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Steel Designers' Manual - 6th Edition (2003)
266 Fracture and fatigue
Fig. 7.11 An example of an F2 cracked weld
7.6.5 Variable-amplitude loading
For constant-amplitude loading, the permissible stress range can be obtained
directly from Fig. 7.10 by considering the required design life. In practice it is more
common for structures to be subjected to a loading spectrum of varying amplitudes
or random vibrations. In such cases use is made of Miner’s rule. 15
Miner’s rule is a linear summation of the fatigue damage accumulated during the
life of the structure. For a joint subjected to a number of repetitions, ni, each of
several stress ranges Dsi, the value of ni corresponding to each Dsi should be determined
from stress spectra measured on similar equipment or by making reasonable
assumptions as to the expected service history. The permissible number of cycles,
Ni, at each stress range, Dsi, should then be determined from Fig. 7.10 for the relevant
joint class and the stress range adjusted so that the linear cumulative damage
summation does not exceed unity:
i= j
n1
n2
n3
nj
ni
+ + + ... + = Â < 10 .
N N N N N
1
2
3 j i=
1
i
(7.9)
The order in which the variable-amplitude stress ranges occur in a structure is not
considered in this procedure.
An example would be a class F2 weld that is subjected to 3 ¥ 10 7 cycles at a stress
range of 20 N.mm -2 and then the stress range increases to 30 N.mm -2 and the remaining
life needs to be estimated. The lifetime at 20 N.mm -2 is read from Fig. 7.10 and