Short Answer:
Elastic deformation is the temporary change in shape or size of a material when a load is applied, and the material returns to its original shape after the load is removed.
Plastic deformation, on the other hand, is the permanent change in shape or size of a material when the applied load exceeds the elastic limit. In this case, even after removing the load, the material does not return to its original shape. Elastic deformation occurs in the elastic region, while plastic deformation occurs beyond the yield point of the material.
Detailed Explanation:
Difference between Elastic and Plastic Deformation
When a material is subjected to an external force, it undergoes deformation — meaning its shape or size changes. The type of deformation depends on the magnitude of the applied load and the mechanical properties of the material. The deformation can be elastic or plastic, depending on whether the material returns to its original form after the removal of the load.
Understanding these two types of deformation is essential in the field of mechanics of materials, as they help in designing structures, machines, and components that can safely bear loads without failure.
Elastic Deformation
Definition:
Elastic deformation is the reversible deformation of a material that occurs when the applied stress is within the elastic limit. When the stress is removed, the material regains its original shape and size completely.
Explanation:
When a force is applied to a material, the atoms in the material are displaced slightly from their equilibrium positions. If the force is small (within the elastic limit), these atomic bonds stretch but do not break. Once the force is removed, the atoms return to their original positions, and the material regains its original dimensions.
The relationship between stress and strain during elastic deformation follows Hooke’s Law, which states that:
where,
- = stress
- = strain
- = Young’s modulus of elasticity
This means that in the elastic region, the stress is directly proportional to strain.
Example:
- A steel spring when stretched under small loads.
- A rubber band when pulled slightly.
- A mild steel rod under low tension.
Characteristics of Elastic Deformation:
- Temporary and reversible deformation.
- Stress is proportional to strain (Hooke’s Law valid).
- No permanent change in dimensions.
- Occurs up to the elastic limit.
- Stored strain energy can be fully recovered when the load is removed.
Plastic Deformation
Definition:
Plastic deformation is the permanent deformation of a material that occurs when the applied stress exceeds the elastic limit. The material does not regain its original shape even after the removal of the load.
Explanation:
When the applied stress crosses the yield point, the atomic bonds in the material begin to break, and atoms start to move to new stable positions. This results in a permanent displacement of the atomic structure.
In this stage, Hooke’s Law no longer applies because the relationship between stress and strain becomes nonlinear. The material continues to deform even if the stress is not increased further.
The plastic region in a stress-strain curve starts from the yield point and continues until the fracture point. The total strain in this region consists of both elastic and plastic components, but only the elastic part is recoverable.
Example:
- Bending a metal wire permanently.
- Forging or pressing metal sheets.
- Plastic deformation during metal forming or machining.
Characteristics of Plastic Deformation:
- Permanent and irreversible deformation.
- Hooke’s law does not hold.
- There is a permanent change in the shape and size of the material.
- Occurs beyond the elastic limit (in the plastic region).
- Some energy is dissipated as heat and internal friction.
Key Differences between Elastic and Plastic Deformation
| Property | Elastic Deformation | Plastic Deformation |
| Nature | Temporary and reversible | Permanent and irreversible |
| Stress-Strain Relationship | Linear (follows Hooke’s law) | Non-linear (does not follow Hooke’s law) |
| Occurs in | Elastic region (below yield point) | Plastic region (beyond yield point) |
| Recovery | Material returns to original shape | Material does not return to original shape |
| Energy | Fully recoverable elastic energy | Partial recovery; rest converted into heat |
| Atomic Behavior | Atomic bonds stretch but don’t break | Atomic bonds break and reform in new positions |
| Example | Small extension of a spring | Permanent bend in a metal rod |
Stress-Strain Curve Representation
In the stress-strain diagram of a ductile material like mild steel:
- Elastic Region:
- Starts from the origin (O) to the yield point (A).
- Stress and strain are proportional.
- Deformation is reversible.
- Plastic Region:
- Starts from yield point (A) to fracture point (B).
- Stress and strain are not proportional.
- Deformation becomes permanent.
This graphical representation helps visualize how materials behave under different loading conditions.
Practical Importance of Elastic and Plastic Deformation
- Design of Components:
- Engineers design parts such that they remain within the elastic limit under normal operating loads to avoid permanent damage.
- Metal Forming:
- Plastic deformation is used in metalworking processes such as rolling, forging, extrusion, and drawing.
- Safety and Reliability:
- Knowing the elastic limit helps prevent failure in structures like bridges, machines, and buildings.
- Energy Absorption:
- Elastic materials can absorb impact energy and return to their original form, useful in springs and shock absorbers.
- Material Testing:
- The stress-strain relationship helps determine material properties like yield strength, ductility, and toughness.
Examples in Real Life
- Elastic Deformation Examples:
- Stretching of a rubber band within limits.
- Small deflection in a loaded beam.
- Slight twist in a steel shaft under torque.
- Plastic Deformation Examples:
- Bending of a nail.
- Permanent dent in a car body.
- Shaping of metal sheets in industry.
Conclusion
Elastic deformation is temporary and completely reversible, occurring within the elastic limit where Hooke’s law is valid. Plastic deformation is permanent and irreversible, taking place beyond the elastic limit when the material yields.
Understanding the difference between elastic and plastic deformation is crucial in mechanical engineering because it helps in predicting how materials will behave under different loading conditions. This knowledge ensures that structures and machine components are designed safely to avoid permanent damage while utilizing controlled plastic deformation for manufacturing and shaping purposes.