Elasticity
9.0 The Stress – Strain Curve
9.0 The Stress – Strain Curve
When a load stretches a wire, it is seen that for small value of load, the extension produced in the wire is proportional to the load. As shown in the figure it is represented by $OA$ portion of the graph. In this region Hooke’s law is obeyed i.e. stress is proportional to strain. The point $A$ is called the limit of proportionality. On removing the load, the wire returns to its original length. If the strain is increased a little bit i.e. in the region $A$ to $B$, the strain is not proportional to strain. However, the wire still returns to its original length when the load is removed. The point $B$ is called the elastic limit.
As we go beyond the point $B$, then even for very small increase in stress, the strain is produced is very large. This is behavior is observed around point $C$ and at this stage the wire begins to flow like a viscous fluid. The point $C$ is called yield point. If the stress is further increased, then the wire breaks off at a point $D$ called the breaking point. The stress corresponding to this point is called breaking stress or tensile strength of the material of the wire.
A material for which the plastic range $CD$ is relatively large is called ductile material. These materials get permanently deformed before breaking. The materials for which plastic range is relatively small are called brittle materials. These materials break as soon as elastic limit is crossed.
