Ethan Clarke
Elasticity and plasticity are two properties of matter that describe how materials respond to external forces. Elasticity is the ability of a body to return to its original shape and size after being deformed by external forces. It is a reversible process, and the deformed object returns to its original state once the deforming forces are removed. On the other hand, plasticity refers to the loss of elasticity, where a body undergoes permanent deformation and does not return to its previous structure when the deforming forces are removed. This is an irreversible process. The transition from elastic behaviour to plastic behaviour is known as yielding, and it occurs when the stress applied exceeds the elastic limit of the material.
| Characteristics | Values |
|---|---|
| Elasticity | The ability of a body to return to its original configuration (shape and size) once deforming forces are removed |
| Elastic deformation | Deformation that subsides when the external forces that caused the change and the stress connected with it are removed |
| Plasticity | The quality of a body that causes it to lose its elasticity and develop a permanent distortion after the deforming force is removed |
| Plastic deformation | A persistent deformation or change in the shape of a solid body caused by a sustained force |
| Elastic limit | The point beyond which a material undergoes plastic deformation |
| Ductile materials | Materials that have a fair amount of plastic deformation before breaking |
| Brittle materials | Materials that can't stretch or bend much without breaking |
| Microscopic mechanisms | The mechanisms responsible for plasticity differ for different materials |
| Stress-strain curve | A graphical representation of the relationship between stress and strain for a material |
| Yield strength | The threshold beyond which a material exhibits plastic deformation |
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What You'll Learn
Elastic deformation
The concept of elastic deformation is crucial in engineering, where it is applied to materials used in mechanical and structural engineering, such as concrete and steel. By understanding the elastic properties of these materials, engineers can design structures that can withstand specific loads without permanent deformation. For example, when designing a bridge, engineers must consider factors such as traffic load, the weight of the bridge, and the force of the wind to ensure that the bridge does not bend excessively or break.
The elastic limit, or elasticity limit, is the point at which elastic behaviour ends and plastic deformation begins. It is important to note that the elastic limit is not a sharp point but rather a range of values. Once the load exceeds this limit, the material undergoes permanent deformation and does not return to its initial shape and size, even when the load is removed.
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Plastic deformation
The amount of force or pressure required to cause plastic deformation varies depending on the material. Ductile materials, such as metals, can withstand large amounts of stress before experiencing plastic deformation, while brittle materials like glass and ceramics will break rather than bend.
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Stress-strain diagrams
The stress-strain diagram of a ductile metal under a load will show a linear relationship between stress and strain at first, which ends at the linearity limit. Beyond this point, the relationship becomes nonlinear but remains elastic. As the load is increased further, the material will eventually reach its elastic limit, where elastic behaviour ends and plastic deformation begins. Plastic deformation is a permanent change in the shape of a solid body caused by a sustained force.
The elastic region of the stress-strain diagram provides data that is used in product design, assuming that the shape will not deform. On the other hand, the plastic region provides data for processing and durability. The elastic modulus, or Young's modulus, can be calculated from the slope of the linear range within the elastic region of the diagram. A higher elastic modulus indicates that the material is more rigid and less likely to deform under a given load.
The modulus of resilience is calculated as the area under the stress-strain curve from the origin to the elastic limit. It represents the ability of a material to absorb energy without creating a permanent distortion. Similarly, the modulus of toughness is calculated as the area under the entire stress-strain curve and represents the ability of a material to absorb energy without breaking.
The stress-strain diagram is a useful tool for understanding the mechanical behaviour of materials and designing products and structures. For example, when designing a bridge, engineers must consider factors such as traffic load, the weight of the bridge, and wind forces to ensure that the bridge does not bend or break.
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Elastic limit
Elasticity and plasticity are two properties of matter. Elasticity is the ability of a body to return to its original configuration (shape and size) once deforming forces are removed. Plasticity is the quality of a body that causes it to lose its elasticity and develop a permanent distortion after the deforming force is removed.
Elastic deformation or elasticity is the deformation that subsides when the external forces that caused the change and the stress connected with it are removed. Plastic deformation or plasticity is a persistent deformation or change in the shape of a solid body caused by a sustained force. Elastic deformation is reversible, but plastic deformation is irreversible.
The elastic limit is a fundamental mechanical property of solid materials that defines the maximum stress a material can sustain without undergoing permanent (plastic) deformation. It marks the boundary between the elastic deformation region—where the material will return to its original shape when the load is removed—and the plastic deformation region, where the material will undergo irreversible changes in shape or size. Stress beyond the elastic limit causes the material to yield, resulting in permanent structural changes.
The elastic limit is the greatest stress that can be applied to a material without causing plastic (permanent) deformation. When a material is stressed to a point below its elastic limit, it will return to its original length once the stress is removed. Once a material is stressed to a point exceeding its elastic limit, it begins to permanently yield, and when the stress is removed, the material will not fully return to its original length.
The elastic limit is difficult to accurately determine using a universal testing machine, which is why it is generally used for educational purposes rather than in practice by the materials testing industry. The proportional limit of a material is the point on a stress/strain curve where the linear, elastic deformation region transitions into the non-linear, plastic deformation region. A material's elastic limit can be equivalent to its proportional limit for certain materials, but for others, the stress/strain relationship will become nonlinear before reaching the proportional limit.
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Plastic crystallinity
The degree of crystallinity in plastics is an important factor in determining the material's properties and performance. Plastics can be broadly categorized as semi-crystalline or amorphous. Semi-crystalline polymers contain sections of ordered structure, while amorphous polymers have an unorganized, loose structure. The degree of crystallinity in a polymer typically ranges between 10% and 80%, and those that are highly crystalline are often referred to as "semi-crystalline".
Amorphous plastics have no definite order of molecular chains, and their molecules form no patterns. They soften gradually as they are heated and have no sharp melting points. During processing, amorphous plastics are normally in an amorphous state. When heated, they may become brittle unless modified with certain additives. Amorphous plastics exhibit a relatively consistent modulus over a temperature range, but as the temperature approaches the glass transition temperature, a sharp decline in modulus is observed.
Crystalline plastics, on the other hand, have a sharp melting point and do not soften gradually with increasing temperature. They remain hard until a sufficient amount of heat is absorbed, after which they rapidly change into a low-viscosity liquid. Crystalline plastics require tighter process control during fabrication to prevent warping and shrinking. Crystalline plastics are strong, stiff, and have excellent resistance to abrasion, heat, chemicals, and fatigue. They also have a low coefficient of surface friction, making them ideal for mechanical products.
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. Crystallization affects the optical, mechanical, thermal, and chemical properties of the polymer. Crystallization of polymers occurs through various mechanisms, including cooling from melting, mechanical stretching, and solvent evaporation. During crystallization, molecular chains fold together to form ordered regions called lamellae, which then compose larger spheroidal structures called spherulites.
Overall, the crystallinity of plastics plays a crucial role in their performance and mechanical properties. Understanding the implications of crystallinity is essential for material selection, part design, and processing. By manipulating the degree of crystallinity and the presence of amorphous regions, engineers can tailor the properties of plastics to suit specific applications and requirements.
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Frequently asked questions
Elasticity is the ability of a body to return to its original configuration (shape and size) once deforming forces are removed.
Plasticity is the quality of a body that causes it to lose its elasticity and develop a permanent distortion after the deforming force is removed.
Elastic deformation is reversible, whereas plastic deformation is irreversible.
The elastic limit is the point at which elastic behaviour ends and plastic deformation begins.
A solid piece of metal being bent or pounded into a new shape is an example of a plastic body as permanent changes occur within the material itself.
