Designing Adhesive Joints for Less Stress

This basic design using a perpendicular butt joint is the least favorable for adhesive bonding. The main stress is cleavage which is highly unfavorable 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.

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.

The ultimate lap joint incorporates a "scarf" to the double lap. This provides the same in-plane benefits as a double lap joint, but the "scarf" now provides additional strength when cleavage forces are present.

Each of those improvements provides better strength and stress normalization in a lap joint assembly. However, each improvement also adds complexity, cost and time. Yet another consideration is the format of the adhesive relative to the geometry. If the joint is designed with three dimensions of adhesive coverage, as with mortise-and-tenon or double lap joints, the application is often restricted to using a liquid adhesive. Since tapes may provide advantages in production efficiency and throughput, it’s important to consider all aspects when designing an adhesive joint to optimize for the specific application.

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.

The ultimate lap joint incorporates a "scarf" to the double lap. This provides the same in-plane benefits as a double lap joint, but the "scarf" now provides additional strength when cleavage forces are present.