## Incompressible hyperelastic materials

For an w:incompressible material $J:=\det {\boldsymbol {F}}=1$ . The incompressibility constraint is therefore $J-1=0$ . To ensure incompressibility of a hyperelastic material, the strain-energy function can be written in form:

$W=W({\boldsymbol {F}})-p~(J-1)$ where the hydrostatic pressure $p$ functions as a Lagrangian multiplier to enforce the incompressibility constraint. The 1st Piola-Kirchhoff stress now becomes

${\boldsymbol {P}}=-p~{\boldsymbol {F}}^{-T}+{\frac {\partial W}{\partial {\boldsymbol {F}}}}=-p~{\boldsymbol {F}}^{-T}+{\boldsymbol {F}}\cdot {\frac {\partial W}{\partial {\boldsymbol {E}}}}=-p~{\boldsymbol {F}}^{-T}+2~{\boldsymbol {F}}\cdot {\frac {\partial W}{\partial {\boldsymbol {C}}}}~.$ This stress tensor can subsequently be converted into any of the other conventional stress tensors, such as the Cauchy Stress tensor which is given by

${\boldsymbol {\sigma }}={\boldsymbol {P}}\cdot {\boldsymbol {F}}^{T}=-p~{\boldsymbol {\mathit {1}}}+{\frac {\partial W}{\partial {\boldsymbol {F}}}}\cdot {\boldsymbol {F}}^{T}=-p~{\boldsymbol {\mathit {1}}}+{\boldsymbol {F}}\cdot {\frac {\partial W}{\partial {\boldsymbol {E}}}}\cdot {\boldsymbol {F}}^{T}=-p~{\boldsymbol {\mathit {1}}}+2~{\boldsymbol {F}}\cdot {\frac {\partial W}{\partial {\boldsymbol {C}}}}\cdot {\boldsymbol {F}}^{T}~.$ For incompressible w:isotropic hyperelastic materials, the w:strain energy density function is $W({\boldsymbol {F}})={\hat {W}}(I_{1},I_{2})$ . The Cauchy stress is then given by

${\boldsymbol {\sigma }}=-p~{\boldsymbol {\mathit {1}}}+2\left[\left({\cfrac {\partial {\hat {W}}}{\partial I_{1}}}+I_{1}~{\cfrac {\partial {\hat {W}}}{\partial I_{2}}}\right){\boldsymbol {B}}-{\cfrac {\partial {\hat {W}}}{\partial I_{2}}}~{\boldsymbol {B}}\cdot {\boldsymbol {B}}\right]$ 