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.
Snapshot 1: no applied stress
Snapshot 2: a stress state that would lead to yield and plastic deformation
Snapshot 3: a stress state that would not lead to yield or plastic deformation
"Von Mises Yield Criterion"
Wolfram Demonstrations Project
Published: January 24 2008