Strain Hardening Definition
Strain hardening (also called work-hardening or cold-working) is the process of making a metal harder and stronger through plastic deformation.
In the plastic region, the true stress increases continuously i.e when a metal is strained beyond the yield point, more and more stress is required to produce additional plastic deformation and the metal seems to have become more stronger and more difficult to deform. This implies that the metal is becoming stronger as the strain increases. Hence, it is called "Strain Hardening". The plastic portion of the true stress-strain curve (or flow stress curve) plotted on a log-log scale gives the n value as the slope and the K value as the value of true stress at true strain of one.
log(ø) = log(K)+ n x log(e)
For materials following the power law, the true strain at the Ultimate Tensile Strength is equal to n. when you plot the log-log plot, use data points after the yield point (to avoid elastic points) and before instability (necking).
A material that does not show any strain hardening (n=0) is classed as perfectly plastic. Such a material would show a constant flow stress irrespective of strain. K can be found by substituting n and a data point (from the plastic region) in the power law or from the y-intercept.
Strain hardening reduces ductility and increases brittleness. e.g. Cold-working can be easily demonstrated with piece of wire or a paper clip. Bend a straight section back and forth several times. Notice that it is more difficult to bend the metal at the same place. In the strain hardened area dislocations have formed and become tangled, increasing the strength of the material. Continued bending will eventually cause the wire to break at the bend due to fatigue cracking.