Short Answer:

Elasticity is the property of a material by which it returns to its original shape and size after the removal of an external force. It helps materials like steel and rubber regain their shape when stretched or compressed within their elastic limit.

Plasticity, on the other hand, is the property of a material by which it undergoes permanent deformation when the applied load exceeds the elastic limit. Materials such as lead, copper, and aluminum show high plasticity. Elasticity is important for reversible deformation, while plasticity is important for permanent shaping and forming processes.

Detailed Explanation:

Elasticity and Plasticity

Meaning and Definition of Elasticity:
Elasticity is one of the most important mechanical properties of materials. It is defined as the property of a material to regain its original shape and size when the deforming force is removed, provided the deformation remains within the elastic limit.

When an external load or force is applied to a body, it tends to deform. If the material is elastic, it will develop internal restoring forces that try to bring it back to its original condition once the external load is removed. This ability to recover its shape and dimensions is known as elasticity.

For example:

• A rubber band returns to its original size after stretching.
• A steel spring regains its shape when the applied load is removed.

Elastic behavior is essential for components such as springs, beams, shafts, and machine parts that experience repeated loading and unloading during operation.

Elastic Limit and Elastic Behavior:

Every material has a certain limit up to which it behaves elastically. This limit is called the elastic limit. When the stress applied to a material is below this limit, the strain (deformation) is directly proportional to the stress, and the material follows Hooke’s Law.
However, if the applied load exceeds the elastic limit, the material cannot return to its original shape after the removal of the load. Beyond this point, permanent deformation starts, leading to plastic behavior.

In engineering, the modulus of elasticity (E) or Young’s Modulus is used to measure the stiffness of an elastic material. The higher the modulus of elasticity, the stiffer the material is, and the smaller the deformation under load.

Formula:

Examples of Elastic Materials:

• Steel
• Aluminum (within small deformation range)
• Rubber (shows high elasticity but nonlinear behavior)
• Glass (in small deformation range)

Meaning and Definition of Plasticity:

Plasticity is the property of a material to undergo permanent deformation without breaking when the applied stress exceeds the elastic limit. Once the load is removed, the material does not regain its original shape and size.

In simple terms, plasticity describes how easily a material can be permanently reshaped under force. This property is very important in manufacturing processes such as forging, rolling, extrusion, and stamping, where metals are permanently deformed into desired shapes.

For example:

• When a copper wire is bent, it remains bent even after removing the force.
• When a lead rod is compressed, it stays deformed without returning to its original size.

Thus, materials that exhibit high plasticity can be easily shaped or molded without cracking.

Plastic Deformation and Yield Point:

When the applied load exceeds the elastic limit, the material enters a stage called the yield point. Beyond this point, any further load causes plastic deformation — a permanent change in shape that remains even after the load is removed.

Plastic deformation occurs because the internal atomic structure of the material is rearranged permanently. The atoms slide over one another, and new positions are formed, leading to a change in the shape or dimension of the body.

Plasticity is desirable in processes where materials need to be formed into various shapes without failure, such as in metal forming industries.

Comparison between Elasticity and Plasticity

1. Nature of Deformation:
   • Elasticity: Temporary and reversible deformation.
   • Plasticity: Permanent and irreversible deformation.
2. Behavior after Load Removal:
   • Elasticity: Material regains its original shape.
   • Plasticity: Material retains deformed shape.
3. Stress Level:
   • Elasticity: Below the elastic limit.
   • Plasticity: Above the elastic limit (after yielding).
4. Examples:
   • Elastic Materials: Steel, Rubber.
   • Plastic Materials: Lead, Copper, Aluminum.
5. Application:
   • Elasticity: Used in springs, machine parts, beams, and shafts.
   • Plasticity: Used in shaping processes like forging, rolling, and extrusion.

Importance in Engineering Applications

Elasticity:

• Used in designing components that experience repeated loading and unloading.
• Helps in predicting how much a structure will deform under load without permanent damage.
• Ensures safety and reliability in mechanical and civil structures.

Plasticity:

• Used in manufacturing industries for shaping and forming operations.
• Helps in metal processing like forging, bending, and rolling.
• Important for predicting the permanent deformation behavior of materials under high load conditions.

Both properties are critical for engineers to understand material selection and design. Elastic materials are used for components that must return to their original shape, while plastic materials are used for permanent shaping and forming.

Elastic and Plastic Region in Stress-Strain Curve

On a stress-strain curve, the initial straight portion represents the elastic region, where Hooke’s Law is obeyed. Beyond the yield point, the curve becomes nonlinear, showing the plastic region where permanent deformation occurs.

• Elastic Region: Material returns to its original shape.
• Plastic Region: Material remains permanently deformed even after the load is removed.

The transition between these two regions defines the yield point — the dividing line between elasticity and plasticity.

Conclusion:

Elasticity and plasticity are two fundamental mechanical properties that describe how materials behave under applied loads. Elasticity allows materials to return to their original form when the load is removed, while plasticity allows materials to undergo permanent deformation when the load exceeds the elastic limit. Both properties are essential in engineering design and manufacturing — elasticity ensures safety and recovery in structures, while plasticity enables shaping and forming of materials in production processes. Understanding both helps engineers select suitable materials for specific applications and ensure performance and reliability.