The model rigid, plastic with linear strain hardening material behaviour shows 2 different parts. First, in the initial part of the curve, the behaviour is rigid, therefore deformations are null until yield stress σ0 is reached. Above yield stress, generated strains are always plastic (permanent) and proportional to the difference between current stress and yield stress.
The mechanical model that describes this behaviour is a parallel configuration connection of a linear spring with constant E and a friction plane with constant σ0. With this configuration, strain levels reached by both elements are the same, while load (stress) applied to the system is distributed between both elements. If the stress is under reference σ0 , then the friction plane supports all the load with null deformation. Once yield stress is reached, the load on the friction plane remains σ0, while a deformation proportional to the load on the spring (σ - σ0) is generated. When it is always plastic, the strain generated remains permanent.