Page 232 - Aircraft Stuctures for Engineering Student

2 16 Principles of stressed skin construction
testing programmes and has the detailed specification of:

%Cu %Mg %Si %Fe %Ni %Ti %A1
Minimum 2.25 1.35 0.18 0.90 1.0 - Remainder
Maximum 2.70 1.65 0.25 1.20 1.30 0.20

Generally, CMOO 1 is found to possess better overall strength/fatigue characteristics
over a wide range of temperatures than any of the other possible aluminium alloys.
The latest aluminium alloys to find general use in the aerospace industry are the
aluminium-lithium alloys. Of these, the aluminium-lithium-copper-manganese
alloy, 8090, developed in the UK, is extensively used in the main fuselage structure
of GKN Westland Helicopters’ most recent design EH101; it has also been qualified
for Eurofighter 2000 (now named the Typhoon) but has yet to be embodied. In the
USA the aluminium-lithium-copper alloy, 2095, has been used in the fuselage
frames of the F16 as a replacement for 2124, resulting in a fivefold increase in fatigue
life and a reduction in weight. Aluminium-lithium alloys can be successfully welded,
possess a high fracture toughness and exhibit a high resistance to crack propagation.

7.1.2 Steel

The use of steel for the manufacture of thin-walledy box-section spars in the 1930s has
been described previously in this section. Clearly, its high specific gravity prevented its
widespread use in aircraft construction, but it has retained some value as a material
for castings for small components demanding high tensile strengths, high stiffness
and high resistance to wear. Such components include undercarriage pivot brackets,
wing-root attachments, fasteners and tracks.
Although the attainment of high and ultra-high tensile strengths presents no
difficulty with steel, it is found that other properties are sacrificed and that it is
difficult to manufacture into finished components. To overcome some of these
difficulties types of steel known as maraging steels were developed in 1961, from
which carbon is either eliminated entirely or present only in very small amounts.
Carbon, while producing the necessary hardening of conventional high tensile
steels, causes brittleness and distortion; the latter is not easily rectsable as machining
is difficult and cold forming impracticable. Welded fabrication is also almost impos-
sible or very expensive. The hardening of maraging steels is achieved by the addition
of other elements such as nickel, cobalt and molybdenum. A typical maraging steel
would have these elements present in the proportions: nickel 17-19 per cent, cobalt
8-9 per cent, molybdenum 3-3.5 per cent, with titanium 0.15-0.25 per cent. The
carbon content would be a maximum of 0.03 per cent, with traces of manganese,
silicon, sulphur, phosphorus, aluminium, boron, calcium and zirconium. Its 0.2 per
cent proof stress would be nominally 1400N/mm2 and its modulus of elasticity
180 000 N/mm2.
The main advantages of maraging steels over conventional low alloy steels are:
higher fracture toughness and notched strength, simpler heat treatment, much
lower volume change and distortion during hardening, very much simpler to weld,
easier to machine and better resistance to stress corrosion/hydrogen embrittlement.