Development of Ultra-High Temperature-use Extensometer and Prediction for Creep- and Fatigue-Life of Ceramics at High Temperatures

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The ultra-high temperature-use extensometer was newly developed. Then this was equipped with the high temperature push-pull fatigue test system. The cyclic axial loaddisplacement response was measured in the smooth specimen of silicon-nitride ceramics under push-pull loading at load ratio of -1.0 at 1200℃ and 1300℃. The great amount of inelastic strain was detected, which was greatly dependent on stress rate. Such an inelastic strain is easier to generate under tensile loading than under compressive one. The stress-inelastic strain hysteresis loop was characterized by the fatter shape on the tensile stress side than on the compressive stress side, since much larger inelastic strain occurs during loading and unloading excursions on former than on the latter.The relationship between maximum stress and equivalent life time which was calculated through SCG (Slow Crack Growth) theory was obtained from push-pull fatigue tests at 1200℃. The equivalent life time of fatigue tests were larger than life time obtained from static fatigue tests at same maximum stress level in low mean stress and high stress amplitude region. This result showed that silicon nitride ceramics was strengthened by the cyclic loadings in these test conditions.