I'm sorry you're having such awful luck with your wheels. This does give something interesting for the rest of us to think about though.
Why would a relatively new spoke break?
One reason would be if the spoke was put under enough tension that it exceeded its yield and snapped! Kind of like pulling a rubber band until it breaks. This is highly unlikely/impossible for several reasons. Spokes are placed under tension when the wheel is built and the failure would have happened then. During normal use, a wheel's spokes go through cycles where the tension is reduced (not increased). And lastly, a normal aluminum rim will deform and break (i.e., spoke rips through eyelet) before the spoke will yield.
The second reason is a fatigue failure. This can occur because of the continuous loading and unloading of the spoke tension while you are riding. These stress cycles cause normal (microscopic) voids and nucleation sites inside the spoke to grow until a crack works its way through the spoke and it breaks. Fatigue failures are greatly accelerated if the spoke becomes totally slack under load or is too close to yield. Again, too close to yield would mean a busted rim, so it's more likely that the spokes don't have enough tension. This should be easy to check.
The last reason I can think of is that the spokes themselves are of a poor quality. That is, they have residual stresses due to the manufacturing. Or there are material defects, impurities and inclusions. All these things could also lead to accelerated fatigue.
Your spokes are breaking regularly which means that they are all near the end of their fatigue life whether it's poor tension or crappy spokes. I'd say rebuild the wheel with new spokes and all should be well...
John Swanson
www.bikephysics.com
