Inelastic deformation and its related life under cyclic/creep loadings in Si 3N 4-monolithic and Si 3N 4/SiCw-composite ceramics at elevated temperature

Abstract

Push–pull low cycle fatigue and creep tests were performed in monolithic-Si 3N 4 and composite-Si 3N 4/SiCw materials at 1573 K. Then cyclic stress–strain response and creep strain were measured with a newly developed extensometer. The inelastic strain, which is greatly dependent upon stress/strain rate, was detected in both the materials. It was found from the shape of the cyclic stress–strain hysteresis loop that inelastic strain was easier to generate under tensile than under compressive loading; the width of the hysteresis loop is larger on the tensile stress side than on the compressive stress side. Such a characteristic cyclic stress–strain response is much more enhanced in the composite-Si 3N 4/SiCw than in the monolithic-Si 3N 4. The low cycle fatigue life plotted against inelastic strain range is much longer in the former than in the latter, although the difference is reduced with decreasing the controlled strain range. Moreover, greater creep resistance is attained in the former than in the latter at the lower stress level, while the opposite is the case at the higher stress level.