When the strain is small (region OP) stress is proportional to strain. This is the region where the so called Hooke’s law is obeyed. The point P is called limit of proportionality and slope of line OP gives the Young’s modulus Y of the material of the wire. Y = tan θ.
- Point E known as elastic limit or yield-point.
- Between EA, the strain increases much more.
- The region EABC represents the plastic behaviour of the material of wire.
Stress-strain curve for different materials, are shown in following figure.
Brittle material
The plastic region between E and C is small for brittle material and it will break soon after the elastic limit is crossed.
Ductile material
The material on this type have a good plastic range and such materials can be easily changed into different shapes and can be drawn into thin wires.
Elastomers
For ealstomers the strain produced is much larger than the stress applied. Such materials have no plastic range and the breaking point lies very close to elastic limit. Example : rubber.
