Designing Adhesive Joints for Less Stress

This basic design using a perpendicular butt joint is not ideal for adhesive bonding. The main stress is cleavage which is highly unfavourable in adhesive bonding. Modifying the original design adds an additional component to reinforce bonding of the original components. This redistributes some of the cleavage stress into a shear stress, strengthening the bond. A mechanical component of the outer reinforcement may also prevent impact forces. However, this joint requires the use of a second piece of material and would likely require a two-step bonding process.

A refined design improves the design for performance and production. It is better than the second design because now the joint does not require additional parts and work. This joint requires two pieces of material with minimal machining complexity and is assembled in one step. It is also stronger than the other designs because the cleavage forces are completely replaced by a compression force, which is the most preferable joint stress for adhesive bonding.

Lap joints typically place the adhesive in shear – a strength for adhesives. While a perfect scenario keeps the adhesive in shear at all times, what happens if your lap joint is experiencing cleavage or the shearing is not perfectly in plane?

A simple overlap joint, while very common with adhesive bonding, may not always provide the strength needed. A single lap joint is often stressed in shear. However, the shear is applied slightly out-of-plane which, as the joint extends, begins to transform into cleavage. This concentrates some stress on the leading edges of the lap joint.

To improve the joint, a "joggle" can be designed into one of the substrates which now places the stress in-plane and helps maintain shear in the adhesive. The adhesive is slightly out of plane of the stress, however, which may still concentrate some cleavage stress as the joint extends.
Further improvement involves "double lap" joints. Now, both substrates are machined or molded with both substrates lapping each other. This keeps the stress and the adhesive in-plane as the joint is being sheared. If cleavage forces are present along the moment arm of the assembly, there may still be some stress concentration.