Elasticity is a material's ability to return to its original dimensions after a force is removed. Plasticity is its ability to undergo permanent deformation without breaking, while ductility is plastic deformation specifically under tensile loading.
The Reasoning
When a force causes deformation, an elastic material recovers its original shape once that force is removed, provided its elastic limit has not been exceeded. For example, a steel spring stretches under load and then returns to its original length.
If loading exceeds the elastic limit, the material enters the plastic region and remains permanently deformed after unloading. Plasticity is important in manufacturing processes such as moulding, pressing, and forging, where a lasting change of shape is required.
Ductility is a more specific property. It describes how far a material can plastically deform under tensile force before fracture, allowing it to be drawn into a wire. Copper is highly ductile, which makes it suitable for electrical wiring.
| Property | Behavior after force removal | Design Technology example |
|---|---|---|
| Elasticity | Returns to its original dimensions if the elastic limit is not exceeded | Spring steel in a suspension spring |
| Plasticity | Retains a permanent change in shape | Thermoplastic formed into packaging |
| Ductility | Extends plastically under tension before fracture | Copper drawn into electrical wire |
A common misconception is that plasticity and ductility are identical. Ductility involves plastic deformation, but it refers specifically to deformation under tension; plasticity is the broader ability to deform permanently without fracture. A material may therefore show plastic behavior without being highly ductile.
Exam Technique
For an IB Design Technology distinguish question, define each property and make the contrast explicit: recovery for elasticity, permanent deformation for plasticity, and permanent tensile deformation for ductility. Link each property to a suitable material, manufacturing process, or product application rather than giving definitions alone.