On the correct interpretation of measured force and calculation of material stress in biaxial tests

Nolan, David; McGarry, J. Patrick

doi:10.1016/j.jmbbm.2015.08.019

Cited by 34 publications

(28 citation statements)

References 23 publications

(33 reference statements)

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“…The distortive factors are discontinuous, loading, and unknown contralateral forces. Although a clamped sample is loaded more continuously than in other methods, significant amounts of contralateral forces remain unmeasured . These contra‐lateral forces create a stress shield for the region of interest causing an artificial increase in the stiffness.…”

Section: Discussionmentioning

confidence: 99%

Comparison of tensile properties of xenopericardium from three animal species and finite element analysis for bioprosthetic heart valve tissue

et al. 2019

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Bioprosthetic heart valves still have poor long‐term durability due to calcification and mechanical failure. The function and performance of bioprostheses is known to depend on the collagen architecture and mechanical behavior of the target tissue. So it is necessary to select an appropriate tissue for such prostheses. In this study, porcine, equine, and bovine pericardia were compared histologically and mechanically. The specimens were analyzed under light microscopy. The planar biaxial tests were performed on the tissue samples by applying synchronic loads along the axial (fiber direction) and perpendicular directions. The measured biaxial data were then fitted into both the modified Mooney‐Rivlin model and the anisotropic four parameter Fung‐type model. The modified Mooney‐Rivlin model was applied to the modeling of the bovine, equine, and porcine pericardia using finite element analysis. The equine pericardium illustrated a wavy collagen bundle architecture similar to bovine pericardium, whereas the collagen bundles in the porcine pericardium were thinner and structured. Wavy pericardia may be preferable candidates for transcutaneous aortic valves because they are less likely to be delaminated during crimping. Based on the biaxial tensile test, the specimens indicated some degree of anisotropy; the anisotropy rates of the equine specimens were almost identical, and higher than the other two specimens. In general, porcine pericardium appeared stiffer, based on the greater strain energy magnitude and the average slope of the stress–stretch curves. Moreover, it was less distensible (due to lower areal strain) than the other two pericardial tissues. Furthermore, the porcine model induced localized high stress regions during the systolic and diastolic phases of the cardiac cycle. However, increased mechanical stress on the bioprosthetic leaflets may cause tissue degeneration and reduce the long‐term durability of the valve. Based on our observations, the pericardial specimens behaved as anisotropic and nonlinear tissues—well‐characterized by both the modified Mooney‐Rivlin and the Fung‐type models. The results indicate that, compared to bovine pericardium, equine tissue is mechanically and histologically more appropriate for manufacturing heart valve prostheses. The results of this study can be used in the design and manufacture of bioprosthetic heart valves.

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Section: Discussionmentioning

confidence: 99%

Comparison of tensile properties of xenopericardium from three animal species and finite element analysis for bioprosthetic heart valve tissue

et al. 2019

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“…In general, clamps, sutures (threads), or hooks (sometimes joined into rakes) are used as gripping elements introducing the load into the specimen in biaxial testing (see Figure 1). While hooks are applied most frequently and considered as a standard gripping method (see among others [21][22][23][24][25][26][27][28]), clamps were rejected as unacceptable by some authors [8][9][10][11]. However, this rejection was substantiated by numerical or experimental analyses done only for broad clamps used one per specimen edge, while two or more narrow clamps per edge wereaccording to our knowledgepresented by our group only [18,29].…”

Section: Introductionmentioning

confidence: 78%

“…While hooks are applied most frequently and considered as a standard gripping method (see among others ), clamps were rejected as unacceptable by some authors . However, this rejection was substantiated by numerical or experimental analyses done only for broad clamps used one per specimen edge, while two or more narrow clamps per edge were – according to our knowledge – presented by our group only .…”

Section: Introductionmentioning

confidence: 86%

“…Then, the axial P a and circumferential P c first Piola–Kirchhoff stresses

P_{a} = \frac{F_{normala}}{L_{c} T}; P_{c} = \frac{F_{normalc}}{L_{a} T},

were computed from the recorded forces and the previously measured specimen dimensions L c , L a and T , and the corresponding stretches in both directions were computed from the positions of markers on the deformed and undeformed specimen. Similarly, we computed shear strains to verify that they are negligible compared to normal strains and cannot decrease the accuracy of the results . Note that they were below 5% of the values of normal strains in all cases, and their impact on magnitude of normal strains was on the order of 10 −3 ; thus they were neglected.…”

Section: Methodsmentioning

confidence: 99%

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Analysis of Accuracy of Biaxial Tests Based on their Computational Simulations

Slažanský

Polzer

Man

et al. 2016

Strain

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Design of biaxial tension testing rig has a significant influence on quality of the measured stress–strain data. The paper analyses the effect of test parameters on the accuracy of results. Finite element simulations of biaxial tension tests were realized, and the resulting stress–strain curves (evaluated in the same manner as in real experiments) compared with the input ones by means of coefficient of determination R2. The investigated parameters were type (hooks or narrow clamps), number (2÷5) and size (0.25÷5 mm) of gripping elements, specimen size (18÷33 mm), testing protocol (different displacement ratios) and specimen material (healthy artery, aneurysm tissue, elastin and intraluminal thrombus). Numerical results were compared with experiments on porcine aortas carried out with two clamps and four hooks per edge. Experiments were evaluated with respect to the obtained quality of fit and maximum achievable stresses under equibiaxial load. Two or three hooks and two narrow clamps per edge give the best accuracy for small specimens. Larger hook diameter increases the accuracy for low numbers of hooks. Non‐equibiaxial protocols show worse accuracy than equibiaxial ones. For large specimens, four and five hooks are the best choice. The experiments revealed that higher stresses can be induced by two clamps setup (median 433 versus 257 kPa in circumferential direction and median 454 versus 333 kPa in axial direction). Quality of the fit was slightly but consistently higher with the four hooks setup (mean R2 = 0.984 versus 0.9797). The number of gripping elements should be chosen with respect to the need to ensure uniform force distribution along the specimen edge. Both narrow clamps and hooks achieved a comparable accuracy regardless of material. High numbers of hooks and wide clamps should be avoided for small specimens. Compared with hooks, use of narrow clamps allowed us to reach higher load until specimen rupture.

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“…During these tests, the specimens are generally clamped to the machine actuator using mechanical clamps (Perez et al, 2014, Hu et al, 2014, Nolan and McGarry, 2016. The interpretation of biaxial test results is complex and the direct use of material parameters in computer simulation may be complex (Tian et al, 2015, Delgadillo et al, 2015.…”

Section: Discussionmentioning

confidence: 99%

Biaxial quantification of passive porcine myocardium elastic properties by region

Ṋemavhola¹

2017

10.5267/j.esm

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Considering accurate constitutive models is of the utmost importance when capturing the mechanical response of soft tissue and biomedical materials under physiological loading conditions. This paper investigated the behaviour of porcine myocardium in passive rested hearts. This was done by applying biaxial loads on the myocardium. The main objective of this research was to investigate the cardiac mechanics of various regions of a healthy passive rested porcine heart. The biaxial mechanical properties of myocardial tissue samples were captured using a biaxial testing system. The porcine heart was divided into three regions, namely, left ventricle (LV), septum and right ventricle (RV). In these regions, 18 × 18 mm2 equal samples were cut from six porcine passive hearts. For the LV sample, the biaxial elastic modulus in the fibre direction was 33.3% larger than in the cross fibre direction, for the mid-wall sample it was 18.8% larger, and for the RV sample it was 33.3% larger. It was concluded that the cardiac mechanics of LV, septum and RV exhibit mechanical behaviour that differs considerably. In developing adequate computational models, these data could be applied to estimate the material parameters of the myocardium.

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On the correct interpretation of measured force and calculation of material stress in biaxial tests

Cited by 34 publications

References 23 publications

Comparison of tensile properties of xenopericardium from three animal species and finite element analysis for bioprosthetic heart valve tissue

Comparison of tensile properties of xenopericardium from three animal species and finite element analysis for bioprosthetic heart valve tissue

Analysis of Accuracy of Biaxial Tests Based on their Computational Simulations

Biaxial quantification of passive porcine myocardium elastic properties by region

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