Aluminium Design and Construction John Dwight

CHAPTER 12
Fatigue
12.1 GENERAL DESCRIPTION
It is well known that seemingly ductile metal components can fail in a
brittle manner at a low load, far below their static strength, when this
load is applied many times. Aluminium is more prone to this problem
than steel.
The phenomenon, known as fatigue, results from the presence of
localized details or irregularities in zones carrying tensile stress, especially
at welds. These act as stress-raisers and although they have no effect on
static resistance, they become critical under repeated load. Elastic analysis
predicts a peak stress at such positions that greatly exceeds the basic
stress found using conventional stress formulae. The ratio of peak to
basic stress, the stress-concentration factor, can reach a value of 3 or
more. The peak stress, which is highly localized, causes a microscopic
crack to form (‘initiate’) at a relatively low level of basic stress, which
then grows (‘propagates’) each time the load is applied. At first the rate
of propagation per load cycle is minute, but after many cycles it speeds
up, eventually leading to catastrophic failure.
In non-welded construction, a fatigue crack may form at a bolt or
rivet hole, at a sudden change of cross-section, or at any other geometric
irregularity. Just the very slight surface roughness of the aluminium
itself, well away from any joint or change of section, may be sufficient
to cause fatigue. Welded components fare worse. Even when the welding
is to the highest standard, there are still inevitable stress-raisers at the
toe or root of a weld, and also in the ripples on the weld surface. These
all lead to an inferior performance in fatigue. With lower standards of
fabrication, the welds are likely to contain additional unintended defects
(micro-cracks, undercut, lack of penetration), which will reduce the
fatigue strength still further. The level of inspection specified to the
fabricator can be crucial.
The number of cycles N to failure (the endurance) at a given detail is
found to relate mainly to the stress range (fr ), especially for cracks initiating
at welds. In other words, what matters is the difference between maximum
and minimum stress in each cycle. Modern design rules for fatigue are
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