Ethan Clarke
Plastic deformation is a common phenomenon that can be observed in many everyday situations. It occurs when a material undergoes a permanent change in shape or dimension due to stress exceeding its yield point or elastic limit. This can be seen in ordinary activities like bending a paperclip, leaving a dent in a car, stretching chewing gum, or moulding clay. Plastic deformation is not limited to everyday objects but is also prevalent in engineering and metalworking processes. For example, metals like steel, copper, silver, and gold exhibit ductility, allowing them to withstand stress without breaking. Understanding the yield point is crucial in differentiating elastic behaviour, where objects return to their original shape, from plastic deformation, where changes become irreversible.
| Characteristics | Values |
|---|---|
| Plastic deformation | An inelastic process |
| Plastic deformation range | Large in soft thermoplastics, ductile metals (copper, silver, gold, steel), and wet chewing gum |
| Plastic deformation range | Minimal in hard thermosetting plastics, rubber, crystals, ceramics, and cast iron |
| Plastic deformation in metals | Slip and twinning |
| Slip | A shear deformation that moves atoms through many interatomic distances relative to their initial positions |
| Twinning | Plastic deformation that takes place along two planes due to a set of forces applied to a metal piece |
| Plastic deformation in alloys | Pseudoelasticity |
| Plastic deformation in crystals | Caused by two modes of deformation in the crystal lattice: slip and twinning |
| Plastic deformation in polycrystals | Voids and cracks can form between individually deforming grains |
| Plastic deformation in materials | Requires progressively increasing levels of applied stress |
| Plastic deformation in materials | Permanent structural alteration |
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What You'll Learn
- Plastic deformation is observed in ductile metals such as copper, silver, and gold
- It is also seen in soft thermoplastics and chewing gum
- Plastic deformation is an inelastic process where chemical bonds are broken and energy is lost
- It is caused by two modes of deformation in the crystal lattice: slip and twinning
- Plastic deformation is irreversible, unlike elastic deformation
Plastic deformation is observed in ductile metals such as copper, silver, and gold
Ductile metals like copper, silver, and gold have a large plastic deformation range. Steel also falls into this category, but cast iron does not. Metals with a crystalline structure tend to be ductile, and their repeating crystal structure allows them to absorb strain through translation or transformation of the crystal. Heating metals can increase their ductility, making them more pliable and workable.
Plastic deformation in metals is often the result of the glide of dislocations driven by shear stresses. This process involves the movement of atoms over significant distances from their initial positions, leading to a new equilibrium. The chemical bonds between atoms are broken and reformed, converting elastic energy into chemical potential energy or heat. This deformation is irreversible, and the energy imparted may be lost through mechanisms other than the direct recovery of strain.
The plasticity of metals can be enhanced by applying very large plastic strains to create fine-grain structures, increasing strength. This technique is used in metal-working processes such as hot and cold deformation, which involve techniques like rolling, extrusion, and wire-drawing. These processes can alter the microstructure of the metal, affecting its mechanical properties.
The ductility of metals is influenced by factors such as the number of independent slip systems, which facilitate dislocation migrations. Crystals with at least five independent slip systems can sustain significant plastic deformation, while those with fewer slip systems may exhibit limited ductility or cracking. Additionally, the von Mises criterion helps determine whether a material has yielded and undergone plastic deformation.
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It is also seen in soft thermoplastics and chewing gum
Plastic deformation is observed in soft thermoplastics and chewing gum. Chewing gum is the only food that intentionally contains plastic. The "gum base" ingredient in chewing gum is largely made of polyvinyl acetate, a type of plastic. The Food and Drug Administration (FDA) has reviewed and deemed 46 substances safe for use in chewing gum bases, including plastics.
Thermoplastics are a type of plastic that becomes soft and pliable above a certain temperature, known as the glass transition temperature (Tg). At room temperature, some thermoplastics are below their Tg and are hard, while others are above their Tg and remain soft. The Tg varies for each plastic, and additives can be added to make a plastic softer and more pliable.
Thermoplastic elastomers, a type of thermoplastic, exhibit both elastic and inelastic behaviours under mechanical stress. They can undergo substantial deformation and return to their original shape, a property known as shape recovery. This behaviour is characterized by highly nonlinear rate-dependent hyperelastic-viscoplasticity, where energy dissipation is accompanied by shape recovery and softening.
The softening behaviour, known as the Mullins effect, has been extensively studied, and micro- and macroscopic mechanisms have been proposed. In situ X-ray scattering measurements on deformed polyurethanes, polyureas, and their chemically modified counterparts have revealed a microstructural breakdown, particularly in the hard domains, leading to substantial softening and hysteresis.
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Plastic deformation is an inelastic process where chemical bonds are broken and energy is lost
Plastic deformation is an irreversible process that occurs when a material is subjected to forces beyond its elastic limit, leading to a permanent change in shape. This phenomenon is commonly observed in materials such as metals, ceramics, and crystalline structures. During plastic deformation, the material undergoes inelastic processes where chemical bonds are broken, and energy is lost or transformed.
In the context of metals, plastic deformation plays a crucial role in their structural integrity. Metals are often bonded to stiff substrates, and when subjected to mechanical loading, they experience both elastic and plastic deformation. The deformation is not one-dimensional due to the differing tendencies of lateral contraction between the metal and the substrate. This results in lateral stresses and the formation of slip planes or slip bands, which are groups of parallel slip planes that can be observed microscopically.
Ceramics, on the other hand, exhibit different behaviour during plastic deformation. Unlike metals, ceramics have higher strength under compression but struggle to break and reform their bonds. This results in ceramics being harder yet more brittle compared to metals, which are tough and resilient. The ability of metals to undergo rapid and easy disruption and reformation of bonds through metallic bonding contributes to their superior performance in withstanding substantial plastic deformations.
The process of plastic deformation involves dislocation motion, vacancy motion, twinning, phase transformation, or viscous flow of amorphous materials. Dislocations are defects in the crystal lattice that allow atoms to slide past each other more easily. When a material is stressed, dislocations move through the lattice, facilitating plastic deformation. Twinning occurs when a portion of the crystal lattice shifts, creating a mirrored region of the crystal structure. Elevated temperatures generally increase the mobility of dislocations, making materials more ductile and easier to deform plastically.
Plastic deformation is a fundamental concept in materials science and engineering, especially in the automotive industry. Car manufacturers design vehicle components to deform plastically in controlled ways during collisions, enhancing safety and minimizing damage to passengers. By understanding and utilizing plastic deformation characteristics, engineers can select appropriate materials for specific applications, ensuring that components can withstand operational stresses and deformations without failure.
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It is caused by two modes of deformation in the crystal lattice: slip and twinning
Plastic deformation is a fundamental concept in materials science and engineering that refers to the permanent change in shape that occurs when a material is subjected to forces beyond its elastic limit. It is observed in most materials, especially metals, soils, rocks, concrete, and foams. The physical mechanisms that cause plastic deformation vary, but it is primarily caused by two modes of deformation in the crystal lattice: slip and twinning.
Slip is the most common mode of plastic deformation, where dislocations move along specific planes and directions known as slip planes and slip directions. In other words, it is the sliding of crystal blocks over one another along various crystallographic planes. The choice of slip system is influenced by the crystal structure of the material. For example, Face-Centered Cubic (FCC) crystals have multiple slip systems, making them highly ductile, while Body-Centered Cubic (BCC) crystals have fewer slip systems and are generally less ductile.
Twinning occurs when a portion of the crystals adopts an orientation that is clearly and symmetrically connected to the direction of the remaining untwined lattice. It is determined by defects, crystal axis shift, visibility, and the occurrence threshold value in stress slip. Twinning causes planes to slip more by affecting the plane's orientation, contributing to plastic deformation.
Both slip and twinning are shear deformations that move atoms through many interatomic distances relative to their initial positions. They are essential in understanding the behaviour of materials under stress and the ability of materials to withstand pressure-induced distortion, known as plasticity.
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Plastic deformation is irreversible, unlike elastic deformation
Plastic deformation is an irreversible process, unlike elastic deformation, which is temporary and reversible. When a material undergoes plastic deformation, it does not return to its original state after the force is removed. This is because the stress applied exceeds the material's elastic limit or yield point, leading to a permanent change in its shape or size.
Plastic deformation occurs when the applied stress surpasses the yield strength of the material. This can be tensile or compressive stress. During this process, the atomic bonds within the material are broken, and new bonds are formed as atoms move to new positions. This disruption of the material structure results in a dimensional change that persists even after the initiating stress is removed.
Elastic deformation, on the other hand, is reversible. In this case, atomic bonds are stretched but not broken, and the material returns to its original shape once the stress is removed. Elastic deformation occurs below the material's yield strength, while plastic deformation happens only when the yield strength is surpassed.
The distinction between elastic and plastic deformation is important in understanding the behaviour of materials under stress. For example, ductile materials can sustain large plastic deformations without fracture. However, even ductile metals will eventually fracture when the strain becomes large enough, leading to work hardening and embrittlement.
While plastic deformation is typically irreversible, there are certain cases where it can be reversible. On the nanoscale, primary plastic deformation in simple face-centered cubic metals can be reversible if there is no material transport in the form of cross-slip. Additionally, shape-memory alloys like Nitinol wire exhibit a reversible form of plasticity, known as pseudoelasticity.
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Frequently asked questions
Plastic deformation can be observed in many daily situations. Some common examples include bending a metal paperclip, leaving a dent in a car's body panel, stretching a piece of chewing gum, and moulding clay.
Materials that undergo plastic deformation include ductile metals such as copper, silver, and gold, as well as soft thermoplastics. Steel also undergoes plastic deformation, but cast iron does not.
Plastic deformation occurs when the applied stress exceeds the elastic limit or yield point of a material. This leads to a slip or dislocation mechanism at the atomic level, causing a permanent dimensional change that doesn't disappear even when the stress is removed.
