Ductile materials
Materials - OCR A-Level Physics
Key Definition
Ductile material
A material that undergoes a large amount of plastic deformationPermanent deformation in which the material does not return to its original shape when the deforming force is removed. before fracture, so it can be drawn into a wire. Examples: copper, mild steel, aluminium.
A material that undergoes a large amount of plastic deformationPermanent deformation in which the material does not return to its original shape when the deforming force is removed. before fracture, so it can be drawn into a wire. Examples: copper, mild steel, aluminium.
Strong, stiff, tough
Strong: high ultimate tensile stressThe maximum stress a material can withstand before breaking. (UTS). Stiff: high Young modulusThe ratio of stress to strain for a material in the elastic region. A measure of stiffness. Measured in pascals (Pa). (small strain for a given stress). Tough: large area under the stress-strain curve (absorbs a lot of energy before fracture). A material can be one without being the others.
Diagram pending
Stress-strain curve for a ductile material (copper). Steep linear elastic region to the limit of proportionality P, elastic limit E close by, yield point Y where strain rises sharply, a long plastic region rising gently to the ultimate tensile stress (UTS) peak, then necking and fracture F at lower stress but higher strain.
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- Ductile materials (e.g. copper, mild steel) show a long plastic region after the yield point. The wire stretches a lot before snapping, which makes drawing into thin wires possible.
- Necking happens after the UTS: the wire thins at the weakest point, cross-sectional area drops, true stress rises and the wire breaks at that local thin spot.
- Worked comparison: copper (Young modulus $\approx 120 \text{ GPa}$, UTS $\approx 220 \text{ MPa}$, breaks at strain $\approx 0.45$) versus cast iron (similar stiffness but brittle, breaks at strain $\approx 0.005$ with no plastic region).
- A nylon climbing rope is tough but not very stiff: it stretches a lot, absorbing the falling climber's kinetic energy through a large area under its force-extension curve.
Common Mistake
MEDIUM
Wrong: Using 'strong', 'stiff' and 'tough' as synonyms. Writing that cast iron is tough because its UTS is high.
Right: Strong $=$ high UTS. Stiff $=$ high Young modulus. Tough $=$ large area under stress-strain curve. Cast iron is strong and stiff but brittle, so it is not tough (small area under curve, snaps without warning).
Right: Strong $=$ high UTS. Stiff $=$ high Young modulus. Tough $=$ large area under stress-strain curve. Cast iron is strong and stiff but brittle, so it is not tough (small area under curve, snaps without warning).
Examiner Tips and Tricks
- When asked to identify a ductile material from a stress-strain graph, point to the long curved plastic region after the yield point.
- The phrase 'drawn into wires' is mark-scheme language for ductile behaviour.