Further deformation of the material causes the stress state to remain on the yield surface , even though the surface itself may change shape and size as the plastic deformation evolves , this is because stress states that lie outside the yield surface are non-permissible .
Further deformation of the material causes the stress state to remain on the yield surface , even though the surface itself may change shape and size as the plastic deformation evolves . 
 This is because stress states that lie outside the yield surface are non-permissible .