CYCLIC FATIGUE OF HOT ISOSTATICALLY PRESSED SILICON-NITRIDE AT ELEVATED-TEMPERATURES

Cyclic fatigue properties of a hot isostatically pressed silicon nitri de were investigated at 1150, 1260 and 1370-degrees-C in ambient air. The uniaxial tensile tests were conducted under various cyclic loading wave forms and frequencies. The correlation of stress-life relations between cyclic and static fatigue results was evaluated. At 1150-1370- degrees-C, cyclic loading caused less damage than static loading, as e videnced by the longer failure time under cyclic loading versus static loading with the same maximum applied stresses. The cyclic loading ef fect was more pronounced in high frequency tests at 1260 and 1 370-deg rees-C and might be related to the viscoelastic behaviour of the inter granular phase. Microstructural analyses and macroscopic cyclic stress -strain and strain-time relations indicated that cyclic loading/unload ing may inhibit the normal accumulation of creep damage.