Design implications of adhesive bonding in car body construction J. Daniels

The genera/principles of unitary construction car body design are reviewed and related to the requirements of conventional spot welding techniques by reference to three current high-volume production cars. The possible evolution of new designs for body assembly using adhesive bonding technology is considered through an analysis of sill and floor joint details. These developments are then discussed in the context of the production opportunities and constraints in the motor industry.

Key words: adhesive-bonded joints; jo int design; automobile bodies.

Why is it that adhesives, their technology and advantages well-proven and documented, are virtually ignored by the world's major car manufacturers7 To understand this, one has to look at the current shape and attitudes of the motor industry: the answers are there. Yet they beg another question, more important in the long-term. What effect would the large-scale adoption of adhesives have on car design and construction7

First, the mass-manufacturing side of the motor industry remains closely tied to pressed sheet-steel con- struction. Steel is cheap, not least because the over-capacity of the world industry has led to widespread cost-price (and less) selling. Beyond that, the technology of steel body production is extremely well established - and represents an enormous capital investment. The total value of sheet- steel handling machinery in the industry almost defies com- prehension. A significant proportion of that machinery is used to build up car bodies by the spot-welding together of individual shells. Thanks to the application of computer design and stressing programs which themselves called for no small investment, such shells are remarkably light and stiff, with good energy-adsorption properties to meet crash safety requirements.

Spot-welding, as a technique, suffers from considerable disadvantages which ought to form a target for the propo- nents of adhesives. It calls for easy access to both sides of the joint - which is a significant design constraint. It leaves behind a poor surface f'mish which must be expensively hidden, either by trim (usually) or by exceptionally careful filling and painting. It is an energy-intensive process, involving the electrical generation of enough heat to melt a local area of metal, while the resulting spark scatter poses a significant safety problem. Finally, it has proved extremely difficult to achieve consistent quality with spot welding - to the extent where cars have often been designed with a gross overprovision of welds than are theoretically required, to allow for the ill effect of poor spacing and

misalignment on body strength. Such problems assumed a greater significance in the light of three strong trends of the late 1970s: the need to reduce costs in order to remain competitive; to reduce weight to improve fuel economy; and to improve car body life in response to consumer pressure.

This situation might have seemed tailor-made for the adoption of adhesives for the joining of car body panels. Instead, starting about ten years ago, the high-volume motor industry began to invest heavily in computer-controlled robot welding. However high the cost, the result is far better consistency of quality which has, in turn, permitted a reduction in the number of welds per body - a trend further extended by the ingenuity of designers. As a result, the number of welds applied to a typical 1984 car body is about 3000 compared with over 5000 for its 1974 equivalent; and perhaps 90 per cent of today's welds are robot-applied.

The important point about all this is that the motor industry, when faced with the triple challenge of cost, weight, and longevity, elected to invest in robotics in order to remain, in all other respects, with the technology it knew. There was then no need for designers to adjust their fundamental attitudes, for computer-stressing programms to be re-written, or for safety standards to be based on new data.

Adhesive applications The only area where adhesives have made any impact at all in structural assembly is among the smaller, more specialized manufacturers who use plastic matrix composite materials rather than sheet steel - and who are therefore prohibited from welding. Lotus, for example, makes its car bodies in two halves, upper and lower, using its VARI (vacuum assisted resin injection) system, and afterwards

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INT.J.ADHESION AND ADHESIVES VOL.4 NO.1 JANUARY 1984 5

bonds the two together to form an extremely strong monocoque shell.

Yet, where composites have been used in larger-volume production, especially in the USA where RIM (reaction- injection moulding) polyurethane is the main material for 'soft ' front ends of cars, attachment is usually mechanical - by means of bolts, self-tapping screws or 'Christmas tree' plastic fasteners. The potential of modem adhesives for joining dissimilar materials has been largely ignored.

Metal design techniques

The modern car design approach has to be considered in more detail before one can reach firm conclusions about the way in which adhesives might be used either as a spot- welding substitute, or to bring about improvements in overall design. It is as well, therefore, to consider three modem car designs in some of their aspects. The main example is the Citroen BX, introduced in 1982 and built on a largely robotized line at Rennes in Brittany. To this example key points have been added from the Austin Metro, launched in 1980 and assembled in the new automated facility at Longbridge, and the Fiat Uno, announced in 1983, its body assembled in Fiat's 'Robogate' facility at Rivalta, Turin -p robab ly the most advanced robot-welding shop (and superb example of flexible manu- facturing facility) in the world.

It is a widespread assumption that modem cars are true monocoque designs which take all their stiffness from the fact that they are semi-enclosed boxes whose faces are stressed diaphragms. This comforting theory is some way

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from the truth. Given the number of cut-outs needed - for doors, windows, and even the increasingly popular rear hatch - the plain box is never stiff enough. It has therefore become the practice to concentrate loads to a considerable extent in closed box-sections which run along the joints of the main shell. These sections play a major role in determining beam-stiffness and impact resistance, and the joints between them are probably the most important factor in the torsional stiffness of the shell as a whole.

As can be seen from the Citroen example, shown in Fig. 1, the box-sections extend all round the cabin area where stiffness is vital for the preservation of 'living space' after a severe impact. The members forward of the cabin are designed far more with the aim of absorbing impact energy through controlled deformation - its pattern usually initiated through the use of pre-positioned folds or notches. At the same time, this forward structure has also to carry the main mechanical loads (the Citroen, like the other two examples - and indeed most modern, medium-sized European cars -uses front wheel drive) and disperse them into the cabin shell as evenly as possibly.

Of all the box-sections around the cabin, the most important are the sill-sections on either side of the floor. These accept the bulk of longitudinal loads and also provide the footing for the centre door pillar. They are also, it should be noted, in a very exposed position from the point of view of moisture and debris thrown up by the front wheels, and some form of extra protection is vital if premature corrosion is to be avoided. Since it is impracticable to seal seams completely when they are spot-welded, this area is usually given extra layers of ex-

Fig. 1 Position and detail of box-section joints in the Citroen BX

6 INT.J.ADHESION AND ADHESIVES JANUARY 1984

Fig. 2 Sill and flow edge construction of the Austin Morris Metro

Fig. 3 Floor and sill detail of the Fiat Uno

ternal protection (such as sprayed-on l'VC) while the interior of the box-section is treated with a wax-oil rust inhibitor. Such measures have become vital if manu- facturers are to offer the six-year warranty against body rusting which is rapidly becoming the European norm; but they are expensive.

A close study of the three sections through the sills of the selected examples shows some interesting detail variations. For instance, the Metro floor edges (shown in Fig. 2) are turned upwards, while those of the Fiat Uno (Fig. 3) and Citroen turn downwards - which might have implications for resistance to water ingress. The Citroen structure is built up around a completely fiat central web, while the other two use a combination of deep-drawn pressings.

On first impression, from the adhesive point of view, the sections look clumsy because all the edges are turned outwards. Immediate thoughts of tidying-up the structure by turning the edges inwards instead (because there is no need when using adhesives to allow welding-jaw access to both sides of the joint) are, however, misplaced. The problem is that all metal pressings - or, indeed, composite mouldings - must avoid significant overhangs in section if they are to be taken from the die. At least one of the edges, therefore, must still open outwards. Even then, by using adhesive jointing for one inward-turning edge, a significant improvement can be obtained (as indicated in Fig. 4(b) for the three cases). By extending the floorpan the full width of the car, a much cleaner underside is obtained, with con- sequent aerodynamic advantages and a reduced risk of

water leakage and corrosion. The retention of one 'open' joint also means that tack-welds can still be used to position the parts relative to one another before curing of the ad- hesive.

However, adhesives open up still further possibilities for the car body structure designer. Whereas the use of spot- welding virtually precludes the use of completely closed, extruded sections in body manufacture, they are entirely compatible with adhesives. On this consideration, sill structures can be still further modified so that the floor edge simply wraps around the underside of a pre- formed, closed section beam (Fig. 4(c)). This solution has the added advantage that it permits a far greater area of adhesive contact; on the other hand, some other means would have to be found of holding parts in the jigged position prior to curing. The layout has one further ad- vantage, in that extra strength could be added with a suitably positioned doubler (Fig. 4(c)).

This stage is by no means the end of adhesive possi- bilities. The use of sandwich materials, formed by applying adhesive to either side of a honeycomb filling and adding top and bottom external layers, can also be explored. It is, Of course, well known that such honeycombs are extremely stiff and strong - but existing forms are also expensive, though widely used in aerospace applications, for instance. For automotive use, further research is certainly needed into low-cost fiUings, possibly in the form of expanded-mesh or even a very simple deep-drawn punch- pressing. If then, the floor/sill junction is considered, it becomes clear that a filling could be positioned on the floor panel, adhesive applied and an upper panel added as shown in Fig. 4(d) to complete a structure undoubtedly strong enough to exist as a rigid 'platform' in its own right.

Thus far only straightforward closed cross-sections have been considered for the possible application of adhesive. However, adhesives appear to offer even more advantages where structural joints, such as between the sill section and

Local T Austin reinforcement

Fiat

~ Fabricated closed section with two adhesive joints

Citroen

a Spot-welded b Adhesive joint c W'flh closed d With honeycomb section sandwich

Fig. 4 Possible evolution of f low and sill section design for adhesive assembly

INT.J.ADHESION AND ADHESIVES JANUARY 1984 7

the centre door pillar (the B-pillar in motor industry parlance) are concerned. Here, the 'both-side-access' limit- ation on spot-welding is a major problem. The standard technique for overcoming it is to weld up the sill and pillar outer skins (giving rise to a rather flimsy component, dif- ficult to handle) first, and adding the inner skin afterwards. Given the use of adhesives, a pillar could easily be nested into place on a central spur, possibly using the kind of spur- joint already widely employed, in plastic form, by mass- market furniture manufacturers. Such a system would have the advantage of being self-jigging, which would be a con- siderable advantage in large-scale car manufacturing terms.

The same general principles could be applied through- out the car. For instance, the attachment of the roof panel to the header rails would be a mirror-image of the floor/sill junction. Floor cross-members, which also contribute heavily to overall structural stiffness as well as providing seat rail mountings, could be easily and reliably positioned. It would still be possible to bolt-on outer panels, such as front wings, where the car manufacturer felt this was desir- able in order to ease the repair of minor accident damage. Other repairs to the structure could certainly be carried out using cold-cure adhesives, with no more difficulty than is now involved in cutting away spot-welded structures and welding new metal into place. For factory production, however, one has to assume (on the basis of today's tech- nology at least) that hot-cure adhesives would be needed to achieve realistic curing times for high production rates. Curing itself presents no problem, since current car paint lines run at around 170°C. Any problem which did arise would be more likely to stem from the need to jig (and preferably self-jig) all body components prior to curing.

Implications for car manufacturers

On this basis, it might appear that adhesives are unlikely to bring about a total revolution in the way cars are manu- factured. That is probably a point in their favour; the major car manufacturers are extremely wary of revolutions. If adhesives can be presented as overcoming some of their production headaches without calling for a wholesale replacement of machinery and re-training of staff, so much the better. The fact that adhesives ought to work equally well with sheet steel or with plastic is a long-term point in favour of adopting such a system. As to the actual means of application, there is no doubt that today's welding robots such as those shown in Fig. 5, would be equally capable of administering exactly the right dose of adhesive in exactly the right place, and with a lot less fuss than is involved in spot-welding.

Even so, there are some significant potential advantages in the use of adhesives, especially for the smaller-volume and more specialized manufacturers. Sections of structure which embodied closed box-sections, or honeycomb, could much more easily stand in their own right as sub-

Fig. 5 Robot welding line for the Alfa Romeo 33. The plant could be readily adapted for assembly with adhesives

assemblies. This should open the way for two major advances. One would be in flexible manufacture, enabling cars to be built on shorter, less comprehensive production lines. The other would be virtually a corollary of this, since cars could be designed on a modular basis which would allow several different models to be built up with ease from a limited range of sub-assemblies.

Conclusion

All of this begs one further question. If the potential is there, why have the motor manufacturers not adopted adhesives with enthusiasm? The sad fact is that present-day manufacturers face an enormous range of worries and possible solutions to them. The best way - probably - for the adhesives industry to present its own case would be to commission a detailed redesign of a typical modern car to demonstrate what could be achieved in the way of weight reduction, improved stiffness, and lower production cost by using its materials. Data alone is not enough, though one suspects there is by no means enough of that to satisfy the ultra-cautious directors of production engineering: demonstration is a more promising (though expensive) way forward.

Author

Mr Daniels is a technical consultant.

8 INT.J.ADHESION AND ADHESIVES JANUARY 1984