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.
