Bonded vehicle structures – exactly what do rivets do?

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by Andrew Marsh,  via AutoIndustryInsider.com |

For an aluminium intensive body shell through to a hybrid steel and aluminium alloy shell, the joining system of choice is structural adhesive. The advantages are:
  • More efficient load distribution in the joint compared with welding.
  • No change in the panel properties around the joint zone due to heat during the joint formation.
  • Ability to join materials which do not ordinarily weld.

So, why use rivets? The bonding agents needs time and stability between adjacent panels to build strength. The rivets pin the shell together to allow the bonding agent to fully cure while the assembly is sealed, painted and dressed into a complete vehicle during the manufacturing process. The body assembly using bonded joints cannot be completed at anything like the same pace as for a welded structure without the use of rivets. Just as with welding processed, access to both sides of the panel joint is not always possible and has produced an ever increasing array of rivets – some of which do not even use rivets but mechanically lock panels together.

The primary task of the rivet is to clamp panels together along a joint flange, and as such perform a relatively minor role in the final joint strength. Whilst the OEMs who blazed the trail for aluminium intensive mass production vehicle structures  have and continue to share technology information with suppliers of sheet metal / bonding agents / rivets, and in so doing have done some pretty impressive things to improve repairability, but  the bottom line is there are too many rivet variations. Given the present and rapidly increasing variety of rivets , this product complexity is hard to understand apart from a relatively narrow manufacturing perspective.

The issue? OEM support of rivet identification via dealer outlets in the after market has remained poor to average. Add to that the number of different types of rivet forming tools required, and then add to that the limited number of product approvals by each OEM together with a relatively small (but increasing) number of applications.

The result? Massive proliferation of dedicated tools which exist mainly because of proliferation of rivet types and limited OEM approvals. For most common repairs (body side, front and rear rials) there should only be two to three different types of rivet. Indeed this after market reputational ‘drag’ will become the main as future vehicle generations complete their production life cycles. We can see this with the increased steel content in the 2015 TT (all new body structure than happens to use aluminium and steel alloys), rapid expansion of the JLR aluminium intensive vehicle lines and the hybrid structures on offer in the Mercedes-Benz W205 C class as well as the W222 S class. Indeed the biggest volume player so far – the new 2015 Ford F-150, is set to bring many more repairers face to face with structural bonding.

The solution? Pure bonded structures. But whilst the bonding agent technology already exists and is even used in things like buses let alone aircraft, it would require a complete re-think about how vehicle bodies are manufactured. For the repairer it would be almost the same as welding in new panels – except paying close attention to degreasing / surface matching / temperature control / humidity control to ensure maximum bond strength once cured.

So, what we have here is an evolving mass production technology which will purge less successful joint types in coming product generations. Until then, an element of chaos reigns in the after market.