Invar 36 Fatigue Strength 1

I just posted this thread at another forum but was thinking this one might be good for this too..

I'm a Mech. Eng. working on Optical systems. We are using Invar in a couple of our systems and would like to figure out lifetime expectancy.

Does anyone know anything about the fatigue strength of Invar or any related data?

Thanks!
Jay (aka username)