Bonding exercise
Rivets and welds work well for many applications, but Colin Chapman of Henkel, maker of Loctite brand products, argues that adhesives provide a viable alternative to mechanical methods of fastening.
Mechanical methods of joining substrates are acceptable in many circumstances - but they do present difficulties which can be readily overcome with adhesives. For instance, rivets provide the opportunity for easy inspection, yet any ‘hole’ in a substrate has the potential to generate a leak path. Such paths increase the likelihood of corrosion and, as a result, additional sealing or substrate protection may become necessary - increasing costs and introducing complexity into the construction. In addition, if different metals are riveted together, galvanic corrosion can occur. And rivets concentrate stress on the material at their point of use - possibly leading to premature material fatigue.
Welding worries; adhesive advantages
Aesthetically, seam welding can produce a better finish than that of riveting - and it does create a strong and immediate joint. Nevertheless, it can distort or heat warp the metal and weaken the alloys. And welding can only be used on similar materials. Further, welding is a specialised activity and the labour costs can reflect the skill that is needed to complete such joints.
Employing an adhesive provides distinct advantages over rivets and welds. For a start, adhesives can be readily used on dissimilar materials - thus providing more flexibility for designers to incorporate the most suitable substrates for the finished product. Beyond that, an adhesive allows stresses to be distributed across the joint area. As a result, the adhesive bonded joint is more resistant to flex and vibration than, for example, a riveted joint. In turn, this makes the assembly less prone to failure.
Tests reveal that, overall, higher loads can be often sustained through the use of bonded joints. However, it should be considered that adhesives are often employed, not because the ultimate strength is greater than that of a weld or a rivet, but for the reason that the fatigue strength is better, especially when the component is subjected to high cyclic loads
Similarly, a bonded joint - because the adhesive is applied as continuous bead - produces a stiffer structure. However, if increased stiffness is not necessary, the weight of the structure can be decreased through the use of adhesives, even while maintaining the required rigidity.
Also the parts are not heated, as in welding so there is no distortion. This enables components with different masses and dimensions to be readily assembled. In the same way, adhesives allow the joining of irregular shaped surfaces - to produce a lighter assembly with little, if any, alteration to the dimensions or geometry of the relevant parts.
It’s also worth remembering that an adhesive will act as a seal - preventing the ingress of moisture or harmful fluids that can lead to corrosion; and it also helps to kill the unwanted transmission of noise throughout a structure.
Beyond the production paybacks, there are cost benefits. For example, because one adhesive can be used for a variety of applications, the inventory of parts such as rivets can be substantially reduced - if not eliminated. Further economies are also possible where adhesives allow for a down-gauging in the dimensions of metals.
Additional cost savings are possible on production operatives. Although the products need to be applied with care and precision, it is easier to train personnel to use an adhesive - and even dispensing equipment - than to teach them to weld. Finally, adhesive bonding provides an improvement in finished assembly appearance. The bonded joints are smoother (there are no protruding fasteners), and need little or no finishing.
Making the change
For many engineers, adhesives are the first - often, the only - choice. Experience has shown them the considerable benefits of using bonding technology. Of course, the right adhesive for each application needs to be employed - there is no such thing as the totally universal glue. And that’s where the help desk of any good supplier will be of assistance. The various adhesive technologies allow a choice between different strength levels and the best adhesion properties.
