Aluminium Design and Construction John Dwight

of fatigue, but the member had a low natural frequency of vibration
and, in the unclad condition, wind-excited oscillations caused it to fail
by flexural fatigue after a few weeks.
Another non-obvious type of fatigue failure is that due to transverse
stressing at the welds in slender plate-girders. If the web operates in
the post-buckled condition, due to a very high d/t ratio, it will flex in
and out each time the load is applied, causing repeated flexure about
the axis of the web/flange joint and hence fatigue in the weld.
The treatment of fatigue presented in this chapter is based on that in
BS.8118, which was largely the work of Ogle and Maddox [30] at the
TWI. The data provided for welded details refers specifically to arcwelded
joints (MIG, TIG). Friction-stir welding is still in its infancy, but
preliminary results suggest the FS process produces joints which are
much better in fatigue than those made by MIG or TIG.
12.2 POSSIBLE WAYS OF HANDLING FATIGUE
There are three possible approaches for checking a proposed design
against failure by fatigue:
1. safe life method;
2. fail-safe method (‘damage-tolerant’ approach);
3. testing.
The usual method (1), which is entirely done by calculation, is the one
explained in this chapter. It essentially consists of estimating the range
of stress fr , arising in service at any critical position, finding the
corresponding endurance N from the relevant fr-N curve, and then
checking that the resulting life is not less than that required.
In method (2), the safety margins in design are lower than those
required in a safe-life design. This is permissible because regular inspection
is carried out, enabling the growth of any fatigue cracks to be monitored
during the life of the structure. If the size of a crack or the rate of crack
growth exceeds that allowed, the structure is taken out of service and
the critical component repaired or replaced. Obviously, it is essential
that all potential fatigue sites should be easily inspectable if this method
is to be adopted, and considerable expertise is needed. Inspection methods,
the time between inspections, acceptable crack lengths and allowable
rates of crack growth must all be agreed between the designer and the
user of the structure. When fatigue is critical, the fail-safe method will
tend to produce a lighter structure than method (1). It is the approach
most used in aircraft design. British Standard BS.8118 does not cover
the fail-safe method, and it is beyond the scope of this book.
Fatigue testing (3) should be employed when it is impossible to apply
method (1), due to problems in verifying a design by calculation alone.
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