26 April 2011

Space Frame Joining Techniques - Mechanical Fasteners

There is a large range of mechanical fasteners available of which self piercing rivets and clinch joints have been identified as having a high potential for use in the automotive industry as a viable alternative to spot welding and to supplement the use of adhesive bonding for joining structural frameworks.

They are both essentially cold forming operations which can join two or more pieces of material mechanically. See below: -

This figure illustrates the self piercing rivet process. Ref. Barnes T, Pashby I, (1998)

The rivet is designed to both pierce and form a permanent fastening within the materials being joined. Having pierced the upper sheet of the material, the rivet expands in the lower sheet, usually without piercing it, to form a mechanical interlock. This is a process widely employed by Audi in the production of ASF (Audi Space Frame) structures.

The clinch joint is very similar in that it involves the deformation of the material being joined to form a mechanical interlock. The main difference is that clinching does not use a rivet.

This figure illustrates the clinching process. Ref. Barnes T, Pashby I, (1998)

In both processes the advantage of not piercing all the way through means that the integrity of the joint is maintained with respect to moisture ingress.

Also both processes compare favourably to spot welding with respect to production criteria, in that they share similar limitations and advantages but have the added advantage of being considered low energy, safe processes.

The main drawback of riveting compared to spot welding is that it introduces additional consumable items and therefore weight into the process, but on the positive side dissimilar metals can be joined. Clinching also has this characteristic without adding a further consumable but is considered to provide a slightly less effective joint in terms of overall strength.

Ref. Barnes T, Pashby I, (1998)

Jaguar have used over 3000 self piercing rivets in the body of the Jaguar XJ aluminium riveted and adhesive bonded monocoque bodied luxury car with no spotwelds. This was achieved by developing a relationship with Henrob, a specialist producer of riveting solutions.

The process development between Jaguar and Henrob claims to have produced some world firsts in terms of riveting techniques, these included the joining of multi-layer material combinations using a comprehensive range of riveting equipment and the introduction of lightweight riveting tools for favourable robotic handling.

Together Henrob and Jaguar went through a series of prototype builds and worked on experimental joining solutions which included static, fatigue and corrosion testing of joints whilst also identifying cost effective rivet coating solutions that would address problems associated with meeting new European legislation on the “end of life” for vehicles.

The final result was a process that produces highly consistent joint properties as well as giving enhanced fatigue properties that can be monitored non-destructively by an integral process monitoring system thus helping to streamline production and reduce inspection intervals.

Ref. Anon (2002)

The use of mechanical fastening techniques combined with adhesive bonding techniques does seem to have provided a solution to the specific problems associated with aluminium component assembly and in some respects suggests that this method of assembly has the potential to provide a superior product in comparison to the traditional steel spot welded monocoque designs both in terms of process and of product performance.

In Jaguars design, the retention of an aluminium monocoque design is significant in that it differs from the approach taken by the other main producers of aluminium bodies i.e. Audi and Honda, it would be interesting to explore the reasons behind this a little further.

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