Pre-stress

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 BS8110 and related codes $k_e=0.8$ and $\varepsilon_{off}=0.005$.