High-cycle fatigue properties of structural materials for FBR (2). High-cycle fatigue tests of 316FR steel at elevated temperature. Interim report

In liquid metal cooled Fast Reactors (LMFBRs), thermal fluctuation due to fluid mixing induces high-cycle fatigue damages on structural materials. Such a cyclic thermal stress is estimated to repeat approximately from 108 to 109 cycles during a plant service period. Therefore, it is essential to investigate the fatigue property in a very-high cycle region. In this study, high-cycle fatigue tests at 550degC, 600degC, and 650degC under strain controlled conditions were carried out for the 316FR steel. The following results are obtained until now. (1) Adopting experimental techniques using laser level meter, high-cycle fatigue tests were performed at elevated temperature in the repetition frequency of 10-60 Hz, and the high cycle fatigue fracture data up to 107 cycle regions were obtained. (2) The number of cycles to failure observed in the high-cycle fatigue tests up to 107 cycle regions was larger than that calculated from simple extrapolation of average fatigue failure formula given in the material strength standard (DDS) for a design. Moreover, these lives were also conservative in comparison with those estimated from the high-cycle fatigue failure formula given by KOM-MSS WG. (3) The influence of frequency on fatigue life was not almost significant in the range from 0.1 to 60Hz. In contrast, the influence of temperature on fatigue life was observed. (4) The fisheye fracture was observed in the high-cycle fatigue test specimens failure in 107 cycle regions. (author)