Von Mises Yield Criterion

In a plane stress condition (i.e., one of the principal stresses is zero), the von Mises yield criterion can be plotted as a function of the other two principal stresses. For stress states where the von Mises equivalent stress is less than the yield strength (points that lie inside of the ellipse), plastic deformation will not occur. For stress states where the von Mises equivalent stress is greater than the yield strength (points which lie outside of the ellipse), plastic deformation will occur.