Composite cohesion

With the increasing use of hybrid construction in automotive applications, traditional welding techniques are being replaced by mechanical fastening and adhesive bonding.

The requirement for weight savings, lower fuel consumption and reduced emissions has led to a growing trend in automotive manufacture to combine different materials such as steel, aluminium, or polymer composites.

But according to a recent government report on the subject, although the advantages are potentially huge, this approach – known as hybrid construction – raises considerable technical challenges when compared to the manufacture of vehicles made from just one material.

Indeed, with choice of joining technique having an effect on both structural performance, ease of manufacture and weight, the joining of dissimilar materials is seen by many as the key to the widespread adoption of hybrid materials. Effective joints between materials and components are vital to ensure that the whole structure is efficient, can carry design loads and, in some cases, contribute to impact resistance and passenger safety.

This May, the DTI dispatched a team of UK industry experts to gauge the current hybrid construction practice in France and Germany, generally considered to be the most technically advanced techniques in this area.

Following visits to companies such as Airbus, Renault, DaimlerChrysler and Smart, the team found that a number of new joining methods are increasingly being used to replace traditional welding techniques – these include laser beam welding, mechanical fastening and adhesive bonding.

The growing use of adhesives in automotive applications is backed up by a new piece of market research carried out by analyst MBD. This says that adhesive manufacturers are making inroads at the expense of alternative bonding methods such as industrial fasteners and welding. It also reveals that by 2003 the vehicle manufacturing sector accounted for almost five per cent of volume demand for adhesives.

The big advantages of using adhesives in car manufacture are that they allow considerable improvements to a vehicle’s torsional stiffness (its resistance to twisting) while adding very little to the overall weight. Adhesives are currently typically used in conjunction with mechanical fasteners. For instance, the report found that Renault and DaimlerChrysler were both using structural adhesive combined with resistance spot welding to improve safety in a crash, vehicle refinement and dynamics. Clearly, if a car is to be held together by glue alone then there are significant quality and durability assurance issues that must be addressed.

A major recommendation of the report concerned the virtual testing and modelling of bonding performance. It suggested that no single computer-aided engineering (CAE) technique is likely to be able to cope with accurate modelling of so many joint types and that new CAE techniques need to be developed that can assess this performance. Increased use of robots to carry out adhesive bonding was also singled out as an important area of development.

But perhaps one of the most troublesome hurdles for the adhesives industry will be created by various EU ELV (end of life vehicle) directives, which set car manufacturers a recycling target of 95 per cent by 2015.

These directives have led to a slight conflict between the requirement to produce ‘permanent’ joints that are cost-effective, durable and robust, and the need to be able to dismantle such joints economically at end-of-life.

Currently, most automotive manufacturers meet their recycling requirements through shredding, but as soon as they start trying to shred vehicles made from mixed materials it can become very difficult to separate materials and re-use them. Clearly, this problem is compounded if the materials in question are joined with adhesive.

In addition to the obvious technical challenges it’s perhaps also worth pointing out that in these early days of hybrid structures, engineers from previously disparate fields may suddenly find themselves working together on the same project. As a result, adhesives manufacturers, fastener suppliers and automotive manufacturers may need to consider each other’s needs more closely than ever before.

A full copy of the DTI Hybridmat2 global watch mission report can be found <link>here=</link>.

<b>Futuristic approach</b>

The DTI team, in its fact-finding tour, discovered several technologies that could lead the way to full hybrid construction. Here are a few of them.

DaimlerChrysler presented a new robotic laser-welding system which the group claimed could take laser technology to the next level, rendering existing robotised spot welding guns on car assembly lines obsolete.

Plastic Omnium – a company that spends six per cent of its turnover on R&D – has developed hybrid thermoplastic composite (HTPC) beams and showed the team an almost complete composite bumper beam system, said to be the first of its kind.

Renault Technocentre demonstrated a laser-welding technique that uses mirrors to direct the laser beam so that the structure itself doesn’t have to be moved.

At Germany’s aerospace research centre, DLR, the team was shown a full-size aerospace fuselage demonstrator showing a variety of different joints and novel sandwich structures.

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