Alexei Volkov
In engineering, deformation is the change in size or shape of an object. Deformation can be elastic or plastic. While elastic deformation is reversible, plastic deformation refers to the irreversible change in shape that occurs when a material is subjected to forces beyond its elastic limit. This phenomenon is observed in most materials, especially metals, soils, rocks, concrete, and foams.
| Characteristics | Values |
|---|---|
| Type of deformation | Permanent deformation |
| Other names | Plastic deformation |
| Occurrence | When a material is subjected to forces beyond its elastic limit |
| Nature of change | Irreversible change of shape |
| Applicability | Most materials, particularly metals, soils, rocks, concrete, and foams |
| Mechanical strains | Caused by mechanical stress |
| Ductility | Ductile materials can sustain large plastic deformations without fracture |
| Hardening | Strain hardening makes the material stronger through the movement of atomic dislocations |
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What You'll Learn
Plastic deformation is irreversible
When an object is subjected to a load or stress that causes it to change shape or size permanently, it is referred to as plastic deformation. This is in contrast to elastic deformation, where an object can return to its original shape once the load or stress is removed. Plastic deformation is irreversible, which means that the object will not return to its original shape or form even after the load or stress is released.
The key characteristic of plastic deformation is that it involves the development of irrecoverable strains or permanent displacements within the material's crystal structure. This occurs because the atomic bonds between the particles or molecules of the material are broken and rearranged under the influence of the applied load or stress. As a result, the material's macrostructure and geometry alter permanently.
There are various factors that influence the occurrence of plastic deformation, including the type of load applied (tensile, compressive, or shear), the amount of load or stress exerted, the temperature, and the characteristics of the material itself, such as its crystal structure, grain size, and pre-existing defects. The extent of plastic deformation can vary, and it is typically characterized by parameters such as strain, which measures the relative deformation or change in shape, and stress, which refers to the force applied per unit area.
Materials like metals and alloys frequently undergo plastic deformation during manufacturing processes such as rolling, forging, and extrusion. This is done to achieve desired shapes and properties. However, it's important to note that not all materials undergo plastic deformation in the same way. For example, brittle materials like ceramics and certain polymers may fracture or yield without undergoing significant plastic deformation.
The capacity of a material to withstand plastic deformation is determined by its ability to withstand irreversible changes in its crystal structure. Materials with strong interatomic bonds and a high resistance to dislocation motion are less prone to plastic deformation. On the other hand, materials with weaker bonds or a higher propensity for dislocation motion may exhibit more significant plastic deformation at lower stress levels.
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Temporary deformation is recoverable
In engineering, deformation is the change in size or shape of an object. Deformation can be elastic or plastic. Temporary deformation, also called elastic deformation, is recoverable, meaning that the object returns to its original shape after the force is removed. This type of deformation involves stretching of the bonds, but the atoms do not slip past each other. The relationship between stress and strain is generally linear and reversible until the yield point, after which the deformation becomes plastic.
Plastic deformation, or permanent deformation, is irreversible. The deformation stays even after the removal of applied forces. Plastic deformation occurs when a shear stress exceeds a critical value and causes permanent changes in atomic positions. This can be visualized as a stress-strain curve, with the strain set on the horizontal axis and stress on the vertical axis. As the sample is elongated, the stress variation is recorded until the sample fractures.
The transition from elastic to plastic deformation is known as yielding. An example of this transition can be seen when a tensile load is applied to a ductile metal sample. Each increment of load is accompanied by a proportional increment in extension. When the load is removed, the piece returns to its original size. However, once the load exceeds a threshold, the yield strength, the extension increases more rapidly, and some degree of extension will remain when the load is removed.
Soft thermoplastics, ductile metals such as copper, silver, and gold, and steel have large plastic deformation ranges. On the other hand, hard thermosetting plastics, rubber, crystals, and ceramics have minimal plastic deformation ranges. Materials with a large plastic deformation range, such as wet chewing gum, can be stretched to many times their original length.
In summary, temporary deformation, or elastic deformation, is recoverable, while permanent deformation, or plastic deformation, is irreversible. The study of temporary deformation is important in mechanical and structural engineering, where materials such as concrete and steel are subjected to very small deformations.
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Plastic deformation ends in fracture
In physics and materials science, plasticity, or plastic deformation, refers to a solid material's ability to undergo permanent, irreversible deformation—a change in shape in response to applied forces. This is distinct from temporary, elastic deformation, where a material returns to its original shape after the removal of applied forces.
Plastic deformation involves the breaking and remaking of atomic bonds. It may occur through slip, twinning, or a combination of both methods. Slip occurs when atomic dislocations cause defects in the crystal structure of a material. Twinning, on the other hand, refers to a change in the normal growth mechanism, resulting in a twinned region that separates the crystal into two regions, one being a mirror image of the other relative to the twin plane.
While ductile materials can sustain large plastic deformations without fracture, even these materials will eventually fracture when the strain becomes large enough. This is due to work hardening, which causes the material to become brittle. Brittle materials, on the other hand, show little to no plastic deformation before fracture. Their fractures are characterised by the rapid propagation of cracks with minimal energy absorption.
In the context of engineering, deformation can be elastic or plastic. Plastic deformation under tensile stress is characterised by a strain hardening region and a necking region, which precedes fracture. During strain hardening, the material strengthens as atomic dislocations move. Necking begins after the material reaches its ultimate strength and can no longer withstand maximum stress, leading to a rapid increase in strain. This process culminates in the fracture of the material.
In summary, plastic deformation ends in fracture due to the accumulation of forces beyond the material's ability to withstand them. The transition from elastic to plastic deformation marks a critical stage where the material undergoes irreversible changes, and further stress ultimately results in its rupture.
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Ductile materials can sustain large plastic deformations
Plastic deformation refers to the ability of a solid material to undergo permanent, irreversible changes in shape in response to applied forces. This phenomenon is observed in various materials, particularly metals, soils, rocks, concrete, and foams.
Ductile materials, such as mild steel, aluminum, copper, silver, and gold, are known for their ability to sustain large plastic deformations without fracture. This means they can withstand significant changes in shape without breaking. The ductility of a material refers to its ability to deform plastically, and ductile materials typically exhibit post-elastic strain (plastic strain) greater than 5%.
The plasticity of ductile materials can be explained by the theory of dislocations. Dislocations are defects in the crystal structure of materials that allow for slip, or the movement of planes of atoms relative to each other, resulting in plastic deformation. In ductile materials, the presence of multiple slip systems facilitates deformation by providing multiple directions for dislocation motion.
However, even ductile materials have their limits. When subjected to large enough strains, ductile materials will eventually fracture due to work hardening, which causes the material to become brittle. Work hardening occurs through processes such as grain refinement, precipitation strengthening, solid-solution strengthening, and the continued motion of dislocations, which increase the yield strength of the material at the cost of decreased ductility.
Heat treatments, such as annealing, can restore the ductility of a worked piece by relieving the effects of work hardening, allowing for further shaping and deformation. Additionally, heating metals generally increases their ductility by providing sufficient thermal energy to overcome the energy barrier to dislocation motion, making deformation more feasible.
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Plastic deformation is a fundamental concept in materials science
Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams. It is characterised by a strain-hardening region and a necking region, followed by fracture. During the strain-hardening phase, the material strengthens through atomic dislocation movement, which involves the breaking and reformation of atomic bonds. This movement is influenced by factors such as temperature, impurities, and alloying elements.
Ductile materials, such as copper, silver, and gold, can sustain large plastic deformations without fracture. However, even ductile metals will eventually fracture when the strain becomes too large due to work hardening, which causes them to become brittle. Heat treatments can restore ductility, allowing further shaping. Brittle materials like concrete, rock, and bone exhibit plasticity due to slippage at microcracks or cell rearrangements.
The initiation of plasticity around yielding contributes to high Acoustic Emission (AE) levels. Factors like high strength, strain rate, and low temperature generally increase the relative amplitude of AE signals. The "Kaiser Effect" states that additional AE occurs only when the stress level exceeds previous levels, although this can be broken in fibre-reinforced plastic materials, leading to the "Felicity Effect".
Plastic deformation in metals involves macroscopic changes to the geometrical shape of samples and structures. The response to imposed stresses and strains depends on the material's microstructure and texture. Deformation structures are influenced by the presence of second-phase particles, which can alter the evolution of deformation microstructures. Understanding these relationships is crucial for optimising material properties in engineering applications.
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Frequently asked questions
A non-plastic deformity is called an elastic deformation. Elastic deformation is recoverable, meaning that the object will return to its original shape after the force is removed.
An example of elastic deformation is a rubber band. When you stretch a rubber band, it changes shape, but when you let go, it returns to its original form.
Materials that undergo elastic deformation include rubber, crystals, and ceramics. These materials have minimal plastic deformation ranges and tend to return to their original shape after the applied force is removed.
