= Extensional strain rate
{title2=$\dot\epsilon$}
The <extensional strain rate> is the rate of relative stretching along a specified material direction. If a short material line of <length> $\ell$ follows a principal stretching direction, $\dot\epsilon=d\log\ell/dt$ is the corresponding <eigenvalue> of the <rate-of-strain tensor>. For the planar model in <extensional flow>, $\dot\epsilon=\partial u_x/\partial x$; the two transverse rates are $-\dot\epsilon$ and zero. For <uniaxial extensional flow>, each transverse rate is $-\dot\epsilon/2$.
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