Alexei Volkov
Plasticity is a property of solid materials that allows them to undergo permanent deformation or a non-reversible change of shape when subjected to external forces or stress. This property is observed in various materials, including metals, soils, rocks, concrete, and foams. In the field of earth science, plasticity helps explain the behaviour of rocks in the Earth's mantle, which undergoes slow and continuous convective motion. This phenomenon is influenced by crystal defects known as 'disclinations', which are located at the boundaries between mineral grains. By studying plasticity in geological contexts, scientists gain insights into the mechanics of solids and advance the field of materials science.
| Characteristics | Values |
|---|---|
| Definition | The ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces |
| Other Names | Plastic deformation |
| Materials Exhibiting Plasticity | Metals, soils, rocks, concrete, foams, glass, molten rock, alloys, polymers |
| Factors Influencing Plasticity | Ductility, malleability, temperature, pressure, deformation speed, crystal defects, dislocations, grain boundaries, mechanical stress |
| Types | Perfect plasticity, microplasticity, neuroplasticity, structural plasticity |
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What You'll Learn
Plasticity in metals
In physics and materials science, plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. This is also known as plastic deformation. Plasticity differs from elasticity, which is the ability of a solid to change shape temporarily under stress before reverting to its original form.
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 important in forming, shaping, and extruding operations on metals. Most metals are rendered plastic by heating and are then shaped. Crystalline materials contain uniform planes of atoms that may slip past each other along their close-packed directions, resulting in a permanent change of shape within the crystal and plastic deformation.
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). Copper is a very ductile metal with a high degree of plasticity, which is why it is often used in wiring. Even the most ductile metals will harden and become brittle if they undergo cold working, although this effect can be reversed through a slow-heating process called annealing.
Perfect plasticity is a property of materials to undergo irreversible deformation without any increase in stresses or loads. Plastic materials that have been hardened by prior deformation may need increasingly higher stresses to deform further. Plastic deformation is also dependent on the deformation speed, with higher stresses usually required to increase the rate of deformation.
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Plasticity in rocks and minerals
In physics, plasticity is the ability of solid materials to deform or change shape permanently without breaking when subjected to external forces. This is also known as plastic deformation. Plasticity is observed in most materials, particularly metals, soils, rocks, concrete, and foams.
The concept of rock plasticity is based on soil plasticity, which differs from metal plasticity. In soil plasticity, the relative movement of microscopic grains is observed, whereas in metal plasticity, the size of a dislocation is smaller, occurring at the sub-grain level. Rock mass plasticity models assume that rocks have an elastic range and elastic limit, beyond which plastic strain increases while stress remains on the yield surface.
The Boudinage experiments provide evidence of localized plasticity in certain rock specimens that have failed in shear. Compression and tension tests on rock specimens also demonstrate necking and lip formation, indicating plasticity. Additionally, tests conducted by Robertson and other researchers show that plasticity in rocks occurs at high confining pressures, and the higher the confining pressure, the greater the ductility observed.
The effect of temperature on rock plasticity has also been explored, revealing that peak stress decreases with temperature. Extension tests show that intermediate principal stress and strain rate impact the strength of rocks, and increasing the strain rate can make rocks stronger but also more brittle in appearance.
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Plasticity in soils
In physics, plasticity is the ability of solid materials to deform or change shape permanently without breaking when subject to external forces. This is different from elasticity, which is the ability of a solid to change shape temporarily under stress before reverting to its original form. Plasticity can be observed in metalworking when a piece of metal is heated and moulded to form a new shape.
The plasticity of soil is typically measured using Atterberg limits, which are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. The plastic limit (PL) is determined by rolling out a thread of the fine portion of soil on a flat, non-porous surface. If the soil is at a moisture content where its behaviour is plastic, the thread will retain its shape down to a very narrow diameter. The liquid limit (LL) is the water content at which the behaviour of a clayey soil changes from a plastic state to a liquid state. The plasticity index (PI) is a measure of the plasticity of soil and is the difference between the liquid and plastic limits. Soils with a high PI tend to be clay, those with a lower PI tend to be silt, and those with a PI of 0 tend to have little or no silt or clay.
The plasticity of soil is important in civil engineering, where it is used to design structures. Tests are used to ensure that the soil will have the correct amount of shear strength and not too much change in volume as it expands and shrinks with different moisture contents.
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Plasticity in glass
In physics, plasticity is the ability of solid materials to deform or change shape permanently without breaking when subjected to external forces. It is important to note that plasticity differs from elasticity, which is the ability of a solid to change shape temporarily under stress before reverting to its original form.
In the field of glassmaking, silicon dioxide and several other substances are heated to the point of plasticity, moulded into a desired shapes, and then hardened. However, finished glass displays no plasticity, as any sufficient force will cause it to break.
A recent study in Nature Communications has confirmed the existence of topological defects in glasses and their crucial role in plasticity. Identifying these defects in glasses is challenging due to their amorphous and disordered structure, which lacks the long-range order found in crystalline solids. However, these defects are essential for understanding when and where the material will break.
Furthermore, glasses may exhibit soft spots with abnormally low elastic constants and increased mobility, similar to dislocations in crystalline systems. These weak zones are believed to play a role in the plastic flow of the material.
While most metallic glasses exhibit negligible plasticity, some novel bulk metallic glass (BMG) forming systems have demonstrated extreme compressive plasticity. The origin of this plasticity has been linked to nano-scale heterogeneities, specifically the presence of homogenously dispersed nanocrystals formed during fast quenching rates. These nanocrystals are responsible for initiating and regulating plastic deformation.
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Plasticity in geology
In geology, plasticity refers to the ability of rocks and other solid materials to change shape or flow permanently without breaking when subjected to external forces. This phenomenon is known as plastic deformation, and it occurs when the stress exerted on a material exceeds its yield strength, resulting in irreversible changes to its structure.
In the context of geological processes, plasticity can be observed in various materials, including rocks, soils, and molten rock, also known as magma or lava. Rocks, for example, can exhibit plasticity due to movement along microscopic cracks or through processes like rock folding and rock flow under the Earth at high temperatures and pressures.
The plasticity of rocks and other geological materials is influenced by their composition and structure. For instance, the presence of crystal defects or dislocations within the material can enhance its plasticity. In the case of Earth's mantle, which is primarily composed of the mineral olivine, researchers have discovered that little-known crystal defects called "disclinations" play a crucial role in explaining the plasticity and deformation of the rocks in this layer.
Additionally, the temperature and pressure conditions can significantly impact the plasticity of geological materials. For example, most metals exhibit increased plasticity when heated, becoming more malleable and easily shaped. Similarly, the extreme temperatures and pressures found within the Earth can cause molten rock to display plastic deformation, contributing to the formation of igneous rocks.
Understanding the plasticity of geological materials is essential for comprehending various geological processes, including the formation of rock structures, the behaviour of soils, and the dynamics of Earth's mantle. By studying plasticity, scientists can gain insights into the complex interactions between external forces and the deformation of solid materials in the natural world.
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Frequently asked questions
Plasticity in earth science refers to the ability of certain solids to flow or change shape permanently when subjected to stress. This is different from elasticity, where a solid returns to its original shape after the load is removed. Plasticity is observed in geological processes such as the flow of molten rock under the Earth.
Plasticity is observed in rocks, soils, concrete, and foams. In rocks, plasticity occurs due to movement along microscopic cracks, while in soils, the causes of plasticity can be complex and dependent on various factors.
Ductility is the ability of a solid to stretch under stress, while malleability is the ability to be shaped without breaking. Plasticity is directly proportional to both ductility and malleability. Metals, which are highly ductile, can undergo significant plastic deformations without breaking.
