Page 233 - Aircraft Stuctures for Engineering Student

7.1 Materials of aircraft construction 21 7

On the other hand, the material cost of maraging steels is three or more times greater
than the cost of conventional steels, although this may be more than offset by the
increased cost of fabricating a complex component from the latter steel.
Maraging steels have been used in: aircraft arrester hooks, rocket motor cases,
helicopter undercarriages, gears, ejector seats and various structural forgings.
In addition to the above, steel in its stainless form has found applications primarily
in the construction of super- and hypersonic experimental and research aircraft,
where temperature effects are considerable. Stainless steel formed the primary struc-
tural material in the Bristol 188, built to investigate kinetic heating effects, and also in
the American rocket aircraft, the X-15, capable of speeds of the order of Mach 5-6.

The use of titanium alloys increased significantly in the 198Os, particularly in the
construction of combat aircraft as opposed to transport aircraft. This increase has
continued in the 1990s to the stage where, for combat aircraft, the percentage of
titanium alloy as a fraction of structural weight is of the same order as that of
aluminium alloy. Titanium alloys possess high specific properties, have a good fatigue
strength/tensile strength ratio with a distinct fatigue limit, and some retain consider-
able strength at temperatures up to 4O0"-50O0C. Generally, there is also a good
resistance to corrosion and corrosion fatigue although properties are adversely
affected by exposure to temperature and stress in a salt environment. The latter
poses particular problems in the engines of carrier-operated aircraft. Further dis-
advantages are a relatively high density so that weight penalties are imposed if the
alloy is extensively used, coupled with high primary and high fabrication costs,
approximately seven times those of aluminium and steel.
In spite of this, titanium alloys were used in the airframe and engines of Concorde,
while the Tornado wing carry-through box is fabricated from a weldable medium
strength titanium alloy. Titanium alloys are also used extensively in the F15 and
F22 American fighter aircraft and are incorporated in the tail assembly of the
Boeing 777 civil airliner. Other uses include forged components such as flap and
slat tracks and undercarriage parts.
New fabrication processes (e.g. superplastic forming combined with diffusion
bonding) enable large and complex components to be produced, resulting in a reduc-
tior. in production man-hours and weight. Typical savings are 30 per cent in man-
hours, 30 per cent in weight and 50 per cent in cost compared with conventional
riveted titanium structures. It is predicted that the number of titanium components
fabricated in this way for aircraft will increase significantly and include items such
as access doors, sheet for areas of hot gas impingement etc.

7.1.4 Plastics

Plain plastic materials have specific gravities of approximately unity and are therefore
considerably heavier than wood although of comparable strength. On the other hand,