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The two most important material properties required to ensure a cycling helmet lining will crush in the event of a crash is yield strength and fracture toughness. Helmets should have a relatively high fracture toughness so that they do not crack under the force of the crash and enough yield strength to take the force of the impact without deforming.

The best material group for this application is Polymers (specifically polystyrene and polycarbonate). This is because polymers have the right amount of yield strength and fracture toughness to absorb a lot of energy and provide sufficient protection in a bike crash as seen in figures …. They are also easily moulded to fit any type of head.

Figure 1- Fracture toughness vs yield strength of all materials

Figure 2- Fracture toughness vs yield strength of all materials (zoomed in)...