Stress - Strain Curve For Mild Steel

 Curve OABCDEF in figure represents the stress-strain curve when a mild steel specimen is subjected to a gradually increasing tensile load.

OAB - represent a straight line curve (stress is proportional to the strain, i.e., it obeys Hooke's law)                       

A - proportional limit

B - represents the elastic limit

C - upper yield point (In many of the structural steel hot rolled sections, the upper yield  point is not obtained due to residual stresses from the hot rolling  process. Hence, it has no practical significance)

D - lower yield point (The stress corresponding to the lower yield point 'D' is the yield stress with a typical magnitude of 250 N/mm²  for mild steel.)

DE - represents plastic yielding (It is the strain which occurs after the yield point, with no increase in stress)                                                        

EF - represents strain hardening region

F - represents the ultimate stress (After reaching the ultimate tensile stress, a localized  reduction of cross section of the specimen take place.)

FG - represents necking region (It is also knows as minimum ultimate tensile strength. Stress falls with rapid increase in strain till the specimen breaks)

G - represents breaking point