According to Hooke's law, the stress produced in a body is proportional to the strain. But this proportionality exists for small strains only.
When a load is applied to a damped wire, the length of wire goes on increasing part $O A$ of the graph is straight line, indicating that upto point $A$ increase in length is directly proportional to load.
After point $A$ on further increasing the load, the increase in length is no longer proportional to load, however till point $B$, the elastic property exists in the wire. The stress at point $B$ is called elastic limit.
Beyond $B$ is the wire does not return to its original position, but its length is permanently increased.
