# Polynomial (hyperelastic model)

The polynomial hyperelastic material model [1] is a phenomenological model of rubber elasticity. In this model, the strain energy density function is of the form of a polynomial in the two invariants ${\displaystyle I_{1},I_{2}}$ of the left Cauchy-Green deformation tensor.

The strain energy density function for the polynomial model is [1]

${\displaystyle W=\sum _{i,j=0}^{n}C_{ij}(I_{1}-3)^{i}(I_{2}-3)^{j}}$

where ${\displaystyle C_{ij}}$ are material constants and ${\displaystyle C_{00}=0}$.

For compressible materials, a dependence of volume is added

${\displaystyle W=\sum _{i,j=0}^{n}C_{ij}({\bar {I}}_{1}-3)^{i}({\bar {I}}_{2}-3)^{j}+\sum _{k=1}^{m}D_{k}(J-1)^{2k}}$

where

{\displaystyle {\begin{aligned}{\bar {I}}_{1}&=J^{-2/3}~I_{1}~;~~I_{1}=\lambda _{1}^{2}+\lambda _{2}^{2}+\lambda _{3}^{2}~;~~J=\det({\boldsymbol {F}})\\{\bar {I}}_{2}&=J^{-4/3}~I_{2}~;~~I_{2}=\lambda _{1}^{2}\lambda _{2}^{2}+\lambda _{2}^{2}\lambda _{3}^{2}+\lambda _{3}^{2}\lambda _{1}^{2}\end{aligned}}}

In the limit where ${\displaystyle C_{01}=C_{11}=0}$, the polynomial model reduces to the Neo-Hookean solid model. For a compressible Mooney-Rivlin material ${\displaystyle n=1,C_{01}=C_{2},C_{11}=0,C_{10}=C_{1},m=1}$ and we have

${\displaystyle W=C_{01}~({\bar {I}}_{2}-3)+C_{10}~({\bar {I}}_{1}-3)+D_{1}~(J-1)^{2}}$

## References

1. ^ a b Rivlin, R. S. and Saunders, D. W., 1951, Large elastic deformations of isotropic materials VII. Experiments on the deformation of rubber. Phi. Trans. Royal Soc. London Series A, 243(865), pp. 251-288.