Helmets
Helmets have existed since early times to protect people in
battle. Before the introduction of steel and aluminium, they were
made of iron and beaten into shape. These days helmets are not
only used by the police and army, but have also seen widespread
use in more peaceful areas like building, cycling, horse riding,
snowboarding, boxing etc. Modern helmets are mainly of polymer
construction, but their design varies depending on the
application.
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Impact resistance
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.
Hard Hats
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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Devise a test to compare
the impact resistance of different polymers like
polystyrene foam, PMMA (also known as Perspex)
and polyethylene. Investigate how easy they are
to break if damaged with a small cut.
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Climbing Helmets
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). |
Cycling Helmets
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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Manufacturing
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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Aesthetics
Compare and describe the
aesthetics (colour, texture, pattern) of many
different types of helmets, and discuss how the
market influences the style and appearance of
each design.
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