interesting! actual breaking torques as well as pictures! nice find!!
The breakages appear to be pure-tensile strength failures, which is why they are breaking in the long spiral which is where the maximum tensile stress is (45 degrees to torsion axis). As opposed to the nearly square snap offs usually seen in actual axle breakages that seem to originate from cracks and propagate along the maximum shear stress plane which is perpendicular to the axle axis, as far as I understand. This is because these types of tests are really testing single-overload failures and not fatigue and crack propagation. So not very meaningful unless crack formation/propagation and fatigue are not playing a role in why your axles are breaking.
The twist angles they note are also not really important as far as understanding or improving axle design. Twist deflection angle is a result of length and shear modulus, not tensile strength or shear strength, which are both separate properties, and determine when the axle will break. Double the length and you get double the deflection angle for the same torsion. Also, shear modulus is the same between all steels regardless of temper, hardening, or alloying, so its not really something that could be changed anyway, even if it did have an impact on breaking strength, which it doesn't.
For instance, a 6al4V titanium shaft would have about half the shear modulus of any steel shaft, and so, for the same length and torque, deflect twice as much. But its tensile strength and shear strength would both be much higher than a 1018 steel shaft, and so it would break at a much higher torsion, even thought it was deflecting far more. This is just to illustrate that its possible to have much higher deflection and also a much higher breaking point, when comparing two materials. In reality I doubt anybody makes axles using 1018. Most materials I have seen mentioned for axle shafts use stronger steels that will be superior to 6al4v in tensile strength and shear strength.