Shear in Planar Biaxial Tests using Rakes: Non-intuitive results

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Planar biaxial testing of soft biological tissue using rakes: a critical analysis of protocol and fitting process

Mechanical characterization of soft biological tissue is becoming more and more prevalent. Despite the growing use of planar biaxial testing for soft tissue characterization, testing conditions and subsequent data analysis have not been standardized and vary widely. This also influences the quality of the result of the parameter fitting. Moreover, the testing conditions and data analysis are often not or incompletely reported, which impedes the proper comparison of parameters obtained from different studies. With a focus on planar biaxial tests using rakes, this paper investigates varying testing conditions and varying data analysis methods and their effect on the quality of the parameter fitting results. By means of a series of finite element simulations, aspects such as number of rakes, rakes׳ width, loading protocol, constitutive model, material stiffness and anisotropy are evaluated based on the degree of homogeneity of the stress field, and on the correlation between the experimentally obtained stress and the stress derived from the constitutive model. When calculating the aforementioned stresses, different definitions of the section width and deformation gradient are used in literature, each of which are looked into. Apart from this degree of homogeneity and correlation, also the effect on the quality of the parameter fitting result is evaluated. The results show that inhomogeneities can be reduced to a minimum for wise choices of testing conditions and analysis methods, but never completely eliminated. Therefore, a new parameter optimization procedure is proposed that corrects for the inhomogeneities in the stress field and induces significant improvements to the fitting results. Recommendations are made for best practice in rake-based planar biaxial testing of soft biological tissues and subsequent parameter fitting, and guidelines are formulated for reporting thereof in publications.publisher: Elsevier articletitle: Planar biaxial testing of soft biological tissue using rakes: A critical analysis of protocol and fitting process journaltitle: Journal of the Mechanical Behavior of Biomedical Materials articlelink: http://dx.doi.org/10.1016/j.jmbbm.2016.01.011 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe

Development of an improved parameter fitting method for planar biaxial testing using rakes

A correct estimation of the material parameters from a planar biaxial test is crucial since they will affect the outcome of the finite element model in which they are used. In a virtual planar biaxial experiment, a difference can be noticed in the stress calculated from the force measured experimentally at the rakes and the actual stress at the center of the sample. As a consequence, a classic parameter fitting does not result in a correct estimation of the material parameters. This difference is caused by the boundary conditions of the set-up and is among others dependent on the sample material. To overcome this problem, a new parameter fitting procedure is proposed that takes this difference into account by calculating a finite element-based correction vector. This paper describes the methodology to apply this new parameter fitting procedure on real experimental data from a planar biaxial test using rakes. To this end, image processing is used to extract the experiment characteristics. This information is used to construct a finite element model. Two variations of the new parameter fitting procedure are investigated using two human aortic samples: a basic approach and an image-based approach. The performance of the method is assessed by the difference between the force measured at the rakes during the experiment and the force at the rakes obtained from the finite element simulation. Both approaches of the new parameter fitting procedure lead to an improved estimation of the sample behavior compared with the classic approach.status: Published onlin

How to measure the deformation during planar biaxial testing of biological soft tissue?

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How important is sample alignment in planar biaxial testing of anisotropic soft biological tissues? A finite element study.

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.status: publishe

Stretching the limits of biaxial testing: Material characterization of biological tissues with subcritical dimensions

Biaxial testing, planar or extension inflation, is the method of choice to test soft biological tissue. In some cases, sample dimensions prohibit the use of the aforementioned setups. For cerebral bridging veins, samples with a diameter as small as 0.5 mm [1] can be obtained. To successfully characterise tissues of that size, a new test setup was developed. This setup applies a combination of uniaxial and simple shear deformations to obtain transversely isotropic, fibre reinforced (GOH) [2] material properties for this type of tissue. To validate our method, a finite element (FE) model was built in Abaqus 6.14-1. A virtual testing sample was modelled as a 2D planar deformable shell, with GOH material parameters. Homogeneous boundary conditions were imposed, allowing uniaxial extension followed by simple shear deformation. The reaction forces and strain data were used as virtual experimental data and given as input to a fitting procedure to determine the GOH material parameters. Using a least squares nonlinear optimization algorithm in Matlab R2015a (lsqnonlin), the difference between experimental and model stress for each loading direction was minimized. Figure1 shows the applied deformations in the FE model and Table 1 shows the results of the fitting procedure. Future work will be to adapt the FE models to more realistic conditions, upon which this methodology will be applied to actual biological tissue, in casu human bridging veins.status: publishe

Biaxial testing of soft tissue: a finite element analysis of protocol and fitting process

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Biomechanical evaluation of the (un)reinforced Ross procedure

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Combination of uniaxial tensile testing and two rail shear testing for small samples of biological soft tissue: Feasibility study

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Determination of layer-specific properties from planar biaxial tests on intact aortic wall

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