The upper yield point is the starting point for plastic deformation. This is because the dislocations in the crystalline structure begin to move. After a while, though, the dislocations become too many and obstruct each other's movement. This is known as strain hardening, and the lower yield point is when strain hardening begins. Plastic deformation occurs beyond the elastic limit, and strains are not fully recovered. When the load is removed, there will be permanent deformation or set. Upper and lower yield points are the terms used to describe these two positions.