Time and time again at work we see people come in and ask for a 'stronger' bolt ie highest tensile strength possible for their car, bike etc...
The tensile rating of a bolt is exactly that, it tensile strength, the power it gas to resist being pulled or stretched. An application where high tortional load eg/ something that is required to be torqued up and therefore a higher clamping load is where 10.9 or 12.9 (grade8 and 9) should be used. There is often more surface area in a high torque joint which helps elliviate pressure or force coming perpendicular to the bolt.
8.8 (grade 5) is used where the required torque wont be succicient to strip the thread, but also where there is a varying and constant side load (force coming perpandicular to the bolt).
The metric system of rating also tells you what forces the bolt is capable if holding.
Hopefully this can help:
These are grade 4.6 and 8.8. The first digit relates to the ultimate strength of the material, whilst the second is the ratio of yield stress to ultimate strength. Thus grade 4.6 bolts have an ultimate material strength of 400N/mm2 and the uield (or proof) stress is 60% of the ultimate strength. Simlarly grade 8.8 bolts have an ultgimate strength of 800 N/mm2 and a ratio of yield/proof stress to ultimate strength of 80%. "
Basically that second number is what we're interested in at the moment. The easier way to look at it is that it tells you how much stretch/deformation it has before it fails, so at 80% strength it will stretch 20% before it snaps.
10.9 (grade 8) and 12.9 (grade 9) has 90% strength which means they will only stretch/deform 10% before breaking.
This means that a sideways force only has to prove less tortal movement in the higher tensile bolts before they fail. And it doesn't have to all come at once
I have sold bolts for cranes, bulldozers, custom hot rods, cars, bikes, and kids scooters. I have seen first hand what happens people ignore our advice and put grade 9 bolts in the flywheel of their car rather than use G5 as per what is in there stock then try to sue us when those bolts have snapped and ruined his engine. Kids scooters have 8.8 (G5) bolts for their axles that strip threads from over tightening them, but if they use a 10.9 or 12.9 (g8, g9) they snap after 2 days of use going over bumps.
I may seem to be ranting here but using a higher tensile bolt in this application where there is not much surface area being joined to provid enough surface friction to help take the the contant side load these bolts will be under.
I sell bolts as my job. For 5 or so years and there is a reason for the different grades other than cost. I have read your posted links and am not convinced they are right. The tests they proved didn prove anything other than the high grade bolts are 'stronger' whch of course they are, but not under every operational condition.
It is far better to have an 8.8 (g5) bolt stretch and be see to be wiggly before it fails, like your rivet, rather than have it fail catastrophically with little to no warning.
My 2c. sorry if I havent explained myself very well, easier talking about it face to face than writing!