Damage evaluation method for 316FR stainless steel based on micro-crack growth behavior has been developed. The effect of grain size on crack growth was investigated on smooth specimens considering the actual component material. Micro-crack growth was observed in the early stage of life in the coarse grain material even in the lower strain ranges. In the fine grain mateirla. however, crack growth started in the later stage of life in the lower strain ranges. As a typical component of Fast-Breeder Reactor plants, thermal fatigue tests for Y-piece models were also conducted for total evaluation of structural integrity assessment methods. The relationship between damage evaluation based on micro-crack length and inelastic analysis was discussed. regarding macroscopic cracking, crack growth behavior from an artificial notch was also observed and it was confirmed that crack growth evaluation using a simplified J-integral analysis method gives a reasonable and conservative estimation.
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