Maya Ellison
Plasticity, in physics, is the ability of solid materials to deform or change shape permanently without breaking when subjected to external forces. Plastic deformation is irreversible and remains even after the removal of the applied forces. It is a property of ductile and malleable solids. The transition from elastic behaviour to plastic behaviour is known as yielding. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams. The plasticity of a material is directly proportional to its ductility and malleability.
| Characteristics | Values |
|---|---|
| Composition | Synthetic or semi-synthetic compounds, mainly polymers |
| Texture | Smooth, shiny, rough, matte, transparent |
| Flexibility | Rigid or flexible |
| Electrical Conductivity | Non-conductive |
| Durability | Durable |
| Strength | Strong |
| Weight | Lightweight |
| Cost | Low-cost |
| Ease of Production | Easy to produce |
| Resistance | Chemical resistance, impact resistance, wear resistance |
| Toxicity | Low toxicity |
| Environmental Impact | Negative environmental impact |
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What You'll Learn
Plastic deformation
In crystalline materials, uniform planes of atoms are organised with long-range order. These planes can slip past each other, resulting in a permanent change in the crystal's shape and plastic deformation. Dislocations are defects where an extra half-plane is inserted into the lattice, and they play a crucial role in plastic deformation by facilitating the slipping of planes.
The fundamental mechanism of plastic deformation in metals is slip movement, where blocks of crystals slide over each other along different crystallographic planes called slip planes. This occurs when the applied stress exceeds the critical resolved shear stress of the material. Another mechanism, twinning, occurs when a portion of the crystals takes up an orientation related to the rest of the lattice symmetrically.
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Elasticity and plasticity
In physics, elasticity is the tendency of solid objects and materials to return to their original shape and size after the removal of external forces (load) causing deformation. The physical reasons for elastic behaviour vary among materials and depend on their microscopic structure. For instance, the elasticity of polymers and rubbers is caused by the stretching of polymer chains under an applied force, whereas the elasticity of metals is caused by the resizing and reshaping of crystalline cells of the lattices.
Plasticity, also known as plastic deformation, 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. However, the physical mechanisms causing plastic deformation vary. At a crystalline scale, plasticity in metals is usually due to dislocations, which are defects that are typically rare in most crystalline materials but are numerous in some and are part of their crystal structure.
The transition from elastic behaviour to plastic behaviour is known as yielding. When stress is gradually increased beyond the elastic limit, the material undergoes plastic deformation. This behaviour can be visualised using a stress-strain diagram, where the relationship between stress and strain is plotted. Each material has its own characteristic strain-stress curve.
Rubber-like materials show an increase in stress with increasing strain, eventually reaching a fracture point where they break. In contrast, ductile materials like metals show a gradual decrease in stress with increasing strain, becoming easier to deform as stress-strain values approach the breaking point.
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 require higher stresses to deform further. Plastic deformation is also dependent on the deformation speed, with higher stresses typically needed to increase the rate of deformation.
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Stress and strain
Plasticity, in the context of physics and materials science, refers to the ability of a solid material to undergo permanent deformation—a non-reversible change in shape—when subjected to external forces or loads. This phenomenon is also known as plastic deformation. When a material undergoes plastic deformation, it does not return to its original shape and size, even after the removal of the load.
According to Hooke's law, in the elastic region, the stress and strain are directly proportional. This means that as the load increases, the deformation increases proportionally, and when the load is removed, the material returns to its original shape. However, if the load exceeds the elastic limit, the material enters the plastic region, and permanent deformation occurs. In this region, the stress-strain relationship becomes nonlinear, and the material may undergo irreversible changes, failing to return to its original shape and size.
The transition from elastic behaviour to plastic behaviour is known as yielding. After a material yields, it begins to experience a high rate of plastic deformation. The yield point is the point on the stress-strain curve where the material starts to deviate from Hooke's law, and the stress-strain relationship becomes nonlinear. The region between the yield point and the ultimate strength of the material is where strain hardening occurs, increasing the material's strength.
The stress-strain curve is a graphical representation of the relationship between stress and strain for a particular material. It illustrates the elastic and plastic regions of the material and is used to predict its behaviour under load. The curve typically starts at the origin, with no load and no deformation, and then progresses to the yield point, where the material yields and deviates from linear behaviour. Beyond the yield point, the curve enters the plastic region, where permanent deformation occurs, and eventually reaches the ultimate strength, where the material can no longer withstand the load and fails.
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Plasticity in metalworking
Plasticity in the context of physics and materials science refers to the ability of solid materials to undergo permanent deformation when subjected to external forces, without breaking. This is distinct from elasticity, where materials can return to their original shape after the stress is removed.
Plasticity is commonly observed in metalworking processes, where metals are heated and shaped. Various methods such as forging, rolling, and extrusion utilise plasticity to manipulate metals. Ductile metals like copper exhibit significant plasticity, while more brittle materials like cast iron do not undergo plastic deformation effectively.
In crystalline materials, plasticity is facilitated by the presence of uniform planes of atoms that can slip past each other, resulting in a permanent change of shape within the crystal. This phenomenon is particularly evident in metals, where dislocations and defects in the crystal lattice allow for slip and twinning deformations. At the atomic level, plasticity in metals occurs through the movement of atoms along atomic slip planes, resulting in dislocations and glitches in the atomic stack-ups.
The plasticity of a material is influenced by its ductility, or its ability to stretch under stress, and its malleability, or its capacity to be shaped without breaking. Nanostructured metals, for example, typically exhibit high plasticity but low ductility due to their low strain hardening capability. Increasing the strain hardening rate through microstructural modifications can enhance ductility in these materials.
Understanding the plastic behaviour of metals is crucial in manufacturing. From raw materials to finished products, plastic deformation plays a role in shaping and forming metals into desired shapes and geometries. Engineers employ constitutive models, such as the Johnson-Cook (JC) model, to simulate plasticity in metals and optimise manufacturing processes.
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Plasticity in crystals
Crystal plasticity assumes that any deformation applied to a material is accommodated by the process of slip, where dislocation motion occurs on a slip system. This slip is a shear deformation that moves atoms through many interatomic distances relative to their initial positions. Each slip system can undergo different amounts of shearing, and the accumulated strain updates the critical resolved shear stress according to various hardening models.
In a crystal of pure metal, plasticity is primarily caused by two modes of deformation in the crystal lattice: slip and twinning. Twinning is the plastic deformation that takes place along two planes due to a set of forces applied to a given metal piece. On an atomistic level, plastic deformation of a metal requires the generation and motion of crystal dislocations. This motion of dislocations is called a crystallographic glide, which is confined to a few crystallographic planes and directions, usually the most densely packed planes and directions in a crystal.
Crystal plasticity is of great importance in understanding the flow of multi-phase magmas, which is crucial for interpreting eruption dynamics and assessing volcanic hazards. It has also been used to study the behaviour of magma at volcanic temperatures and load conditions, which can help determine a volcano's eruptive style.
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Frequently asked questions
Plasticity is the ability of certain solids to flow or change shape permanently when subjected to stresses of intermediate magnitude. It is also known as plastic deformation.
Elastic deformation is reversible, meaning the deformation disappears after the removal of applied forces. Plastic deformation, on the other hand, is irreversible and remains even after the removal of applied forces.
Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams. It is also seen in metalworking, glassworking, and geological processes such as the flow of molten rock.
The plasticity of a material is influenced by its ductility, the ability to stretch under stress, and its malleability, the ability to be shaped without breaking. It is also affected by the deformation speed, with higher stresses required to increase the rate of deformation.
