= Natural state of a hyperelastic material
{title2=$DW(F)=0$}
An unconstrained natural state is a stress-free homogeneous <deformation gradient>. For $J>0$, its equivalent conditions are $DW(F)=0$ and $J^{-1}DW(F)F^T=0$. An interior differentiable minimum of the <strain energy density> is a natural state, although a stationary state need not minimize the energy. Constraints require allowing their reaction stress; the unconstrained equivalence must then be modified. Under strict <Baker-Ericksen inequalities>, an isotropic natural state is a dilation times a proper rotation.
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