Aluminium Design and Construction John Dwight

component is introduced and the failure load thereby reduced. Secondly,
depending on the relative stiffnesses of the aluminium (in tension) and
the adhesive (in shear), there is a variation in the shear stress in the
adhesive along the length of the joint, with a peak at either end. This
produces a further drop in the failure load, because the adhesives used
are not generally ductile enough to allow full redistribution of stress to
occur, as would occur with a welded connection. The standard test is
thus a convenient means for comparing different adhesives, but does not
give the true intrinsic shear strength. It only provides a rough indication.
Possible tests for obtaining the intrinsic shear strength are the thick
adherend shear test and the butt torsion test (Figure 11.14). Both eliminate
the secondary effects inherent with the standard specimen. The ultimate
shear stress obtained, using either, can be as much as 40% more than
that found in the standard lap test.
Another kind of test employs a specimen composed entirely of adhesive,
known as a bulk tension specimen, enabling the tensile properties of the
adhesive material to be found. The ultimate tensile stress thus obtained
is of fairly limited interest. But the value obtained for the Young’s modulus
E is useful, because it enables the shear modulus G to be deduced, the
latter being needed when a joint is to be checked by finite element
analysis. Such a test also provides the percentage elongation of the
adhesive material at failure, which gives an indication of its ductility in
shear, a significant factor in relation to redistribution of stress in a
practical joint.
11.4.9 Glue-line thickness
Another factor affecting the performance of a bonded joint is the glue-line
thickness, namely the thickness of the adhesive in the final joint as made.
Ideally, for maximum strength, this should not exceed 0.3 mm for nontoughened
and 0.6 mm for toughened adhesives. Precise data on the
Figure 11.14 Tests for obtaining intrinsic shear strength of adhesive: (a) thick-adherend
test; (b) butt torsion test.
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