Ascending aortic dissection (AD) is a potentially fatal vascular disease associated with degradation and fragmentation of the elastic fibers in the aortic media, increasing low-stress distensibility, and a dilated aorta may lead to dissection. In this study, a Fung-type hyperelastic model was formulated incorporating the initial tangent moduli (ITM) of stress-strain curves as an index of low-stress distensibility. ITM were correlated with the material constants by linearizing incompressible stress-strain relationships at zero strain. For uniaxial loading tests, the robustness of the material constants was examined in the stress ranges of 0-200, 0-180, and 0-160 kPa and to the ITM values of 100%, 95%, and 90%. Examination revealed stable changes in the material constants of 80% of the specimens. For equibiaxial stretch tests, the material constants were determined for each curve of the isotropic and anisotropic deformation groups by pre-identifying the ITM and minimizing fitting errors using isotropic or anisotropic models. The errors for all groups were <6% using a transversely isotropic model, and <10% for an orthotropic model. Comparisons with experimental curves showed that Fung-type models described both the ITM and significant stiffening at high stress levels. The mechanical characteristics of the aorta in the stage prior/posterior to dissection is such that while hardening occurs under both low- and high-stress levels with an increase in collagen content as an aging response, softening occurs under low-stress conditions due to histological abnormalities such as elastin deficiency and fragmentation. Numerical simulations using Fung-type models implied that elastic fiber degeneration and fragmentation in AD tissues reduced not only the low-stress stiffness but also the elastic stiffness with superimposed shear. The latter stiffness was modulated by the stiffening at high stress levels in tensile deformation behavior and normal-strain state under physiological loading conditions, and therefore provides further insight into wall rupture.
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