handlebar failure

Fatigue.
Stress concentration due to severe cross sectional change. Look at historic handlebar design for methods of reducing the cross-sectional change by adding darts to the gooseneck. Looking down on a gooseneck, you see something like this:

Most such failures I have looked at were fatigue, often from scratches caused during assembly, combined with poor design and high stresses in service. I have also seen SCC and corrosion related failures although these have been more common on seat posts.

Hello Clinton,
I am the OP of the discussion Rick refers to. Unfortunately I know little about Mechanical Engineering, and have not been able to find any useful information on the web to clarify your post. Would it be possible for you to explain what you are saying in a more layman-compatible format, as I am really quite interested to understand what you mean.
Specifically, I am not sure what you mean by "severe cross-sectional change", "darts", "gooseneck", nor what your ASCII diagram represents.
Thanks in advance, Dani (asqui)

Very shiny bike. Is this suspiciously new for a fatigue failure ?

Yes, seems so to my eyes too. I believe the cross section is constant through the broken area. The bars are broken at the stem opening (about 1/2 inch from center), yet the bar's OD is constant for about 2 inches from center.
Yes, these bars show such a "smooth" tapered OD in the region away from the break, near the blue colored handlebar tape. See the first photo.
Not mentioned is that the handle bar has a nominal OD of 26mm, and the stem a nominal ID of 25.8mm. I wonder if such a small mismatch could cause local plastic deformation, maybe at the pinch closure, and initiate a crack?
Many 26.0mm handlebars are installed in even smaller 25.4mm stems, though this hardly means they won't also fail someday...

Yes. 6 months and 2700miles since I bought it. (I also changed the handlebar tape and wiped the bike down with a rag on the day before those photos were taken :)
Dani / asqui

Not if there is a problem with the bars or the usage.
The pictures are not much use, but the one labelled shear seems to show some marks on the tube surface near the fracture.

A picture face on to both sides of the fracture would help.
Looks like it's failed in High Cycle Fatigue where you'd expect it to in a bending mode.
Can I ask five questions?
a) Have you ever crashed it on that side? b) Are the roads you ride on very bumpy? c) Do you ride with a high saddle and tend to put a lot of weight on the bars? d) What's the typical ambient temperature when you go out riding? e) We can't see from the pictures, but is the tube inside the clamp distorted or show any evidence of being over-tightened?
I'm sure I could think of some more if I tried.

No. I dropped it once though.
Not usually. It's a pretty subjective call, but generally no.
My riding style is pretty horizontal, but I weigh less than 70kg and usually pedal hard enough to take most of the weight off the bars. If I'm relaxing I ride with my hands in the hoods or on top.
Varies with the seasons. In the summer I once went out on melting tar at 35 degrees Centigrade, and in the winter down to near-zero. Not much below zero though.
No to my eye, but it did have a similar scratch on the other side of the clamp, on the top/front of the tube. Presumably this was caused by the edge of the clamp, but I'm not sure.
Shoot when ready.
Dani / asqui

Probably not enough to cause damage that would initiate a fatigue failure.
Yeah, that one would be a bit subjective.
So you're not Colonel Blimp and you haven't caused the bars to yield! ;-) Rules another one out
How deep was the scratch? I would have thought that minor scratches wouldn't be enough to initiate a failure. Could you define it as a 'score' rather than a scratch?
Could still do with some more photo's of the fracture face.

It's difficult to say as I only had a momentary look at it. It was wide and deep enough to catch my fingernail in it, so it was definitely more than a minute superficial surface stratch.
I can't work out what would have caused that though -- surely there's not much movement there, and certainly not any rotational movement (the clamping mechanism has a series of ridges which lock with ridges on the handlebars, eg. -- this also elliminates the possibility of a mechanic twisting the bars in a relatively tight clamp).
I'm afraid that will not be possible as the bars are now in the posession (or garbage bin, as the case may be) of Modolo. They had to send them off in order for me to get a warranty replacement, and I never got them back. I even had the bike back the very next day, but I've not actually had a chance to ride it yet because of the spontaneous snow-storm! I hope to go out for the club ride tomorrow morning though, weather conditions permitting.
Dani / asqui