Aluminium Design and Construction John Dwight

i.e. a joint extending in the same direction as the applied stress, as for the
web/flange connection in a beam. For such a joint, it is reasonable to
assume that there is little adverse effect due to the presence of the adhesive,
and take a class approaching that for the basic wrought metal (class 60).
Secondly, consider the case when fatigue loading acts transverse to
a bonded joint. Here there are two possibilities: tensile failure in the
aluminium, and shear failure in the adhesive. The former can be handled
in the same way as if the component were monolithic (no joint), taking
due account of the stress concentration caused by the geometry. The
latter is more difficult, due to lack of generally available data, and
testing becomes necessary.
12.6 ENDURANCE CURVES
British Standard BS.8118 provides endurance curves in the form of
stress range fr plotted against endurance N (number of cycles to failure),
covering the nine classes of fatigue detail. These are presented on a
log-log plot, where they appear as a series of straight lines, the general
form being shown diagramatically in Figure 12.6. Different curves are
provided for constant and variable amplitude loading:
1. Constant amplitude curves. For any given class, line A is defined by
the class number (=fr in N/mm2 at N=2×106 ) and the reverse gradient
1/m (as specified in Table 12.5). This line continues down to a horizontal
cut-off at N=107. 2. Variable amplitude curves. For a given class, this uses the same line A,
but only down to N=5×106 . There is then a transition line B of reverse
gradient 1/(m+2) going from N=5×106 to 108 , followed by a (lower)
horizontal cut-off.
For any given class, the equations to lines A and B are as follows, where
the values of kA and k B appear in Table 12.5:
(12.4a)
(12.4b)
Also given in the table are the cut-off values (f oc and f ov ) for each class,
the idea being that a repeated stress range of lower magnitude is nondamaging.
The variable amplitude case has a lower cut-off, because
occasional stress ranges occurring above f oc will produce some propagation,
thus causing lower stress ranges to become damaging.
The resulting BS.8118 endurance curves are presented in Figure 12.7.
Comparison of these with certain European data might suggest that at
high N the British Standard endurance curves for welded details are too
low. In fact, it would be wrong to conclude that the British Standard is
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