Crack/fissure in the headtube My 2007 Ghost AMR 5700 developed a crack or fissure at the upper end of the headtube. I do not know whether it developed over time or whether there was a specific event leading up to it; it looked like that when I discovered it first.

Still, everything else on it is in good condition. The bike obviously is not worth its weight in gold, but it would be a bummer just to throw it away.

Questions:


How on earth can a crack develop at that point (it's about 10:30 o'clock when viewed from the sattle)? I am riding slow and methodically on single trails, I don't race downhill, and am pretty sure that I never once "punched through" the front suspension (sorry, don't know how you would call that in english); I do tend to not put my full body weight on the handlebar anyways for the sake of my wrists. The location just seems a bit arbitrary. Or is that a common place for these kinds of injuries, for some reason?
The bike is the entry level of the AMR line from Ghost, it cost about 1700€ (?) in 2007, and the parts are certainly not high-end, but appropriate to my needs; I like the bike as much today as in the past, and have not tried other fullies since then, so have no comparison anyways. That said, would you say it makes sense economically to look for a new frame (Ghost or different brand) and move everything over? I do have a family member who built his own road bike for a world tour from scratch, so the necessary know-how and maybe even most of the tools should be around, although he has no experience with suspension. 

But I have no idea how to select a proper frame, and frankly don't want to make this my "project" for the next year. I guess I'm asking how hard it is to find a fitting frame, or how easy it is to botch the choice. It would only make sense if the result would be able to run single trails again (not crazy downhill with lots of jumping, I'm not getting younger).
I don't even dare to ask, but will do so just for completeness: would a competent welder be able to fix that crack, and would you trust your own life to it afterwards?


The crack itself: 

The specs from their 2007ish paper catalog:
Q: "[W]ould a competent welder be able to fix that crack[?]"

No, that frame is done for. By attempting to weld such a crack, a welder would prove their incompetence.

Even if you fix that crack you might find another crack on the right hand side, mirroring this crack. What is more, the crack will go considerably deeper.

I had a crack at the same spot and we asked the manufacturer (Corratec). According to their frame builder welding would warp the head tube too much to seat the head set properly.

Q: "[Why] can a crack develop at that point[?]"

Aluminium is susceptible to fatigue under light dynamic loads.

The upper bearing of the headset is pressed into the headtube at this point.

The position of the crack suggests it is caused by a force pushing the upper bearing forward. Due to the great mechanical advantage of the fork/headset system forces pushing the wheel backward require the upper support to bear a force pointing forward. This strains the upper support. (The lower support is compressed).

When you pull a ring from the inside it will typically break at the same points your tube cracked. (If I remember correctly from undergrad engineering ±2/3π from the direction of the force.) (Try it for example with a bagel.)

If your headtube showed cracks at any other place it would suggest machining tolerances not met. A head tube that is too tight cracks when the upper bearing race is pressed in. These cracks would then grow, for example, due to stress corrosion cracking.

Q: New frame?

Have you tried to ask Ghost? Corratec have given me a new frame when their ten year old frame failed in the same mode.

PS: Do not ride your cracked bike! Not even a single metre: it might fail catastrophically without warning.