Originally Posted by cyclezen

I've seen a few rim failures over the many years, but almost all could be ascribed to rim damage done prior and just a few likely due to rim material failure.
Never seen one where the rim was worn enough to part, except in one case where the rider had their pads improperly angled, and the very hard pads had cut the braking surface closer , at the lower/inner part of the brake surface.
Pics don;t really tell us too much - but a hard thing to do without hands on.
A possible is that there was a flaw in the rim, and possibly an undetect 'hit' to that one side, and a highly inflated tire, might cause the rim to bulge and peel outward.
Alu has a tendency to fracture, then continue that fracture along the stress lines - in this case being any light grooves worn into the braking surface, by the pads.
What pressures do you run your tires?.
There seems to be plenty of sidewall material in the visible section, if it's 1.5 to 2 mm, that's plenty material.
I'd mic sections in that failure area and determine if there are any 'thin' spots. Not that that will mean anything... You're still gonna need a new rim...
Ride On
Yuri