= Coaxiality of isotropic elastic stress
{title2=$\sigma B=B\sigma,\quad B=FF^T$}
The <polar decomposition in continuum mechanics> and <material isotropy> remove the reference rotation from an elastic <Cauchy stress tensor>. Half-turns around principal stretch axes leave the diagonal <left stretch tensor> fixed, so <material frame indifference> forces the stress's off-diagonal entries to vanish in those axes. Rotations within repeated-stretch eigenspaces also force isotropic stress there. Consequently stress and the <left Cauchy-Green deformation tensor> are <coaxial symmetric tensors>.
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