Paper presented at Institution of Mechanical Engineers Seminar on Plastics and Polymers, September 2002.
The combination of two or more joining technologies (hybrid joining) in many instances can offer enhanced benefits by virtue of their complementary nature. The four most common systems, weld-bonding, riv-bonding, clinch bonding and' glue and screw' are used extensively in a wide variety of industrial sectors. Although purists may argue that an all-bonded structure is aesthetically and structurally superior, or that a bolted structure is the simplest and most effective solution, both approaches have their shortcomings. The hybrid joint offers the benefits of both systems e.g. by utilizing self-jigging and high load carrying capacity and increasing the fatigue properties of the structure.
This is the second event organised by the IMechE addressing the issues associated with the joining of plastics and polymer composites.
The first event, held over two days in 1991 covered four main topic areas (18 papers):
- Bolted joints (six papers)
- Adhesive joints (four papers)
- Repair (five papers)
- Joining of Plastics (three papers).
However, one topic that was not covered in any significant detail was that relating to hybrid joints i.e. joints which consist of a combination of two or more joining methods. The most common examples are those that include an adhesive and an alternative joining technology including fasteners and localised welds. The primary types of hybrid joining are:
- Weld-bonding (spot weld + adhesive)
- Riv-bonding (rivet + adhesive)
- Clinch-bonding (clinch + adhesive)
- 'Glue and screw' (threaded fastener + adhesive)
The extensive use of weld bonding of metals, especially in the automotive sector has been a driving force in promoting hybrid joining technology as a whole. Weld-bonding has limited applications within the plastics/composites area due to the fact that only thermoplastic systems of the same or fusion compatible material can be considered. In these instances it may be argued that a fully welded joint would be preferable to a hybrid joint.
Hybrid joining - a definition
Before one goes into any detail about hybrid joining, it is important to stress that the use of the word hybrid in this context relates to the joint and the combining of two or more joining processes. This is not the same as using the term to describe a structure composed of different materials; such references have lead to confusion in the past.
As a result of the content within earlier papers at the conference, the audience will be fully appraised of the 'continuous' methods for joining plastics and polymer composites i.e. by bonding and welding. The design issues associated with bolted structures will be covered in detail in a subsequent paper. This paper will provide an overview of the key issues associated with the hybrid joint.
Continuous vs discontinuous joining - the pros and cons
As has been presented previously, there are a number of benefits associated with continuous joining technologies:
- elimination/reduction of stress concentrations
- enhanced fatigue characteristics
- fully sealed system
- enhanced stiffness (light-weighting opportunities)
- cosmetic benefits
- little/no damage to substrates
- in the case of adhesive bonding little/no thermal distortion.
There are of course some disadvantages:
- difficulty with regards to disassembly
- matching materials (fusion compatibility for welding, surface preparation for bonding)
- susceptibility to impact damage/failure (especially for bonded systems)
- careful jigging is required for both welding and bonding to ensure necessary accuracy and tolerance
- capital expenditure on welding equipment.
Many converse arguments exist for mechanically fastened joints, benefits including:
- rapid accurate assembly and disassembly
- high level of visible quality control and perceived high performance
- ease of joining dissimilar materials
- significantly higher load transfer capacity for thick substrates.
But on the downside:
- holes to drill or form which can result in localised damage to the substrate material (very likely for anisotropic composite materials)
- no ability to form a seal to contain liquids or gases
- cosmetic implications
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.
Some of the best examples of hybrid joining can be found in the aerospace and automotive sectors, which will be considered later in this paper. Both industries employ extensive adhesive bonding in a wide range of areas, either on its own or as part of a hybrid system. However, the demands required of the joints differ greatly:
- aerospace structures are designed to withstand high loading conditions over long periods of time, it is essential that the joints form an effective seal
- automotive structures are required to be assembled quickly in high volumes and the joints must be capable of performing adequately under impact loading.
Such differences are accommodated through the use of hybrid joining.
It is interesting to note that hybrid joints can have very well defined properties, especially when high modulus structural adhesives are employed. In studies carried out for metal hybrid joints the load displacement curve for a single lap joint is an additive combination of an adhesive joint and a mechanically fastened one with the adhesive 'profile' leading the fastener profile. There is a thickness effect too where a thin metal substrate (<1.2mm) is seen to plastically deform prior to adhesive failure whereas thicker materials give rise to higher peel stresses and earlier adhesive failure.
A more complex situation will arise where the modulus of the adhesive selected is low, thereby enabling the fastener to 'feel' some of the loading within the joint due to the higher strain values experienced. The distribution of stresses within the joint throughout loading will be highly dependent upon the loading conditions, the adhesive properties and the properties of the fastening systems (dimensions, pitch, hole size, placement, etc.). This particular area has seen little in the way of published data perhaps due to its complexity. The attraction of the simpler extreme situations of either a high modulus adhesive (load all with the adhesive until failure then with fastener) or a very low modulus adhesive (load all with the fastener until failure, adhesive only there to seal) may be the primary explanation for this.
Polymers and composites
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. Ideally holes should be eliminated and substituted with an adhesive joint but this is not often possible and the best compromise is to employ hybrid joints where the adhesive is factored into the design calculations. The effect of the adhesive varies according to the load conditions placed upon the joint and the dimensions of the materials to be joined. For example, thin section material is particularly susceptible to bearing stress failure around fastener holes whereas thick section material can experience loads far in excess of what can be tolerated by an adhesive joint on its own.
Hybrid joints comprising of rivets and threaded fasteners (both can be produced in a variety of materials including metals such as steel, titanium, and light alloys and composites) are now being used to join composite to composite and composite to metal in a vast range of applications and industry sectors.
Aircraft have been employing bonded structures for many years; indeed the very early planes were fully bonded structures out of wood and canvas. As aluminium alloys took over as the primary material for aircraft construction (in terms of fuselage and wings), design considerations and conservatism caused the mechanical joint to predominate and the adhesive function was reduced in many cases to the level of sealant. Rivets and other fasteners were a known quantity and the design of ever more complex structures could not tolerate the 'uncertainty' of a bonded joint, especially in terms of durability. This situation was exacerbated by the need to define a surface pre-treatment process that would ensure high strength, durable adhesively bonded joints. Despite the existence of key processes (predominantly acid etch and anodise) derived from hundreds of man-years of research and development, there is still a reluctance to rely on a fully bonded aircraft structure. Certainly, structural adhesives are used and they can be shown to function over the time periods required e.g. the tail sections on all Airbus aircraft are completely composed of fully bonded carbon fibre composite. However, much of the design is still based upon the load carrying capacity of the rivets and other threaded fasteners rather than the adhesive.
In many instances conventional civilian aircraft utilize hybrid joints in which only one component of the joint, the fastener, is 'trusted'! However, the new generations of aircraft that are being built (e.g. Airbus 380 series) and planned craft for the future, are using more and more composite materials within the primary structure, i.e. fuselage and wings. Indeed the need for mixing such materials with metallics is essential to achieve the weight savings and structural performance that will dictate the future of aircraft design. Even today hybrid bonding is found in a variety of locations around the aircraft fuselage. Examples include ribs and stringers to skin structures in the wing, and skin to skin joints.
In contrast, the automotive sector is driven by completely different requirements to the needs of the aerospace industry. The consequences of high levels of impact, is by far the most significant factor in vehicle design and the poor performance of structural adhesives in such conditions has been a major factor in limiting their application. In addition, adhesive bonding is a relatively slow joining method compared to spot welding or mechanical fastening, a significant factor in high volume vehicle manufacture. Despite such shortcomings adhesives are steadily being used within structural and semi-structural areas of cars including hem flanges, windscreens, polymer/polymer composite panels, and areas of the sub-chassis. With the exception of windscreen bonding, the joints employed are hybrid in nature either weld-bonding or riv-bonding. By employing hybrid joints and placing fasteners in areas susceptible to peel, an extremely robust and durable joint can be formed that will fail in a controlled manner in an impact situation. The need for appropriate surface preparation prior to adhesive application is always a concern for any industry. The automotive sector's approach has been to encourage the adhesive manufacturer to develop systems that are sufficiently robust to bond to contaminated steel surfaces with a minimum of cleaning and by and large this has been achieved. More care is required for aluminium and polymers but simple pre-treatments are still being pursued.
The increasing use of new and varied materials within the car has reduced the scope of weld-bonding but appropriate mechanical fastening in the form of self-pierce rivets, self tap screws, clinching, mechanical pop-fit etc. are readily available to fill the gaps.
Adhesives serve a range of functions within the hybrid joint from being the primary load carrier as in the Lotus Elise, Jaguar XJ220, Lister etc. to a noise/vibration damper within hem flanges and body cavities.
AdhFAST ® - An innovative future
Recently TWI has been looking at expanding the hybrid joining system to simplify the whole assembly process. The end result has been called AdhFAST ®. It entails using a modified fastener containing an internal spacer (between the substrates) that allows adhesive to be injected through or down the side of the body of the fastener into the bond space. The spacer controls the bond thickness and eliminates the possibility of dry contact areas where adhesive cannot be introduced. By enabling the structure to be dry assembled first and then introducing the adhesive, the process can be broken into two discrete stages which can be carried out in different locations or at different times. The AdhFAST ® system has been shown to produce effective bonded structures with the potential to precisely control fillet profiles and fill complex joint cavities. The failure results show more consistency than manually produced joints and there is the added benefit of minimal operator exposure to uncured adhesive material.
The very evident benefits of hybrid joining systems have been presented in conjunction with examples from two very different industry sectors, aerospace and automotive. The former requiring a fastener component to transmit very high loads in thick section material whilst the latter needs the combination of both fastener and adhesive to provide predictable structural performance under impact loading.
The load displacement behaviour of an all-metallic lap shear hybrid joint under tensile loading was presented. It showed that when a rigid structural adhesive was used the profile was linear i.e. the joint exhibited adhesively bonded properties which reverted to fastened properties after adhesive rupture. Very little or no overlap of properties was observed. The effect of lowering the modulus of the adhesive was discussed together with effect of replacing one or both substrates with plastic or composite, the latter possibly introducing additional failure modes. The lack of published data was highlighted.
Finally the AdhFAST ® hybrid joining system was presented which offers the additional benefits of adhesive injection, bondline control, improved joint quality and minimal operator exposure to uncured adhesive.