I have just been Googling torque figures, and for a low grade M14 x 1.5 nut the tightening torque is 79Nm
(~58ft/lbs).
But this does not take into consideration the fact that the axle thread is incomplete due to the two flats which have no thread whatsoever:
According to my calculations, machining the flats will have actually removed almost 50% of the threaded section of the axle, therefore it would be a bit unfair to expect it to withstand the full recommended torque.
As half the thread has been removed, I would therefore suggest that the recommended torque should also be reduced by 50% too.
This would give a more realistic recommended tightening torque of just 39.5Nm, or 28.76 foot pounds.
As I mentioned in
a previous post, a longer
(deeper) nut will be able to withstand a much greater torque without stripping because the applied load is spread over more thread.
If a 10mm deep nut strips at just over 36 foot pounds, then a 20mm deep nut should withstand just over 72 foot pounds
(97.61Nm) before it strips.
I must admit that I usually tighten my axle nuts by feel, because after 40 plus years of tightening nuts and bolts, I now have a pretty good "feel" for the correct tightness of wheel nuts etc. and often have more confidence in my own judgement than the accuracy of some torque wrenches that I have used in the past.
I previously used a torque wrench to check exactly how much I had tightened my axle nuts up, in order to answer
a similar question.
I positioned the torque wrench so it was just about level under its own weight and then slackened the nut. I then carefully retightened the nut until the released torque wrench was in the same position as it was before I slackened the axle nut.
The torque required to tighten the nut back to the same position as before was ~80Nm
(59 ft/lbs).
Unfortunately, I am unable to explain why your axle threads appear to be unable to withstand the higher torque that mine are tightened to, unless they had been weakened by over-tightening them initially.
Here is my preferred tool for tightening the axle nuts, a 7/16 Whitworth ring spanner:
I would expect a standard Magic Pie to exert a maximum torque of around 50Nm through the axle, and most of this torsional force would be counteracted by the dropouts and the tabbed torque washers rather than the axle nuts.
Theoretically, the two axle nuts should be able to withstand this amount of torque without the assistance of the axle flats and torque washers etc., provided they were both tightened to
at least 25Nm.
I therefore feel that the 39.5Nm, or 28.76 ft/lbs tightening torque calculated above should be more than adequate, especially when combined with the axle flats in the dropouts, torque washers and an additional torque arm.
Alan 
