Tuesday, December 16, 2014

Behavior of a Wire under Increasing Load

We want to study how a wire behaves when we apply the force continuously on the body. By increasing the stress on the body continuously the corresponding strain is noted down and a graph is drawn between the stress and strain as shown. Here the stress is taken and y-axis and the strain is taken on x-axis.

When we do not apply any force on the body there is no stress and hence there is no strain. With the increase of the stress the strain also increases proportionately up to certain extent and the point up to where it happens is called as elastic limit. Many times this limit is also called as proportionality limit, it meant to say that for many bodies proportionality limit and the elastic limit coincides with each other.

When the stress is applied about this point the stress is not exactly directly proportional to strain but the particle will still have elastic nature. That means if you revert back the stress that we have applied strain also comes back and the graph retraces back to the origin.

Once if this elastic limit is crossed ,with respect to the increase of the stress the strain also increases but the proportionality is no more there. From this point we cannot retrace the graph which mean to say that when you decrease the stress strain is not going to completely disappear rather a small portion of the strain will exists forever. This kind of the arrangement is called permanent set. We can calculate the value of the permanent set has the product of the original length of the wire and the shift of the graph. This point is generally called as yielding point.

If the force is applied beyond the yielding point there will be a increase of the strain even when the stress is not increases significantly and  the wire start becoming thin. This can happen up to some extend and finally at a particular point the wire will break .This  point where the wire breaks is called as breaking point.

If there is a good gap between the yielding point and breaking point, that kind of the material is called ductile  materials and there are very much useful in making thin and long wires. Gold and copper are simple examples of this kind of the materials.

If there is no big gap between yielding point and the breaking point they cannot be molded into wires and this kind of the material is called as brittle materials. Glasses a typical example of a brittle material.

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