Plasticity (physics)
Plasticity in physics refers to the ability of solid materials to undergo permanent deformation when subjected to external forces, without breaking. This property contrasts with elasticity, where materials can return to their original shape after stress is removed. Key factors influencing plasticity include a material's ductility (how much it can stretch) and malleability (its capacity to be shaped). Plasticity is commonly observed in processes like metalworking, where metals are heated and shaped, as well as in geological phenomena, such as the flow of molten rock beneath the Earth's surface.
When a solid reaches its elastic limit under stress, it enters a phase where permanent deformation occurs; this threshold is known as yield strength. Unlike elastic deformation, which can be quantified using Young's modulus, plastic deformation involves energy dissipation. Various methods, such as forging, rolling, and extrusion, utilize plasticity to manipulate metals, which tend to become more malleable when heated. While ductile metals like copper exhibit significant plasticity, more brittle materials, such as cast iron, do not undergo plastic deformation effectively. The study of plasticity is essential for understanding material behavior in both engineering and natural processes.
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Plasticity (physics)
In physics, plasticity is the ability of solid materials to deform or change shape permanently without breaking when subject to external forces. Plasticity differs from elasticity, which is the ability of a solid to change shape temporarily under stress before reverting to its original form. The plasticity of a material is affected by its ductility—the ability to stretch under stress—and its malleability—the ability to be shaped without breaking. Plasticity can be observed in metalworking when a piece of metal is heated and molded to form a new shape. Under very high temperatures, plasticity also occurs in glassworking and in geological processes such as the flow of molten rock under the earth.
Background
Many solid materials undergo some physical change when subjected to stress. In physics, stress is the force per unit area applied to an object. The amount of stress can be measured by dividing the force exerted on an object by the cross-sectional area of the object. The ability of a material to return to its original shape after it has been deformed through stress is known as its elasticity. This property is best illustrated in the way a rubber band or coiled spring can be stretched but snaps back into shape when the force is removed. Denser objects such as metals also exhibit elasticity, however, considerably greater force is required to affect such objects and the results are far less noticeable. The energy used in the deformation of a material is completely recovered when the force is removed.
The measure of a material's elasticity is determined by a formula called Young's modulus, which in simple terms can be described as elasticity equals stress divided by strain. Strain is a measure of how much an object stretches or changes relative to its original length. If too much stress is applied to a material, it will break or rupture. A material's ultimate strength is the amount of stress it can withstand before it breaks. Metal and rock display high values of ultimate strength while a rubber band has a lower value.
Overview
If a material is subjected to an amount of stress great enough to permanently deform it but not great enough to break it, the material is said to have reached its elastic limit. The measure of force needed to deform a material permanently is called its yield strength. The property of material solids to deform permanently under stress is known as plasticity. Unlike elasticity, the energy expended in the plastic deformation of a material is dissipated in the process. In plastic deformation, there are no easily measured values that can be determined by a simple formula such as Young's modulus.
Nineteenth-century French engineer Henri Tresca, who is sometimes called the father of the science of plasticity, developed a standard to determine when plastic deformation would occur. This standard, called the Tresca criterion, is built on the principle that plasticity occurs when the maximum shear stress over all planes of a material reaches a critical value. Shear stress is a unit of external force that causes two contacting planes or layers to slide upon each other in opposite directions parallel to the plane of contact. A simple example of shear stress is using a pair of scissors to cut a thick material as the movement of the two blades of the scissors exert shear stress on the object.
In metals and other crystalline materials, plasticity occurs at the microscopic level because of dislocations or movement in the boundaries between the tightly packed grains of the material. In granular material, such as sand, plasticity occurs because of an irreversible rearrangement and crushing of individual particles. When a solid piece of metal is pounded into a new shape, it undergoes plastic deformation because of physical changes taking place within the material. In more fragile materials, such as rocks, plasticity usually occurs because of movement along microscopic cracks.
Since metals are more ductile materials, they can undergo significant plastic deformations without breaking or fracturing. Most metals exhibit increased plasticity when they are heated to a sufficient temperature. This allows them to become highly malleable and can more easily undergo plastic deformation by such processes as forging, rolling, and extrusion. Forging involves heating metal in a fire, and then pounding and shaping it with a hammer. Rolling is the process of shaping metal by forcing it through rotating rollers. Extrusion shapes heated metal by pushing it through a mold or die. Metals can also be heated to a melting point and poured into a mold. A process called cold working uses high pressure without heat to change the shape of a metal. The process is most often used on steel, aluminum, and copper.
Copper is a very ductile metal and displays a high degree of plasticity. This aspect is one of the reasons copper is often used in wiring. Cast iron—a hard alloy made of iron, carbon, and silicon—is extremely brittle and cannot be plastically deformed. Even the most ductile metals will harden and become brittle if they undergo cold working, which is also known as work hardening. The effect can be reversed through a slow-heating process called annealing.
Finished glass displays no plasticity, as any sufficient force will cause it to break. In the glassmaking process, however, silicon dioxide and several other substances are heated to the point of plasticity, molded into a shape, and hardened. Similar to glass, molten rock melted by the extreme pressures within the earth also display properties of plastic deformation.
Bibliography
Bell, Terence. "What Is Cold Working?" The Balance, 17 Mar. 2017, www.thebalance.com/what-is-cold-working-2340011. Accessed 22 June 2017.
Campbell, Allison, et al. "Elasticity vs. Plasticity." Energy Education, energyeducation.ca/encyclopedia/Elasticity‗vs‗plasticity. Accessed 22 June 2017.
Coleman, Lawrence B. "Elasticity and Plasticity." University of California, Davis, 14 June 1998, smartsite.ucdavis.edu/access/content/user/00002774/Sears-Coleman%20Text/Text/C11-15/12-4.html. Accessed 22 June 2017.
Goodier, J.N., and P.G. Hodge, Jr. The Mathematical Theory of Elasticity and the Mathematical Theory of Plasticity. Dover Publications, 2016.
Kachanov, L.M. Fundamentals of the Theory of Plasticity. Dover Publications, 2004.
Kelly, Piaras. "Introduction to Plasticity." University of Auckland, homepages.engineering.auckland.ac.nz/~pkel015/SolidMechanicsBooks/Part‗II/08‗Plasticity/08‗Plasticity‗01‗Introduction.pdf. Accessed 22 June 2017.
Lubliner, Jacob. Plasticity Theory. 1990. Dover Publications, 2008.
Martinez, Tara. "Types of Metal Strength." Monarch Metal Fabrication, 24 Aug. 2016, www.monarchmetal.com/blog/types-of-metal-strength/. Accessed 22 June 2017.