A high profile adhesive failure has hit the news in the last few days. The story has been widely reported – for example: Nearly All Cybertrucks Have Been Recalled Because Tesla Used the Wrong Glue. Reports suggest that Tesla will recall 46,000 Cybertrucks to repair a stainless steel trim panel that could come off the truck while driving. There are about 150 warranty claims that could be traced to this fault.
In an official filing, Tesla say: “The recalled component uses only a structural adhesive to join the assembly. This structural adhesive has been found to be susceptible to environmental embrittlement. The remedy component uses a different structural adhesive not prone to environmental embrittlement to join the assembly, which is reinforced with a stud welded to the stainless panel with a nut clamping the steel panel to the vehicle structure.”
The clear implication here is that Tesla chose the wrong adhesive.
Structural adhesives are used to reliably and robustly bond stainless steel all the time – not just in cars, but aircraft too. We don’t know which type of adhesive or adhesive product was used in these vehicles. Two-part epoxy or acrylic (MMA) structural adhesives are most commonly used due to their durability and high strength on metals like stainless steel. The words “environmental embrittlement” may give us a clue as to the potential failure mechanism. Through temperature cycling, some adhesives (typically epoxies) will get harder and more brittle as they undergo further cross-linking. Toughened epoxies have better resistance to this. There is potential for sunlight (UV) exposure to cause adhesive embrittlement, although it looks like the adhesive was hidden behind the stainless steel panel, so less likely. Maybe moisture from rain or car washing encouraged the failure.
The point is, the adhesive should have been thoroughly tested in the lab and in real life conditions before being put into production. There are well understood methodologies for doing this, and the automotive industry has got pretty good at testing for adhesive bond durability (fatigue, thermal cycling, humidity, corrosion exposure) and environmental aging (resistance to fluids, temperature, UV). It looks like this was not done properly in this case.
It is not the adhesive’s fault.
