Abstract
The DFC and DFC2 subcommittees in the Japan Welding Engineering Society developed a new fatigue analysis method with support from the Japanese utilities. This fatigue analysis method had several modifications made and has been incorporated into the JSME Environmental Fatigue Evaluation Method 2022 Edition (JSME S NF1). Also, research on effects of surface finishing and mean stress on fatigue lives of notched specimens have been conducted by DFC3 and DFC4. In this paper, the test plan and overview of the outcomes of the DFC3 and DFC4 are introduced.
In this series of research, effects of surface finishing on fatigue strength and relation between a fatigue strength reduction factor and mean stress correction have been investigated using notched specimens, of which materials are carbon steel (CS:JIS STPT370) and low alloy steel (LAS:JIS SQV2A). The applicability of the Ksf (factor for the effect of surface finishing) equation of the JSME S NF1 to notched specimens has been investigated and confirmed based on the experimental results. The results of the tests for the effects of surface are presented in PVP2024(Ref.[11]).
The order of consideration of the fatigue strength reduction factor and mean stress correction may not need to be considered if the Smith-Watson-Topper (SWT) approach can be applied to mean stress correction. Hence, the applicability of the SWT approach with stress concentration and mean stress have been investigated and confirmed based on the experimental results.
Finally, we evaluated the simultaneous effects of stress concentration due to notching and mean stress on fatigue life was discussed on LAS by comparing the smooth and notched specimens. The shape of the notched specimen affects stress concentration factors (Kt). Additionally, elastic-plastic finite element analysis (FEA) was conducted to confirm the behavior of the notch bottom when stress concentration due to the notch and mean stress act simultaneously. The applicability of the SWT approach was also confirmed using FEA results.
In the fatigue test of the notched specimen, it was confirmed that the effect of the with or without of the mean stress on the fatigue life was small, which was consistent with the results of the analysis. In this evaluation, four methods were evaluated as SWT approach. The Cases 1 to 3 is evaluated by setting the stress and strain values of the notched bottom obtained by FEA, and the Case 4 is simply evaluated by multiplying the nominal stress by the stress concentration factor.
It is confirmed that the SWT approach is on the conservative for design base. Therefore, the application of the SWT approach to the design is judged to be appropriate.