Stress-strain diagram for mild steel

The diagram is obtained from a tension test on a mild steel specimen (nominal stress $= P/A_0$ against strain $= \Delta L/L_0$).

 stress
   |            D (ultimate)
   |           ,-'''-.
   |       B C/       '.  E (fracture)
   |      ,-'''           x
   |     /  yield
   |    /  plateau
   |   /
   |  /  elastic
   | /A  (Hooke's law)
   |/__________________________ strain
   O
 A=proportional limit, B=upper yield, C=lower yield
 D=ultimate stress, E=breaking point

Salient points

1. O to A, proportional limit: stress is proportional to strain (Hooke's law); the slope is $E$.
2. Elastic limit: just beyond A; material returns to its original length on unloading.
3. Upper yield point (B) and lower yield point (C): the stress drops slightly and the specimen elongates considerably at nearly constant stress (yield plateau). Mild steel has $\sigma_y \approx 250\ \text{N/mm}^2$.
4. Strain hardening (C to D): the material regains strength and the curve rises to the maximum.
5. Ultimate stress (D): the maximum nominal stress; after this, necking starts.
6. Breaking (fracture) point (E): the nominal stress falls as the section necks, and the specimen breaks. The true stress, based on the reduced area, actually keeps rising to fracture.