Complex sentence: 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.
Split into simpler sentences: 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.