I checked units made for go-karts and similiar light weight applications and they were still too heavy and inconvenient sizes. It's never bent and is a practical weight. I probably would have had a better chance of it holding. Sounds like something more high-tech that what I came up with. I really didn't expect the 10mm axles to hold up, but I've showed no mercy, and it's held up well to any kind of abuse I've figured out how to dish out.
Wheel diameter plays an important part in stress-calculations for an axle.
The highest stress could occur on the long straights. With carts, driver fatigue is probably more important for part life than metal fatigue. If I would have to design my own axle, I would probably glue or pressfit a 12 mm inhexbolt in a tube, and use a anti-compression spacer between the bearings in the hub.