Ethan Clarke
Plastic deformation is an inelastic process where a material changes shape in response to applied stress, resulting in permanent deformation. It is an important concept in metal-working processes, as it allows engineers to predict the behaviour of metals under load. The ability of metals to withstand substantial plastic deformations makes them attractive structural materials, especially under tension. Plastic deformation in metals typically occurs through shearing, where lattice planes slide over each other, allowing macroscopic changes without disrupting the atomic arrangement. This is in contrast to elastic deformation, where the material returns to its original shape and size when the applied stress is removed. While not all metals are expected to conform to a specific functional form, the work hardening rate tends to decrease as strain increases, approaching a plateau.
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What You'll Learn
Plastic deformation is an inelastic process
Plastic deformation is a common phenomenon in metals, and it occurs when there is a change in the size or shape of an object. This deformation is not simply undone by removing the applied force, and the object will only return partially to its original shape. This is in contrast to elastic deformation, where the object returns to its original shape once the stress is removed.
The most common metal-processing techniques, such as drawing, rolling, extrusion, explosive forming, stamping, and forging, involve some form of permanent deformation. This deformation can be analysed using the first law of thermodynamics, which considers the change in internal energy, mechanical work done, and the heat effect associated with the deformation. The mechanical work done on a sample undergoing a shape change can be determined using the volume integral over the strain range.
The continuation of plastic deformation generally requires progressively increasing levels of applied stress, as seen in a stress-strain curve. This effect is termed "work hardening" or "strain hardening". It occurs due to the creation of more dislocations and their interactions, leading to reduced mobility. The plasticity characteristics of a material can be significantly altered by thermal or mechanical treatments or exposure to different environments. Predictive modelling and characterisation of metal-working processes help engineers understand and utilise the plastic deformation behaviour of metals in various applications.
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Plastic deformation requires progressively increasing stress
Plastic deformation is an inelastic process, and the energy imparted to a material may be lost through mechanisms other than the direct recovery of strain. This deformation is not inherently undesirable, as it allows plastic materials to reduce stresses and withstand substantial mechanical work before failure. This capability makes metals attractive structural materials, especially under tension.
Plastic deformation of metals occurs predominantly as a result of the glide of dislocations driven by shear stresses. The movement of dislocations results in a progressive refinement of the microstructure, leading to a reduction in the size of subgrains and grains. This deformation is accompanied by a temperature rise, with most of the mechanical energy supplied being dissipated as heat. The energy required for deformation is always higher than the minimum energy needed to deform a material due to redundant work, which results in energy dissipation and storage.
The continuation of plastic deformation requires progressively increasing stress, as seen in a stress-strain curve. This effect, known as "work hardening" or "strain hardening," arises from the creation of more dislocations and their interactions, leading to reduced mobility. The work hardening rate tends to decrease progressively with increasing strain and may eventually plateau. This behaviour is influenced by the competition between creating new dislocations and inhibiting their mobility through processes that enhance their organisation and annihilation.
The specific plasticity characteristics of a material depend on factors such as crystal structure, grain size, crystallographic texture, composition, and phase constitution. These characteristics can be altered by thermal or mechanical treatments or exposure to different environments. The work hardening of metals can be characterised using equations such as the Ludwik-Hollomon and Voce equations, which consider applied stress, yield stress, plastic strain, and the work hardening coefficient and exponent.
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Dislocations and glide movement cause plastic deformation
Plastic deformation of metals is a common phenomenon observed in metal-processing techniques such as drawing, rolling, extrusion, explosive forming, stamping, and forging. It is an inelastic process where the material undergoes a non-recoverable deformation, reducing the stresses applied to it. This makes metals attractive structural materials, especially under tension, as they can withstand substantial mechanical work before failure.
Dislocations and glide movements play a crucial role in causing plastic deformation in metals. A dislocation is a linear crystallographic defect or irregularity within a crystal structure, characterised by an abrupt change in the arrangement of atoms. In simpler terms, it is a linear defect that defines the boundary between regions of a material that have slipped and those that have not. These dislocations occur due to the creation and movement of many dislocations within the crystal structure. The movement of these dislocations allows atoms to slide over each other at low stress levels, which is known as glide or slip.
The glide movement of dislocations can be understood through the concept of slip planes and the Burgers vector. Dislocations can slip or glide in planes containing both the dislocation line and the Burgers vector. The Burgers vector represents the distance and direction of movement caused to the atoms as the dislocation moves through the lattice. For screw dislocations, the dislocation line and Burgers vector are parallel, allowing slipping in any plane containing the dislocation. On the other hand, edge dislocations have a perpendicular relationship between the dislocation and the Burgers vector, resulting in slipping in only one plane.
Additionally, the presence of jogs and kinks in the dislocation line can impact glide movement. Jogs are steps in the dislocation line that are not in the glide plane, impeding dislocation movement by requiring vacancy diffusion to move through the lattice. Kinks, on the other hand, facilitate glide by acting as nucleation points, reducing the energy barrier for slip. As more dislocations are created and interact with each other, they form jogs and tangles, leading to a decrease in their mobility and an increase in the yield strength of the material, known as work hardening.
In summary, dislocations and their glide movements are fundamental to understanding plastic deformation in metals. The creation and movement of dislocations, along with the interactions between dislocations and the presence of jogs and kinks, collectively contribute to the plastic deformation observed in metals during various metal-working processes.
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Plastic deformation is important in metal processing
Plastic deformation is an intrinsic part of most metal processing techniques. It involves applying external stress to a metal to achieve a shape change, while also causing a controlled alteration in its mechanical properties. This process is commonly known as "work hardening" or "strain hardening".
The ability to withstand substantial plastic deformation is what makes metals attractive structural materials, especially under tension. For example, plastic flow can limit the rate of spread of Mode I fracture, where tensile forces act to pull apart fracture surfaces. This is in contrast to materials like ceramics, which, despite showing higher strength under compression, cannot easily break and reform their bonds to undergo plastic deformation.
The most common metal-processing techniques, such as drawing, rolling, extrusion, explosive forming, stamping, and forging, involve some form of permanent deformation. The deformation needed to introduce a required shape change in a sample of material can be analysed using the first law of thermodynamics, where ΔU is the change in internal energy of the body, W is the mechanical work done on the sample, and Q is the heat effect associated with the deformation.
Plastic deformation is also important in metal processing as it allows engineers to predict the plastic response of a material to a load. This predictive capability is crucial in a high-technology society, where machinery operates at elevated temperatures, increasing the likelihood of plastic flow even in normally brittle materials.
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Plastic deformation can be modelled using the Ludwik-Hollomon and Voce equations
Plastic deformation of metals is a common phenomenon that occurs due to the glide of dislocations driven by shear stresses. This process is irreversible and can cause disruption to the material's structure. However, it is not inherently undesirable as it allows materials to withstand substantial mechanical work before failure. The ability of metals to undergo plastic deformation makes them attractive structural materials, especially under tension.
> \\[\sigma = \sigma _{\rm{Y}} + K\varepsilon ^n \qquad \qquad \qquad (6)\\]
Where \\(\sigma \) is the (von Mises) applied stress, \\(\sigma _{\rm{Y}} \) is its value at yield, \\(\varepsilon \) is the plastic (von Mises) strain, K is the work hardening coefficient, and n is the work hardening exponent. This equation is particularly useful for characterising the work hardening behaviour of metals.
The Voce equation is another powerful tool for modelling plastic deformation:
> \\[\sigma = \sigma _{\rm{s}} - (\sigma _{\rm{s}} - \sigma _{\rm{Y}})\exp \left( {\frac{{ - \varepsilon }}{{{\varepsilon _0}}}} \right) \qquad \qquad \qquad (7) \\]
Here, \\(\sigma \)s is a saturation level, and \\(\varepsilon \)0 is a characteristic strain for the exponential approach of the stress towards this level. The Voce equation provides better estimates of yield strength and tensile strength compared to the Hollomon and Swift equations. It is also well-suited for describing stress-strain curves in large strain regions and accurately predicting ultimate tensile strength.
Both the Ludwik-Hollomon and Voce equations are valuable tools for engineers and scientists working with metals and alloys to predict and understand their deformation behaviour under various conditions.
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Frequently asked questions
Plastic deformation is an inelastic process where a material deforms and does not return to its original size and shape. This is different from elastic deformation, which is non-permanent.
Plastic deformation occurs as a result of the glide of dislocations, driven by shear stresses. The two types of dislocation are edge and screw. During glide, dislocations move along a surface defined by their burgers vector, and during climb, they move outward from the glide surface.
Elastic deformation occurs when the external stress does not exceed the yield strength of the material. In this case, the material tends to return to its original size and shape when the stress is removed. Plastic deformation occurs when the stress increases beyond the yield point, causing the material to deform quickly and permanently.
Understanding plastic deformation is crucial for predicting the response of metals to applied loads. This is especially important in high-technology applications where machinery operates at elevated temperatures, increasing the likelihood of plastic deformation in normally brittle materials. Additionally, plastic deformation allows metals to withstand substantial mechanical work before failure, making them attractive structural materials, particularly under tension.
