Designing Adhesive Joints for Less Stress

There are tradeoffs in optimising joint design for adhesive and tape use. The amount of material used, the machining cost, or the process friendliness play important roles in “optimisation”. The two examples below show how strength should be weighed with convenience or cost when improving a joint for adhesives.

Corner Joint

  • graphic of bonding failure due to cleavage stress in a traditional corner joint

    Basic corner joint under cleavage stress

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

  • graphic of reinforced corner joint with dispersed cleavage stress

    Reinforced corner joint

    By adding an additional component, the basic design is modified to reinforce bonding of the original components. This design 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.

  • graphic of optimal corner joint design with cleavage stress replaced by compression.

    Optimal corner joint design

    The corner joint design is further improved for performance and production. This alternative is better than the second design as now the joint does not need additional parts and work. Instead, it requires two pieces of material with minimal machining complexity, which can be 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 Joint

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?

  • graphic of bonding failure due to cleavage stress in a basic lap joint

    Lap Joint in shear and cleavage stress

    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.
  • graphic of improved lap joint design using a joggle

    Joggle Lap Joint reduces cleavage stress

    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.

  • graphic of double lap joints to further reduce cleavage stress.

    Double Lap Joint further reduces cleavage stress

    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.

  • graphic of double scarf lap best for greater bonding strength

    Use the Double Scarf Lap Joint for ultimate strength

    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.

Additional lap joint design considerations

Each of those improvements provides better strength and stress dispersion in a lap joint assembly. However, each improvement also adds complexity, cost and time. 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 optimise for the specific application.

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