Alexei Volkov
Plastic strain refers to a type of irreversible strain that causes a material to distort and not return to its original size or shape. It is caused by a rearrangement of atoms within the material when subjected to high stress. Plastic strain is observed in both crystalline and amorphous materials, such as polymers and metals, and can lead to deformation or fracture. The accumulation of plastic strain needs to be monitored to prevent unstable fractures or collapses, especially in structures like steel tubes. The transition from elastic to plastic deformation is defined by critical resolved shear stress, which varies with temperature and the structure of the material.
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What You'll Learn
- Plastic strain is caused by the rearrangement of atoms in a material
- It is a type of inelastic strain that is not reversible
- Plastic skeleton-strain is defined by the absolute values of plastic tension and compression
- Plastic deformation occurs when the bending moment exceeds the fully plastic moment
- Accumulated plastic strain can lead to unstable fracture or plastic collapse
Plastic strain is caused by the rearrangement of atoms in a material
Plastic strain refers to the inelastic strain that occurs in a material when it is subjected to high stress. This type of strain is not recovered when the material is unloaded and is often observed in metals, soils, rocks, concrete, and foams.
Plastic strain is caused by the rearrangement of atoms within the material, leading to irreversible deformation. This deformation occurs when the stress applied exceeds the maximum stress the material can withstand, causing the atoms to move from their initial positions and establish a new equilibrium. The chemical bonds between atoms are broken and reformed, resulting in a disruption of the material's structure. While this deformation may seem undesirable, it allows plastic materials to reduce stress and withstand substantial mechanical work before failure.
In crystalline materials, plastic deformation is typically associated with dislocations, which are defects in the crystal structure. These dislocations facilitate the movement of planes of atoms relative to one another, known as slip. Slip allows the crystal structure to be retained while leaving the material displaced along a slip plane. Additionally, plastic deformation can occur through the realignment of the crystal structure itself due to shear stresses or phase transformations.
The ductility of metals, which is their ability to undergo plastic deformation without fracture, is influenced by their crystalline structure. Amorphous metallic glasses, despite having similar metallic bonding, exhibit decreased ductility due to the absence of a repeating crystal structure. In contrast, polymers and glasses can undergo plastic deformation through a time-dependent process called viscous deformation, which involves the rearrangement of atomic and molecular bonds.
The extent of plastic strain in a material can be quantified using various methods, such as stress-strain curves and critical resolved shear stress calculations. By understanding the mechanisms of plastic deformation and strain, engineers can design materials with specific properties, ensuring that the amount of plastic loading is acceptable for the intended application.
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It is a type of inelastic strain that is not reversible
Plastic strain refers to a type of inelastic strain that is not reversible. When a material is subjected to stress, it can experience elastic strain, but if the stress exceeds the material's yield strength, plastic strain occurs, and the material undergoes irreversible deformation. This deformation persists even after the removal of the applied stress, and the material does not return to its original size and shape.
In the context of engineering and physics, plastic strain is a critical concept. It is observed in various materials, including metals, polymers, and ceramics. The behaviour of a material under plastic strain is characterised by its plasticity or ductility. During plastic deformation, the atoms or molecules within the material rearrange, leading to a permanent change in structure.
The distinction between elastic and plastic strain lies in the reversibility of deformation. Elastic strain is temporary and reversible, whereas plastic strain results in irreversible changes. This distinction is crucial in understanding the behaviour of materials under stress and designing structures that can withstand specific load conditions.
Accumulated plastic strain refers to the cumulative effect of plastic deformation over time. It is particularly important in engineering applications, such as the fabrication and installation of steel tubes in umbilicals. To ensure the safety and integrity of structures, engineers must consider the allowable accumulated plastic strain to prevent unstable fractures or plastic collapse.
Plastic strain is influenced by factors such as temperature, stress levels, and the unique characteristics of the material. In crystalline materials, the transition from elastic to plastic deformation is defined by the critical resolved shear stress (τCRSS), which initiates dislocation migration along slip planes. At high temperatures, plastic flow can occur due to thermally activated deformation mechanisms, even with relatively low τCRSS values.
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Plastic skeleton-strain is defined by the absolute values of plastic tension and compression
Plastic strain refers to the irreversible deformation of a material when stress is applied that exceeds the yield point. This is in contrast to elastic deformation, which is recoverable. Plastic strain is caused by the rearrangement of atoms in the material.
Plastic skeleton-strain is a specific type of plastic strain that occurs in ductile metals under tensile loads. When a ductile metal is subjected to finite deformations, it will exhibit identical stress-strain behaviour in tension and compression if true stress is plotted against true strain. This behaviour can be observed in a stress-strain curve, where the skeleton curve is formed by adding the strain corresponding to the stress that exceeds the maximum stress in the preceding cycle, on both the tension and compression sides. The plastic skeleton-strain is then defined as the larger of the absolute values of the plastic strain components of these skeleton curves.
The plastic skeleton-strain is an important parameter in evaluating the performance of structural materials under repeated loading. For example, research has shown that the reduction in fracture toughness of structural steels subjected to repeated loading can be related to plastic skeleton-strains. Accumulated plastic strain, which is the sum of plastic tension and compression strains, can also impact the yield stress and yield/ultimate stress ratio of a material.
Furthermore, plastic skeleton-strain can influence the ductility and toughness of a material. Special strain aging and toughness testing must be carried out when accumulated plastic strain exceeds certain limits to ensure that the material properties do not become substandard. This is particularly relevant for applications where the material is subjected to large loads, such as metal forming, machining, and other manufacturing processes.
In summary, plastic skeleton-strain is a critical concept in understanding the behaviour of ductile metals under tensile loads and plays a significant role in evaluating the performance and suitability of materials for various engineering applications.
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Plastic deformation occurs when the bending moment exceeds the fully plastic moment
Plastic strain refers to the permanent deformation of a material that occurs when it is subjected to stresses beyond its yield strength. This can be caused by tensile, compressive, bending, or torsion forces, resulting in irreversible changes such as elongation, compression, buckling, bending, or twisting.
The plastic theory of bending considers the behaviour of structures under bending moments and the subsequent formation of plastic hinges. A plastic hinge forms when a joint or point in a structure rotates under a bending moment without any increase in the applied load. Collapse occurs when sufficient plastic hinges are formed to create a "mechanism". For example, in a portal frame under vertical and horizontal loads, plastic hinges may form at various points, and collapse is reached when enough hinges are formed to cause beam collapse or another form of failure.
Plastic deformation can be observed in various materials, including metals and ceramics. It is characterised by large strains, with ductile materials capable of withstanding plastic strains exceeding 100%. This is in contrast to elastic strains, which are typically limited to 1-2%. The behaviour of a material under bending moments depends on its mechanical properties, such as its strength coefficient and strain-hardening exponent.
The plastic deformation of a beam can be analysed by considering the stress-strain distribution within the beam. When subjected to a bending moment, the outer regions of the beam experience stress up to a certain limit, while the strain continues to increase linearly with distance from the neutral axis. Upon removal of the applied moment, the beam retains a residual curvature due to residual stresses, which can be calculated using force and moment balance equations.
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Accumulated plastic strain can lead to unstable fracture or plastic collapse
Plastic strain refers to the type of strain caused by the rearrangement of atoms in a material and is not reversible. It is caused by higher stress and is characterised by some amount of inelasticity. Structural materials may suffer elastic, anelastic, and plastic deformations when exposed to complex stresses due to varied external forces.
Accumulated plastic strain is defined as the sum of plastic strain increments, irrespective of sign and direction. It is calculated from the point where the material stress-strain curve deviates from a linear relationship and is computed from the time of fabrication to the end of the lifetime of the material. It is limited to ensure that the material properties do not become substandard, especially in terms of fracture toughness. Accumulated plastic strain can occur in the steel tubes of an umbilical during fabrication and installation.
The area under the stress-strain curve represents the energy required for deformation and fracture, and this is used to grade a material as brittle or ductile. A higher m value, which indicates a higher total elongation to fracture, can lead to viscous flow in fluids and metals and ceramics at high temperatures and low strain rates.
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Frequently asked questions
Plastic strain is a type of irreversible deformation in which the distorted body does not return to its original size and shape after the force causing the deformation is removed.
Plastic strain is caused by the rearrangement of atoms in a material.
Elastic strain is reversible, whereas plastic strain is not.
An example of plastic strain is the accumulated plastic strain that can occur in the steel tubes of an umbilical during fabrication and installation.
The formula for plastic strain is not provided in the sources, however, it is related to the stress-strain curve, which represents the energy required for deformation and fracture.
