If all else fails …

Citation

Kella, E. (2008), "If all else fails …", Assembly Automation, Vol. 28 No. 4. https://doi.org/10.1108/aa.2008.03328daa.002

If all else fails …

Article Type: Viewpoint From: Assembly Automation, Volume 28, Issue 4

The authorEwen Kellar is based at the Adhesives, Composites and Sealants Section, TWI Ltd, Great Abington, Cambridge, UK.

One of the first things I encountered when I first entered the world of adhesives over 13 years ago was the apparent struggle that existed between bonding and the other joining technologies such as welding and fastening. On one hand, adhesives appeared to be more than capable of providing all of the joining needs for an engineer, especially with respect to superior fatigue resistance, ability to join dissimilar materials, low temperatures for curing and enhanced stiffness; surely this technology would take over! Then there were the doubters who would point out that adhesives are very weak in peel and cleavage, not very good in conditions of high temperature and/or fire and inferior in strength to welded or bolted structures. It was almost as if one had to take sides!

Obviously the reality lies somewhere in between, with some applications benefitting clearly from a bonding approach and others finding solutions with fasteners, welding or other joining processes. Often the trick is to understand exactly where the benefits would lie and to show the end-user that the price of change may be more than compensated for in terms of productivity, performance or innovation. Another option is to take the best of both worlds and combine forces to produce hybrid joints where two or more joining technologies co-exist.

Probably, the largest exponent of hybrid joining is the automotive sector where for example weld-bonding has been used for many years to produce structures which utilize the presence of adhesive to impart both strength and stiffness, and the spot welds to both assist with assembly and to arrest bond failure during a crash. Ironically adhesives were originally introduced as a means to reduce noise and vibration and seal joint areas, the structural benefits were secondary! In contrast, performance vehicles both on the road and the track are adopting a much more adhesive-only approach. Examples include the Lotus Elise and the Aston Martin Vanquish where fasteners are employed purely for assembly and crack arrest at impact and Formula 1 cars where adhesive-only solutions are the norm. In addition, all modern road vehicles employ only adhesive bonding for direct glazing (in itself a structural element of the car) and riveted brake shoes are a thing of the past.

Another early adopter or adhesives is the aerospace industry, where the first planes relied totally upon adhesive bonding to achieve the necessary lightness through different material combinations. Since that time, bonding has been a major feature in all aircraft, although the structural properties have been superseded by the need to create fully sealed structures such as the wing-boxes which now serve as fuel tanks. Conservatism and the avoidance of risk has led to a dual back up system where sufficient fasteners must be in place to fully duplicate the mechanical performance of the adhesive. Whilst this appears to be completely rational for all-metallic structures where the incorporation of holes into the metallic elements is well understood, the recent move by the commercial sector to increase significantly composite materials in the fuselage and wings of current and next generation aircraft, raises the spectre of compromising the benefits of lightness and strength through the need to drill so many holes!

It is understandable that aerospace engineers adopt such a stance as they require to minimize risk. A fastener is easily rated in terms of function, failure is understood and can be mitigated against whereas an adhesively bonded joint may only be as good as the process and people that made it. However, composite materials really only come into their own when damage during fabrication (i.e. holes) can be eliminated and adhesive bonding can be adopted as the primary joining solution.

Unfortunately, it is still not possible to completely inspect an adhesive joint non-destructively so to succeed with structural adhesive bonding it is essential that a rigid set of process controls at all stages of fabrication is put into place. In particular, surface treatment and the avoidance of contamination of the substrate surfaces prior to bonding is critical.

So 13 years on and the debate still continues but at the end of the day the only thing that truly matters is fitness for purpose. Adhesives and fasteners have their benefits and can be used singly or in combination as the application dictates. But whatever is done, process control must never be sacrificed and as my previous boss used to tell me “if all else fails, use bloody big nails!”

Ewen Kellarbased at the Adhesives, Composites and Sealants Section, TWI Ltd, Great Abington, Cambridge, UK.