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Plasticity (physics)
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== Physical mechanisms == [[File:PlasticityIn111Copper.jpg|thumb|alt=A large sphere on a flat plane of very small spheres with multiple sets of very small spheres contiguously extending below the plane (all with a black background)|Plasticity under a spherical nanoindenter in (111) copper. All particles in ideal lattice positions are omitted and the color code refers to the von Mises stress field.]] === In metals === Plasticity in a crystal of pure metal is primarily caused by two modes of deformation in the crystal lattice: slip and twinning. Slip is a shear deformation which moves the atoms through many interatomic distances relative to their initial positions. Twinning is the plastic deformation which takes place along two planes due to a set of forces applied to a given metal piece. Most metals show more plasticity when hot than when cold. Lead shows sufficient plasticity at room temperature, while cast iron does not possess sufficient plasticity for any forging operation even when hot. This property is of importance in forming, shaping and extruding operations on metals. Most metals are rendered plastic by heating and hence shaped hot. ====Slip systems==== {{main|Slip (materials science)#Slip systems}} Crystalline materials contain uniform planes of atoms organized with long-range order. Planes may slip past each other along their close-packed directions, as is shown on the slip systems page. The result is a permanent change of shape within the crystal and plastic deformation. The presence of dislocations increases the likelihood of planes. ====Reversible plasticity==== On the nanoscale the primary plastic deformation in simple [[face-centered cubic]] metals is reversible, as long as there is no material transport in form of [[Cross Slip|cross-slip]].<ref>Ziegenhain, Gerolf; and Urbassek, Herbert M.; "Reversible Plasticity in fcc metals" in ''Philosophical Magazine Letters'', 89(11):717-723, 2009 [https://dx.doi.org/10.1080/09500830903272900 DOI 10.1080/09500830903272900]</ref> [[Shape-memory alloy]]s such as Nitinol wire also exhibit a reversible form of plasticity which is more properly called [[pseudoelasticity]]. ====Shear banding==== The presence of other defects within a crystal may entangle dislocations or otherwise prevent them from gliding. When this happens, plasticity is localized to particular regions in the material. For crystals, these regions of localized plasticity are called [[shear band]]s. ====Microplasticity==== Microplasticity is a local phenomenon in metals. It occurs for [[stress (physics)|stress]] values where the metal is globally in the [[Elasticity (physics)|elastic]] domain while some local areas are in the plastic domain.<ref name="Maaร2018">{{cite journal |last1=Maaร |first1=Robert |last2=Derlet |first2=Peter M. |title=Micro-plasticity and recent insights from intermittent and small-scale plasticity |journal=Acta Materialia |date=January 2018 |volume=143 |pages=338โ363 |doi=10.1016/j.actamat.2017.06.023|arxiv=1704.07297 |bibcode=2018AcMat.143..338M |s2cid=119387816 }}</ref> === Amorphous materials === ====Crazing==== In [[amorphous]] materials, the discussion of "dislocations" is inapplicable, since the entire material lacks long range order. These materials can still undergo plastic deformation. Since amorphous materials, like polymers, are not well-ordered, they contain a large amount of free volume, or wasted space. Pulling these materials in tension opens up these regions and can give materials a hazy appearance. This haziness is the result of ''[[crazing]]'', where [[fibrils]] are formed within the material in regions of high [[hydrostatic stress]]. The material may go from an ordered appearance to a "crazy" pattern of strain and stretch marks. === Cellular materials === These materials plastically deform when the bending moment exceeds the fully plastic moment. This applies to open cell foams where the bending moment is exerted on the cell walls. The foams can be made of any material with a plastic yield point which includes rigid polymers and metals. This method of modeling the foam as beams is only valid if the ratio of the density of the foam to the density of the matter is less than 0.3. This is because beams yield axially instead of bending. In closed cell foams, the yield strength is increased if the material is under tension because of the membrane that spans the face of the cells. === Soils and sand === {{main|critical state soil mechanics}} Soils, particularly clays, display a significant amount of inelasticity under load. The causes of plasticity in soils can be quite complex and are strongly dependent on the [[microstructure]], chemical composition, and water content. Plastic behavior in soils is caused primarily by the rearrangement of clusters of adjacent grains. === Rocks and concrete === {{main|rock mass plasticity}} Inelastic deformations of rocks and concrete are primarily caused by the formation of microcracks and sliding motions relative to these cracks. At high temperatures and pressures, plastic behavior can also be affected by the motion of dislocations in individual grains in the microstructure.
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