Omar Khalid
Elastic and plastic materials are terms used to describe the behaviour of objects under stress. Elastic materials are those that return to their original shape and size after the removal of external forces. This behaviour is observed when the stress applied is within the elastic limit of the material. On the other hand, plastic deformation is permanent and irreversible. This occurs when the stress applied exceeds the elastic limit, causing the material to undergo a non-reversible change and resulting in a new shape and size. Understanding the distinction between elastic and plastic deformation is crucial in fields such as engineering and materials science.
| Characteristics | Values |
|---|---|
| Elasticity | The tendency of solid objects and materials to return to their original shape after the external forces (load) causing a deformation are removed |
| Plasticity | The ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces |
| Elastic deformation | Temporary and reversible deformation, where the material returns to its original form after the stress is removed |
| Plastic deformation | Permanent and irreversible deformation, where the material does not return to its original shape once the stress is removed |
| Yielding | The transition from elastic behaviour to plastic behaviour |
| Elastic limit | The maximum stress that a material can withstand without undergoing permanent deformation |
| Proportionality limit | Stress values lower than this limit are proportional to strain |
| Elastic modulus | A parameter that determines the elasticity of a material; a high elastic modulus is typical for materials that are hard to deform |
| Elastic-plastic deformation | A type of deformation that includes elastic deformation; it is often referred to as "elasto-plastic deformation" or "elastic-plastic deformation" |
| Perfect plasticity | A property of materials to undergo irreversible deformation without any increase in stresses or loads |
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What You'll Learn
Elastic materials return to their original shape after deformation
Elasticity is the tendency of solid objects and materials to return to their original shape after deformation. When the external forces causing deformation are removed, elastic materials will return to their original size and shape. This elasticity is due to the microscopic structure of the material. For example, the elasticity of polymers and rubbers is caused by the stretching of polymer chains under an applied force. On the other hand, the elasticity of metals is caused by the resizing and reshaping of crystalline cells in their lattice structures.
The two parameters that determine the elasticity of a material are its elastic modulus and elastic limit. The elastic modulus describes how much force is required to deform a material, with a high elastic modulus indicating a material that is hard to deform. The elastic limit, or yield strength, is the maximum stress a material can withstand without undergoing permanent deformation. When the stress on a material exceeds its elastic limit, it enters a state of plastic deformation and will not return to its original shape.
Plastic deformation is a permanent and irreversible change in the shape of a material in response to applied forces. It occurs when the stress applied exceeds the elastic limit of the material. In engineering, the transition from elastic to plastic behaviour is known as yielding. Plastic deformation is observed in various materials, particularly metals, soils, rocks, concrete, and foams, and is an important concept in manufacturing and construction.
The difference between elastic and plastic deformation is crucial in engineering and materials science. Materials designed to undergo elastic deformation, such as springs, must return to their original shape after being stressed. On the other hand, materials intended to undergo plastic deformation, such as metals used in construction, must be designed to withstand the expected stresses without failure.
While elastic deformation is temporary and reversible, it is important to note that it is an approximation. The quality of elastic deformation depends on the timeframe considered and the loading speed. Additionally, most materials exhibit a range of elastic and plastic deformation, depending on the amount of force or pressure applied. If the force remains within the elastic limit, the deformation is temporary and reversible. However, if the force exceeds the elastic limit, the material undergoes plastic deformation and changes shape permanently.
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Plastic deformation is permanent and irreversible
Elasticity is the tendency of solid objects and materials to return to their original shape after the external forces causing deformation are removed. An object is elastic when it returns to its original size and shape when the load is no longer present. Elastic deformation is thus temporary and recoverable.
Plastic deformation, on the other hand, is an irreversible process. It occurs when a material is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength, causing it to elongate, compress, buckle, bend, or twist. This deformation is permanent and does not disappear even after the removal of the initiating stress or load. The material will retain a permanent deformity because it has exceeded its elastic limit.
The distinction between elastic and plastic deformation lies in the degree of stress applied to the material. When the stress is within the elastic limit, the material exhibits elastic behaviour and returns to its original shape once the stress is removed. However, when the stress exceeds the elastic limit, the material undergoes plastic deformation and experiences permanent changes in atomic positions, resulting in a new equilibrium location.
Plastic deformation can be observed in various materials, including metals, ceramics, polymers, and glasses. In metals, plastic deformation occurs through dislocation plasticity, while in brittle materials like concrete, rock, and bone, it occurs due to the slippage of microcracks. The deformation may lead to fracture or rupture if the deformation continues and cracks start to form and propagate.
While plastic deformation is irreversible, it is not inherently undesirable. By deforming in response to applied stresses, plastic materials can reduce these stresses and withstand substantial mechanical work before failure. This property of enduring deformation under pressure, known as plasticity, allows materials to distribute stress and prevent sudden failure.
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Plastic deformation occurs when stress exceeds the elastic limit
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. In other words, an object is elastic when it snaps back to its original size and shape once the load is no longer present. The two parameters that determine the elasticity of a material are its elastic modulus and its elastic limit. A high elastic modulus is typical for materials that are hard to deform; in other words, materials that require a high load to achieve a significant strain.
Plastic deformation, on the other hand, occurs when stress exceeds the elastic limit of a material. This means that the material deforms irreversibly and does not return to its original shape and size, even when the load is removed. In engineering, 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.
The mathematical theory of plasticity, or flow plasticity theory, uses a set of non-linear, non-integrable equations to describe the set of changes in strain and stress with respect to a previous state and a small increase in deformation. If the stress exceeds a critical value, the material will undergo plastic, or irreversible, deformation. This critical stress can be tensile or compressive.
Plastic deformation of metals involves macroscopic changes to the geometrical shape of samples and structures. The way in which the material responds to imposed stresses and strains is determined by its microstructure and texture. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams. However, the physical mechanisms that cause plastic deformation can vary widely.
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Plasticity is observed in metals, soils, rocks, concrete, and foams
Elasticity is the tendency of solid objects and materials to return to their original shape after the external forces (load) causing a deformation are removed. An object is elastic when it returns to its original size and shape when the load is no longer present. Plasticity, on the other hand, is observed when stress exceeds the elastic limit of a material, causing it to undergo irreversible plastic deformation. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams.
Metals
Plasticity in metals is usually a consequence of dislocations in their crystal structure. Dislocations are defects that are relatively rare in most crystalline materials but are common in some metals, such as ductile metals, and are part of their crystal structure. At high temperatures, plastic behaviour can also be influenced by the motion of dislocations in individual grains in the microstructure. Additionally, plasticity in a crystal of pure metal is caused by two modes of deformation in the crystal lattice: slip and twinning. Slip is a shear deformation that moves atoms away from their initial positions, while twinning is plastic deformation that occurs along two planes due to a set of forces applied to a given metal piece. Most metals exhibit greater plasticity when hot than when cold. For example, lead displays sufficient plasticity at room temperature, while cast iron does not exhibit enough plasticity for any forging operation, even when heated.
Soils
Soils, particularly clays, exhibit significant inelasticity under load. The causes of plasticity in soils are complex and depend on factors such as microstructure, chemical composition, and water content. Plastic behaviour in soils is primarily caused by the rearrangement of clusters of adjacent grains.
Rocks
Rocks can exhibit plasticity through localized plasticity, as observed in certain rock specimens that have failed in shear. Plasticity in rocks is typically observed through techniques such as compression and tension tests, which show necking of rock specimens, and wedge penetration tests, which reveal lip formation. The transition from elastic to plastic behaviour in rocks may indicate the transition from softening to hardening. The effect of temperature on rock plasticity has been studied, with findings suggesting that an increase in temperature enhances the rate effect in the plastic behaviour of rocks.
Concrete
Concrete is a brittle material, and plasticity in concrete is predominantly caused by slip at microcracks. Inelastic deformations in concrete are primarily caused by the formation of microcracks and sliding motions relative to these cracks.
Foams
Plasticity in foams, such as liquid foams or cellular materials, is mainly due to bubble or cell rearrangements, specifically T1 processes. Foams can be made of any material with a plastic yield point, including rigid polymers and metals. The plasticity behaviour in foams depends on whether they are open-cell or closed-cell foams. In open-cell foams, plasticity occurs when the bending moment is exerted on the cell walls, causing them to yield axially. In closed-cell foams, the yield strength increases when the material is under tension due to the membrane that spans the face of the cells.
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Elastic deformation is temporary and reversible
Elastic deformation is a temporary and reversible process. When an object is subjected to external forces or loads, it may undergo a change in shape or size, but if this change is elastic, the object will return to its original form once the load is removed. This behaviour is characteristic of elastic materials and objects.
The elasticity of a material depends on its microscopic structure. For example, the elasticity of polymers and rubbers is caused by the stretching of polymer chains under an applied force. On the other hand, the elasticity of metals is caused by the resizing and reshaping of crystalline cells of lattices, which are the material structures of metals.
Each material has its own elastic limit, which is the maximum stress a material can withstand without undergoing permanent deformation. As long as the stress applied to a material is within its elastic limit, it will deform in a reversible manner. This means that the material will return to its original shape and size once the load is removed.
Plastic deformation, on the other hand, is permanent and irreversible. When the stress applied to a material exceeds its elastic limit, it undergoes plastic deformation and does not return to its original shape when the stress is removed. In other words, the material has been permanently deformed and will take on a new shape and size.
The distinction between elastic and plastic deformation is crucial in engineering and materials science. Materials designed to undergo elastic deformation, such as springs, must be able to return to their original shape after being stressed. On the other hand, materials intended to undergo plastic deformation, such as metals used in construction, must be designed to withstand the expected stresses without failure.
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Frequently asked questions
Elasticity is the ability of solid objects and materials to return to their original shape after the external forces (load) causing a deformation are removed.
Plastic deformation is when a material permanently changes shape due to stress beyond its elastic limit.
Elastic deformation is temporary and reversible, as the material returns to its original form after the stress is removed. Plastic deformation, on the other hand, is permanent and irreversible.
Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams. For example, rolling steel into a particular shape involves plastic deformation, as a new shape is created.
Understanding the concepts of elastic and plastic deformation is crucial in fields like engineering and materials science. Materials that are intended to undergo plastic deformation, such as metals used in construction, must be designed to withstand the stresses they will be subjected to without failure.
