Ethan Clarke
Plasticity is a term used in physics and materials science to describe the ability of a solid material to undergo permanent deformation, or a non-reversible change of shape, in response to applied forces. This phenomenon, also known as plastic deformation, occurs when the stress on a material exceeds a critical value, leading to irreversible deformation. In engineering, the transition from elastic behaviour to plastic behaviour is known as yielding, and it is a crucial part of design. The understanding and application of plasticity are essential in various technological operations, such as metal-forming processes, where materials are forced to accept new forms through techniques like stamping, rolling, and drawing through a die.
| Characteristics | Values |
|---|---|
| Definition | Plasticity is the deformation of a material undergoing non-reversible changes of shape in response to applied forces. |
| Other Names | Plastic deformation |
| Materials Exhibiting Plasticity | Metals, soils, rocks, concrete, foams, liquid foams, biological tissues, bone, glass |
| Factors Influencing Plasticity | Load, time, temperature, ductility, malleability, stress, strain, load, temperature |
| Types of Deformation | Slip, twinning, shear deformation |
| Plasticity in Crystals | Caused by defects in the crystal structure, such as dislocations |
| Importance in Engineering | The transition from elastic to plastic behaviour is crucial in design and applied calculations |
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What You'll Learn
Plastic deformation
In engineering, plasticity refers to the deformation of a material that undergoes non-reversible changes in shape in response to applied forces. This is also known as plastic deformation. It is the ability of a solid material to undergo permanent deformation, which is often observed in metals, soils, rocks, concrete, and foams.
The physical mechanisms that cause plastic deformation can vary. In metals, plasticity is usually a result of dislocations, which are defects in the crystal structure. In brittle materials like rock or concrete, plasticity occurs due to slippage at microcracks. In cellular materials such as liquid foams or biological tissues, plasticity arises from bubble or cell rearrangements.
Technological operations often involve plastic deformation, such as stamping, rolling, or drawing through a die, where materials are forced to take on new forms. Understanding the plasticity of materials is essential to ensure that products function properly. Even small amounts of deformation can cause misalignment and lead to product failure.
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Technological operations
In engineering, plasticity is a term used to describe the mechanical behaviour of a material when subjected to external forces. This behaviour, known as plastic deformation, occurs when a material undergoes a permanent change of shape, also referred to as irreversible deformation.
Plastic deformation is observed in a variety of materials, including metals, soils, rocks, concrete, and foams. The ability of a material to be moulded or altered in this way is a result of its plasticity. This is distinct from elastic behaviour, where a material returns to its original shape once the external force is removed.
The technological operations that utilise plasticity are those that involve substantial plastic deformation to achieve the desired shape and properties. For example, metal-forming processes such as rolling, forging, extrusion, and drawing rely on the plastic behaviour of metals. By applying heat, most metals become more plastic and can be shaped using these methods.
The transition from elastic to plastic behaviour is critical in engineering and is known as yielding. Beyond the yield point, the material enters the plastic phase of deformation, which includes processes such as yielding, strain hardening, necking, and ultimately, failure of the material.
Understanding the plasticity of materials is essential for engineers to predict and control the behaviour of these materials in various technological operations. By studying the stress-strain relationship and utilising computational methods, engineers can optimise manufacturing processes and improve the performance of materials for specific applications.
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Stress and strain
Stress is defined as the force applied to a material per unit area. It is a tensor quantity, represented by three normal stresses and six shear stresses at each point within a body. The normal stresses refer to the forces that act perpendicularly to the surface, while the shear stresses refer to those that act parallel to the surface. Stress can be measured indirectly through experimental testing techniques such as strain gauges, extensometers, and computational simulations. The stress equation (σ=F/A) is then used to calculate stress.
Strain is a measure of the deformation or change in shape of a material under the influence of an external force. It represents the amount of deformation that occurs in a material when subjected to stress. Strain is defined as the ratio of the change in length or other dimensions of a material to its original length or dimension. It is often expressed as a unitless quantity or as a percentage. For example, if a material with an original length of 10 units is subjected to stress and changes to a length of 12 units, the strain would be expressed as 0.2 or 20%.
The relationship between stress and strain is not always straightforward and can vary depending on the material. This relationship can be visualized through a stress-strain curve, which is obtained by gradually applying load to a test sample and measuring the corresponding deformation. The curve typically has two parts: the elastic region, where the material returns to its original shape after the load is removed, and the plastic region, where the material undergoes permanent deformation. The stress-strain curve can reveal important properties of materials, such as strength, ductility, and toughness, which are crucial for designing applications and performing operations like extrusion, rolling, and bending.
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Slip and twinning
In engineering, plasticity is a material's propensity to undergo enduring deformation under load when compressed. It is the quality or state of being plastic, especially the capacity for being moulded or altered. Plastic deformation occurs when external forces cause stress within an object that exceeds its elastic limit. The object then undergoes a permanent change in shape.
Twinning is another mechanism of plastic deformation, occurring when there are few slip systems in place. In twinning, a portion of the crystals adopts an orientation that is clearly and symmetrically connected to the direction of the remaining untwined lattice. Twinning results in a mirrored orientation of a crystal portion, with local atoms rearranging across a twinning plane as mirror copies of one another. Atoms parallel to the twin plane travel along the lattice during twinning, causing the lattice within the twinned region to distort. Each atomic plane's separation from the twin plane directly relates to the quantity of movement. Twinning causes planes to slip more by affecting the plane's orientation, which adds to plastic deformation.
The transition from elastic to plastic behaviour of solids is crucial in engineering applications and design. Technological operations often require an elastic-to-plastic transition to force a material to accept a new form, such as stamping, rolling, or drawing through a die.
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Ductility and malleability
Plasticity is a material's ability to undergo enduring deformation under load when compressed. It is the quality or state of being plastic, especially the capacity for being molded or altered. The plasticity of a material is directly proportional to its ductility and malleability. Ductility and malleability are material properties that describe the ability of a material to deform.
Ductility is a physical property of matter that can be observed without bringing about a chemical change. It is associated with the ability of a material to be stretched into wires without breaking. Ductility is the process of measuring the tensile stress on a metal. Tensile stress is the force that pulls the two ends of an object away from each other. Metals with high ductility include gold, which can be drawn into extremely thin wires without losing its structural integrity.
Malleability is the ability of a material to deform without breaking under compression. A malleable metal can be beaten or rolled into thin sheets. Tin, for example, is a highly malleable metal that can be flattened into thin sheets without breaking. Its malleability allows for the production of tin foil and cans.
Ductile metals are crucial in industries where materials must be drawn into wires or undergo significant plastic deformation under tensile stress. Malleable materials, on the other hand, are extensively used in industries where materials need to be hammered or rolled into sheets, such as in the construction sector.
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Frequently asked questions
Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. In engineering, the transition from elastic behaviour to plastic behaviour is known as yielding.
Elasticity enables a solid to return to its original shape after the load is removed. Plastic deformation occurs when the deformation force exceeds the limit of elasticity, causing the material to be damaged.
Plastic deformation occurs in many metal-forming processes (rolling, pressing, forging) and in geologic processes (rock folding and rock flow). Metals, soils, rocks, concrete, and foams are among the materials that exhibit plastic deformation.
