Page 230 - Aircraft Stuctures for Engineering Student

214 Principles of stressed skin construction

of the group, has a typical composition of: 4 per cent copper, 2 per cent nickel, 1.5 per
cent magnesium, the remainder being aluminium and was covered by Specification
DTD 58A issued in 1927. Its most important property was its retention of strength
at high temperatures, which meant that it was a particularly suitable material for
aero engine pistons. Its use in airframe construction has been of a limited nature
only. Research by Rolls-Royce and development by High Duty Alloys Ltd produced
the ‘RR’ series of alloys. Based on Y alloy, the RR alloys had some of the nickel
replaced by iron and the copper reduced. One of the earliest of these alloys, RR56,
had approximately half of the 2 per cent nickel replaced by iron, the copper content
reduced from 4 to 2 per cent, and was used for forgings and extrusions in aero engines
and airframes. Specification DTD 130, issued in 1930, listed minimum mechanical
properties for RR56 of 0.1 per cent proof stress 310N/mm’, tensile strength of
400 N/mm2 and elongation of 10 per cent.
The third and latest group depends upon the inclusion of zinc and magnesium for
their high strength. Covered by Specification DTD 363 issued in 1937, these alloys had
a nominal composition: 2.5 per cent copper, 5 per cent zinc, 3 per cent magnesium and
up to 1 per cent nickel with mechanical properties, 0.1 per cent proof stress 510N/mm2,
tensile strength 585 N/mm2 and an elongation of 8 per cent. In modern versions of
this alloy nickel has been eliminated and provision made for the addition of
chromium and further amounts of manganese.
Of the three basic structural materials described above, namely wood, steel and
aluminium alloy, only wood is no longer of significance except in the form of
laminates for non-structural bulkheads, floorings and furnishings. Most modern
aircraft, for example Concorde, still rely on modified forms of the high strength
aluminium alloys which were introduced during the early part of the 20th century.
Steels are used where high strength, high stiffness and wear resistance are required.
Other materials, such as titanium and fibre-reinforced composites first used about
1950, are finding expanding uses in airframe construction. All these and some
additional materials are now discussed in detail.

7.1.1 Aluminium alloys

We have noted that airframe construction has depended for many years on the three
groups of aluminium alloys: (i) the nickel free duralumins, (ii) the derivatives of Y
alloy and (iii) the aluminium-zinc-magnesium group. Alloys from each group
have been used extensively for airframes, skins and other stressed components, the
choice of alloy being influenced by factors such as strength (proof and ultimate
stress), ductility, ease of manufacture (e.g. in extrusion and forging), resistance to
corrosion and amenability to protective treatment, fatigue strength, freedom from
liability to sudden cracking due to internal stresses and resistance to fast crack
propagation under load. Clearly, different types of aircraft have differing require-
ments. A military aircraft, for instance, having a relatively short life measured in
hundreds of hours, does not call for the same degree of fatigue and corrosion
resistance as a civil aircraft with a required life of 30 000 hours or more.
Unfortunately, as one particular property of aluminium alloys is improved, other
desirable properties are sacrificed. For example, the extremely high static strength of