That's great news for you, the carbon frames have been pretty rock solid.The dealer JE James in Sheffield is going to RMA the frame. If they don't have a hydro-aluminium frame in stock in the UK, they will upgrade it to carbon fibre.
Not surprising as that's always going to be the weak point. There's not much to go wrong with a tube, and it can be repeatability manufactured (unlike carbon). The welds are not as repeatable if done by hand, and will always cause a change for the worse in the base metal making it less ductile, which then requires a specific heat treatment to mitigate. Lots of room for issues there in comparison with the rest of the frame.Poor welding seems to be a theme in so many Alu frame failures.
To be honest that whole area looks a mess to me. The welded joint is only a "bulge" at the bottom of the tube section with no proper weld visible..........meaning it has likely been machined down.........and yes, that weakens a joint. Otherwise a proper weld is not the weak point. The weak point is the tube just beyond the weld if it has not been properly heat managed or if the weld has undercut the wall of the tube at that point ( poor weld/too many amps).Not surprising as that's always going to be the weak point. There's not much to go wrong with a tube, and it can be repeatability manufactured (unlike carbon). The welds are not as repeatable if done by hand, and will always cause a change for the worse in the base metal making it less ductile, which then requires a specific heat treatment to mitigate. Lots of room for issues there in comparison with the rest of the frame.
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