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.

15 April 2011

Space Frame Joining Techniques - Adhesive Bonding

Adhesive bonding tends to be considered as a low cost option for joining spaceframes, but process automation requirements and the need for viscosity compensation to provide consistent application tend to contradict this view.

There are however a number of advantages associated with the use of adhesives for the joining of structural components that leads to the use of this method being retained particularly with respect to joining aluminium spaceframes.

  • No distortion as would typically result from arc welding
  • Improved joint stiffness due to the continuous bond, as opposed to local joint contact, and more uniform stress distribution.
  • Good energy absorbing characteristics combined with noise and vibration dampening properties.
  • Dual purpose, provides mechanical strength and seals against moisture and debris ingress.
  • Smooth joints reduce stress concentration at the joint edges providing good fatigue resistance.
  • High strength in shear.
  • Dissimilar metals can be joined without leading to galvanic corrosion.
The limitations associated with adhesive bonding are numerous but the main factors that effectively rule it out as a stand alone process relate to the inability to use non destructive methods to check the strength of the bond and the unknown effects of exposure to different environments over time.

Clearly it would be very risky to rely entirely on the adhesive bond without knowing its real strength or understanding how it will endure in all potential environments it might be exposed to. There are also further limitations as listed below: -

  • Epoxy or solvent based adhesives typical of the type used can be hazardous to health and require suitable fume extraction systems, protective clothing and storage facilities to protect against the risk of fire.
  • Investment in equipment is risky because there is a possibility that these substances may be banned in the future.
  • Heat curing is a necessary stage in the process.
  • Limited shelf life of adhesives requires that adequate batch management procedures are in place.
  • Adhesive dispensers require regular routine maintenance to keep them clean.
  • For aluminium, surfaces need to be carefully prepared to ensure a good bond is achieved.
Despite all the limitations adhesive bonding is still being adopted, however automobile manufacturers have typically elected to provide some sort of mechanical re-enforcement that can combine with the use of adhesives and provide solutions to the inherent problems associated with this method of joining.

Audi for example, among other methods, use a joining technique called roller type hemming where rollers secured to a robot arm bend an outer panel over an inner panel which when combined with a hem-bonding adhesive provides a powerful connection. These adhesive joints are then hardened using a process called ‘inductive gelling’ which uses an electric field to target the specific hem-bonded zones.

Ref. Youson M (2002)