I guess it is as always, a combination of reasons…
1.) As mentioned before, UTS is not the answer to all structural questions. Ductility can be a desirable quality as well, and in steel it is often one of the two, not both.
2.) The weld looks ok from afar (just talking about the shape of the beads…), do you know what filler was used, and what does the inside of the HT look like? You never know, maybe there was a tiny weld prep error that initiated the crack…
3.) Is 120kg/ASTM3 a usual combination of parameters? 120kg sounds like a lot…
yes, this is a good point and prob the issue here with the 853 TT.
I’ll double check - Reynolds has some specific filler guidelines for 853 so I’ll check with them as well, but yeah, could be that a small error with the mitre or prep played a role here…
I think I may have overdone it here too - my frame is designed as a fully rigid, off-road “adventure bike”, but there’s no way anyone would be (intentionally) hitting ASTM 3 limits (60cm drops) with a 120kg total weight. Probably 100kg (unloaded) at ASTM 3 or 120kg (with a touring load) ASTM 2 (15cm drops) would be more appropriate… But here in Europe, if you only get the gravel (ASTM 2) certification, I would have to put warnings on my bike that it’s “not intended for off road use”, which it completely antithetical to its concept.
Based on the photos/video EFBE provides, can you tell where the crack initiated? Does it correspond to where you tacked the tubes together or started/ended a weld segment?
Yes, it looks like it initiated right in the weld at the bottom of the TT where it meets the HT. Could be related to tacking, but the more I dig into it, the more I’m leaning towards switching to a 631 TT for (what sounds like would be) a material that is a little less finicky during welding to avoid brittleness in the HAZ (?). Either that, or I’ll just bite the bullet and go with a gusset!