| Abstract | Background: The elastic behavior of arteries is nonlinear when subjected to large deformations. In order to measure their anisotropic behavior, planar biaxial tests are often used. Typically, hooks are attached along the borders of a square sample of arterial tissue. Cruciform samples clamped with grips can also be used. The current debate on the effect of different biaxial test boundary conditions revolves around the uniformity of the stress distribution in the center of the specimen. Uniaxial tests are also commonly used due to simplicity of data analysis, but their capability to fully describe the in vivo behavior of a tissue remains to be proven. In this study, we demonstrate the use of inverse modeling to fit the material properties by taking into account the non-uniform stress distribution, and discuss the differences between the three types of tests. Methods: Square and cruciform samples were dissected from pig aortas and tested equi-biaxially. Rectangular samples were used in uniaxial testing, as well. On the square samples, forces where applied on each side with hooks, and strains where measured in the center using optical tracking of ink dots. On the cruciform and rectangular samples, displacements were applied on grip clamps and forces were measured on the clamps. Each type of experiment was simulated with the finite element method. Mooney-Rivlin and Ogden constitutive models were adjusted with an optimization algorithm so that the simulation predictions fitted the experimental results. Results: Higher stretch ratios (> 1.5) were reached in the cruciform and rectangular samples than in the square samples before failure. Therefore, the nonlinear behavior of the tissue at large deformations was better captured by the cruciform biaxial test and the uniaxial test, than by the square biaxial test. Uniaxial and biaxial behaviors were more consistent when predicted by the Mooney-Rivlin than by the Ogden model. Conclusion: Advantages of cruciform samples over square samples include: 1) higher deformation range; 2) simpler data acquisiton and 3) easier attachment of sample. However, the nonuniform stress distribution in cruciform samples requires the use of inverse modeling adjustment of constitutive model parameters. |
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