Page 132 - Handbook of Adhesives and Sealants

Joint Design 101

Thicker sections could change the curing properties and result in in-
creased internal stresses and non-optimal physical properties.
The substrates should be as parallel as possible, thus requiring uni-
formity in adhesive thickness across the bonded area. If the substrates
are not parallel, the loading will not remain aligned and this condition
could translate into cleavage stress on the adhesive.
There are several methods used for maintaining a constant, prede-
termined adhesive thickness. These methods include:

Adjusting the viscosity of the adhesive
Application of a precalculated amount of pressure during cure
Using fixturing that is specifically designed for the application
Application of a shim or insert within the bond-line so that a uni-
form, predetermined thickness can be maintained

These techniques will be reviewed in Chapter 18.

3.3.3 Effect of bond area geometry
For a given adhesive and adherend, the strength of a joint stressed in
shear depends primarily on the width and depth of the overlap and
the thickness of the adherend. Adhesive shear strength is directly pro-
portional to the width of the joint. Strength can sometimes be in-
creased by increasing the overlap depth, but the relationship is not
linear. Since the ends of the bonded joint carry a higher proportion of
load than the interior area, the most efficient way of increasing joint
strength is by increasing the width of the joint.
A plot of failure load versus overlap length for brittle and ductile
adhesives is shown in Fig. 3.5. Increasing overlap length increases the
joint strength to a point where a further increase in bond overlap
length does not result in an increase in load carrying ability. This
curve is the result of the capability of ductile adhesives resisting non-
uniform loads more than brittle adhesives.
Since the stress distribution across the bonded area is not uniform
and depends on joint geometry, the failure load of one joint design
cannot be used to predict the failure load of another design having a
different geometry. The results of a particular test pertain only to
joints that are exact duplicates of one another. This means that the
results of laboratory tests on lap shear specimens cannot be directly
converted to more complex joint geometries.
To compare different geometries, curves showing the ratios of over-
lap length to adherend thickness, l/t, are sometimes useful. Figure 3.6
shows an example of the effect of l/t ratio on aluminum joints bonded