How much torque can a lego axle handle?
In this forum thread I have found the following tidbits:


"We all know that XL motors can twist axles to their breaking point" This means 40 Ncm, according to Philo.
"If you're looking to get max torsional stiffness, then stacked 2L axles + connectors would definitely be much stiffer in torsion than a single long axle."
Axles can twist without breaking in two


Besides these, I couldn't find an exact number.