[Note: this is #9 of a series of 20]
Back in the 1970's, aluminum nipples were rare. We had superlight tires (<200g), superlight rims (<260g), and superlight spokes (butted 15 ga), but no one thought about aluminum instead of traditional brass for nipples because the weight savings is small (~20g/wheel) and a nipple failure is just as bad as spoke breakage. The only aluminum nipples available were made from inferior alloys and didn't impress serious riders and builders.
In the 1980's, companies like DT and Wheelsmith set their sights on making high quality aluminum nipples. In Wheelsmith's case, the solution was four-fold. One, use 7075 aluminum, which has superior hardness. Two, make the nipple with 30% more threading so the chance of stripped threads is reduced. Three, forge the nipple rather than machine it to shape, to increase metal integrity. Lastly, rely upon recently developed thread compounds (Spoke Prep) to reduce friction and prevent corrosion, both a much greater concern with aluminum than brass.
These strategies succeeded but the move to aluminum for high end wheels was overwhelming. Both DT and Wheelsmith were surprised. Even with the enhancements listed above, aluminum nipples are still structurally inferior to brass. They like to corrode and fracture. The fracture is typically right where the saucer shaped head transitions to the cylinder body of the nipple. The head snaps off and the cylinder, with a healthy spoke inside, drops away.
Corrosion plays a role, but so does nipple alignment. Rims can provide nipple locations that aim directly at the incoming spoke. A nipple itself can, also, orbit in its seat to better adjust, but rims with holes too small constrain the alignment. And many rims do nothing to support nipple angle. These conditions continue today. However, hurray, there's a 100% solution to aluminum nipple failure. Use a longer spoke.
Correct spoke length.
When your spoke passes through the nipple's weak spot, the transition from head to shaft, it reinforces the structure. The nipple can't break if the spoke fills its length. For most builds, this adds one millimeter to the length. You'll have to experiment a bit, but freedom from nipple breakage is more than worth the effort.
From now on, don't let any of your wheels lose aluminum nipples because they broke at their heads. There's simply no excuse!