Aluminium Design and Construction John Dwight

Figure 11.19 (a) Peaking of the stress at the ends of a conventional bonded lap-joint, (b), (c)
Methods for making the shear stress more even.
stress pattern increases with the rigidity of the adhesive layer. Therefore,
although increasing t g above the optimum value decreases the actual
intrinsic strength of the adhesive, this will partly be offset by the attainment
of a less peaky stress pattern, having a more favourable value for the
ratio q 1 /q. The need for strict control of glue-line thickness thus tends to
be less critical than might at first appear.
Figure 11.19(c) shows another method of reducing the peakiness, as
used by the British aluminium designer Ron Cobden. The metal is cut
back at each edge of the bonded area, as shown, thereby increasing the
thickness of the adhesive and making it more flexible in shear (at the
critical points). This device and also the tapering method are easy to
incorporate with extrusions.
(b) Limiting stress (pv )
An appropriate value to take for the limiting shear stress pv for the
adhesive depends on many factors and is difficult to arrive at. One
possibility is simply to accept the value t provided by the maker, based
on the standard lap test. This is often a reasonable approach. Admittedly,
the maker’s listed value is only typical and not a guaranteed minimum.
But against this, the standard test is clearly pessimistic and produces a
strength value for the adhesive well below its intrinsic value. The second
effect can be expected to more than compensate for the first, and the
value thus obtained for pv will probably be a safe one. Alternatively, for
a more sophisticated assessment, the designer might call for special
tests to be conducted to obtain the real intrinsic strength of the adhesive,
using one of the specimen types in Figure 11.14.
Whatever the method used, it is obviously essential to allow for
possible factors arising in service that might degrade the strength of
the adhesive, such as: (a) operation at elevated temperature; (b) immersion
in water; and (c) sustained loading (creep).
(c) Load factor
British Standard BS.8118 takes � m =3 for bonded joints, this being the
value called for when a joint is validated by testing. This relatively
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