Yield is the name given to the point in the stress-strain curve where the material’s response to an applied force switches from an elastic to an inelastic behavior [39]. A body unloaded under a plastic regime keeps some strain (permanent deformations). Thus, a future loading starts with an offset and the elastic regime is retained until the prior unload point is reached, as sketched in Fig. 2.10. This means that the yield point shifts accordingly to the load history of the material. Moreover, the yield strength increases and the material becomes less prone to plastic deformations. This phenomenon is known as hardening. It is behind the manufacturing of several engineering products, such as the creation of blades. When a smith hits the metal, hardening takes place and it becomes tougher to deform the metal permanently in any subsequent load, especially during the use of the blade.
| Figure 2.10.: | Material plasticity in the stress-strain curve. Unloads in the work hardening region retain the deformation history and any subsequent load will relate linearly with the strain until the original unload point is reached (“New” Yield strength). Beyond the work hardening region the material enters a perfect plastic regime, where increases to the yield strength can be considered (most of the time) negligible. |