Sofia Martinez
In physics and materials science, plasticity, or plastic deformation, is a material's ability to undergo irreversible changes in response to applied forces. This occurs when stress surpasses the elastic limit, causing the material to exceed its elasticity and enter the plastic region. In this region, the material does not return to its original size or shape, resulting in permanent deformation until it reaches the fracture point. The transition from elastic to plastic behaviour, known as yielding, is observed in various materials, including metals, soils, rocks, concrete, and foams. The plastic zone, an area of deformation and failure, is influenced by factors such as stress, temperature, and crystalline structure.
| Characteristics | Values |
|---|---|
| Definition | Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. |
| Materials | Ductile materials such as metals, soils, rocks, concrete, and foams. |
| Plastic deformation | Occurs when the load exceeds a threshold, the yield strength, and the extension increases more rapidly than in the elastic region. |
| Plastic zone | The region ahead of a crack tip where a ductile material is plastically deformed due to the local stress field exceeding the yield strength. |
| Plastic flow | In single crystals and polycrystals, it is defined by a critical/maximum resolved shear stress (τCRSS), which initiates dislocation migration along parallel slip planes. |
| Critical resolved shear stress | Comprised of athermal (τa) and thermal (τ*) shear stresses, arising from the stress required to move dislocations and the resistance of point defect obstacles, respectively. |
| Temperature regions | Low temperature (T ≤ 0.25Tm), moderate temperature (0.25Tm < T < 0.7Tm), and high temperature (T ≥ 0.7Tm). |
| Plastic deformation behaviour | In engineering, the transition from elastic behaviour to plastic behaviour is known as yielding. |
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Plastic deformation
In the context of plasticity, ductile materials like metals exhibit a gradual decrease in stress as strain increases. This means that as stress-strain values approach the breaking point, these materials become easier to deform. When a load is applied to a ductile metal, it initially behaves elastically, with each increment of load accompanied by a proportional increase in extension. However, once the load exceeds the yield strength, the extension increases more rapidly, and upon removing the load, some degree of extension remains. This behaviour is known as "elasto-plastic deformation".
The transition from elastic to plastic deformation can be defined by critical resolved shear stress (CRSS), which is influenced by factors such as temperature and the angle between the slip plane direction and the applied force. In low-temperature regions, achieving high CRSS requires a high strain rate, and both athermal and thermal shear stresses come into play. As the temperature increases, the thermal shear stress component diminishes, and in the moderate temperature region, time-dependent plastic deformation mechanisms, such as solute-drag, become relevant.
The plastic zone is a critical concept in understanding plastic deformation. It refers to the region ahead of a crack tip where ductile materials undergo plastic deformation due to local stress exceeding the yield strength. The size and shape of the plastic zone are influenced by factors such as the maximum principal stress, with larger plastic zones indicating more severe deformation and potential hazards.
Understanding plastic deformation is crucial in engineering to predict and prevent failures in structures. By considering the characteristics of different materials and their responses to applied forces, engineers can design structures that can withstand and distribute stresses effectively, ensuring the safety and longevity of various constructions.
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Plastic behaviour
The transition from elastic behaviour to plastic behaviour is known as yielding. In ductile materials such as metals, plasticity occurs due to dislocations, which are defects in the crystal structure. These dislocations allow for slip planes along which deformation can occur. As stress increases, the material becomes easier to deform, and beyond the yield strength, the extension increases more rapidly, resulting in plastic deformation.
The plastic zone is a region ahead of a crack tip where a ductile material undergoes plastic deformation due to the local stress field exceeding the yield strength. The size and shape of this zone depend on the maximum and minimum principal stresses. A larger plastic zone indicates a greater extent of deformation and potential failure.
The plastic behaviour of materials can be modelled through stress-strain diagrams. Each material has its own characteristic curve, with the linear behaviour ending at the limit of proportionality. Beyond this point, the relationship becomes nonlinear but still elastic until the elastic limit is reached. Further increases in load result in plastic deformation, with the material continuing to deform until it reaches the fracture point.
The microscopic mechanisms responsible for plasticity vary across different materials. For example, in brittle materials like rock and concrete, plasticity is caused by slip at microcracks, while in cellular materials like liquid foams, it is a result of cell rearrangements.
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Plastic zone
In physics and materials science, plasticity, or plastic deformation, is the ability of a solid material to undergo permanent deformation—a non-reversible change of shape in response to applied forces. This occurs when the load on an object exceeds the elastic limit, causing it to acquire a permanent deformation and not return to its original size or shape when the load is removed.
The plastic zone is a region ahead of a crack tip where a ductile material is plastically deformed due to the local stress field exceeding the yield strength. The size of the plastic zone can be determined by the characteristic radii, with the largest of the three radii representing the maximum plastic zone size. When the regional maximum principal stress changes, the plastic zone shape changes from circular to elliptical, and the maximum radius of the plastic zone is on the longitudinal coordinate axis, where the deformation is the largest and the destruction is most severe.
In roadway engineering, the plastic zone is used to determine the location of potential hazards. A large plastic zone area calculated in theory will deform and cause serious destruction in the corresponding position in the actual roadway. By determining the maximum principal stress directions and using Equation 33, the potential hazard areas can be identified, and measures can be taken to prevent disasters.
The Dugdale-Barenblatt model represents the plastic zone by a constant closure stress equal to the flow stress acting on the tip of a fictitious crack over a distance equal to the plastic zone size. This model is valid for thin sheets in plane stress and is used to describe the plastic zone for perfectly plastic materials in mode III.
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Plastic crystallinity
The crystallization of polymers is a process associated with the partial alignment of their molecular chains. These chains fold together to form ordered regions called lamellae, which then compose larger spheroidal structures called spherulites. Polymers can crystallize upon cooling from melting, mechanical stretching, or solvent evaporation. Crystallization affects the optical, mechanical, thermal, and chemical properties of the polymer. The degree of crystallinity typically ranges between 10% and 80%, and crystallized polymers are often called "semi-crystalline". The properties of semi-crystalline polymers are determined not only by the degree of crystallinity but also by the size and orientation of the molecular chains.
Plastics can be broadly categorized as semi-crystalline or amorphous. Amorphous plastics have no crystalline structure, and their molecules form no patterns. They usually soften gradually as they are heated and may become brittle unless modified with certain additives. Amorphous plastics have no sharp melting points and are usually glassy and transparent. Examples include acrylonitrile-butadiene-styrene (ABS), acrylic (PMMA), polycarbonate (PC), polystyrene (PS), and polyvinyl chloride (PVC).
Semi-crystalline polymers, on the other hand, contain sections of ordered structure bounded by unorganized amorphous regions. They are ductile above the glass transition temperature and can deform plastically. The crystalline regions of these polymers are linked by the amorphous regions, and tie molecules prevent the two phases from separating under an applied load. When a tensile stress is applied, the molecular chains of the amorphous phase stretch, leading to the onset of plastic deformation of the crystalline regions. This deformation involves dislocation motion, resulting in crystalline fragmentation and yielding.
Crystalline plastics, such as polyethylene, exhibit increased resistance to creep compared to amorphous plastics. They tend to have higher melting temperatures and require tighter process control during fabrication to prevent shrinkage and warping. Crystalline plastics do not soften gradually but remain hard until a sufficient amount of heat is absorbed, after which they rapidly change into a low-viscosity liquid. This property of crystalline plastics can affect the processing and performance of the final product if not properly controlled.
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Plastic flow
Plasticity, or plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. This phenomenon is observed in most materials, especially metals, soils, rocks, concrete, and foams.
In crystalline materials, the transition from elastic to plastic deformation behaviour is defined by a critical or maximum resolved shear stress (τCRSS), which initiates dislocation migration along parallel slip planes of a single slip system. The critical resolved shear stress for single crystals is given by Schmid's law: τCRSS=σy/m, where σy is the yield strength and m is the Schmid factor, which depends on the angles between the slip plane direction and the applied tensile force.
At low temperatures (T ≤ 0.25Tm), a high strain rate is required to achieve the τCRSS necessary for plastic flow. In this region, the critical resolved shear stress has two components: athermal (τa) and thermal (τ*) shear stresses, which arise from the stress needed to move dislocations in the presence of other dislocations and the resistance of point defect obstacles, respectively. As the temperature increases to the moderate region (0.25Tm < T < 0.7Tm), the thermal shear stress component disappears, indicating the elimination of point defect impedance to dislocation migration.
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Frequently asked questions
The plastic region is when an object or material exhibits plastic behaviour and undergoes plastic deformation. This occurs when stress is larger than the elastic limit.
In the plastic region, the object or material does not return to its original size or shape when the stress is removed. Instead, it acquires a permanent deformation.
Plastic deformation is the ability of a solid material to undergo a non-reversible change of shape in response to applied forces. This means that when a load is applied, the material is permanently deformed and does not return to its initial state when the load is removed.
Many materials can undergo plastic deformation, particularly metals, soils, rocks, concrete, and foams. Ductile materials, such as metals, become easier to deform as stress-strain values approach the breaking point.
The plastic region can be modelled using methods such as the Dugdale-Barenblatt model, which represents the plastic zone by a constant closure stress acting on the tip of a fictitious crack. The size and shape of the plastic zone can vary depending on factors such as the regional maximum principal stress.
