Static fatigue of Hi‐Nicalon™‐S fiber at elevated temperature in air, steam, and silicic acid‐saturated steam

A facility for testing SiC fiber tows in static fatigue and creep at elevated temperatures in air and steam was developed. Static fatigue of Hi-Nicalon (TM)-S fibers was investigated at 800 degrees C-1100 degrees C at applied stresses between 115 and 1250 MPa in air, in Si(OH)(4)(g)-saturated steam, and in unsaturated steam. Fibers tested in Si(OH)(4)(g)-saturated steam and in air had silica scales throughout the test sections, but those tested in unsaturated steam did not develop scales near the steam injection point. Fiber lifetimes in static fatigue were shortest in unsaturated steam, intermediate in Si(OH)(4)(g)-saturated steam, and longest in air. Failure strains did not exceed 0.3%. Steady-state strain rates and static fatigue lifetimes are modelled empirically by the Monkman-Grant relationship. Failure mechanisms are discussed.