Plastic deformation occurs when one has exceeded the Yield Stregnth of the Material. Below that Stress the material will return to its original shape after the Stress is removed. Plastic deformation also introduces dislocations or disruptions in the lattice pattern that does cause the material to resist elastic deformtion or 'harden' i.e it becomes brittle like the wire example given but and this is the big but, the lack of elastic deformation means that the material is subject to brittle failure where a low stress can cause the material to fail catastrophically! Obviously one wants to stay in the elastic deformation region for steel in gun barrels and once the material has entered the platic deformation region, catastrophic failure can occur at a stress lower than the normal stress from firing a factory load.-Dick
