Aluminium Design and Construction John Dwight

when the design life is low or when a detail occurs in a zone of low
stress. For such a detail, it is possible to calculate the required class,
defined as the class of the lowest endurance curve that would produce
the required life. By specifying this class instead of the normal (maximum
possible) one for the detail considered, it becomes acceptable to relax
the inspection requirements and hence save money. British Standard
BS.8118 suggests the following procedure:
1. Determine those regions of the structure where the required class is
equal to or higher than class 24.
2. At all details in such regions, the Fat-number shown on the drawing
should indicate the required class, rather than the maximum possible
class.
3. The fabricator then tailors his level of inspection to the Fat-number
actually indicated.
An obvious example of where it is pointless to put the maximum possible
Fat-number on the drawing is when a potentially high-class detail occurs
adjacent to one of lower class. In such a case, the high-class detail may
be marked with the same Fat-number as that for the low-class one next
door to it, with nothing lost.
12.8 IMPROVEMENT MEASURES
If a critical detail fails to satisfy its fatigue check, the designer has
essentially two options. One is to increase the section, thus reducing
the level of stress. This increases weight and cost, and may be highly
inconvenient if the design is far advanced. The alternative is to carry
out an improvement measure, thus raising the fatigue class of the detail.
The following are some of the possibilities:
1. Redesign the detail. The original low-class detail is replaced by one of
higher class.
2. Weld-toe grinding. Grinding the toe of a transverse weld to a smooth
profile reduces the stress-concentration effect. If the crack is toe initiated,
the resistance to fatigue will be enhanced.
3. Weld-toe peening. Peening at the toe of a transverse weld introduces
compressive residual stress, which again improves the performance
when initiation is at the toe.
4. Cold expansion of a bolt hole, using a suitable drift. Like (3), this is helpful
because it produces compressive residual stress at the point needed.
5. Static overload. Another way of introducing beneficial compressive
residual stresses at potential fatigue sites is to give the whole
component a controlled static overload to a stress beyond the elastic
limit.
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