Page 100 - Welding of Aluminium and its Alloys

Welding design 89

of the base materials. Longitudinal bow in welded beams should preferably
be done cold by pressing, and buckled plate may be pressed flat.
As a last resort, local spot or line heating may be used to heat-shrink
items that have been distorted by the welding of, for instance, stiffeners.
Some examples of how these techniques can be applied are illustrated in
Fig. 5.24. The high thermal conductivity of aluminium means that local
heating with an oxy-gas torch is not very effective. If this technique is to be
used then electric induction heating is the most effective method of intro-
ducing sufficient heat into the component.
If heat must be used then this should not exceed 400°C for the non-heat
treatable alloys. Remember, though, that temperatures over 250°C will
produce full softening in the alloy if it is in the work-hardened condition.
The age-hardened heat-treated alloys should not be heated to more than
150°C as this will cause softening due to overageing of the precipitates.
Whenever these techniques are used then reference must be made to the
design engineer to ensure that the potential loss of strength is taken into
account.

5.8 Fatigue strength of welded joints
Fatigue, as the name suggests, is a failure mechanism where the component
fails after a period of time in service where it sees a repetitive cyclic stress.
Failure may occur even if this stress is substantially below the yield strength
of the metal as the other factor in causing failure is the number of stress
cycles that the component experiences. Failure always occurs normal to the
principal applied stress and the fracture surface is characterised by so-called
‘beach marking’ where changes in the stress level give different rates of
crack propagation. This gives the surface a rippled appearance similar to a
beach when the tide has ebbed away. The rate of crack propagation is pro-
portional to the stress range and the crack length. Cracks in the early stages
of growth tend to be very small and to grow slowly, making it easy for them
to be missed during in-service inspection.
To be able to predict the fatigue life of a structure the designer needs
accurate details of the full service loading conditions and accurate fatigue
data on the performance of the component parts of the structure.The most
common sites for initiation are weld toes, both root and face, drilled holes,
machined corners and threaded holes. Of these sites the most significant are
welds. Since welding has such a significant effect on fatigue life it will be
necessary to specify welding details and controls rather more closely than
for a statically loaded structure. This will, inevitably, have an effect on the
cost of fabrication.