8500 psi is 60Mpa I think (so about 100MPa tensile - 6 tons per square inch), so pretty cheesy stuff. I think that may have a safety factor of 2 in that, otherwise fatigue could fail it.
Not sure how you do the sums for square stuff, but for round I came up with figures as follows for the torque to cause it to start to take a permanent set. I read that the shear yield stress is about 58% of the tensile yield stress.
1/2 inch round, mild steel (215 Mpa Tensile Yield) = 40 lb.ft 3/4 inch round, mild steel = 125 lb.ft 3/4 inch round, carbon steel (guessing 350 Mpa tensile yield) = 215 lb.ft 3/4 inch round, carbon heat treated to 1000MPa tensile yield = 590 lb.ftIf you can use the ultimate tensile strength instead of the yield stress (for a once off emergency), then I get the following
1/2 inch round, mild steel (360 Mpa UTS) = 60 lb.ft 3/4 inch round, mild steel = 210 lb.ft 3/4 inch round, carbon steel (guessing 600 Mpa tensile yield) = 350 lb.ft 3/4 inch round, carbon heat treated to 1000MPa tensile yield = 590 lb.ft (uts and yield stress are pretty much the same for HT steel).Looks like we are in the same ball park.
However Dave has been up half the night torturing a bit of half inch square steel and says it is happy with 120 lb.ft - so either its not mild, or our sums are wrong (and this is NOT my strong suit !). Though it is interesting to sort out and may come in useful later.
Steve