Fatigue fracture through the rivet hole, where the stress is the highest:
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But where's the control?
so have you had one that has failed there, or are you just pointing out the most likely place? I have never seen one sheared like that, often when people say the chain has snapped, it is where the side plate has pulled away from the rivet,
Not sure how you could have a control - I would guess with most bikes, first chain gets to 1.25% / 1.5% starts to slip, and the over worn chain has ruined the cassette,
A careful mechanically minded cyclist will probably get 2 may be even 3 chains out of one cassette, providing they change the chain a bit before 1% (0.75% ideally) but even with this care eventually the cassette becomes too worn to accept a new chain
I'm just curious that if I can match the chains wear matches to cassette wear - how stretched could a chain become and still function correctly?
I did manage to get one out to 2.4% and I am eiteher trying to better that or just recreate that - there will be a limit