Smith chapter 1 problem 5, Materials for protective sports helmets
a) Important
criteria include
- Need
an inexpensive material and production process both, something like
injection molding to produce net shape in one step.
- Solid
up to a high enough temperature so hot water or heat inside a car in the
summer won’t melt it.
- Light weight,
low density
- Stiffness
(elastic modulus) not too low and not too high. At least part of the helmet needs to be compliant
and energy absorbing when hit hard.
Viscoelastic would be good.
- Tough,
fracture resistant so it won’t crack in two, even under extreme impact.
- Chemically
rather inert so it won’t oxidize in air or be damaged by rain or
perspiration or sunlight.
- Strong
enough so that dropping it or minor impact does not make a dent or ugly
looking damage to it.
b) Materials
satisfying the above criteria:
- hard
plastic shell
- soft
foam rubber or plastic foam interior
c) Solid
metal is not a good choice because it would be way too expensive. One could, for example make the outside of a
football helmet out of aluminum alloys, but it would cost a lot. And still one would need a foam rubber or
soft plastic liner.