Idealized stress-strain diagram of steel

The real stress-strain curve is idealised for design (IS 800:2007).

Mild steel (E250)

 stress
  fu |            ___------.
     |         _-/          \
  fy |-------+==============+   <- yield plateau
     |      /|
     |     / |
     |    /  |  E = 2x10^5
     |   /   |
     |__/____|______________|____ strain
        0.00125   ~0.014    ~0.2

• Elastic line up to $f_y$ with slope $E=2\times10^5$ N/mm²; yield strain $\varepsilon_y=f_y/E=0.00125$ for $f_y=250$.
• Constant stress $f_y$ (yield plateau) up to roughly 10-15 times $\varepsilon_y$ (strain hardening starts at about 0.014).
• Strain hardening up to the ultimate stress $f_u$ ($\approx410$ N/mm²) at a strain of about 0.15-0.2, then necking and fracture.
• For design the curve is idealised as elastic-perfectly plastic (bilinear): $\sigma = E\varepsilon$ up to $f_y$, then $\sigma=f_y$.

High tensile steel (E350-E450, cold-worked)

 stress
  fu |           ___---
     |        _-/
 0.2%|------_/
 proof      /
     |     /
     |    /
     |___/____|__________ strain
       0.002 offset

• No definite yield point or plateau; the stress rises smoothly.
• The yield stress is taken as the 0.2% proof stress (found by a line parallel to the elastic part, drawn from 0.002 strain).
• Idealised as elastic up to the proof stress and then flat (or rising slightly). Ductility is lower than that of mild steel.