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The most important design consideration for helmets is their ability to survive impacts. These impacts can vary from falling objects to head-on collisions. The helmet lessens the impact by absorbing energy - but it turns out that the best way of doing this, and the best materials, depends on the type of impact.
Workers on a building site or in dangerous factories must wear a hard hat. This is to prevent injury caused by impacts from small falling objects such as tools or small stones dropped by those working overhead - no helmet will protect from a falling girder! The key requirement here is that the helmet must not break under the impact. The materials used must have good toughness so they do not fracture (crack), and enough strength to take the maximum load without deforming.
The selection chart helps identify materials that have high toughness and sufficient strength. Composites, most metals, and many polymers look good. Another requirement for hard hats is that they should be inexpensive and low weight. Polymers best meet all these needs and are also easy to manufacture. In fact hard hats are mainly made from polycarbonate or ABS and fitted via adjustable polyethylene straps inside the shell.
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The
requirements for a climbing helmet are similar to those
of a hard hat (although a climber must be able to rely on
their helmet even after it has suffered an impact). The
first designs for climbing helmets were, therefore, not
much different from a hard hat. Since then, considerable
effort has been put into making climbing helmets more
comfortable and lighter in weight. The shell is often
still made of polycarbonate, but as cost is less
of an issue new materials like glass fibre or carbon
fibre reinforced polymer (GFRP/CFRP) composites are
now being used to reduce weight. Helmets can be made more
comfortable by producing a wider range of shapes and
including a liner (usually made of a nylon fabric). |
As well
as protecting from small knocks, a cycle helmet must
protect against the large impact on the head received
during a crash. In this case it doesn't matter if the
helmet is permanently damaged (all manufacturers
recommend replacement after an accident), but it must
absorb lots of energy. The best materials for this are
foams - they absorb lots of energy when they are crushed.
The main impact absorbing material in a cycling helmet
is, therefore, a moulded block of polystyrene foam,
usually with a polycarbonate covering. These
materials are easily shaped - cycling helmets often have
striking shapes to improve their aerodynamic performance.
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All helmets are made from a number of components: an outer
shell, an inner liner (for good fit and comfort), fitting straps,
and perhaps a decorative covering.
Helmet shells are made by blow moulding, vacuum moulding or
injection moulding. In many cases the relatively thin section of
the outer shell lends itself to sheet process technologies.
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