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Just guessing here: driveshaft whip is what we're trying to avoid. It's basically the driveshaft acting like a jump rope. With aluminum this can cause work hardening. This can make it weaker to other stresses, such as torsion. If your driveshaft twisted in the middle I bet it was likely work hardened from spinning too fast.
I'd expect if a healthy driveshaft were going to fail in torsion it would be at one end or the other rather than in the middle. The leverage of the length of it causing some bending to add to the torsional shear. I'm guessing here.
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