Ethan Clarke
Plastic deformation is the irreversible and permanent distortion of a material when it is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength. It is observed in a variety of materials, particularly metals, soils, rocks, concrete, and foams. The transition from elastic to plastic deformation is marked by yielding, where the load exceeds a threshold and some degree of extension remains after the load is removed. This process generates heat, with the greatest temperature changes observed in the second stage of deformation, and the amount of heat generated depends on the level of crystallinity. The heat generation associated with plastic deformation is a volumetric phenomenon, occurring in the subsurface rather than on the surface. The plastic deformation process involves the breaking and re-establishing of interatomic bonds, requiring higher stresses to increase the rate of deformation.
| Characteristics | Values |
|---|---|
| Definition | Plastic deformation is the permanent and irreversible distortion of a material. |
| Causes | Tensile, compressive, bending, or torsion stresses that exceed a material's yield strength. |
| Effects | Elongation, compression, buckling, bending, or twisting of the material. |
| Time dependence | Plastic deformation may be time-dependent (creep) or time-independent. |
| Temperature dependence | Creep is traditionally associated with elevated temperatures, but can occur at all temperatures above absolute zero Kelvin. |
| Heat generation | Plastic deformation can generate heat, with the highest temperatures typically occurring in the subsurface. |
| Crystallinity | The level of crystallinity in a material can influence the temperature rise during plastic deformation. |
| Ductility | Plastic deformation is influenced by the ductility and malleability of the material. |
| Plasticity | Perfect plasticity allows for irreversible deformation without increased stresses or loads. |
| Deformation mechanisms | Slip and twinning are two modes of deformation in a crystal lattice that contribute to plasticity. |
| Atomic structure | Plastic deformation involves breaking and re-establishing interatomic bonds, but the overall atomic structure remains locally unchanged. |
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What You'll Learn
Heat generation during plastic deformation
Plastic deformation is the permanent distortion that occurs when a material is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength. Plastic deformation is irreversible and consists of time-dependent and time-independent components. At elevated temperatures, diffusion occurs extensively, and time-dependent plastic deformation (creep) continues for a long period.
The Finite Element Method (FEM) is a valuable tool for analysing heat generation during plastic deformation. FEM models can predict heat generation by considering various impact velocities and material properties. These models have confirmed that heat generation during plastic deformation is a volumetric phenomenon, with peak temperatures generally found in the subsurface region. This contradicts the traditional view that dissipation occurs primarily through surface friction.
In experiments with TRIP steels, the fraction (β) of plastic work converted into heat was analysed. It was found that β increases with plastic deformation, and the latent heat due to phase transformation is a significant factor. The heat generated during plastic deformation can impact the microstructure of the material, potentially leading to changes in the deformation mechanism and affecting workability and energy consumption.
Additionally, the deformation-induced temperature can be predicted based on the corresponding strain, and there is a linear correlation between deformation-induced temperature and plastic strain. This relationship is utilised to establish prediction models for heat generation rates and strain rates.
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The thermodynamics of plastic deformation
Plastic deformation refers to a material's permanent alteration in shape or size when subjected to a certain level of stress or temperature that exceeds its elastic limit. This phenomenon is irreversible and comprises both time-dependent and time-independent components. The time-dependent aspect, known as "creep," involves a slow and continuous deformation over an extended period under load. On the other hand, plastic deformation at room temperature is time-independent and occurs within a short duration.
The heat generation associated with plastic deformation has been investigated, particularly during particle impact. Finite Element Method (FEM) and theoretical models have been employed to predict the heat generated during normal impact. These models have revealed that the highest temperatures are typically found in the subsurface region, indicating that heat generation is a volumetric phenomenon rather than surface friction.
In the context of semi-crystalline polymers, thermal effects during plastic deformation have been examined. An endothermic effect is observed in the initial stage, followed by an exothermic effect during the yield region when the neck is formed. The level of crystallinity influences the temperature rise, with higher crystallinity resulting in increased temperature. Structural changes, such as the decrease in entropy of polymer chains, the formation and growth of cavities, and strain-induced crystallization, contribute to heat generation.
Additionally, the thermodynamics of plastic deformation in metals has been explored. The plastic deformation of metals is predominantly facilitated by shearing, where lattice planes slide over each other, allowing shape changes without significantly affecting the atomic arrangement. The generation and movement of dislocations are crucial, and the density of these dislocations can be determined using thermodynamic principles. The Burgers Vector (b) characterizes the amount of shear produced by dislocation movement.
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Plastic deformation as a source of heat
Plastic deformation is the permanent distortion that occurs when a material is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength. This deformation is irreversible and consists of time-dependent and time-independent components. At elevated temperatures, plastic deformation can continue for a long period, and diffusion can assist this process.
Plastic deformation is a significant source of heat. The heat generated during plastic deformation is a result of the plastic work done on the material. The energy consumed for deformation is converted into heat, leading to a temperature increase. This is particularly noticeable in the second stage of deformation, when the neck is being formed. The degree of crystallinity in polymers, for example, affects the temperature jump during this transition. The heat generated is influenced by the level of plastic deformation, with greater deformation resulting in more heat.
The heat generation associated with plastic deformation has been studied across various materials, including metals, alloys, polymers, and glasses. In metals, the movement of dislocations along crystallographic planes contributes to heat generation. For instance, in the case of particle impact, the highest temperatures are generally found in the subsurface region, indicating that heat generation is a volumetric phenomenon rather than a surface effect.
The finite element method (FEM) and theoretical models can be used to predict the heat generated during plastic deformation. These models are particularly useful in understanding the temperature evolution for different impact velocities and material properties. Additionally, the coefficient of restitution plays a role in accurately estimating the heat generation.
Furthermore, the dynamics and temperature effects on the formation of shear bands in metallic glasses have been a subject of interest. Cryogenic temperatures have been observed to have a stabilizing effect on plastic deformation, and the role of temperature in the failure of these materials has been extensively studied.
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Plastic deformation's impact on enthalpy
Plastic deformation is the ability of a solid material to undergo permanent, irreversible changes in shape in response to applied forces. It is observed in most materials, particularly metals, soils, rocks, concrete, and foams. The physical mechanisms underlying plastic deformation can vary widely and are not fully understood. However, it is known that plastic deformation is associated with heat generation, which can be analysed using methods such as the finite element method (FEM) and theoretical models.
During plastic deformation, the interatomic bonds between atoms are broken and re-established, allowing for the motion of one plane of atoms over another. This process is known as slip and is a shear deformation that moves atoms through many interatomic distances relative to their initial positions. In the case of metals, plastic deformation occurs predominantly by shearing, with lattice planes sliding over each other without significantly affecting the ordering and arrangement of atoms within the structure. This is in contrast to elastic deformation, where only the stretching of interatomic bonds occurs, and the work done by external forces is stored within the deformed body.
The generation of heat during plastic deformation is due to the movement of dislocations along crystallographic planes of a polymer crystal. The greatest temperature and structural changes are observed in the second stage of deformation, when the neck is being formed. An increase in the level of crystallinity will lead to a higher temperature rise. The heat generation associated with plastic deformation is a volumetric phenomenon, with peak temperatures generally found in the subsurface rather than on the surface.
The plastic deformation of materials can be influenced by factors such as temperature, pressure, deformation speed, and the presence of defects within the crystal structure. At elevated temperatures, thermally activated migration of atoms and vacancies, known as diffusion, occurs extensively, leading to time-dependent plastic deformation or creep. Higher stresses are usually required to increase the rate of deformation, and plastic deformation is generally dependent on the generation and movement of dislocations.
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Plastic deformation's effect on energy storage
Plastic deformation is the permanent distortion that occurs when a material is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength. This results in the material elongating, compressing, buckling, bending, or twisting. Plastic deformation is irreversible and consists of time-dependent and time-independent components.
During plastic deformation, energy is consumed and expelled from the system as the microstructure is irreversibly changed. The energy balance equation during elastic-plastic deformation includes the work of plastic deformation, Epl, which accounts for stored energy in the form of dislocations, vacancies, interfaces, and other defects. This stored energy is considered elastic energy and is recoverable, but it is classified as part of plastic work because it is caused by the existence of defects.
The effect of plastic deformation on energy storage is particularly evident in the dynamic recovery and recrystallization processes of metals subjected to severe plastic deformation. The energy stored in the metal during plastic deformation plays a crucial role in these processes and is considered a primary factor in the formation of adiabatic shear bands when metallic materials are deformed at high rates.
The ratio of energy storage to heat dissipation under shock compression has been investigated, revealing that the strain rate significantly influences energy storage and dissipation. For certain orientations, such as [001] copper, the ratio remains relatively constant regardless of the strain rate. In contrast, for [123] copper, a high strain rate is more likely to result in the storage of plastic work in the form of dislocations.
Furthermore, the generation of heat during plastic deformation has been studied, particularly in the context of particle impact and polyethylene. It has been found that heat generation is a volumetric phenomenon, with peak temperatures occurring in the subsurface rather than on the surface. The temperature rise is influenced by the level of crystallinity, with an increasing level of crystallinity leading to a higher temperature increase.
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Frequently asked questions
Plastic deformation is the permanent and irreversible distortion of a material when it is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength.
Plastic deformation affects enthalpy by generating heat. The greatest temperature and structural changes are observed in the second stage of deformation, when the neck is being formed. The heat generated during plastic deformation is a volumetric phenomenon, with peak temperatures generally found in the subsurface rather than on the surface.
The amount of heat generated during plastic deformation depends on the level of crystallinity of the material. In polymers, the temperature jump at the transition to the neck correlates with the degree of crystallinity. An increasing level of crystallinity will result in a higher temperature rise.
The finite element method (FEM) and theoretical models can be used to predict and analyse the heat generation during plastic deformation. These models consider various impact velocities, material properties, and structural changes to determine the temperature profile and heat evolution associated with plastic deformation.
