Difference Between Elastic and Plastic Deformation

Main Difference – Elastic vs Plastic Deformation

Deformation is the action or process of deforming or distorting. When a force is applied to an object, the object will either compress or stretch as a response to the force. In mechanics, the force applied to a unit area is called stress. The extent of stretching or compressing (as a response to the stress) is called strain. Every material responds differently to stress. The response is highly dependent on the chemical bond type of the substance. Deformations can be elastic or plastic based on what happens after the stress is released. Elastic deformation is the deformation that disappears upon removal of the external forces causing the alteration and the stress associated with it. Plastic deformation is a permanent deformation or change in shape of a solid body without fracture under the action of a sustained force. The main difference between elastic deformation and plastic deformation is that elastic deformation is reversible whereas plastic deformation is irreversible.

Key Areas Covered

1. What is Elastic Deformation
     – Definition, Effect on Chemical Bonding, Stress-Strain Curve
2. What is Plastic Deformation
     – Definition, Effect on Chemical Bonds, Example Substances
3. What is the Difference Between Elastic and Plastic Deformation
     – Comparison of Key Differences

Key Terms: Deformation, Elastic Deformation, Elasticity, Force, Plastic Deformation, Plasticity, Strain, Stress

What is Elastic Deformation

Elastic deformation is the deformation that disappears upon the removal of external forces, causing the alteration and the stress associated with it. Elastic deformation is hence reversible and non-permanent. Elastic deformation is best explained by the chemical concept “elasticity”. Elasticity is the ability of a substance to resume the normal state after deformation.  

Elastic deformation depends mainly on the chemical bonding of the substance. If the chemical bonds are able to withstand a high stress by deforming without breakage, that substance can undergo elastic deformation.

In order to fulfil this need, chemical bonds should stretch or bend when a stress is applied to the substance. The stretching and bending of chemical bonds should be temporary. However, atoms do not slip on each other while stretching or bending. But with time, the elastic property of substances degrade, and sometimes the substance becomes brittle losing the ductility.

The best examples for elastic deformation come from elastomers such as vulcanized rubber. Vulcanized rubber has a cross-linked polymer structure. There are sulfur bridges between polymer chains. These sulfur crosslinks enhance the elasticity of rubber by helping it to withstand stress.

Figure 1: A Stress Strain Curve for a Ductile Material

The above image shows the stress-strain curve for a ductile material. Ex: copper metal. The elastic region shows the extent to which the elastic deformation takes place. After the elastic limit, the material will undergo plastic deformation which is a permanent deformation.

What is Plastic Deformation

Plastic deformation is the permanent deformation or change in the shape of a solid body without fracture under the action of a sustained force. This occurs when a large stress is applied on to a substance. Plastic deformation is irreversible and is permanent. Plastic deformation is best explained by the chemical concept “plasticity”. Plasticity is the quality of being easily shaped or moulded permanently.

Plastic deformation occurs due to the breakage of a limited number of chemical bonds between atoms that make up the substance. During plastic deformation, atoms may slip pass on each other. This causes dislocations of atoms; thus, the material stays still after removing the applied stress.

For ductile substances, elastic limit is the initial point of plastic deformation. Elastic limit is the maximum extent to which a solid may be stretched without permanent alteration of size or shape. If the stress is applied beyond the elastic limit, then the substance will undergo plastic deformation.

Figure 2: The Elastic Limit shown in a Stress-Strain Curve

The materials in which plastic deformation can be observed include metals, plastics, rocks, etc. In ductile materials such as metals (ex: copper) plastic deformation takes place when the deformation exceeds the elastic limit. But in brittle substances such as rocks, no elastic deformation can be observed before the initiation of plastic deformation. Plastic deformation is important in making new articles using heat or pressure treatments and moulding.

Difference Between Elastic and Plastic Deformation

Definition

Elastic Deformation: Elastic deformation is the deformation that disappears upon the removal of the external forces, causing the alteration and the stress associated with it.

Plastic Deformation: Plastic deformation is the permanent deformation or change in shape of a solid body without fracture under the action of a sustained force.

Reversibility

Elastic Deformation: Elastic deformation is reversible.

Plastic Deformation: Plastic deformation is irreversible.

Status after Deformation

Elastic Deformation: Elastic deformation is non-permanent. The substance can resume the initial state back.

Plastic Deformation: Plastic deformation is permanent. The substance stays unchanged after removing the stress.

Chemical Bonds

Elastic Deformation: Elastic deformation causes the chemical bonds of the substance to undergo stretching and bending.

Plastic Deformation: Plastic deformation cause some of the chemical bonds of the substance to undergo breakage.

Atomic Behavior

Elastic Deformation: Atoms do not slip pass on each other during elastic deformation.

Plastic Deformation: Atoms slip pass on each other during plastic deformation.

Conclusion

Deformation of a substance can occur due to the application of a stress. This deformation can be elastic or plastic depending on the type of the substance, and the extent of stress applied. The main difference between elastic deformation and plastic deformation is that elastic deformation is reversible whereas plastic deformation is irreversible.

Reference:

1. “What is Elastic Deformation? – Definition from Corrosionpedia.” Corrosionpedia, Available here.
2. “What is Plastic Deformation? – Definition from Corrosionpedia.” Corrosionpedia, Available here.
3. Elastic/Plastic Deformation. Education Resources, Available here.

Image Courtesy:

1. “Stress-strain1” By Moondoggy – (CC BY-SA 3.0) Available here.