Indexed on: 05 Sep '16Published on: 04 Sep '16Published in: Strain
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.