The popular terms used in metallurgical engineering as plastic deformation simply means the deformation of metal that occurs after the removal of stress in its crystal structure. In other words, during heat operation of metal it is very possible for the metal to be deformed without the removing its internal stress such deformation cannot be referred as plastic deformation. It is due to this property, that the metals may be subjected to various operations like rolling, forging, drawing, spinning, etc.
The plastic deformation in crystalline materials occurs at temperatures lower than 0.4 tm (where tm is the melting temperature in kelvin). In this temperature range, the amount of deformation, which occurs after the application of stress is very small and generally ignored. However, the rate at which the material is deformed plays some role in determining the deformation characteristics.
The plastic deformation may occur under the tensile, compressive or torsional stresses. It is a function of stress, temperature and rate of straining. There are two basic modes of plastic deformation, namely:
Slip or gliding and twining.
The slip mode is common in many crystal at ambient and elevated temperatures. At low temperature, the mode of deformation changes over to twining in a number of cases.
Materials that are ductile undergo some plastic permanent strain before fracture, for example if a steel beam is loaded, it will first deflect elastically. The deflection disappears when the load is removed. An overload will permanently bend the beam in the location where the stress exceed the yield strength of the steel. In contrast, on a production line, the yield strength of a sheet of steel may be intentionally exceeded to bend the sheet in to the shape required. During this stage, the metal has yielded but it has not failed because the process in the production requires considerable plastic strain. It is necessary in both production and service to know the following concerning metals;
The critical stress requirements to initiate permanent deformation
The amount of plastic strain available before eventual fracture of a ductile metal