If your library has it, check out "The Bicycle Wheel" by Jobst Brandt. Your testing, while interesting, has little to do with the strength of a properly built bicycle wheel. Assuming the wheel is properly constructed with uniform high tension (per side on the rear wheel), the ability of a spoke to yield elastically will have the largest influence on spoke durability. It turns out that because of this, perhaps counter-intuitively, double butted spokes (thinner in the middle) are stronger than straight gage spokes in bicycle wheel service.

Paul