Associated flow rule
= Associated flow rule
{title2=$D^p=\dot\lambda\,\partial f/\partial\sigma$}
Plastic strain rate is normal to the <yield surface> in the work-conjugate stress variables. With symmetric <stress> variations, the pairing is $D:\delta\sigma$; in two dimensions its off-diagonal contribution is $2D_{12}\delta\sigma_{12}$. This factor is essential when changing to reduced stress coordinates.