Shaft sizing question

Attempting to use the formula:
D = 1.72 (Tmax/σmax)^1/3

In my case I do not know σmax or at least how to calculate it from material properties.
Material is 1045
UTS 84800
YTS 74300
Tmax = 591.55 in-lbs

D = 1.72 (Tmax/σmax)^1/3

Any help would be appreciated.

Regards
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 Recognitions: Science Advisor As a very rough pass, you can use your yield strength as $$\sigma_{max}$$. Your equation is essentially assuming a set relationship for ductile materials and the relationship between shear stresses and normal stresses. This is for static, pure shear only.