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Plasticity (physics)
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{{Short description|Non-reversible deformation of a solid material in response to applied forces}} {{redirect|Plastic material|the material used in manufacturing|Plastic}} <div class=skin-invert-image>{{Metal yield.svg |290px}} {{Stress v strain A36 2.svg |290px}}</div> {{Continuum mechanics|solid}} In [[physics]] and [[materials science]], '''plasticity''' (also known as '''plastic deformation''') is the ability of a [[solid]] [[material]] to undergo permanent [[Deformation (engineering)|deformation]], a non-reversible change of shape in response to applied forces.<ref name="Lubliner">{{cite book |first=Jacob |last=Lubliner |year=2008 |title=Plasticity theory |publisher=Dover |isbn=978-0-486-46290-5 }}</ref><ref>{{cite book |last=Bigoni |first=Davide |title=Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability |publisher=Cambridge University Press |year=2012 |isbn=978-1-107-02541-7 }}</ref> For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself. In engineering, the transition from [[Elasticity (physics)|elastic]] behavior to plastic behavior is known as [[Yield (engineering)|yielding]]. Plastic deformation is observed in most materials, particularly [[metal]]s, [[soil]]s, [[Rock (geology)|rock]]s, [[concrete]], and [[foam]]s.<ref name="Jirasek">{{cite book |first1=Milan |last1=Jirásek |first2=Zdeněk P. |last2=Bažant |author-link2=Zdeněk P. Bažant |year=2002 |title=Inelastic analysis of structures |publisher=John Wiley and Sons |isbn=0-471-98716-6 }}</ref><ref name="Chen">{{cite book |first=Wai-Fah |last=Chen |year=2008 |title=Limit Analysis and Soil Plasticity |publisher=J. Ross Publishing |isbn=978-1-932159-73-8 }}</ref><ref name="Yu">{{cite book |first1=Mao-Hong |last1=Yu |author-link1=Yu Mao-Hong |first2=Guo-Wei |last2=Ma |first3=Hong-Fu |last3=Qiang |first4=Yong-Qiang |last4=Zhang |year=2006 |title=Generalized Plasticity |publisher=Springer |isbn=3-540-25127-8 }}</ref><ref name="Chen1">{{cite book |first=Wai-Fah |last=Chen |year=2007 |title=Plasticity in Reinforced Concrete |publisher=J. Ross Publishing |isbn=978-1-932159-74-5 }}</ref> However, the physical mechanisms that cause plastic deformation can vary widely. At a [[crystalline]] scale, plasticity in metals is usually a consequence of [[dislocation]]s. Such defects are relatively rare in most crystalline materials, but are numerous in some and part of their crystal structure; in such cases, [[plastic crystallinity]] can result. In [[brittleness|brittle]] materials such as rock, concrete and bone, plasticity is caused predominantly by [[Slip (materials science)|slip]] at [[microcrack]]s. In cellular materials such as liquid [[foams]] or [[Tissue (biology)|biological tissues]], plasticity is mainly a consequence of bubble or cell rearrangements, notably [[T1 process]]es. 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. When the load is removed, the piece returns <!--[Absolutely, utterly and completely?:] exactly-->to its original size. However, once the load exceeds a threshold – the yield strength – the extension increases more rapidly than in the elastic region; now when the load is removed, some degree of extension will remain. [[Elastic deformation]], however, is an approximation and its quality depends on the time frame considered and loading speed. If, as indicated in the graph opposite, the deformation includes elastic deformation, it is also often referred to as "elasto-plastic deformation" or "elastic-plastic deformation". Perfect plasticity is a property of materials to undergo irreversible deformation without any increase in stresses or loads. Plastic materials that have been [[work hardening|hardened]] by prior deformation, such as [[cold forming]], may need increasingly higher stresses to deform further. Generally, plastic deformation is also dependent on the deformation speed, i.e. higher stresses usually have to be applied to increase the rate of deformation. Such materials are said to deform [[Viscoplasticity|visco-plastically]].
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