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In physics, plasticity is a property of a material that undergoes
plastic deformation in response to an applied force. Plastic deformation is a non-reversible change in the shape
of an object. A common example of a plastic material is clay. 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.
In 1934, Egon Orowan, Michael Polanyi and Geoffrey Ingram Taylor, roughly simultaneously, realised that
the plastic deformation of ductile materials could be explained in terms of the theory of dislocations.
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