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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
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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.
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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.
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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?
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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. -
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.
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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.
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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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The Science of Adhesion Educational Series is designed to be a comprehensive introduction to Adhesion Science and the use of tapes and adhesives in design applications. Visit the Science of Adhesion to view all articles or select from the featured topics below.
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