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Elasticity and Plasticity

  • Last Updated : 16 Jun, 2021

You’ve undoubtedly heard of the idea of elasticity by now. In layman’s words, it indicates that after being stretched, some substances return to their former form. You’ve experimented with a slingshot. Didn’t you? That is an elastic substance. Let us go into the ideas of elasticity and plasticity to discover more about these 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. When deforming pressures are removed from plastic bodies, they do not tend to return to their previous structure. 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.

What is Deformation?

The action or process of deforming or distorting is referred to as deformation. When you apply a force to an item, it will either compress or stretch in reaction to the force. The force exerted to a unit area is referred to as stress in mechanics. Strain is the amount of stretching or compressing that occurs as a result of a stress reaction. Every material reacts to stress differently.

The reaction is strongly reliant on the substance’s chemical bond type. Based on what happens once the stress is removed, deformations might be elastic or plastic described as:

  1. 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.
  2. Plastic deformation or Plasticity is a persistent deformation or change in the shape of a solid body caused by a sustained force. The primary distinction between elastic and plastic deformation is that elastic deformation is reversible, but plastic deformation is irreversible.

Elasticity

When deformation forces are removed, a body’s property allows it to restore its previous size and shape, that property is known as Elasticity. Elasticity is a phenomenon that occurs at the molecular level.

Elastic deformation is the deformation that dissipates when the external forces that caused the modification and tension linked with it are removed. As a result, elastic deformation is reversible and non-permanent. The chemical notion of “elasticity” best describes elastic deformation. Elasticity is a substance’s capacity to return to its original condition after being deformed.

Elastic deformation is mostly determined by the chemical bonding of the material. That substance can undergo elastic deformation if the chemical linkages can tolerate high stress by deforming without breaking.

When stress is given to a substance, chemical linkages should expand or flex to meet this need. Chemical bond stretching and bending should be only transient. However, atoms do not collide during stretching or bending. However, the elastic quality of material degrades over time, and the substance can become brittle and lose its ductility.

Elastic Substances:

Elastomers such as vulcanised rubber are the greatest examples of elastic bodies. Vulcanized rubber has a polymer structure that is cross-linked. Sulfur bridges exist between polymer chains. These Sulphur cross-links improve rubber flexibility by allowing it to endure stress.

 Stress-Strain curve for a ductile material

The stress-strain curve for a ductile material is seen in the figure above. Consider the metal copper. The elastic zone depicts the extent to which the elastic deformation occurs. After reaching the elastic limit, the material will experience plastic deformation, which is a permanent distortion.

Plasticity

The ability of a body to lose its elasticity and develop a permanent deformation when a deforming force is removed is known as Plasticity.

Plastic deformation is defined as the persistent deformation or change in the shape of a solid body caused by a sustained force. This happens when a great amount of tension is applied to a material. Plastic deformation is permanent and irreversible. Plasticity is the ability to be permanently formed or molded.



Plastic deformation happens as a result of the breakdown of a few chemical bonds between the atoms that comprise the material. Atoms may slip past each other during plastic deformation. As a result of atom dislocations, the material remains motionless when the applied tension is removed.

The elastic limit of ductile material is the beginning point of plastic deformation. The elastic limit of a solid is the utmost amount to which it may be stretched without permanently changing size or form. If the tension is placed beyond the elastic limit, the substance will deform plastically.

Plastic Substances:

Metals, polymers, rocks, and other materials exhibit plasticity. Plastic deformation occurs in ductile materials such as metals (for example, copper) when the distortion surpasses the elastic limit. However, in brittle materials such as rocks, no elastic deformation may be detected prior to the onset of plastic deformation. Plastic deformation is vital in the manufacture of new goods utilizing heat or pressure treatments, as well as molding.

The difference in the behavior of the material is based on its elastic and plastic nature. That is the material reacts to a certain force based on its elasticity. This is the difference between the application of force on a plastic body and an elastic body.

Difference between Elasticity and Plasticity

 

Elasticity 

Plasticity

1.

Elasticity is the property of a solid material that allows it to restore its shape after an external load is removed.

Plasticity is the property of a solid substance that allows it to keep its deformed shape even when the external load is removed.

2.

The amount of external force necessary to bend a solid elastically is quite tiny.



The amount of external force necessary to bend a solid elastically is high.

3.

Hooke’s Law of elasticity is applicable within this elastic region.

Hooke’s Law of elasticity is not applicable within this plastic region.

4.

The stress-strain curve is linear in the elastic region mostly. 

The stress-strain curve is non-linear in the plastic region

5.

Many properties of the solid material remain the same for elastic deformation.

Many properties of the solid material change considerably for plastic deformation.

6.

Elastic deformation occurs first than plastic deformation.

Plastic deformation occurs only after the body undergoes elastic deformation.

7.

Here deformation is reversible.



Here deformation is irreversible.

8.

Elasticity is used in – Machine tool structures, Bridges, other civil frames, many household structures that should retain their shape etc.

Plasticity is used in various forming operations such as rolling, forging, extrusion, etc, sheet metal working, Rivet joining etc.

Applications of Elasticity

  • We know that cranes are used to lift and move heavy loads. The crane is equipped with a thick metallic rope. The maximum load that the crane hold under any circumstances should not exceed the elastic limit of the material of the rope. By knowing this elastic limit and the extension per unit length of the material, the area of cross-section of the wire can be evaluated. From this, the radius of the wire can be calculated.
  • Elasticity is used in estimating the maximum height of a mountain on earth.
  • Elasticity is used in the design of bridges. While designing a bridge, we have to consider some factors like traffic load, the weight of the bridge, the force of winds. The bridge is so designed that it should neither bend too much nor break.

Sample Problems

Problem 1: Define Elastic fatigue and its effect.

Solution:

Elastic fatigue is a condition in which the elasticity of a material is temporarily lost as a result of continual strain.

The effect of elastic fatigue are:

  • Within the elastic limit, a wire can be broken.
  • Without the use of equipment, a wire can be sliced into pieces.
  • After a long period of service, railway tracks and bridges are deemed hazardous.
  • After a long period of usage, spring balances give incorrect readings.

Problem 2: Explain Elastic after effect.

Solution:

Elastic after effect refers to the time it takes for a material to restore its previous length after a deforming force is removed.



  • Elastic after effect for a perfectly elastic body is zero.
  • Elastic after effect for a perfectly inelastic body is infinite.

Problem 3: What happens when force is applied to a body?

Solution:

If we apply force on an elastic body like rubber then it stretches up to the elastic limit. If we apply the same force continuously then it may break. And whenever we stop applying force then it regains its original shape. But coming to the plastic body like an aluminum rod, when we apply force on that plastic body then it may somewhat change its shape but after stopping the force it won’t regain its original shape.

Problem 4: What is a Plastic substance? Also, mention its characteristics.

Solution:

A plastic substance is one that does not exhibit elastic behavior. Putty, chewing gum, lead solder, and wax are some of the examples.

Some of the main characteristic properties of a plastic substance are:

  1. More elastic is a body that is more difficult to strain.
  2. Rubber is less elastic than steel.
  3. Steel, not copper, is used to make springs because steel is more elastic than copper.
  4. Elasticity is increased through hammering, rolling, and adding impurities.
  5. The elasticity of the material is reduced by annealing and increasing the temperature.
  6. Some material’s elasticity does not change as a function of temperature. One example of this is Invar Steel.

Problem 5: What is an Elastic body? Give its examples also.

Solution:

The term “elastic body” refers to a body that exhibits elasticity. Steel and rubber are the two examples.

Some of the approximate examples of a perfectly elastic body are Quartz and phosphor bronze.

Problem 6: A heavy mass is attached to a thin wire and is whirled in a vertical circle. The wire is most likely to break at which point?

Solution:

When heavy mass is attached to a thin wire and is whirled in a vertical circle, the wire will break at lowest point of a vertical wire because tension is more at that point.


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