Page 234 - Aircraft Stuctures for Engineering Student

2 18 Principles of stressed skin construction

their specific gravities are less than half those of the aluminium alloys so that they find
uses as windows or lightly stressed parts whose dimensions are established by
handling requirements rather than strength. They are also particularly useful as
electrical insulators.

7.1.5 Glass

The majority of modern aircraft have cabins pressurized for flight at high altitudes.
Windscreens and windows are therefore subjected to loads normal to their midplanes.
Glass is frequently the material employed for this purpose in the form of plain or
laminated plate or heat-strengthened plate. The types of plate glass used in aircraft
have a modulus of elasticity between 70 000 and 75 000 N/mm2 with a modulus of
rupture in bending of 45 N/mm2. Heat strengthened plate has a modulus of rupture
of about four and a half times this figure.

7.1.6 Composite materials

Composite materials consist of strong fibres such as glass or carbon set in a matrix of
plastic or epoxy resin, which is mechanically and chemically protective. The fibres
may be continuous or discontinuous but possess a strength very much greater than
that of the same bulk materials. For example, carbon fibres have a tensile strength
of the order of 2400 N/mm2 and a modulus of elasticity of 400 000 N/mm2.
A sheet of fibre-reinforced material is anisotropic, that is, its properties depend on
the direction of the fibres. Generally, therefore, in structural form two or more sheets
are sandwiched together to form a lay-up so that the fibre directions match those of
the major loads.
In the early stages of the development of composite materials glass fibres were used
in a matrix of epoxy resin. This glass reinforced plastic (GRP) was used for radomes
and helicopter blades but found limited use in components of fixed wing aircraft due
to its low stiffness. In the 1960s, new fibrous reinforcements were introduced; Kevlar,
for example, is an aramid material with the same strength as glass but is stiffer. Kevlar
composites are tough but poor in compression and difficult to machine, so they were
used in secondary structures. Another composite, using boron fibre and developed
in the USA, was the first to possess sufficient strength and stiffness for primary
structures.
These composites have now been replaced by carbon fibre reinforced plastics
(CFRP), which have similar properties to boron composites but are very much
cheaper. Typically, CFRP has a modulus of the order of three times that of GRP,
one and a half times that of a Kevlar composite and twice that of aluminium alloy.
Its strength is three times that of aluminium alloy, approximately the same as that
of GRP, and slightly less than that of Kevlar composites. CFRP does, however,
suffer from some disadvantages. It is a brittle material and therefore does not yield
plastically in regions of high stress concentration. Its strength is reduced by impact
damage which may not be visible and the epoxy resin matrices can absorb moisture