Page 277 - Aircraft Stuctures for Engineering Student

258 Airworthiness and airframe loads

of the aircraft rather than carry a heavier undercarriage which has the same safe life as
the aircraft.
The fail-safe approach relies on the fact that the failure of a member in a redundant
structure does not necessarily lead to the collapse of the complete structure, provided
that the remaining members are able to carry the load shed by the failed member and
can withstand further repeated loads until the presence of the failed member is
discovered. Such a structure is called a fail-safe structure or a damage tolerant
structure.
Generally, it is more economical to design some parts of the structure to be fail-safe
rather than to have a long safe life since such components can be lighter. When failure
is detected, either through a routine inspection or by some malfunction, such as fuel
leakage from a wing crack, the particular aircraft may be taken out of service and
repaired. However, the structure must be designed and the inspection intervals
arranged such that a failure, for example a crack, too small to be noticed at one
inspection must not increase to a catastrophic size before the next. The determination
of crack propagation rates is discussed later.
Some components must be designed to have a safe life; these include landing gear,
major wing joints, wing-fuselage joints and hinges on all-moving tailplanes or on
variable geometry wings. Components which may be designed to be fail-safe include
wing skins which are stiffened by stringers and fuselage skins which are stiffened by
frames and stringers; the stringers and frames prevent skin cracks spreading
disastrously for a sufficient period of time for them to be discovered at a routine
inspection.

8.7.2 Designing against fatigue

Various precautions may be taken to ensure that an aircraft has an adequate fatigue
life. We have seen in Chapter 7 that the early aluminium-zinc alloys possessed high
ultimate and proof stresses but were susceptible to early failure under fatigue loading;
choice of materials is therefore important. The naturally aged aluminium-copper
alloys possess good fatigue resistance but with lower static strengths. Modern
research is concentrating on alloys which combine high strength with high fatigue
resistance.
Attention to detail design is equally important. Stress concentrations can arise
at sharp corners and abrupt changes in section. Fillets should therefore be
provided at re-entrant corners, and cut-outs, such as windows and access panels,
should be reinforced. Rivets should not be used in areas of high stress and stiffeners
should be bonded to plates rather than attached by rivets. In machined panels the
material thickness should be increased around bolt holes, while holes in primary
bolted joints should be reamered to improve surface finish; surface scratches and
machine marks are sources of fatigue crack initiation. Joggles in highly stressed
members should be avoided while asymmetry can cause additional stresses due to
bending.
In addition to sound structural and detail design, an estimation of the number,
frequency and magnitude of the fluctuating loads an aircraft encounters is necessary.
The fatigue load spectrum begins when the aircraft taxis to its take-off position.