting [12].…”