TENSILE DYNAMIC AND STATIC FATIGUE RELATIONS FOR A HIPED SILICON-NITRIDE AT ELEVATED-TEMPERATURES

Dynamic and static fatigue behavior of a hot-isostatically pressed (HI Ped) silicon nitride was investigated at 1150, 1260 and 1370-degrees-C . Uniaxial tensile tests were conducted over ranges of constant stress es and constant stress rates. Correlation of stress-life relations bet ween static and dynamic fatigue results was evaluated and failure mode s were determined as functions of temperature, stress and stress rate. At 1150-degrees-C the static and dynamic fatigue failures were contro lled by a slow crack growth mechanism for all stresses and stress rate s. Creep rupture was the dominant failure mechanism in static loading at 1260 and 1370-degrees-C. A transition in the dominant failure mecha nism in dynamic fatigue at 1260 and 1370-degrees-C occurred at a stres s rate of 10(-2) MPals. Slow crack growth was the dominant failure mec hanism with stress rates >10(-2) MPa/s while creep rupture was the gov erning mechanism for the failure at stress rates less-than-or-equal-to 10(-2) MPa/s.