As long as the stress is less than the yield value, Hooke's law may be used when the material behaves linearly elastic. When the stress reaches the yield stress, the material starts to yield and keeps deforming plastically at a constant stress level. If the load is removed at any point along the stress-strain diagram, then the unloading path will be along a straight line segment CD parallel to the initial portion AY of the loading curve. The fact that the unloading curve is linear is because the material is linearly elastic.