Cassette & Six Chains

[Ajax Bay]

(better double check I have chain breaker on bike)

My 'bike' one stays permanently in my under-saddle bag. Pressed into service a few times (never my chain though, crosses fingers).

 

[Ming the Merciless]

I don't log how many miles each chain does etc, I've better things to do with my time.

If you register the chain as gear and set it to be automatically included in your ride. Then Garmin Connect will automatically keep track of mileage, no spreadsheet required. Assuming of course you use a Garmin GPS on your rides.

Plus I also register brake pads when I fit new ones.

Plus tyres

 

[Yellow Saddle]

Fatigue fracture through the rivet hole, where the stress is the highest:
View attachment 764943

But where's the control?

That's not fatigue fracture in the true sense of the word. That crack started in the hole of the side plate at a tiny little nick where the hole wasn't perfectly round. The rivet is countersunk with force and puts a radial force on the hole (sideplate) that propagates. Fatigue happens through cyclic stresses close to the elastic limit.

In your case the crack was vertical, I have chains in my collection with cracks in just about every direction, it just depends where the imperfection was.

Electro-plated chains crack more often than non-plated ones because the process causes hydrogen embrittlement.

This didn't happen in the days before flush-peened chains. Modern chains require very high tolerances in manufacturing and hence cost a bomb.

If it's cheap, it probably ain't good.

 

[Yellow Saddle]

Never had a chain snap or fail in any other way than wore out

Where did the crack appear on yours ? inner plate, outer plate, did the crack run to the rivet/pin hole ?
doing a google search for snapped chains shows a lot of chains that have pulled apart (probably due to not being joined together correctly) but I see a few quick links that have snapped / sheared apart

They neither snap nor shear. Snap implies tensile breaks and shear implies a type of break akin to holding a peeled banana in your two hands, thumbs adjacent to each other. You then push one hand forward and shear the banana in half.
Chains simply crack and break.