Google a few terms like "fatigue striations" and "beach marks" and "fatigue crack origin". Look at your fracture surface and see if you see these (beach marks, since striations often require a SEM). I was an engineer before I became a cop, and I did failure analysis on gas turbine engines.
The fracture described here is likely a fatigue crack, it probably originated shortly after the bike was new, and propagated across three plus years of riding. The crack didn't go critical and fully separate until outside the warranty period, but it was there... growing a micron or two each time you landed hard. The edge (or toe) of a weld creates a natural stress riser and is a textbook location for fatigue crack origins, even absent a defect or manufacturing error.
Photo-document the fracture, show it to the manufacturer, and see what they say. If they do nothing for you, explain that you intend to share your experience publicly. I've had companies replace hardware in lieu of seeing my metallurgical analysis made public.
This isn't my picture, but it's representative of a fatigue fracture:
Does your fracture surface look like that?
Obviously, take a hard look at your past use and determine whether you got your money's worth. If you really just flogged the bike too hard, and the company actually delivered a product that did what it was designed for, then acknowledge as much.
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