Rear axle/suspension failure – no known impact. What could cause this?

[JoeHorner]

If its not happened on any others I think the chances of a contribution on what they (and me to be honest) consider to be accident damage, are pretty slim. At the very least you would need an independent engineers inpection at some cost. And even then I dont think he will find in your favour, just based on the photo. The metal looks slightly crumpled around the tear, points to heavy impact to me. My guess would be heavy impact some time back which slightly crumpled the bracket, new wheel or whatever, then the resulting weakness has resulted in a fail at a later date. But as I said, I've not seen it so guess work based on the pic.

Not so sure on the impact - if you look at the tear itself it appears to be consistently clean, bright, metal. Which suggests a relatively sudden failure around the entire bracket rather than existing damage spreading.

And, with a design flaw, there's ALWAYS a "first case" that comes to light.

Absolutely agree that MB will likely push back on it but DVSA would certainly be interested and will inspect for free - it's part of their remit.

As far as MBUK (not the local dealer) are concerned, if there IS an issue it's much better for them to "discover it" and start recalls than to be told to by an outside agency, so letting them know that the next stop will be raising it with DVSA before anything's repaired might just be enough for them to suddenly feel the goodwill kicking in!

 

[ALFAitalia]

If it was bent, causing weakness, it would be silver all around the tear as it let go sometime after an impact that first weakened it. Good luck to the OP, but i think his chances are slim. That car model has been out for seven years, if corrosion was not involved, which it doesn't not look like, there would have been other failures by now. Only way you will know for sure is to get a proper metal structures engineer to look at it....and be prepared to lose his fee if his findings dont fall in your favour.
The stress and buckle marks, arrowed, look a lot older than the tear to me. You need to zoom in.

 

[JoeHorner]

If it was bent, causing weakness, it would be silver all around the tear as it let go sometime after an impact that first weakened it. [...] The stress and buckle marks, arrowed, look a lot older than the tear to me. You need to zoom in.
View attachment 98857

I can see your point there but that buckling could simply be the last part to give up getting distorted as it failed - possibly during the RAC loading?

Even if it was previous damage, the engineer in me can't help wondering if it should cause such sudden and complete failure as has obviously happened across the majority of that tear. In design terms that would suggest cutting safety margins far closer than they should be.

Would be interesting to see it in the flesh - photos are notoriously hard to judge this out of thing from. But I'd certainly say there's enough there to invite DVSA to take a look. Or at least to suggest to MB that you intend to!