Alexei Volkov
Plastic strain is a phenomenon that occurs when a material is subjected to stress beyond its yield point, resulting in permanent deformation. Unlike elastic deformation, where objects return to their original shape after the stress is removed, plastic deformation is irreversible. This is because the stress applied exceeds the energy required to break molecular bonds, causing a permanent change in the material's structure. While some materials, such as concrete, exhibit predominantly elastic behaviour, others like soft thermoplastics and ductile metals have a large plastic deformation range. This means they can be stretched well beyond their original length, and the strain is not recoverable.
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What You'll Learn
Plastic strain is irreversible
When a load is applied to a material, it initially exhibits elastic behaviour, where the deformation is proportional to the stress and is completely recoverable. This is because the applied stress does not exceed the energy required to break molecular bonds, allowing the material to deform reversibly. However, once the yield point is reached, the material undergoes plastic deformation, characterised by a strain-hardening region and a necking region. During the strain-hardening phase, the material strengthens through the movement of atomic dislocations, making it harder and less ductile.
In the necking phase, the cross-sectional area of the material decreases, and it can no longer withstand the maximum stress, leading to a rapid increase in strain. This results in irreversible plastic deformation as the material fractures or ruptures. The extent of plastic deformation depends on the type of material, with soft thermoplastics and ductile metals like copper, silver, and gold exhibiting larger plastic deformation ranges compared to hard thermosetting plastics, rubber, crystals, and ceramics.
While it was once believed that plastic deformation was entirely unrecoverable, recent studies have suggested the concept of reverse plasticity. In certain rare cases, it has been observed that after the removal of stress, the plastic strain does not recover in the negative direction but continues to accumulate in the forward direction. This non-monotonic recovery challenges the traditional understanding of plastic deformation and suggests that material memory effects may play a role in strain recovery.
The distinction between recoverable and unrecoverable strain is crucial in engineering and material science. By understanding the yield point and the behaviour of materials beyond this point, engineers can design structures that can withstand complex stresses without failing. Additionally, the accumulation of plastic strain in materials like pipes can impact their material properties, toughness, and susceptibility to stress corrosion cracking, highlighting the importance of managing and limiting plastic deformation in certain applications.
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Elastic deformation is recoverable
Elastic deformation is the property of materials with a high degree of cross-linking and porous structures that can deform under external forces and recover their original shape once those forces are removed. Elastic deformation is recoverable, and the degree of recovery depends on the elastic modulus of the material. This deformation occurs at short timescales and is transmitted at phonon velocities. It encompasses elastic deformation that arises from bond stretching and twisting, and anelastic deformation that arises from atomic reconfigurations, such as defect motions.
Elastic deformation is instantaneous and completely recoverable, while plastic deformation is gradual and irreversible. Plastic deformation occurs when a material is subjected to a load beyond its yield point, causing it to permanently deform and not return to its original shape. The yield point is the maximum load that a material can withstand without experiencing plastic deformation. If the load exceeds this point, the material will undergo permanent deformation, and the strain will not be recoverable.
In the context of structural materials, such as pipes, it is important to consider the accumulated plastic strain to ensure that the material properties do not deteriorate. This is particularly relevant for maintaining fracture toughness and preventing stress corrosion cracking. By limiting the accumulated plastic strain through strain aging testing, the risk of failure can be mitigated.
The distinction between elastic and plastic deformation is crucial in engineering to prevent failures and disasters. Elastic deformation is reversible, allowing materials to return to their original state, while plastic deformation leads to permanent changes that can have significant consequences, such as in the examples of collapsed bridges, burst boilers, rail accidents, and aircraft crashes.
The recoverable nature of elastic deformation is essential for designing resilient structures and materials that can withstand external forces without permanent deformation. By understanding the elastic properties of materials, engineers can ensure that structures can recover from temporary deformations caused by loads or stresses, maintaining their integrity and functionality over time.
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Plastic deformation is permanent
The concept of plastic deformation is important in engineering and materials science. It refers to the behaviour of a material when subjected to an applied load or stress beyond its yield point or yield strength. The yield point is the maximum stress or load that a material can withstand without experiencing permanent deformation. When the yield point is exceeded, the material undergoes plastic deformation, leading to a permanent change in its shape or structure.
The extent of plastic deformation depends on various factors, including the type of material, its microstructure, and the magnitude and direction of the applied load. Different materials have different yield strengths, and their ability to withstand plastic deformation varies. For example, soft thermoplastics and ductile metals like copper, silver, and gold have a large plastic deformation range, meaning they can undergo significant plastic deformation before fracturing. On the other hand, hard thermosetting plastics, rubber, crystals, and ceramics have minimal plastic deformation ranges.
Plastic deformation can have both desirable and undesirable effects. In some cases, it is intentionally induced to improve the properties of a material. For instance, strain hardening, a technique used in metalworking, involves plastic deformation to increase the strength and hardness of a metal. However, excessive plastic deformation can lead to undesirable outcomes, such as fracture or failure of the material.
Additionally, plastic deformation is associated with the accumulation of residual stresses within the material. These residual stresses can have significant implications for the material's performance and susceptibility to corrosion or cracking. For example, in pipes, plastic deformation can impose high residual stress, increasing the risk of stress corrosion cracking in the presence of certain substances, such as H2S. Therefore, it is crucial to monitor and control plastic deformation in engineering applications to ensure the integrity and functionality of structures and components.
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Plastic deformation occurs under constant load or stress
Plastic deformation is the permanent distortion that occurs when a material is subjected to stresses beyond its yield strength. This can cause the material to elongate, compress, buckle, bend, or twist. Plastic deformation occurs when the deformation is in the elastic limit, and the load is too high compared to the amount of strain. This is known as the plastic limit stress (PLS) or the plastic limit strain.
The irreversible process of plastic deformation occurs when shear stress exceeds a critical value, causing permanent changes in atomic positions. This is assumed to occur at a constant volume, as is generally the case for metals. However, ceramics, which can densify under stress at high temperatures, may be an exception to this assumption.
When the load on a material is removed after plastic deformation, it will retain a permanent deformation because it has exceeded its elastic limit. This is known as residual strain or residual stress. Materials that can recover their original shape without any residual stress after the load is removed are called ductile materials.
The concept of recoverable strain refers to the strain recovered once a stress is removed from a body, in the direction opposite to the imposed stress. Polymer chains, for example, will recoil after the stress is removed. Any unrecoverable strain is attributed to irreversible plastic deformations.
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Plastic deformation increases the yield stress of the material
Plastic deformation is an irreversible process, and any unrecoverable strain is attributed to this phenomenon. When a material undergoes plastic deformation, it does not return to its original form even after the stress is relieved. This is because the deformation caused by plastic flow is permanent. The material has then moved into the region referred to as plastic deformation, and the yield strength is equal to the stress at which noticeable plastic deformation has occurred.
The increase in yield stress due to plastic deformation is essential in various applications, especially in designing and selecting materials for specific purposes. For example, in the case of pipelines, the accumulated plastic strain increases the yield stress of the material, improving its ability to withstand higher stresses without failure. This is particularly relevant in reeling installation methods, where the pipeline may be exposed to more than 2% accumulated plastic strain.
The mechanisms behind plastic deformation involve processes such as dislocation motion, vacancy motion, twinning, phase transformation, and viscous flow of amorphous materials. These processes can increase the yield strength of the material by hindering dislocation motion, such as through grain refinement, precipitation strengthening, and solid-solution strengthening. While these processes increase yield strength, they typically come at the cost of decreased ductility.
While plastic deformation is generally considered irreversible, recent studies have explored the concept of recoverable strain from reverse plasticity. These studies suggest that, in some cases, strain recovery can occur through 'reverse plasticity', where the strain recovers in the negative direction, opposite to the previously imposed stress. However, this behaviour is not always observed, and further research is ongoing to understand the mechanisms behind it.
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Frequently asked questions
Plastic strain is not recoverable because it is a permanent deformation that stays in place even after the removal of the applied forces.
Plastic deformation is a permanent distortion that remains after unloading. It occurs when the applied stress surpasses the energy required to break molecular bonds, preventing the material from returning to its original shape.
Many materials can undergo plastic deformation, including soft thermoplastics, ductile metals such as copper, silver, and gold, and wet chewing gum.
Recoverable strain is the elastic recovery of a material to its original shape once the applied stress is removed. Polymer chains, for example, will recoil after being stretched when the stress is removed.
Plastic deformation can increase the hardness of a material, making it more susceptible to stress corrosion cracking. It also imposes high residual stress, which can further promote cracking.
