Sofia Martinez
Plastic flow, or plastic deformation, is a phenomenon observed in materials such as metals, soils, rocks, and foams, where they undergo a permanent, non-reversible change in shape when subjected to applied forces. This occurs when the stress on a material exceeds a critical value, causing it to behave like a liquid and flow instead of snapping back to its original shape. In ductile metals, tensile loading can lead to elastic behaviour until a threshold, the yield strength, is reached, after which the material undergoes plastic deformation and retains some degree of extension even when the load is removed. The transition from elastic to plastic behaviour is of significant importance in understanding the structural performance and mechanical processing capabilities of metals.
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What You'll Learn
Plastic deformation
The fundamental mechanism of plastic deformation relies on the movement of dislocations. In the elastic region, since the applied stress is lower than the yield point, it cannot activate dislocation movement, so atomic bonds only stretch temporarily and return to their original state when the stress is removed. Within the plastic region, the applied stress exceeds the yield point, so it can activate deformation movement. Here, atomic bonds both stretch and break, and planes shear over each other, causing the material to deform permanently.
The rate of plastic deformation under constant shear stress is initially high but tapers off to a steady value. If this steady value, the shear strain rate, is plotted against the stress for many different values of applied stress, a curved graph will result. The curve illustrates the process of deformation, which may result from hydrostatic pressure, shock impact, or directed tectonic stress.
The continuation of plastic deformation requires a progressively increasing level of applied stress, apparent in a stress-strain curve. This effect is termed "work hardening" or "strain hardening". It arises because, as more dislocations are created, and as they interact with each other, they become less mobile. The yield stress and work-hardening characteristics have a complex dependence on crystal structure, grain size, crystallographic texture, composition, and phase constitution. Even for a given material, these plasticity characteristics can be dramatically changed by thermal or mechanical treatments or by exposure to various environments.
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Plastic flow in crystalline materials
Plastic flow, also known as plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. In crystalline materials, plasticity is usually a consequence of dislocations, which are defects in the crystal structure. While these defects are rare in most crystalline materials, they are common in some and can lead to plastic crystallinity.
In ductile metals, tensile loading causes the material to behave elastically, with each increment of load accompanied by a proportional increment in extension. However, once the load exceeds the yield strength, the extension increases more rapidly, and when the load is removed, some degree of extension remains. This is known as plastic deformation, and it occurs in many metal-forming processes such as rolling, pressing, and forging.
The transition from elastic behaviour to plastic deformation at the yield point is significant. It represents the upper stress limit for structural performance and the lower limit for mechanical processing. At this point, the material undergoes plastic or irreversible deformation, which can be tensile or compressive. The Tresca and von Mises criteria are commonly used to determine whether a material has yielded, but they are inadequate for a wide range of materials.
The plastic deformation of ductile materials can be explained by the theory of dislocations, which uses a set of non-linear, non-integrable equations to describe the changes in strain and stress. This is known as flow plasticity theory. Additionally, finite strain crystal plasticity is a rigorous non-linear continuum theory that has been studied since the early 1900s. However, certain subtleties of this theory have not been sufficiently tested in critical experiments.
Experiments on the plastic deformation of single crystals of metals and rock salt have yielded results with common characteristics. Glacier flow, for example, involves intracrystalline gliding, where layers within a crystal shear parallelly without disrupting the crystal lattice, and recrystallization. In highly porous metals, plastic deformation remains localized unless another mechanism, such as densification or work hardening, causes the hardening needed to initiate deformation elsewhere.
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Plastic instability
Plastic flow refers to the ability of a solid material, such as metal, to undergo permanent deformation, resulting in a non-reversible change of shape when exposed to applied forces. This phenomenon is observed in various metal-forming processes, including rolling, pressing, and forging. At the yield point, the transition from elastic behaviour to plastic flow is significant as it marks the upper stress limit for structural performance and the lower limit for mechanical processing.
In the context of metal deformation instability, the concept of energy change is crucial. Instability signifies a shift in energy, leading to a transition from one equilibrium state to another or to an unstable state. Shearing is identified as the primary cause of deformation instability. Plastic deformation instability occurs during the deformation process of metal materials, and its manifestation can vary, including local necking, buckling, and wrinkling.
The deformation instability of metal materials is influenced by multiple factors, such as intrinsic material properties, macro/microstructure, geometric structure, ambient temperature, and loading conditions. These factors contribute to the complexity of the deformation instability process, making it a multi-factor coupling phenomenon. The understanding and prediction of deformation instability are essential in engineering applications, especially in fields like aerospace, automobile, and coastal power stations, where material stability is critical.
Furthermore, at very low temperatures, metals exhibit unstable plastic deformation. Discontinuities in the stress-strain curve are attributed to localised temperature rises during deformation. It is theorised that all metals will demonstrate unstable deformation at sufficiently low temperatures. Additionally, studies have explored discontinuous plastic flow in superconducting multifilament composites and the evolution of single shear bands in large-strain deformation of metals.
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Work hardening
Plastic flow refers to plastic deformation, which is a permanent, non-reversible change in the shape of a solid material in response to applied forces. Plastic deformation is observed in many materials, especially metals, and is a common phenomenon in metal-forming processes such as rolling, pressing, and forging.
Cold working techniques, such as squeezing, bending, drawing, and shearing, are commonly used to induce work hardening in metals. These processes are performed at high speed and pressure, typically at temperatures below the metal's recrystallization point. While the initial deformations may weaken the metal, continued deformation leads to increased strength due to the accumulation and interaction of dislocations.
The work-hardening rate is influenced by factors such as the initial dislocation density, the rate of dislocation generation, and the rate of dislocation annihilation. Understanding this rate is crucial for predicting the mechanical behaviour of the material. Work hardening can significantly enhance the strength, ductility, and toughness of metallic materials.
However, excessive work hardening can lead to undesirable outcomes, such as metal fatigue and brittleness. Therefore, specialised alloys and heat treatments are employed for metal objects designed to flex, like springs, to prevent work hardening and maintain their desired characteristics.
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Yield strength
In the context of metals, yield strength refers to the stress level at which the metal begins to deform permanently. This deformation is often observed in metal-forming processes such as rolling, pressing, and forging. At low stress levels, the applied stress is linearly proportional to the induced strain, resulting in elastic deformation. However, when the stress exceeds the elastic region, the relationship becomes nonlinear, and plastic deformation occurs.
The yield strength of metals can be influenced by various factors, including grain size, concentration of inclusions, and previous working history. For instance, exceedingly pure metals and metals at high homologous temperatures can rapidly recover their yield strength during deformation. Additionally, heat treatments can be employed to control the properties of metals by reducing dislocation density without melting them.
The Bauschinger effect is another important consideration in understanding yield strength. It describes the relationship between tensile and compressive yield strengths, where an increase in tensile yield strength leads to a decrease in compressive yield strength in the same direction. This effect is observed in face-centred cubic and body-centred cubic metals.
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Frequently asked questions
Plastic flow is a phenomenon where a material behaves like a liquid and flows instead of bending or breaking when subjected to stress. It is a type of plastic deformation, where the material does not return to its original shape once the stress is removed.
Plastic deformation in metals is mainly caused by the glide of dislocations driven by shear stresses. It occurs when the load exceeds the yield strength, resulting in plastic flow. Work hardening or strain hardening can also contribute to plastic deformation in metals.
Creep is another type of plastic deformation that occurs in materials like ice and metals under constant stress. It involves intracrystalline gliding, where layers within a crystal shear parallel to each other without disrupting the crystal lattice. Plastic flow, on the other hand, is a dislocation-mediated effect and occurs suddenly when the stress reaches a critical point.
Understanding plastic flow in metals is crucial in engineering and metallurgy. It helps determine the suitability of metals for various applications, ensuring they meet ductility requirements. Additionally, it aids in the development of metal-forming processes such as rolling, pressing, and forging, where plastic deformation is commonly employed.
