Omar Khalid
Metals are known for their ability to deform plastically, a characteristic that sets them apart from brittle materials like glass, which tend to fracture when subjected to stress. This plasticity is a result of the rearrangement of the metal's atomic structure, leading to a permanent change in shape. In contrast, elastic deformation is temporary and occurs when the stress applied is below the material's yield point. So, do pure metals behave elastically or plastically? The answer is: it depends. Metals can exhibit both elastic and plastic behaviour, depending on the amount of stress applied. When the stress is within the elastic limit, metals will return to their original shape and size. However, when the stress exceeds this limit, plastic deformation occurs, and the metal remains deformed.
| Characteristics | Values |
|---|---|
| Elastic deformation | Temporary shape change that is self-reversing after the force is removed |
| Plastic deformation | Permanent deformation of the metal |
| Elastic limit | Marks the end of elastic behaviour and the beginning of plastic behaviour |
| Proportional limit | Marks the end of elastic behaviour that can be described by Hooke's law |
| Elastic modulus | The two parameters that determine the elasticity of a material are its elastic modulus and its elastic limit |
| Stress-strain curve | Each material has its own characteristic stress-strain curve |
| Non-proportional elasticity | Exists between the elastic limit and the proportional limit for some materials |
| Strain-energy function | Can be used to predict the behaviour of materials in certain circumstances |
| Elasticity in metals | Caused by resizing and reshaping the crystalline cells of the lattices |
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What You'll Learn
Elastic deformation
In the context of metals, elastic deformation occurs when they are subjected to small loads or stresses. During this process, the bonds between atoms are stretched or bent, but the atoms do not slip past each other. This behaviour is described by Hooke's law, which states that the stress-strain curve can be used to determine Young's modulus, a measure of a material's elasticity.
The ratio between stress and strain under elastic deformation is known as the modulus of elasticity or Young's modulus (E). It is a characteristic of the type of metal and depends on the force of attraction between its atoms. For example, when a steel sheet is bent, the bonds between atoms are only slightly stretched or bent, and the sheet returns to its original shape when the force is removed.
Additionally, the elasticity of a material can be affected by factors such as temperature and the presence of hardening chemicals. To enhance elasticity, softening materials can be added to the mix, allowing the material to bend and give under pressure without permanent deformation. This understanding of elastic deformation is crucial in engineering applications to ensure the safe use of materials within their elastic limits.
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Plastic deformation
The stress required for plastic deformation can be lowered by localizing deformation through line defect movement, rather than sliding the entire lattice plane. This allows for a lower stress application to achieve the same deformation. The force required to break all atomic bonds at once is very high, but by moving dislocations along planes, atoms can slip over each other at a lower stress.
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Elastic limit
The elastic limit of a material is the maximum stress or force per unit area that can be applied to it before the onset of permanent deformation. When the stress is removed, the material returns to its original size and shape. Stresses beyond the elastic limit cause a material to yield or flow, marking the end of elastic behaviour and the beginning of plastic behaviour.
For many metals, the proportional limit is equal to the elastic limit, but this is not always the case. The proportional limit is defined as the point up to which the stress and strain are directly proportional, so that the stress-strain graph is a straight line. For materials where the two limits are not equal, there exists a region of non-proportional elasticity between them.
The elastic limit can be determined by measuring the greatest stress that can be applied to a given sample without causing any permanent deformation. This can be observed as a joggle in the stress-strain curve. However, for some ductile materials like aluminium, titanium, and steel, there is no clear yield point, so the 0.2% offset line is used instead.
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Stress-strain curve
A stress-strain curve illustrates the relationship between stress and strain for a given material. It provides insights into the material's mechanical properties and its behaviour under various stress levels. The curve can be divided into several stages, each indicating how the material responds to stress and whether it undergoes elastic or plastic deformation.
The first stage of the curve is the linear elastic region. In this region, the material's response to stress is elastic deformation, meaning it temporarily changes shape but returns to its original form once the stress is removed. The stress is directly proportional to the strain, and the slope of this region is known as the modulus of elasticity or Young's modulus. This stage ends when the material reaches its yield strength, which is the maximum stress it can withstand without experiencing permanent deformation.
As the stress exceeds the yield point, the curve enters the second stage, known as the strain hardening region. Here, the material starts to undergo plastic deformation, where some atomic bonds break, and the deformation becomes irreversible. The stress continues to increase, reaching a maximum at the ultimate tensile strength (UTS) or ultimate tensile stress. This point represents the maximum stress the material can sustain.
Beyond the ultimate tensile stress, the curve enters the necking and fracture stage. Due to the incompressibility of plastic flow, the cross-sectional area of the material decreases uniformly along its length. This leads to the formation of a 'neck', which becomes more pronounced until the material eventually fractures. The appearance of necking is associated with geometrical instability in ductile materials.
The stress-strain curve is a valuable tool for understanding the mechanical behaviour of materials, including metals. It helps engineers and scientists predict and design materials for specific applications, ensuring they can withstand the expected stresses without failing. The curve also allows for the classification of materials as elastic or plastic, depending on their response to stress and their ability to return to their original shape after deformation.
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Elasticity and plasticity in engineering
Elasticity and plasticity are two important concepts in engineering that describe the behaviour of materials under stress. Understanding these properties is crucial for designing and selecting appropriate materials for various engineering applications.
Elasticity refers to the ability of a material to return to its original shape and size after being subjected to deforming forces. In other words, when the external forces are removed, the material regains its initial configuration. This is often referred to as elastic deformation, where the change in shape is temporary and self-reversing. For example, a slingshot exhibits elasticity as it returns to its original shape after being stretched. The degree of elasticity can be quantified using the Modulus of Elasticity, or Young's Modulus, which represents the ratio of stress to strain.
On the other hand, plasticity describes the behaviour of materials that do not return to their original shape after deformation. Plastic deformation is a persistent change in the shape of a solid body caused by a sustained force. When the stress applied exceeds the material's yield strength, it undergoes permanent distortion, and the atomic bonds break. This is common in metals, where dislocations in the crystal lattice move, causing a limited number of bonds to break.
The distinction between elasticity and plasticity is not always clear-cut, and they can be considered a spectrum. Some materials may exhibit elastic behaviour within a certain range of stresses before becoming plastic. Additionally, ductility and malleability are related concepts. Ductility refers to the ability of a material to be drawn out into wires under tension without breaking, while malleability is the ability to be formed into sheets. Copper, for instance, is highly ductile, making it suitable for wire production.
In engineering, the choice of materials depends on the specific requirements of an application. For structures that need to withstand deformation, elastic materials are preferred. On the other hand, plastic deformation may be desirable in processes like metalworking, where permanent changes in shape are intended. By understanding the elasticity and plasticity of different materials, engineers can make informed decisions to design systems that can withstand specific stress levels and achieve the desired functionality.
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Frequently asked questions
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 comes back to its original size and shape when the load is no longer present.
Plastic deformation is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces.
Yes, pure metals behave elastically until the load exceeds a threshold – the yield strength.
Yes, metals deform plastically when the applied stress exceeds their yield strength. Mercury is an exception to this.
