Ethan Clarke
Plasticity is a term used in physics and materials science to describe the ability of a solid material to undergo permanent, non-reversible deformation in response to applied forces. This phenomenon, also known as plastic deformation, is distinct from elastic behaviour, where a material returns to its original shape after the force is removed. Plastic deformation is observed in a wide range of materials, including metals, soils, rocks, concrete, foams, and even biological tissues. It is a result of the ductility and malleability of the material, and its propensity to flow or change shape permanently under stress. The causes of plasticity vary depending on the type of material, with factors such as temperature, microcracks, and crystal lattice deformations playing a role in the behaviour of different substances.
| 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 |
| Occurrence | Most materials, particularly metals, soils, rocks, concrete, foams, and bone |
| Cause in crystalline materials | Two modes of deformation in the crystal lattice: slip and twinning |
| Cause in brittle materials | Slip at microcracks |
| Cause in cellular materials | Bubble or cell rearrangements, notably T1 processes |
| Cause in ductile metals | Tensile loading |
| Cause in soils | Complex and dependent on many factors, including hardness, corrosion, and physical properties |
| Cause in amorphous materials | Discussion of dislocations is inapplicable |
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What You'll Learn
Plastic deformation
In ductile metals, tensile loading causes elastic deformation, where the piece returns to its original size when the load is removed. However, once the load exceeds the yield strength, the extension increases rapidly, and some degree of permanent deformation remains, leading to plastic deformation. This transition from elastic to plastic behaviour is known as yielding.
The plasticity of a material is directly proportional to its ductility and malleability. Materials with higher ductility and malleability can undergo larger plastic deformations before failure. Additionally, most metals exhibit greater plasticity when heated, making it easier to shape them through processes like forging and extrusion.
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Slip and twinning
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. Plastic deformation occurs in many materials, particularly metals, soils, rocks, concrete, and foams.
Plastic deformation in metals involves slip and twinning. Slip is the primary deformation mechanism and occurs when shear stress exceeds a critical value, following Schmid's law. It is the sliding of crystal blocks over one another along various crystallographic planes referred to as slip planes. When dislocations move through the crystal beyond its elastic limit, it results in a step formation on the crystal surface. Slip is the most common mode of plastic deformation.
Twinning occurs when slip is not possible and results in a deformed mirrored grain. It is another deformation mechanism that occurs in BCC and HCP crystals under high shear stresses. Twinning involves the parallel displacement of planes of atoms in the same direction, resulting in a mirrored atomic arrangement across the twinning plane. In twinning, a portion of the crystals adopts an orientation that is clearly and symmetrically connected to the direction of the remaining untwined lattice. Atoms parallel to the twin plane travel along the lattice during twinning, causing the lattice within the twinned region to distort.
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Yielding
Plastic deformation occurs in most materials, particularly metals, soils, rocks, concrete, and foams. However, the mechanisms that cause plastic deformation vary widely. For example, in crystalline materials, plasticity is caused by dislocations in the crystal lattice, which are relatively rare. In contrast, in metals, dislocations are numerous and are part of their crystal structure.
In brittle materials, such as rock, concrete, and bone, plasticity is caused by slip at microcracks. On the other hand, in cellular materials like liquid foams or biological tissues, plasticity is a result of bubble or cell rearrangements.
The yield strength of a material is the critical stress value at which it transitions from elastic to plastic deformation. Various criteria are used to determine whether a material has yielded, such as the Tresca and von Mises criteria. However, these criteria may not be suitable for all materials, and other yield criteria are also used.
Perfect plasticity is a concept that describes the mechanical behaviour of materials that do not exhibit an increase in stress with an increase in strain during yielding. In other words, the yield surface remains constant during plastic deformation. Prager (1949) proposed that for a material characterised by plastic behaviour, the stress state must remain on the yield surface to ensure an accurate description of the physical process.
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Ductility and malleability
Ductility
Ductility is the ability of a material to be drawn out into a thin wire. It is the opposite of brittleness and is associated with metals that can be stretched without breaking. Ductility can be used to qualitatively rank materials. For example, it is easy to observe 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.
Ductility is largely governed by the strain hardening rate, which is influenced by the microstructure of the material. Nanostructured metals, for example, have low ductility due to their low strain hardening capability. By increasing the strain hardening rate through modifications to the microstructure, ductility can be improved.
Malleability
Malleability is the ability of a material to be hammered or rolled into thin sheets without rupturing. Materials that are malleable can be reshaped without breaking. For example, gold can be beaten into sheets of gold leaf, and dough can be rolled out into a thin crust. If a material can be hammered flat, it is malleable, and if it cannot, it is not.
Most materials that are malleable are also ductile. However, some metals, like lead, are significantly less ductile than they are malleable.
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Brittle materials
In physics and materials science, plasticity is a material's ability to undergo enduring deformation under load when compressed. It is the propensity of a material to undergo irreversible changes of shape in response to applied forces.
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Frequently asked questions
Plasticity in materials science is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces.
Plastic deformation is observed in most materials, especially metals, soils, rocks, concrete, and foams.
The word plastic has its origin in the Greek word plastikos, meaning to shape or to form.
