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In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces.〔J. Lubliner, 2008, Plasticity theory, Dover, ISBN 0-486-46290-0, ISBN 978-0-486-46290-5.〕〔Bigoni, D. Nonlinear Solid Mechanics: Bifurcation Theory and Material Instability. Cambridge University Press, 2012 . ISBN 9781107025417.〕 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 elastic behavior to plastic behavior is called yield. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, foams, bone and skin.〔M. Jirasek and Z. P. Bazant, 2002, Inelastic analysis of structures, John Wiley and Sons.〕〔W-F. Chen, 2008, Limit Analysis and Soil Plasticity, J. Ross Publishing〕〔M-H. Yu, G-W. Ma, H-F. Qiang, Y-Q. Zhang, 2006, Generalized Plasticity, Springer.〕〔W-F. Chen, 2007, Plasticity in Reinforced Concrete, J. Ross Publishing〕〔J. A. Ogden, 2000, Skeletal Injury in the Child, Springer.〕〔J-L. Leveque and P. Agache, ed., 1993, Aging skin:Properties and Functional Changes, Marcel Dekker.〕 However, the physical mechanisms that cause plastic deformation can vary widely. At a crystalline scale, plasticity in metals is usually a consequence of dislocations. 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 brittle materials such as rock, concrete and bone, plasticity is caused predominantly by slip at microcracks. 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 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 with hardening necessitate increasingly higher stresses to result in further plastic deformation. 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 visco-plastically. ==Contributing properties== The plasticity of a material is directly proportional to the ductility and malleability of the material. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Plasticity (physics)」の詳細全文を読む スポンサード リンク
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