Discuss the behaviour of wire under increasing load .

Explanation

According to Hooke's law, the stress produced in a body is proportional to the strain. But this applies for small strains only.
When load is applied on wire which is gradually increased, length of wire increases as :
`(i)` When the strain is small `( lt 2%)` (i.e., in region OP stress is proportional to strain. The point `P` is called limit of propotionality and slope of line OP gives the Young's modulus `Y` of the material of the wire.
`(2)` If the strain is increased a little bit i.e., in the region PE, the stress is not proportional to the strain. However, wire still regains its original length after the removal of stretching force. This behaviour is shown upto point `E` known as elastic limit.
`(3)` If the wire is stretched further between `E` and `A`, the strain increases much more rapidly and if the stretched force is removed the wire does not come back to its natural length and length of wire increases permanently.
`(4)` If the stress is increased further, by a very small increase in it a very large increase in strain is produced (region AB) and after reaching point `B`, the strain increases even if the wire is unloaded. The region EABC represents the plastic behaviour of the material of the wire.
`(5)` Once the neck is formed, the wire goes on stretching even if the load is reduced, until the breaking point `C` is reached, where the wire breaks.