Choosing for the job

Choosing a material starts from the job, not from the material. Write down what the part must do (the requirements), then look for the material that meets them at an acceptable cost. A few examples:

• Electrical cable: it must carry current with little loss, bend without breaking, and be affordable. Copper conducts very well and is easy to draw into wire. Aluminium conducts less well but is lighter and cheaper, so it is used in long overhead power lines.
• Window: it must let light through and keep out wind and rain, so it must be transparent. Glass does that, which is why it is chosen despite being brittle.
• Frying pan: the base should spread heat evenly (a good heat conductor such as aluminium or copper), while the handle should stay cool (a poor conductor such as plastic or wood).
• Spring: it must be stretched or squeezed and return to its shape many times, so it needs a very high elastic limit. Spring steel is made for exactly that.

There is no “best material”, only the best compromise. A bicycle frame shows the trade-offs:

• Steel: strong, tough and easy to repair, but heavier.
• Aluminium alloy: much lighter than steel for a stiff frame, but less forgiving of repeated loading.
• Carbon fibre composite: very light and can be tuned to be stiff in the right directions, but costly and harder to repair.

A cyclist who wants a cheap commuter and a racer who wants every gram saved choose differently, and both are right for their job.

Look at a cycling helmet. The outer shell is a thin hard plastic, but most of the helmet is a crushable foam. In a crash the foam is squashed permanently. That takes a little time and distance, and it absorbs the energy of the impact, so less force reaches the head. A tough, rigid helmet with no foam would stop the head more suddenly and be worse. Here the “soft” material is the clever choice.