Pre-stressing a torsion bar

[JacobP12345]

I am looking to extend the fatigue life on a torsion bar I am designing. Currently it is tempered to the highest rating possible and is made of 4140, and has a diameter of .5''. I don't have much knowledge on the theory behind pre-stressing or pre-torquing a torsion bar and considering the price it costs for the bar to be treated at the rating it is treated at, I would prefer not to learn from trial and error. Does anyone have any knowledge on the theory or simple trade secret relationships I can use to estimate a pre-torque angle when the cyclic twist rate will be between 0 and 90 degrees?

 

[Tmoose]

For decades Chrysler corp used torsion bars from ~ 1957 - 1992.
I believe after the first year or two they were shot peened while twisted/preloaded in the same direction they would be twisted in service, thus there were Right and Left parts.
Their design precluded any bending loads, and the fixation details ensured the highest stress was in the main length of the bar.

https://p-s-t.com/images/F142751418.jpg

They also got a better paint job than typical under-car components of that era, to keep the highly finished working OD protected from corrosion.