Paper published in Assembly Automation, vol.25, no.2., 2005.
Adhesives have been used in component joining for many years, with a current annual world business in excess of $20billion. This equates to an approximately twice round the earth in total length for bonded joints. In the past 30years, however, adhesives have been a significant enabling technology in the assembly of modern multi-component transport and commodity goods.
The primary benefits for the use of adhesives can be summarised as:
- Reduced production costs
- Increased production speed
- Design for manufacture
- Better production sequences
- Low capital costs
- Low production costs
- Improved product features
- Modularity options
Compared with conventional welding technologies, however, adhesives offer significant advantages in the joining of dissimilar substrates. Provided adhesive selection, surface pre-treatment and curing processes are adequate then sufficient stress transfer will be achieved across the bond and failure will occur in one or both of the substrate structures.
By far the most difficult task facing industry in the design and manufacture of an adhesively bonded component or product is to ensure that an acceptable level of reliability and consistency is achieved in the bonded joint. Inspection of the final product is expensive, not always applicable and too late.
Reliability and quality can be reached through process control but the many different adhesive systems mean that it is difficult to develop a uniform approach.
Quality assurance is particularly invisible: it is not something that can be seen or heard. It is more of a philosophy than a system, i.e. it is not a set of procedures to be slavishly followed; rather it is an attitude that affects every aspect of organisational activity, including manufacturing. Quality assurance is making sure that you give the customer what the customer wants.
So, bearing all this in mind, where is industry with the adoption of adhesive bonding? In many industry sectors adhesive bonding is commonplace as a primary joining technology where welding or fastening technologies are not a viable option.
In performance road cars, where high grade aluminium alloys are employed adhesives bonding as a single joining technology are now commonplace, with widespread use in performance vehicles such as the Lotus Elise & the Aston Martin Vanquish. The welding of aluminium is problematic and adhesive bonding has offered a beneficial route to quality joining.
Whilst holes in metals and other isotropic materials are acceptable and well understood, the same features within anisotropic polymeric composite materials require a completely different set of design criteria. Holes should be regarded as defects and therefore have the potential to lower the performance benefits of such materials.
As a consequence, the automotive sector is leading the way compared with other civil transport vehicles in the use of adhesive bonding of critical load bearing joints as a single joining technology in composite structures. F1 and other Motorsport vehicles manufactured from significant polymer composite components have used structural adhesives as a single joining technology for some time, following the lead of military aircraft, but it is with vehicles such as the Mercedes McLaren SLR, where bonding technology is making headway towards mainstream automotive vehicles.
In automotive and other sectors we have also seen the use of hybrid joining such as riv-bonding and weld-bonding. The hybrid joint can offer the best of both worlds by allowing both joining technologies to complement each other. Through careful design and planning it is possible to minimise the negative aspects of either process. The adhesive enables an evenly stressed, fully sealed joint to be formed which, with the aid of fasteners can be assembled quickly and accurately and can resist the effects of adverse loading (e.g. crashes) and extreme environmental conditions (e.g. fire). The need for some holes to be formed is unavoidable but the numbers are reduced and the potential for localised high stresses is minimised through the action of the adhesive, which is the primary load carrier. It is for these reasons that the hybrid joint has been adopted within a wide range of applications. The removal of fastening or spot welding all together would require significant changes in the current production process, which doesn't offer a big enough economic benefit to justify the additional cost.
So, where are the future challenges for engineers in expanding the use of adhesive bonding?
Inspection in general as both a post-manufacture quality control measure and as an in-service defect detection technique is not trivial for adhesively bonded joints. Disbonding at the adhesive-adherend interface is relatively easy to detect with NDT and automated systems exist for the inspection of large composite structures; and smaller, more portable systems for the examination of individual joints. It is widely recognised, however, that the main factor inhibiting industry's more widespread use of adhesively bonded joints is the inability of NDT methods to detect poor adhesion. This ultimately creates an issue for manufacturers and assemblers to correlate inspection data with the effect on mechanical performance of the joint. Whilst there are new techniques being developed which can give greater accuracy of size and position and of defect there is no current correlation between this and joint performance.
In a world where recycling and re-use or components and substrate materials is a growing requirement driven by legislation, the general irreversibility of adhesively bonded joints may hamper efficient separation of components. Design is taking this into account by compartmentalising materials types into structural modules, which are designed to be repeatedly taken apart, both during life and at end of life. Future increasing targets for re-use, however, will require disassembly on command features of some adhesive materials to give clean separation to allow recycling or re-use. Mechanisms already exist in the electronics sector, with shape memory plastics used as fastenings in mobile phones. This and other concepts could easily be scaled for other applications provided production can incorporate it economically. This is likely to occur in areas, initially anyway, where removal of high value or particularly hazardous components is required.
In summary, therefore, adhesive bonding is now successfully used in many complex component assemblies across a breadth of industries and it is part of the mainstream joining technology options readily used in high volume and quality assembly. Provided the challenges of durability and reliability can be advanced there is still considerable mileage, however, in the potential for adhesive bonding in engineering applications.