Thus, the state boundary surface can also be a yield surface like that shown
in Fig. 3.16. If the boundary surface is a yield surface then while the state is on the
surface there are simultaneous elastic and plastic strains, but if the state is brought
inside the boundary surface, by unloading, the strains are assumed to be purely elastic.
This is a highly idealized model for soil behaviour and we now know that there are
inelastic strains when the state is inside the boundary surface.