Aluminium Design and Construction John Dwight

11.4.6 Creep
Shear strength figures provided by adhesive manufacturers are normally
based on short-term tests. When fully loaded over a long period, it is
found that adhesives suffer from creep, characterized by continuing
deformation and a drop in the shear strength. The effect becomes more
pronounced as the temperature increases. Reliable data on this effect is
not generally available, but a rough guide is that constant loading over
a long period should not exceed about one-quarter of the short-term
lap shear strength.
11.4.7 Peeling
Bonded lap joints are inherently vulnerable to premature failure by
peeling, when subjected to loading perpendicular to the plane of the
adhesive. This may occur even when the tension is only a secondary or
accidental effect.
Manufacturers provide figures for the peel strength of their adhesives
based on a standard test (the ‘roller peel test’). This is expressed as the
failure force per unit width of joint and typically ranges from 3 to 10 N/
mm. An engineer should be encouraged to ignore such data and instead
design joints so that peeling cannot occur. With joints between extrusions,
this can be achieved by suitable design of the section. With lap joints
between sheet-metal components, a possible technique is to employ
occasional mechanical fasteners, strategically placed, to act as peel inhibitors,
or else occasional spot welds can be used (through the adhesive).
11.4.8 Mechanical testing of adhesives
The most important mechanical property of an adhesive is its ultimate
strength in shear. This is normally determined by testing a standard
lap-joint specimen with the dimensions shown in Figure 11.13, composed
of 2014A-T6 clad sheet. The shear strength t of the adhesive is taken as
the applied load at failure divided by the lap area.
Such a test is simple, but not ideal. Firstly, as the load builds up the
aluminium bends slightly, as shown, with the result that a peeling
Figure 11.13 Adhesive lap shear test. Standard specimen.
Copyright 1999 by Taylor & Francis Group. All Rights Reserved.