Aluminium Design and Construction John Dwight

12.3.2 Variable amplitude loading
The simple state of affairs covered in Section 12.3.1 is fairly rare. In
most fatigue situations, the loading is more complex, leading to a spectrum
of stress ranges at any critical position. This is known as variable amplitude
loading and the checking procedure runs as follows:
1. Decide on the design life of the structure, referring to Section 12.3.3
as before.
2. Find the number of load cycles during the design life.
3. Obtain the variation of nominal unfactored stress f in each cycle at
the point considered (Figure 12.2). Refer to Sections 12.3.4 and 12.4.
4. Rationalize this stress history by reducing it to a set of specific stress
ranges (f r1 , f r2 , f r3 , etc.), the number of times that each occurs during
the design life being denoted by n 1 , n 2 , n 3 , etc. This provides a stress
range spectrum (Section 12.3.5).
5. Establish the class of the detail at the point considered. Refer to
Section 12.5.
6. Select the appropriate endurance curve, and for each stress range
value (f r1 , f r2 , f r3 , etc.) read off the corresponding endurance (N 1 , N 2 ,
N 3 , etc.) that would be achieved if that stress range were the only
one acting. Refer to Section 12.6.
7. The fatigue resistance at the point considered is acceptable if the
Palmgren-Miner rule is satisfied:
12.3.3 Design life
(12.1)
The nominal design life of a structure is the time for which it is expected
to be in service, and this should be agreed with the client. British Standard
BS.8118 gives a range of typical values for a variety of applications.
The design life, as used in fatigue calculations, is normally taken the
same as the nominal design life. However, the British Standard gives a
designer the option of playing safer, if thought necessary, by multiplying
the nominal life by a fatigue life factor � L (>1). A decision to do this would
hinge on the accuracy of the assumed loading spectrum, whether records
of loading will be kept, or the possibility of a change in use during the
structure’s life. It is fairly rare to step up the design life in this way.
12.3.4 Stress range
The stress range (fr ) is normally taken equal to the nominal stress range,
namely the range over which f varies when nominal (unfactored) loads
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