Passive mechanical response of a healthy ovine aortic tissue: experiments and constitutive modelling

Aortic tissue exhibits highly nonlinear elastic and anisotropic mechanical behaviour. Those characteristics are provided by its constituents such as collagen and elastin. Yet the mechanisms of deformation of this material are not well understood. This study addressed the passive mechanical behaviour of a healthy ovine aortic tissue. Various biaxial and uniaxial tests were conducted to experimentally characterise the passive mechanical properties of ovine aorta tissue, including different biaxiality ratios, to find the influence of tissue microstructure on its anisotropic response. Furthermore, two versions of the orthotropic eight-chain model, i.e. the freely-jointed chain (FJC) and worm-like chain (WLC), were applied in this study in order to capture the anisotropic response of the tissue. The results showed that ovine aortic tissue demonstrated a nonlinear and anisotropic behaviour under biaxial and uniaxial loads. The circumferential direction was found to be stiffer than the longitudinal one, which resulted from the internal tissue structure formed by collagen and elastin. The response of the tissue was anisotropic even at low stretches, and the anisotropy increased with applied deformations. Moreover, the predictions of the orthotropic models for stresses in two perpendicular directions were in a good agreement with experiments. Also, the validity of the model was confirmed by employing the single set of model parameters obtained from the fitting of biaxial data in order to capture the uniaxial stress-stretch response of the tissue. This clearly suggests that the orthotropic eight-chain model is able to capture the anisotropic hyperelastic mechanical behaviour of ovine aortic tissue under different loading conditions with a single set of parameters.