= Principal stress formula for incompressible hyperelasticity
{title2=$t_i=\lambda_iW_i-p$}
For an <isotropic> <hyperelastic material> with <incompressibility>, the <principal stresses> are $t_i=\lambda_i\partial W/\partial\lambda_i-p$. This follows by evaluating the <Cauchy stress tensor> formula $T=F(\partial_FW)^T-pI$ in principal directions. For the reduced <strain energy density> $h(\lambda)=W(\lambda^{-2},\lambda,\lambda)$, the <chain rule> and equality of the tangential derivatives give $t_\theta-t_r=\lambda h'(\lambda)/2$.
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