The big idea: A mechanical property describes how a material behaves when a force is applied.
Each is defined by a test: pulled, squashed, bent, struck, scratched, hammered, released, drawn out. Learn the test and the definition follows.
Every property drawn as its own test. The last step is the four confusions examiners rely on.
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| Property | The test | Where it matters |
|---|---|---|
| Tensile strength | Pull it apart until it breaks | Climbing rope, bridge cable, bike spoke |
| Compressive strength | Squash it until it crushes | Concrete column, brick, chair leg |
| Stiffness | Load a beam and measure the sag | A diving board is strong and deliberately not stiff |
| Toughness | Strike it sharply | A hard hat is tough; glass is not |
| Hardness | Drag a harder point across it | Phone screen, drill bit, worktop |
| Malleability | Hammer it into sheet | Aluminium foil, pressed car panel |
| Elasticity | Stretch it and let go | Rubber band, spring, running-shoe sole |
| Ductility | Draw it into a wire | Copper cable, steel wire rope |
Plasticity, the ninth: Plasticity is the opposite of elasticity: the material deforms under load and stays deformed. A paperclip you bend has passed from elastic to plastic behaviour — which is exactly what makes it useful.
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These four pairs are the most examined confusion in the whole subject. Learn them as pairs, with the example attached.
Strong ≠ stiff
Strength is the force it takes to break it. Stiffness is how much it bends before then. A diving board is very strong and deliberately flexible; a glass sheet is stiff and not strong.
Hard ≠ tough
Hardness resists scratching at the surface. Toughness absorbs energy without fracturing. Glass is very hard and shatters instantly; a hard hat is tough and scratches easily.
Elastic ≠ plastic
Elastic deformation returns; plastic deformation stays. A spring is elastic; a bent paperclip has deformed plastically. Most materials do both — elastic up to a point, then plastic.
Malleable ≠ ductile
Malleable goes into SHEET by hammering or rolling. Ductile goes into WIRE by drawing. Gold is both; lead is malleable and not very ductile.
Mechanical properties at work
Tensile strength
Resists pulling: seatbelts, safety harness straps, climbing ropes and the Kevlar fibres in a puncture-resistant tyre.
Toughness
Absorbs impacts without cracking: a polycarbonate visor, a titanium drinks flask, a cycle helmet shell.
Hardness
Resists scratching: sapphire watch glass and ceramic watch bezels stay unmarked.
Ductility
Can be drawn into wire: copper cable, and the steel wire of a wire-mesh stool.
Thermosets keep their shape
A thermoset plastic stays stiff and keeps its shape when hot, so it is used for pan handles and plug sockets.
Old papers, old labels
Some older papers call hardness and toughness "physical" properties. In this course they are mechanical.
How this is tested — explaining the mechanical properties and distinguishing the pairs that are confused. It comes up two ways:
Paper 1 — multiple choice
- Identify the property a described test measures.
- Pick the property a product requirement needs.
Paper 2 — analysing a product
- Explain the mechanical properties needed by parts of a product.
- Distinguish between two properties students confuse.
The trap: Using strong as a catch-all. Say which strength — tensile or compressive — or say stiffness, toughness or hardness if that is what you mean.
Explain the mechanical properties required by the shell and the strap of a cycle helmet.
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