Describe stress strain graph of a loaded steel wire and hence explain the terms elastic limit, permanent set, tensile strength.

 

The material, for which the stress-strain curve is to be plotted, is stretched by an applied force. The change in length (strain) of the material and the applied force are recorded. The applied force is gradually increased in steps and change in length is noted. The stress-strain graph is plotted between the stress (applied force per unit area) and the strain produced.

 

In the graph region between O to A, the curve is linear. In this region, the material obeys Hook’s law. The material regains its original dimensions when the applied force is removed. In this region, the material behaves like an elastic body.

In the region from A to B, stress and strain are not proportional. The point B in the curve is known as yield point (elastic limit). Corresponding stress is known as yield strength of the material.

 

If the load is increased further, the strain increases rapidly even for a small change in stress. The portion B to D shows this. At a particular point C, the body does not regain its original dimension. In this case, even if we made the stress zero, the strain will not be zero. The material is said to have a permanent set. The deformation is known as plastic deformation. The point D is known as ultimate tensile strength of the material. Beyond this point, even a small applied force causes additional strain to be produced and at the point E, fracture occurs. If points D and E are close, the material is known as brittle. If they are apart, the material is known as ductile.

 

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