Q.Describe the behaviour of a wire under gradually increasing load.

A stretched wire passes through elastic, plastic, ultimate-strength, necking, and fracture stages as the load is steadily increased — captured by the stress-strain curve.

When a wire is loaded gradually and the resulting stress (force per unit area) is plotted against strain (fractional change in length), the following stages are observed:

1. Proportional limit (Hooke's law region): For small loads, stress is directly proportional to strain (stress/strain = Young's modulus, a constant). The wire returns exactly to its original length when the load is removed. This region is a straight line on the stress-strain graph.

2. Elastic limit: Beyond the proportional limit but still within a nearby range, the wire is still elastic (returns to original shape on unloading) but stress is no longer exactly proportional to strain — the graph starts to curve.

3. Yield point (plastic region begins): Beyond the elastic limit, the wire starts to undergo permanent (plastic) deformation — it does not return to its original length when the load is removed. A small increase in load now produces a relatively large increase in strain.

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