Why do materials show plastic behaviour for large stress? 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?
 A: Plastic behaviour is characterised by there being permanent (non-reversible) deformations.  In terms of molecules held together with springs (the bonds) in plastic deformation the springs are broken and the bonds then might the be between different molecules.
A: 
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.
A: This is due to the fact that the elastic energy per unit volume becomes so high at some points that there is a possibility of spontaneous irreversible transformations in the microstructure of the material: in metals, displacement of topological defects (plasticity theory), in materials with microfibers or with the capacity to create internal surfaces, micro-cracking and increased internal damage (continuous damage mechanics). The Gibbs free energy increases with the strain to a point where the deformation becomes a thermodinamically irreversible process.
