Maya Ellison
Plastic flow is a phenomenon observed in materials science, specifically in the study of plastic materials, where the material reaches a point of permanent deformation due to severe stress. This occurs when the stress applied exceeds the material's elastic limit or yield strength, causing it to behave as a Newtonian fluid and leading to irreversible changes in its shape. While high pressure can influence the flow of certain materials, the primary mechanism of plastic flow is often basal slip, which involves the movement of dislocations within the crystal lattice of the material. This understanding of plastic flow is crucial in manufacturing to ensure efficient processes and high-quality products.
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What You'll Learn
High pressure increases the likelihood of plastic flow
Plastic flow occurs when materials are subjected to stress beyond their elastic limit, resulting in permanent deformation. While basal slip is the primary mechanism causing plastic flow in glaciers, high pressure can increase the likelihood of deformation.
High pressure can contribute to plastic flow by increasing the stress on materials. In glaciers, for example, high pressure at greater depths increases the likelihood of plastic deformation in ice. Research in glaciology has shown that when pressure exceeds 100 kilopascals, ice can behave plastically and flow. This phenomenon has been confirmed by extensive studies, highlighting the role of basal slip in facilitating glacier movement under high-pressure conditions.
However, it is important to note that high pressure is not the only factor influencing plastic flow. The direct mechanism for plastic flow in glaciers is the sliding of ice facilitated by meltwater, which acts as a lubricant. Basal slip occurs when ice at the bottom of a glacier slides over the underlying surface, and it is particularly effective at temperatures close to freezing.
In materials such as metals, the plastic flow is influenced by factors beyond just high pressure. For instance, in metal plasticity, the assumption is made that the plastic strain increment and deviatoric stress tensor share the same principal directions, as described by the flow rule. Additionally, the J2 flow model, specifically developed for metal plasticity, suggests that plastic flow in metals is unaffected by pressure. This was experimentally proven by Bridgman in 1949.
Furthermore, the complexity of plastic flow behaviour can vary depending on the material. For instance, in FSW, plastic flow complexity arises from the interplay between variations in strain rates and flow stress, leading to changes in viscosity that affect the flow. Additionally, the presence of dislocations within the crystal lattice of a material can increase the likelihood of plastic flow. At high temperatures and pressures, plastic behaviour can be influenced by the motion of dislocations within individual grains of a material's microstructure.
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Basal slip is a mechanism that causes plastic flow
Plastic flow occurs in materials when they are subjected to stress beyond their elastic limit. This results in the material being permanently deformed. Basal slip is one of the primary mechanisms that cause plastic flow.
Basal slip involves the movement of dislocations within the crystal lattice of the material. In the context of glaciers, basal slip occurs when the ice at the bottom slides over the underlying surface. This movement is facilitated by the presence of water acting as a lubricant, enabling the glacier to move faster. The water can come from melted ice due to the immense pressure exerted by the glacier or from surface meltwater that seeps down to the base.
The presence of lubricating water at the base of a glacier reduces friction, allowing the bottom layer to move faster than the top layer. This difference in speed creates a shearing effect, causing the ice to crack and form crevasses. While crevasses are evidence of movement, they are not the cause of plastic flow.
Research in glaciology has confirmed that basal slip is a leading mechanism for glacier movement. At depths where pressure exceeds 100 kilopascals, ice behaves plastically and can flow. This phenomenon is observed particularly in warm and fast-moving glaciers, where the presence of lubricated bases results in greater surface speeds compared to glaciers frozen to their beds.
In summary, basal slip is a significant mechanism that enables plastic flow by reducing friction and facilitating the movement of materials or glaciers.
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Crevasses are cracks that form in the upper layers of glaciers
Crevasses are deep cracks that form in the upper layers of glaciers. They are a result of the movement and stress associated with shear stress when two semi-rigid pieces above a plastic substrate have different rates of movement. The shear stress causes a breakage along the faces, resulting in crevasses. Crevasses often have vertical or near-vertical walls, which can melt and create seracs, arches, and other ice formations. They can also expose the glacier's stratigraphy. Crevasse size depends on the amount of liquid water present in the glacier.
Crevasses are one of many secondary structures observed on glaciers that result from strain. They are brittle secondary structures, in contrast to ductile secondary structures like foliations or ogives. Crevasses can range from millimetre-scale cracks to several meters wide. Regularly spaced crevasses up to 33 meters wide have been observed in Greenland. Crevasses can also be very deep, with air-filled crevasses in Antarctica reaching depths of 45 meters.
Crevasses form in different ways. One type of crevasse forms when a glacier flow speeds up too fast for the ice to compensate by flowing, similar to how a substance like putty will break if yanked apart suddenly. Another type of crevasse forms when the ice piles up on itself, crunching and cracking, similar to faults in the Earth's crust. These crevasses can occur when the ice flows over obstacles, rounds a bend, or slows down.
Crevasses can be dangerous, as they are often covered by snow bridges, making them invisible and potentially lethal to anyone navigating the glacier. Mountaineers use rope teams and friction knots to minimise the danger of falling into a crevasse. Ground-penetrating radar can be used to identify crevasses and measure their depths by sensing discontinuities in the dielectric properties of near-surface firn.
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Low pressure acts against the conditions needed for plastic flow
Low pressure typically acts against the conditions needed for plastic flow. It reduces the likelihood of deformation and, therefore, plastic flow. This is because low pressure reduces the stress on the material, making it less likely to deform permanently.
Plastic flow occurs in materials when they are subjected to stress beyond their elastic limit. This means that the material is permanently deformed. Basal slip is one of the main mechanisms that cause plastic flow. It involves the movement of dislocations within the crystal lattice of the material. In glaciers, basal slip occurs when the ice at the bottom slides over the underlying surface, facilitated by the presence of meltwater acting as a lubricant.
High-pressure levels increase the likelihood of deformation and can contribute to plastic flow by increasing the stress on the material. However, it is not the only or most direct cause. While crevasses, or cracks in the surface of a material, can be related to plastic flow, they are not the cause of it.
In summary, low pressure typically acts against the conditions needed for plastic flow by reducing the stress on a material and making it less likely to deform permanently. The primary cause of plastic flow, particularly in glaciers, is basal slip, which is facilitated by high-pressure levels and the presence of meltwater.
Therefore, while low pressure may reduce the likelihood of plastic flow, it is important to consider other factors such as basal slip, high pressure, and the presence of lubricants that can also influence the deformation of materials.
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Plastic flow is associated with softer materials
Plastic flow occurs when materials are subjected to stress beyond their elastic limit, causing permanent deformation. It is generally associated with softer materials, particularly in the case of gears, where the hardening process is not initiated.
Basal slip is one of the main mechanisms causing plastic flow. It involves the movement of dislocations within the crystal lattice of the material. In glaciers, basal slip occurs when the ice at the bottom slides over the underlying surface, facilitated by meltwater acting as a lubricant. High-pressure levels increase the likelihood of deformation, but basal slip is the direct mechanism. Crevasses, or cracks in the surface of a material, are a result of plastic flow but are not the cause. Low pressure typically acts against the conditions needed for plastic flow, reducing the likelihood of deformation.
The mathematical theory of plasticity, or flow plasticity theory, uses a set of non-linear, non-integrable equations to describe the changes in strain and stress with respect to a previous state and a small increase in deformation. If the stress exceeds a critical value, the material will undergo irreversible deformation. This critical stress can be tensile or compressive. The mathematical theory of plasticity, however, has limitations and cannot account for all materials. For instance, the causes of plasticity in soils are complex and dependent on factors such as microstructure, chemical composition, and water content. Additionally, at high temperatures, plastic flow can be influenced by thermal activation and dislocation migration.
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Frequently asked questions
Plastic flow is the deformation or movement of a material under stress that leads to permanent changes in its shape without fracture.
Plastic flow occurs when a material is subjected to stress beyond its elastic limit or yield strength. Basal slip, a mechanism where the ice at the bottom of a glacier moves over the underlying surface, is one of the main causes of plastic flow.
Basal slip occurs when the ice at the bottom of a glacier slides over the underlying surface, facilitated by the presence of meltwater acting as a lubricant.
In plastic flow, the material undergoes a permanent deformation and does not return to its original shape once the force is removed. On the other hand, elastic deformation is reversible, and the material regains its original shape when the force is removed.
While high temperatures are not necessary for plastic flow, they can accelerate the process. In glaciers, plastic flow occurs at much lower temperatures compared to materials like polymers, which typically flow when heated and become molten.
