israelkk,
The 0.577 factor is from a yield criterion (known as one of the following: von Mises, Hencky, Distortion Energy, or Octahedral Shear Stress), but fracture behavior is not governed by yield behavior, so your three significant figure number is not accurate. The ultimate shear stress/ultimate tensile strength ratio varies by alloy and condition, with a general rule of thumb being ~ 0.6.
Regards,
Cory
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