Finite element models of biomedical applications increasingly use anisotropic hyperelastic material formulations. Appropriate material parameters are essential for a reliable outcome of these simulations, which is why planar biaxial testing of soft biological tissues is gaining importance. However, much is still to be learned regarding the ideal methodology for performing this type of test and the subsequent parameter fitting procedure.

This paper focuses on the effect of an unknown sample orientation or a mistake in the sample orientation in a planar biaxial test using rakes. To this end, finite element simulations were conducted with various degrees of misalignment. Variations to the test method and subsequent fitting procedures are compared and evaluated.

For a perfectly aligned sample and for a slightly misaligned sample, the parameters of the Gasser-Ogden-Holzapfel model can be found to a reasonable accuracy using a planar biaxial test with rakes and a parameter fitting procedure that takes into account the boundary conditions. However, after a certain threshold of misalignment, reliable parameters can no longer be found. The level of this threshold seems to be material dependent.

For a sample with unknown sample orientation, material parameters could theoretically be obtained by increasing the degrees of freedom along which test data is obtained, e.g. by adding the data of a rail shear test. However, in the situation and the material model studied here, the inhomogeneous boundary conditions of the test set-ups render it impossible to obtain the correct parameters, even when using the parameter fitting method that takes into account boundary conditions.

To conclude, it is always important to carefully track the sample orientation during harvesting and preparation and to minimize the misalignment during mounting. For transversely isotropic samples with an unknown orientation, we advise against parameter fitting based on a planar biaxial test, even when combined with a rail shear test.

生物医学应用的有限元模型越来越多地使用各向异性超弹性材料配方。适当的材料参数对于这些模拟的可靠结果至关重要，这就是为什么对软生物组织进行平面双轴测试变得越来越重要的原因。然而，关于执行这种类型的测试以及随后的参数拟合过程的理想方法学，还有很多东西要学。

本文重点介绍在使用耙子进行的平面双轴测试中未知样品方向或样品方向错误的影响。为此，进行了各种程度的未对准的有限元模拟。比较和评估了测试方法和后续装配程序的变化。

对于完全对准的样品和稍微未对准的样品，可以使用带有耙子的平面双轴试验和考虑边界条件的参数拟合程序，以合理的精度找到Gasser-Ogden-Holzapfel模型的参数。但是，在一定的未对准阈值之后，将不再能找到可靠的参数。此阈值的水平似乎与材料有关。

对于具有未知样品取向的样品，理论上可以通过增加自由度来获得材料参数，例如，通过增加钢轨剪切试验的数据，可以沿着该自由度获得测试数据。但是，在这里研究的情况和材料模型中，即使使用考虑边界条件的参数拟合方法，测试装置的边界条件也不均匀，也无法获得正确的参数。

总而言之，始终重要的是在收获和准备过程中仔细跟踪样品的方向并最大程度地减少安装过程中的未对准情况。对于方向未知的横向各向同性样品，即使与钢轨剪切试验结合使用，我们也建议不要基于平面双轴试验进行参数拟合。