Plastic deformation

Plastic deformation is a non-reversible change in the shape of an object caused by an applied force. For many ductile metals, as a tensile load is applied to a sample it will first behave in an elastic manner. Each increment of load is accompanied by a proportional increment in extension, and when the load is removed, the piece returns exactly to its original size. However, once the load exceeds some threshold (the yield strength), the extension increases more rapidly than in the elastic region, and when the load is removed, some amount of the exension remains. A generic graph displaying this behaviour is below.

Ductile materials can sustain large plastic deformations without fracture. However, even ductile metals will fracture when the strain becomes large enough - often this is as a result of work-hardening of the material. Heat treatment such as annealing can restore the ductility of a worked piece, so that shaping can continue.