Effect of Stress Pattern on Fatigue Strength of Sintered Silicon Nitride by

It is difficult to conduct cyclic tensile fatigue tests for fine ceramics because of its high brittleness, and a little eccentricity of loading axis and gripping device gives inaccurate strength data. So, a major part of cyclic fatigue tests have been carried out under bending load conditions, for the allowance of misalignment of loading axis is generous to some extent in the case of bending fatigue. As is well known, fatigue strength together with other fracture strength is critically affected by pre-existing defects, and consequently, the distribution of fatigue lives is very large even at high stress levels depending on the distribution characteristics of the defects. To evaluate precisely the correlation between the distribution features of the defects and lives, it is preferable to discuss the contribution of the defects and their distribution to the fatigue strength on the basis of the fatigue strength data under tensile load condition to exclude the influence of stress gradient in the specimen. In this paper, the results of static and cyclic fatigue tests under tensile and 3-point bending load conditions are discussed, where self-aligning rings for loading axis developed by the authors are equipped to the gripping devices in the case of tensile fatigue tests. The fatigue data obtained in this study are evaluated with the initial stress intensity factor for pre-existing defects at the fracture origination site, and the effect of stress pattern on fatigue strength is discussed from several points of view.