Ethan Clarke
Deformation is a change in the size or shape of an object due to stress. It can be categorised as elastic or plastic deformation. Elastic deformation is temporary and self-reversing, meaning the object returns to its original shape once the force is removed. This occurs when the stress does not surpass the energy required to break molecular bonds, allowing the material to deform reversibly. On the other hand, plastic deformation is permanent and irreversible. It occurs when the stress is sufficient to permanently deform the material, causing a limited number of atomic bonds to break.
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What You'll Learn
Elastic deformation is temporary and recoverable
Elastic deformation is a temporary change in the shape of a material that is recoverable after the stress is removed. It occurs when the applied stress does not surpass the energy required to break molecular bonds, allowing the material to deform reversibly. In other words, the deformation is still elastic but nonlinear up to the elasticity limit.
Elastic deformation is characterised by the ability of a material to return to its original shape and size after the load or stress is removed. This recovery of the original dimensions is referred to as elastic behaviour. For example, when a material is stretched or compressed within the elastic limit, it will return to its initial state when the stress is removed. This behaviour is observed in various materials, including metals, polymers, and concrete.
The concept of elastic deformation is essential in engineering, particularly in the study of temporary deformation in materials used in mechanical and structural engineering, such as concrete and steel. These materials are designed to undergo small deformations without permanent distortion. By understanding the elastic properties of these materials, engineers can design structures that can withstand specific loads while maintaining their original shape and size.
The elasticity limit, also known as the elastic limit, defines the maximum stress or load that a material can withstand while still exhibiting elastic behaviour. Beyond this limit, the material enters the realm of plastic deformation, where it undergoes irreversible changes. The transition from elastic to plastic deformation depends on factors such as the type of material, size, geometry, and the forces applied.
It is important to note that not all materials exhibit linear elastic deformation. Some materials, like concrete and polymers, respond in a nonlinear fashion, and their behaviour cannot be described by Hooke's Law. However, even in the nonlinear region, if the stress is slowly removed, the material will still exhibit elastic behaviour and return to its original state.
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Plastic deformation is permanent
Plastic deformation is a permanent and irreversible change of shape in response to applied forces. It occurs when the stress applied to a material exceeds the energy required to break its molecular bonds. This results in a disruption of the material's structure, with atoms displaced from their initial positions and chemical bonds broken and reformed. The deformation is considered permanent because the object will not return to its original shape, even after the removal of the applied force.
Elastic deformation, on the other hand, is temporary and reversible. It occurs when the applied stress does not surpass the energy required to break molecular bonds, allowing the material to deform and then return to its original shape once the stress is removed. This type of deformation involves the stretching of bonds, but the atoms do not slip past each other.
The transition from elastic to plastic deformation depends on the material's properties and the forces applied. For example, materials like concrete, steel, and many polymers exhibit elastic deformation under very small deformations. However, when the stress exceeds a certain threshold, known as the yield strength or elastic limit, the material enters the plastic deformation range, and some degree of permanent deformation occurs.
Plastic deformation is observed in a wide range of materials, particularly metals, soils, rocks, concrete, and foams. In metals, plastic deformation is often associated with dislocations in the crystal structure, where the movement of atomic dislocations leads to permanent changes in atomic positions. Soft thermoplastics, ductile metals like copper, silver, and gold, and even wet chewing gum exhibit large plastic deformation ranges.
While plastic deformation may be undesirable in some cases, it can also be advantageous. By deforming in response to applied stresses, plastic materials can reduce these stresses and withstand substantial mechanical work before failure. Additionally, the ability to permanently shape materials through plastic deformation is essential in various forming, shaping, and extruding operations, particularly in metalworking.
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Stress and strain can be graphed
The stress-strain curve can be used to determine the type of deformation a material undergoes, whether it be elastic or plastic. Elastic deformation is temporary and recoverable, with the material returning to its original shape once the stress is removed. This occurs when the stress is below the elastic limit, and the deformation is reversible. On the other hand, plastic deformation is permanent and irreversible. It occurs when the stress exceeds the elastic limit, causing the material to acquire a new shape and size that persists even after the stress is removed.
Each material has its own characteristic stress-strain curve. For example, ductile materials like metals show a gradual decrease in stress with increasing strain, making them easier to deform as the breaking point is approached. Conversely, rubber-like materials exhibit an increase in stress with increasing strain, becoming harder to stretch until they eventually fracture.
The stress-strain curve can also be categorised as either an engineering stress-strain curve or a true stress-strain curve. The engineering curve is based on the original cross-section and gauge length, assuming that the cross-sectional area of the material remains constant during deformation. On the other hand, the true stress-strain curve is based on the instantaneous cross-sectional area and length, acknowledging that the area decreases during deformation due to elastic and plastic deformation.
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Plastic deformation involves breaking atomic bonds
Plastic deformation refers to the permanent change in shape that occurs when a material is subjected to forces beyond its elastic limit. This is in contrast to elastic deformation, which is reversible, and where the material returns to its original shape after the force is removed.
Plastic deformation involves the breaking and reformation of a limited number of atomic bonds, leading to a change in the material's microstructure. This breaking of bonds is facilitated by dislocations, which are defects in the crystal lattice that allow atoms to slide past one another more easily than in a perfect crystal structure. When a material is stressed, these dislocations move through the crystal lattice, enabling plastic deformation.
There are two common types of dislocations: edge dislocations and screw dislocations. Edge dislocations occur when an extra half-plane of atoms is inserted into the crystal structure, creating a distortion in the lattice. Screw dislocations, on the other hand, involve a helical arrangement of atoms around a central axis, resulting in a different type of distortion.
The ability of a material to undergo plastic deformation is influenced by its microstructure, including the size, shape, and distribution of grains and phases within the material. For example, materials with fine grains may exhibit different deformation characteristics compared to those with coarser grains. Additionally, the process of plastic deformation can be utilized in various applications such as forging, rolling, and extrusion to create components with improved mechanical properties and desired shapes or profiles.
It is important to note that the plasticity of a material is dependent on several factors, including the type of bonding and the crystalline structure. For instance, ceramics, with their covalent atomic bonds, typically fail by the extension of flaws rather than by dislocation motion, which would require higher stresses.
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Elastic deformation involves stretching of bonds
Elastic deformation involves the stretching of bonds between atoms. This is a temporary change in the shape of a material, which can be reversed when the force or load causing the deformation is removed. The material returns to its original shape after the stress is removed. This is because, during elastic deformation, the bonds are stretched but the atoms do not slip past each other.
Elastic deformation occurs when the applied stress does not surpass the energy required to break molecular bonds. This means that the material can deform reversibly and return to its original shape. The relationship between stress and strain is generally linear and reversible until the yield point, and the deformation is elastic. However, if the stress is increased above the proportionality limit, the stress is no longer linearly proportional to the strain. Even in this non-linear region, if the stress is slowly removed, the material will still return to its original state.
The maximum value of stress at which the material will remain elastic is called the elastic limit. For stresses above the elastic limit, when the stress is removed, the material will not return to its original state and some permanent deformation sets in. This behaviour is referred to as plastic deformation.
Elastic deformation is similar to the behaviour of a spring, with unloading returning the materials to their original shape. This behaviour is observed in materials such as concrete, grey cast iron, and many polymers.
Elastic deformation is an important concept in engineering, where it is studied in the context of materials used in mechanical and structural engineering, such as concrete and steel, which are subjected to very small deformations.
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Frequently asked questions
Deformation is the change in size or shape of an object. This change can be temporary or permanent. Temporary deformation is called elastic deformation, while permanent deformation is called plastic deformation.
Elastic deformation is a temporary change in the shape of a material that is self-reversing after the force is removed, so the object returns to its original shape. Elastic deformation occurs when the applied stress does not surpass the energy required to break molecular bonds.
Plastic deformation is a permanent change in the shape or size of an object that stays even after the removal of the applied forces. Plastic deformation occurs when the stress is increased beyond the elastic limit of the material.
