Models Torsional couple M expressions Mooney-Rivlin $M=\pi \Psi {a}^{4}\left({C}_{10}+\frac{1}{\lambda }{C}_{01}\right)$ Yeoh $\begin{array}{l}M=4\pi \Psi {a}^{4}\left(\frac{3}{8}{\Psi }^{4}{a}^{4}\lambda {C}_{30}+\frac{1}{6}\lambda \left(2{\Psi }^{2}{C}_{20}+6{\Psi }^{2}\left(\frac{2}{\lambda }+{\lambda }^{2}-3\right){C}_{30}\right){a}^{2}\right)\\ \text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}+4\pi \Psi {a}^{4}\left(\frac{1}{4}\lambda \left({C}_{10}+2\left(\frac{2}{\lambda }+{\lambda }^{2}-3\right){C}_{20}+3{\left(\frac{2}{\lambda }+{\lambda }^{2}-3\right)}^{2}{C}_{30}\right)\right)\end{array}$ Fung $M=\frac{2\pi C}{{\Psi }^{3}\beta }{\mathrm{exp}}^{-3\beta }\left({\mathrm{exp}}^{\beta \left(2+{\lambda }^{3}\right)/\lambda }-{\mathrm{exp}}^{\beta \left({\Psi }^{2}{a}^{2}\lambda +{\lambda }^{3}\right)/\lambda }+{\Psi }^{2}{a}^{2}\beta {\mathrm{exp}}^{\beta \left({\Psi }^{2}{a}^{2}\lambda +2+{\lambda }^{3}\right)/\lambda }\right)$ Gent-Thomas $\begin{array}{l}M=\frac{\pi }{{\Psi }^{3}\lambda }\left(2{K}_{2}\mathrm{ln}\left(1+2{\lambda }^{3}\right)+4{K}_{2}{\lambda }^{3}\mathrm{ln}\left(1+2{\lambda }^{3}\right)+\lambda {K}_{1}{a}^{4}{\Psi }^{4}+2\lambda {K}_{2}{\Psi }^{2}{a}^{2}\right)\\ \text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}\text{\hspace{0.17em}}-\frac{\pi }{{\Psi }^{3}\lambda }\left(2{K}_{2}\mathrm{ln}\left({\Psi }^{2}{a}^{2}\lambda +1+2{\lambda }^{3}\right)+4{K}_{2}{\lambda }^{3}\mathrm{ln}\left({a}^{2}{\Psi }^{2}\lambda +1+2{\lambda }^{3}\right)\right)\end{array}$