Originally Posted by
unterhausen
There is no other place for a crack to start other than the weld. Unless the tube suffered some insult that is not apparent in the picture we have. But it clearly started out of the picture, and probably on the side of the bb just at the edge of the weld. It then traveled perpendicularly to the load path. That weld looks shady to me, like there is poor penetration at the edge. But everyone on the internet is a weld critic.
It looks like that in the first picture, but fine in the second one so I think it must be a trick of the light. What concerns me is whether there's a groove down the middle, just above the proper weld bead, towards the CS, like the guy blew a hole, made a complete dog's breakfast of filling it in and then filed it all down to hide his shame. But it's hard to tell from these pictures so I'm probably being unfair.
As an expert from personal experience in how not to do TIG welding one thing that can catch you out with a joint like that is the arc leaping off to the side when you start up. The HF start is very good at finding its way to earth sometimes via strange routes. It can do that intermittently sometimes and can either burn the tube a bit (there's less argon over there) or even start melting it if you don't notice in time. It takes probably less than a second to liquefy a bit of bicycle tubing with a TIG arc that isn't being fed fresh rod to cool it down especially if it's longer than usual. The welder will crank up the voltage automatically if the arc is long to help ruin your day.