A
yield surface is a five-dimensional surface in the six-dimensional space of
stresses. The yield surface is usually
convex and the state of stress of
inside the yield surface is elastic. When the stress state lies on the surface the material is said to have reached its
yield point and the material is said to have become
plastic. Further deformation of the material causes the stress state to remain on the yield surface, even though the shape and size of the surface may change as the plastic deformation evolves. This is because stress states that lie outside the yield surface are non-permissible in
rate-independent plasticity, though not in some models of
viscoplasticity.