· The constant strain rate engineering stress/strain diagram opposite shows an initial region of elastic deformation followed by a region in which permanent shape change is produced.
· The stress boundary between elastic and plastic deformation occurs at the yield stress, σ_y , but the offset yield stress (proof stress),σ_0.001, provides a more easily measured stress value.
· Beyond the ultimate tensile stress, σ_TS , the material would be unstable under constant load conditions, but the curve can be followed at constant strain rate.