Aluminium Design and Construction John Dwight

For example:
The loading spectrum is unknown and cannot be reliably calculated.
The geometry of the structure makes stress-analysis difficult.
It is not clear to which fatigue class a certain detail should be assigned.
Testing may also be preferred even when method (1) would be possible.
For example with a mass-produced component, built to closely controlled
standards of workmanship, it may be found that fatigue testing of
prototypes would indicate a better performance than that predicted
from the standard endurance curves. Advice on fatigue testing appears
in BS.8118.
12.3 CHECKING PROCEDURE (SAFE LIFE)
12.3.1 Constant amplitude loading
The simplest type of fatigue calculation is when a single load is repeatedly
applied to the structure, so that at any point there is a steady progression
from minimum to maximum stress in each cycle without any intervening
blips (Figure 12.1), referred to as constant amplitude loading. In such a
case, the checking procedure at each potential fatigue site is as follows:
1. Decide on the design life of the structure. Refer to Section 12.3.3.
2. Calculate the number of load cycles n during the design life.
3. Determine the pattern and variation of nominal (unfactored) loading
on the structure in each cycle.
4. Calculate the resulting stress range (f r ) at the position being considered
—generally taken as the difference between maximum and minimum
stress in each cycle. Refer to Sections 12.3.4 and 12.4.
5. Establish the class of the detail at the point considered. Refer to
Section 12.5.
6. Using the endurance-curve appropriate to the class, read off the
predicted number of cycles to failure (N) corresponding to the stress
range f r . Refer to Section 12.6.
7. The fatigue resistance at the point considered is acceptable if N � n.
Figure 12.1 Constant amplitude loading. f r =stress range, f m =mean stress.
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