You misunderstood my comment. I said 11lb weight limit on the back of the subframe. The rear rack is on the back of the subframe. Any way you spin it, the back of these subframes are not designed to hold much weight at all (no matter what rack you have on it). I saw your pictures it's a simple case of too much weight being put on the back of the bike. And yes I am serious. I have seen a lot of rack and subframe failures over the years with those huge boxes. The vibration and the constant pounding the rear of the bike takes ends up with what you are showing in your photos.I never implied he wasn't being reasonable.
But 20lb is an insane amount of weight? Seriously? Guess we have vastly different definitions of that word.
Just to be clear the bike spends most of its life on pavement with the box empty. I live in NYC after all. And otherwise gravel fire roads and such. Also, 20lb is by far the most it ever had in it.
Now, to discuss the actual failure, there are no bends in the rack. Which is what I would expect to see from an excessive amount of weight. The cracks are caused by metal fatigue caused by vibration. Which to me indicates a design issue and so I will have the new rack reinforced locally.
Oh and that 11lb weight limit refers to the stock plastic rack and has nothing to do with the subframe.