Design fatigue curves of new concept were developed in the Subcommittee on Design Fatigue Curve in the Atomic Energy Research Committee in the Japan Welding Engineering Society (JWES). Also, Working Group on Design Fatigue Curves (WG DFC) in the JSME is studying the validity and the applicability of the design fatigue curves developed in the JWES to incorporate into the JSME Environmental Fatigue Evaluation Method.

The developed design fatigue curve consists of the best-fit curves using tensile strength as a parameter, correction of mean stress effect employed the Smith-Watson-Topper approach, surface finish effect and variable loading amplitude effect. This paper discusses the treatment of variable loading amplitude effect in the improved design fatigue curves.

The developed design fatigue curves have employed the method of variable loading amplitude effect in EN 13445. This method uses a fatigue life exponent of −0.1 from N = 2 × 106. Research Group on Fatigue Strength in the JSME performed a series of fatigue tests for variable loading amplitude and the test data showed the fatigue life exponent was −0.07. This outcome of the JSME Research Group can support the fatigue life exponent of −0.1 in EN 13445.

A survey of the procedure of variable loading effect has been performed on a thermal fluctuation transient at mixed zone of T-junction piping of nuclear power plants for which UF is significantly affected by variable loading effect. Based on this study, the design fatigue curve for variable loading effect that starts from 2 × 106 with the slope of 1/10, which comes from EN 13445-3, can be considered to be applicable to the developed design fatigue curves. Also, the difference of UF between up to 1 × 108 and extension with no limit is relatively small, and the maximum number of cycles of 1 × 108 can be employed for the engineering fatigue analysis.

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