where $\sigma 1U$, $\sigma 2U$, and $\tau 12U$ are constants representing the material behavior. Specifically, $\sigma 1U$ is the ultimate strength of the material in the principal material direction (direction of highest material strength, typically that of fiber orientation), $\sigma 2U$ is the ultimate strength of the material in the transverse direction, and $\tau 12U$ accounts for the shear strength of the material. For in-plane artery tests, the preferred principal material direction was assumed to be the circumferential, and the transverse direction was taken to be the axial, since uniaxial testing shows higher circumferential failure stresses compared to axial [42–44]. Therefore, in Eq. (5), *θ* was defined to be the counterclockwise sample angle relative to the circumferential direction, $\sigma 1U$ was the circumferential (0 deg) failure stress, $\sigma 2U$ was the axial (90 deg) failure stress, $\tau 12U$ was the shear stress, and $\sigma x$ was the failure stress in uniaxial extension at a given sample angle. When *θ* = 0 deg, the condition reduces to $\sigma x$ > $\sigma 1U$, and when *θ* = 90 deg, the condition reduces to $\sigma x$ > $\sigma 2U$. The three constants $\sigma 1U$, $\sigma 2U$, and $\tau 12U$ were fit to the experimental data.