Plasticity (physics)
In
physics and
materials science,
plasticity is a property of a material to undergo a non-reversible change of shape in response to an applied force. Plastic deformation occurs under
shear stress, as opposed to
brittle fractures which occur under
normal stress. Examples of plastic materials are
clay and mild
steel. In engineering, this is called
yield.
For many
ductile metals, tensile loading applied to a sample will cause it to 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 extension remains. A generic graph displaying this behaviour is below. See also
Hooke's law.
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Stress-strain1.png |
Ductile materials can sustain large
plastic deformations without
fracture. However, even
ductile metals will
fracture when the
strain becomes large enough - this is as a result of work-hardening of the material, which causes it to become
brittle.
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, realized that the plastic deformation of ductile materials could be explained in terms of the theory of
dislocations.
Some materials, especially those prone to
Martensitic transformations, deform in ways that are not well described by the classic theories of plasticity and elasticity. One of the best-known examples of this is
nitinol, which exhibits pseudoelasticity: deformations which are reversible in the context of mechanical design, but
irreversible in terms of
thermodynamics.
* R. Hill,
The Mathematical Theory of Plasticity (1998), Oxford University Press, USA, ISBN 0-19-850367-9
(Old but classic)* Jacob Lubliner,
Plasticity theory (1990), Macmillan Publishing, New York ISBN 0-02-372161-8
(Provides general overview)
