Ethan Clarke
Plasticity in mechanical engineering is the ability of a solid material to undergo irreversible deformation or a permanent change of shape in response to applied forces. It is also known as plastic deformation and is observed in most materials, including metals, soils, rocks, concrete, and foams. The plasticity of a material is directly proportional to its ductility and malleability. Perfect plasticity is a property of materials that allows them to undergo irreversible deformation without any increase in stress or load. Plastic deformation is often characterized by mechanical properties such as yield stress, ultimate tensile strength, percent elongation to failure, and hardness.
| Characteristics | Values |
|---|---|
| Definition | Plasticity is the ability of certain solids to flow or change shape permanently when subjected to stresses of intermediate magnitude. |
| Other names | Plastic deformation |
| Comparison with elasticity | Unlike elasticity, plasticity is a microscopic phenomenon. |
| Yielding | Yielding is the transition from elastic behaviour to plastic behaviour. |
| Examples | Metals, soils, rocks, concrete, foams, and polymers. |
| Types | Rate-independent plasticity, Rate-dependent plasticity, Elastoplasticity |
| Mathematical criteria | Tresca yield criterion, Von Mises yield criterion |
| Plastic behaviour | Yield stress, ultimate tensile strength, percent elongation to failure, hardness |
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What You'll Learn
Plastic deformation
Plasticity, or plastic deformation, is the ability of a solid material to undergo permanent deformation—an irreversible change of shape in response to applied forces. This is distinct from temporary or elastic deformation, where a material will return to its original shape after the load is removed. Plastic deformation occurs when the applied stress is high enough to cause cracks in the material, which then propagate until it fractures completely.
The plasticity of a material is directly proportional to its ductility and malleability. Ductile metals like copper, silver, and gold have a large plastic deformation range. Metal-forming processes such as rolling, pressing, and forging exploit the plastic deformation of metals to create components with improved mechanical properties and reduced defects. For example, rolling reduces the thickness of metal to achieve desired dimensions, while extrusion forces metal through a die to create complex shapes and profiles.
Understanding plastic deformation is crucial in materials science and engineering. It helps engineers and scientists design and manufacture materials and components that meet the demands of various applications. For instance, in the automotive industry, car manufacturers design components to deform plastically in controlled ways during collisions, enhancing vehicle safety and minimizing damage to passengers.
The study of plastic deformation also includes understanding the factors that influence it, such as temperature and the presence of impurities or alloying elements. At elevated temperatures, the probability of plastic flow increases, even in normally brittle materials like ceramics and glass.
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Yielding
In the field of mechanical engineering, plasticity refers to a solid material's ability to undergo irreversible deformation in response to applied forces. This phenomenon, also known as plastic deformation, is characterised by permanent changes in the material's shape. The transition from elastic behaviour to plastic behaviour is known as yielding.
In ductile metals, tensile loading causes the metal to behave elastically, with each increment of load resulting in a proportional increment in extension. However, when the load exceeds the yield strength, the deformation enters the plastic region, and the material will not return to its original shape. This behaviour is observed in most materials, particularly metals, soils, rocks, concrete, and foams, although the specific mechanisms causing plastic deformation vary.
The plasticity of a material is influenced by factors such as deformation speed and prior deformation history. Generally, higher stresses are required to increase the rate of deformation. Materials that have been hardened by prior deformation, such as cold forming, may require even higher stresses to deform further. Additionally, the plasticity of a material is directly proportional to its ductility and malleability.
Perfect plasticity is a concept describing the behaviour of materials that, while yielding, do not exhibit an increase in stress with an increase in strain. In other words, there is a plateau where further deformation occurs without an increase in stress. This concept has been applied to the analysis of various materials, including geomaterials such as soils and rocks, and metals.
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Ductility and malleability
Ductility is a measure of a material's ability to deform under tensile stress without failing. In other words, it is how well a material can stretch before breaking. Ductility is often used to qualitatively rank materials. For example, it is commonly understood that metal is more ductile than stone. However, ductility can also be quantified using metrics such as elongation at failure, reduction in area, critical bending radius, and fracture-related metrics. Nanostructured metals, for instance, typically have high plasticity but low ductility due to their low strain-hardening capability.
Malleability, on the other hand, is a material's ability to deform under compressive stress without fracturing. It describes how easily a material can be shaped into thin sheets or other forms by processes such as rolling, hammering, or pressing. For example, gold is a highly malleable material that can be beaten into sheets of gold leaf. Malleability is especially important in manufacturing operations like forging, extrusion, and sheet metal forming. Like ductility, malleability is also closely related to plasticity. Highly malleable materials typically have a high degree of plasticity and low internal resistance to dislocation motion. Many metals become more malleable at higher temperatures due to increased atomic mobility.
While ductility and malleability are distinct properties, they are often related. In fact, some sources claim that they are essentially the same thing but in opposite directions. That is, ductility pertains to tension while malleability pertains to compression. It is also generally understood that all metals that are malleable are also ductile, though some, like lead, are significantly less ductile than they are malleable.
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Slip and twinning
Plasticity, or plastic deformation, is the ability of a solid material to undergo a permanent, non-reversible change in shape or size in response to applied forces or loads. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams.
Twinning can occasionally replace slip systems when there are few slip systems to begin with. It involves the reorientation of the crystal lattice, creating a twin boundary across which the atomic arrangement is a mirror image. This twin boundary segments the material into mirrored regions, facilitating further deformation. Twinning occurs across specific crystallographic planes and directions known as twin planes and twin directions. Atoms parallel to the twin plane travel along the lattice during twinning, causing lattice distortion within the twinned region. The quantity of atomic plane separation from the twin plane directly relates to the amount of movement. Twinning is less common than slip but becomes significant in materials where slip is less favored due to their atomic structure.
The choice between slip and twinning as deformation mechanisms is influenced by environmental factors such as temperature and the type of applied stress. For example, at higher temperatures, metals might favor slip due to increased atomic mobility. Understanding these mechanisms is essential for engineers and scientists to innovate and improve material performance, especially in applications where material reliability and performance are critical, such as aerospace components.
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Plastic behaviour
In physics and materials science, plasticity is the ability of a solid material to undergo permanent deformation, or a non-reversible change of shape, in response to applied forces. This is also known as plastic deformation. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams.
Plastic deformation is a property of ductile and malleable solids. The plasticity of a material is directly proportional to its ductility and malleability. Ductile metals, for example, will behave elastically up to a point, but once the load exceeds a threshold, the extension will increase more rapidly, and some degree of extension will remain when the load is removed. This is known as yielding.
In crystalline materials, plasticity is usually a consequence of dislocations, which are defects in the crystal structure. At high temperatures, plastic flow will still occur due to thermally activated high-temperature time-dependent plastic deformation mechanisms.
In engineering, the term 'plastic' is used to describe a specific type of mechanical behaviour in a material. Engineering plastics are a category of thermoplastic materials that have better mechanical properties than commodity plastics. They are often used for structural purposes and in industrial components. Examples include ABS, polycarbonate (PC), polymethyl methacrylate (PMMA), PET, polybutylene terephthalate (PBT), polyamide (PA), and polyoxymethylene (POM).
Engineering plastics are chosen when higher mechanical strength or thermal resistance is required than what is offered by commercial plastics such as polyethylene (PE), polypropylene (PP), or polyvinyl chloride (PVC). They offer superior chemical and wear resistance, excellent machinability, reliable dimensional stability, and long-term performance at high temperatures. They can be easily processed at relatively low temperatures and allow for higher machining feeds and speeds, reducing production time and cost.
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Frequently asked questions
Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. It is also known as plastic deformation.
Elasticity is when a material changes shape in response to an external force but then returns to its original shape when the force is removed. Plasticity, on the other hand, is when a material undergoes a non-reversible change and does not return to its original shape.
Examples of plasticity include a piece of metal being bent or pounded into a new shape, rock folding and rock flow within the earth, and twisting a paper clip.
