Strain sensor designs and strain measurements based on single crystal sapphire fibers with inscribed first order fiber Bragg gratings for applications up to 600°C are presented. We report on all the details of two different sensor designs; for instance we show that the resolution of multimode sapphire fiber Bragg grating (SFBG) strain sensors is about ∆l / l = 10-5 (10µstrain), which is comparable to state-of-the-art high temperature sensors. We apply our sensors for the determination of the thermal expansion coefficients of high temperature steel alloys, showing a good match to known values. Hence we believe that sapphire fiber Bragg grating sensor may represent a promising alternative to currently used non-optic-based strain-detecting devices.