As the stress is increased, the strain increases proportionally up to elastic limit and the material regains its original dimension within elastic limit. When the stress is increased further the material shows a plastic behaviour. What change in the internal structure causes the transformation from elastic to plastic behavior?
What change in the internal structure causes the transformation from elastic to plastic behavior?
It depends on the material. For metals, small elastic strains are just the result of very small changes in the interatomic spacings. When more stress is applied, pre-existing dislocations in the metal start to move, causing re-arrangements in the atomic positions which result in plastic flow behavior. Dislocations are also generated with increasing strain by mechanisms such as Frank-Read sources. As the amount of plastic strain increases, the dislocation density tends to increase.