Park

The initial slope is defined by the elastic modulus,$E$, until the stress reaches $f_{yd}$. The slope is then zero for a short strain range, then rising to a peak stress before failure.

$$\sigma=f_{ud}-(f_{ud}-f_{yd})(\frac{\varepsilon_u-\varepsilon}{\varepsilon_u-\varepsilon_p})^p$$

$$p=E(\frac{\varepsilon_u-\varepsilon_p}{f_{ud}-f_{yd}})$$