Figure 1.

A representative plot of experimental biaxial testing and respective fitting results using a three-fibre family constitutive model including pressure–diameter measurements at different axial stretches and their respective theoretical values (a), axial force–pressure measurements at different axial stretches and their respective theoretical values (b), the stored strain energy calculated based on the estimated parameters (c) and a two-dimensional iso-contour representation of strain energy as a functional of circumferential and axial stretches (d). As shown in (c,d) the strain energy is sensitive to axial stretch and circumferential stretch indicating a stiffening response (increased stored energy), especially for axial stretch above 20% . The black plus symbols indicate experimental data and the red solid lines denote theoretical values based on the best-fit values (sample 1 in table 1).