How materials deform
A material can change shape or length when a force is applied. What happens after the force is removed distinguishes elastic from plastic deformation.
| Behaviour | After the force is removed | Key idea |
|---|---|---|
| Elastic deformation | The object returns to its original shape and length. | The deformation is reversible. |
| Plastic deformation | The object does not return completely to its original shape or length. | There is permanent deformation. |
The elastic limit
The elastic limit marks the boundary beyond which permanent deformation occurs. If a material is stretched beyond this limit and then unloaded, it retains a permanent extension.
The elastic limit is not the same as the limit of proportionality. A material may cease to have force directly proportional to extension and still return to its original dimensions when the force is removed.
- Below the elastic limit, deformation can be recovered when the load is removed.
- Beyond the elastic limit, some deformation remains permanently.
- Plastic behaviour should therefore be identified by permanent deformation, not simply by failure to obey Hooke's law.
Exam Tip: When explaining plastic deformation, state that the material is permanently deformed or has a permanent extension after the load is removed. Saying only that it no longer obeys Hooke's law does not fully describe plastic behaviour.