Page 135 - Handbook of Adhesives and Sealants

104 Chapter Three

Figure 3.8 Interrelation of fail-
ure loads, depth of lap, and ad-
herend thickness for lap joints
with a specific adhesive and ad-
herend. 7

3.4 General Joint Design Rules
The designer should take into consideration the following rules in the
design of adhesive or sealant joints. These rules are the basis for rec-
ommended joint design geometries in the next section.

1. Keep the stress on the bond-line to a minimum.
2. Whenever possible, design the joint so that the operating loads
will stress the adhesive in shear.
3. Peel and cleavage stresses should be minimized.
4. Distribute the stress as uniformly as possible over the entire
bonded area.
5. Adhesive strength is directly proportional to bond width. Increas-
ing width will always increase bond strength; increasing the depth
of overlap does not always increase strength.
6. Generally, rigid adhesives are better in shear, and flexible adhe-
sives are better in peel.
7. Although typically a stronger adhesive material may produce a
stronger joint, a high elongation adhesive with a lower cohesive
strength could produce a stronger joint in applications where the
stress is distributed nonuniformly.
8. The stiffness of the adherends and adhesive influence the strength
of a joint. In general, the stiffer the adherend with respect to the