The permanent change in shape of a metal object as a result of applied or internal forces. This feature permits metals to be formed into pipe, wire, sheet, and so on, with an optimal combination of strength, ductility, and toughness. The onset of noticeable plastic deformation occurs when the applied tensile stress or applied effective stress reaches the tensile yield strength of the material. Continued plastic deformation usually requires a continued increase in the applied stress. The effects of plastic deformation may be removed by heat treatment.