Aluminium Design and Construction John Dwight

Table 12.4 Classification of fatigue details (arc-welded) —propagation through weld
Notes. 1. For cases 34–37 the ends of the weld must be ground flush, using run-on and run-off plates.
2. For case 35 the class depends on how closely the weld profile is controlled (i.e. preventing
the use of excessive reinforcement).
3. For case 35 any change in thickness or width must be gradual, with tan � � 0.25.
For details not covered in these tables the reader may refer to the more
comprehensive data provided in the British Standard.
The classes given in Tables 12.3 and 12.4 specifically refer to arcwelded
joints made by MIG or TIG. They will only be valid if the
fabrication meets a specified standard referred to in BS.8118 as ‘fatiguequality
welding’ (Section 12.7).
12.5.2 Friction-stir welds
At the time of writing, the new friction-stir process is being actively
developed, and there are strong indications that FS welds will prove
far superior in fatigue to those made by MIG or TIG. For example, a
preliminary series of fatigue tests made by Hydro-Aluminium in Norway
on transverse butt welds in 6082-T4 extruded material, 5 mm thick,
suggest that class 50 might be appropriate for such a joint. This compares
with class 24 for a single-V MIG weld with permanent backing bar, or
class 17 if unbacked.
The reason for the better fatigue performance of FS welds is their flat
as-welded profile. To obtain optimum results, it is important to avoid
the small fin that can occur at the flow-zone edge of the nugget, when
the welding parameters are not properly controlled. Provided this fin
is eliminated, it is claimed that a fatigue class equal to 90% of that for
the parent metal is possible.
12.5.3 Bonded joints
Bonded joints are generally superior to welded ones in fatigue. Consider
first the performance of a member containing a longitudinal bonded joint,
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