Why add Adhesive Bonding to your Engineering Toolbox?

Over the years, I helped many vehicles- and industrial manufacturers adopt adhesive bonding in design engineering and operations. It has become apparent that adhesive bonding has a massive disadvantage against traditional joining methods (welding & mechanical fasteners): Design engineers are taught to design with welding or mechanical fasteners and generally don't have access to the required knowledge to do the same job with engineered adhesives. That is precisely what I should do in this article.

Why? The reason it all begins.

Specifically, in metal fabrication, the primary reason for our customers to consider adhesive bonding is to increase their output significantly. One of the limiting factors is the welding time required to produce a part. Despite working in a fully automated environment, car producers aim to reduce spot welds to speed up production. We operate a lot with fabricators and manufacturers of heavy equipment; the same is true for their operations, although many jobs are done manually instead of fully robotic. Based on recent projects, we can demonstrate productivity gains of up to 65%.

The second reason to adopt adhesive bonding is to reduce the post-treatment of assembled parts. Welding produces distortion and visible damage on the surfaces. Therefore, metal fabricators must often apply additional working steps to smoothen surfaces before painting. While some metal fabricators spend hours grinding off welded areas, others spend substantial money for metal putties to smoothen their surfaces before the paint shop. None of these operations are required with adhesive bonding.

Third on my list of reasons why metal fabricators consider adhesive bonding is to extend the durability of finished products. Since adhesive bonding allows stress distribution over the entire bonded area, it results in fewer stress peaks and, thus, more durable joints. In the following section, I want to highlight some of the results published by Miriam Laubrock in her Ph.D. thesis comparing welding and mechanical fasteners with adhesive bonding.

Are Engineered Adhesives strong enough?

That is the most significant concern design engineers have when being introduced to adhesive bonding.

Now, adhesives are polymers, and by that, they will never be "as strong" as welds. In a 1:1 comparison, welds will mainly outperform adhesives. But, if you apply the basic joint design rules for adhesive bonding (e.g., adhesives require enough bonding area), they can outperform welds.

In the field of industrial manufacturing, strong means durable. And durability it is, where adhesive bonding regularly outshines the traditional fastening methods. Adhesive bonding will outlast mechanical fasteners and welds by a factor of 3-4. In applications where the joints are subjected to crash or cyclic loads, adhesives perform significantly better than other joining methods due to their ability to absorb impacts, vibrations, and movements.

Load-bearing adhesive joints vs. welding

Miriam Laubrock published her Ph.D. thesis about load-bearing adhesive joints and has presented the results several times.

The specimens tested and compared were standardized, whereby lap-shear samples of fillet welded-, plug-welded- and adhesively bonded have been prepared to undergo static- and dynamic load testing. The aim was to compare joint strength in relation to fatigue resistance.

Several steel grades have been selected, ranging from high-strength S900MC steel to commonly used, cost-effective S355J2 steel. Several adhesives out of the structural epoxy range have been chosen for comparison tests. They are known to withstand high loads in combination with excellent aging and chemical resistance features.

Based on this configuration, the static load capacity of the welded specimens displays a higher maximum breaking force than the adhesively bonded specimens. Given that the joint surface area is the same for the welded and adhesively bonded specimens, the higher static force is of no surprise at this point.

Despite the lower static strength of adhesively bonded specimens, they outperform the welded joints after approx. 2M load cycles and thus show a significantly better fatigue behavior no matter the adhesive or substrate thickness used.

The high cycle fatigue resistance makes adhesive bonding very attractive to industrial manufacturers, as it significantly prolongs the component's durability during service life, resulting in lower claims and repair costs.

Making a case for the Hybrid Joining

I prefer to be clear that every joining method has certain disadvantages. For this reason, combining different methods (e.g., adhesive bonding + riveting) often brings the best processing and service life durability results.

I have been part of a new project with Andre Siegrist in which a combination of adhesive bonding and riveting is the optimal solution regarding durability and stiffness.

During the development of a new innovative and patented bike rack for the startup NXTGAD GmbH, initial simulations revealed a stress concentration in the welded joint, potentially leading to premature failure during service life. Given that today's bike racks must withstand the substantially higher weight of e-bikes than traditional bicycles, re-inforcing that joint was necessary to comply with the relevant road traffic standards.

To add structural strength to the frame assembly, the solution was to add a patch to the sides for strength increase instead of choosing thicker walled assembly components or more costly high-strength profiles. An additional challenge is, that the mainframe is used for four weight variations, and only one profile geometry is allowed. The test forces from 1471,5 to 2943 N result from these different weight variations.

The assembly frame weighs approx. 3kg, and adding the riveted patches and adhesive bonding only adds 1.1% weight to the structure, which is more than acceptable and does not compromise the total system weight requirements.

Andre and his team built a test set-up closest to the real-life load conditions to compare the maximum force capacity of the three systems (welded, riveted, and hybrid joining).

The initial testing results look promising, as hybrid joining technology substantially increases the structure's load-bearing capacity. While adding riveted patches to the structure increases the load-bearing capacity by +23%, another big jump of approx. 50% could be achieved by adding adhesive bonding to the rivets. Since bike racks undergo high cyclic loads during service life, adhesive bonding is a safety guard for structural integrity and guarantees life expectancy.

Hybrid joining comes in many forms. The most used forms of hybrid joining in industrial manufacturing today are:

• Weld-bonding - Resistance spot-welding + Adhesives | Most commonly used hybrid joining method in the automotive industry.
• Riv-Bonding - Rivets + Adhesives | Provides similar peak strength to weld-bonding but is a cold joining method.
• Clinch-Bonding - Clinching + Adhesives | Provides weaker peak strength than weld-bonding. The only consumable is the adhesive.
• Fasteners + Adhesives | Extensively used throughout industrial manufacturing and can be applied to many base materials (wood, metal, composite)

Hybrid joining methods provide the advantages of both worlds and are generally convincing for three reasons:

• They provide continuous, leak-tight joints.
• They display the highest levels of peak strength and durability.
• It is the most cost-effective way in terms of processing in relation to the quality of the built structure.

Reasons not to use Adhesive Bonding

To end this article, I'd like to explain when you should not consider adhesive bonding for your application. As I mentioned before, it's essential to know about the limitations of all joining methods, and so I would like to bring up a few examples related to adhesive bonding.

1. Service life temperature exceeds acceptable levels.

Adhesives are polymers; thus, their melting point is way below steel or aluminum. While most used metals fare around 600-1500°C, most adhesives will begin to depolymerize close to 300°C. For load-bearing applications, please ensure to consider temperature effects for adhesives.

2. You can't change the joint design.

Adhesives primarily work by having enough surface area to develop the required strength. If you are unable or unwilling to change your joint design from, e.g., butt joint to overlap joints, then adhesive bonding will not work.

3. The assembled part must be disassembled regularly.

Adhesive bonding creates a fixed bond and can be difficult to disassemble. You can disassemble thick, elastic adhesive joints using wires, as in car window replacements. This will not work with highly structural adhesives commonly used in metal fabrication.

4. You are using excessive metal wall thicknesses.

The adhesive strength needs to reflect your metal thickness. Thin metal thickness (0.3-0.8mm) may require lower strength adhesive to avoid the read-through effect.

High wall thicknesses require high-strength adhesives to ensure load-bearing capacity during service life. I don't recommend using adhesive bonding for metal thicknesses over 30-40mm.

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