BS 5400

In tension the initial slope is defined by the elastic modulus, $E$, until the stress reaches $k_ef_{yd}$. The slope then reduces until the material is fully plastic, $f_{yd},$ at $\varepsilon=\varepsilon_{off}+f_{yd/E}$. Post-yield the stress remains constant until the failure strain, $\varepsilon_u$, is reached. For BS5400 $k_e=0.8$ and $\varepsilon_{off}=0.002$.

In compression the initial slope is defined by the elastic modulus, $E$, until the stress reaches $k_ef_{yd}$ or a strain of $\varepsilon_{off}$. It then follows the slope of the tension curve post-yield and when the strain reaches $\varepsilon_{off}$ the stress remains constant until failure.