understand and use the terms elastic deformation, plastic deformation and elastic limit

Elastic and Plastic Behaviour

Elastic Deformation

When a material is stretched or compressed and returns to its original shape once the force is removed, it undergoes elastic deformation. The atoms are displaced from their equilibrium positions but the bonds are not broken.

• Behaviour is reversible.
• Often follows Hooke’s Law: $$F = k\,\Delta x$$ or in terms of stress and strain: $$\sigma = E\,\epsilon$$
• Common examples: a rubber band, a steel spring, or a metal wire within its elastic range.

Plastic Deformation

If the applied force is large enough, the material does not return to its original shape after the load is removed. This permanent change is called plastic deformation. Dislocations in the crystal lattice move and the material retains a new shape.

1. Behaviour is irreversible (or only partially reversible).
2. Occurs after the elastic limit is exceeded.
3. Examples: bending a paperclip permanently, shaping clay, or car body panels in a crash.

Elastic Limit

The elastic limit is the maximum stress (or force per unit area) that a material can withstand while still exhibiting purely elastic behaviour. Beyond this point, plastic deformation begins.

💡 Remember: Elastic behaviour is like a spring – stretch it and it snaps back. Plastic behaviour is like modelling clay – shape it and it stays shaped. The elastic limit is the boundary between the two.