Von Mises yield criterion (source code)

= Von Mises yield criterion
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{title2=$\sigma':\sigma'=2k^2$}
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The <Von Mises yield criterion> depends only on the second invariant of <deviatoric stress>. With shear <yield stress> $k$, its surface is $\sigma':\sigma'=2k^2$; the uniaxial <yield stress> is $\sqrt3k$. An <associated flow rule> gives <plastic strain> rate proportional to $\sigma'$, up to a positive <normalization> of the plastic multiplier.