Session: DA-03-02 Fatigue 2-Research on Effects of Surface Finishing and Mean Stress on Fatigue Lives of Notched Specimens

Paper Number: 122454

122454 - Research on Effects of Surface Finishing and Mean Stress on Fatigue Lives of Notched Specimens - Mean Stress Effect for Notched Specimens of Low Alloy Steel - 

DFC and DFC2 subcommittees in the Japan Welding Engineering Society developed a new approach of fatigue analysis method. This fatigue analysis method has been made several modifications and incorporated into the JSME Environmental Fatigue Evaluation Method 2022 Edition (JSME S NF1-2022). Research on effects of surface finishing and mean stress on fatigue lives of notched specimens have been followed by DFC3 and DFC4 subcommittees. In this paper, overview of the outcomes of DFC3 and DFC4 subcommittees is introduced.in this project, fatigue strength reduction factor for mean stress correction using the Smith-Watson-Topper (SWT) approach. 
In order to evaluate the effect of stress concentration due to notching and mean stress act simultaneously, mean stress on fatigue life was discussed on low alloy steel of SQV2A (equivalent to SA-533 Gr. B Cl.1) by comparing the smooth and notched 8 mm bar specimens. The shape of notched is different stress concentration factors Kt=1.5 and 1.8. The difference in specimen displacement is measured with an extensometer, and a stoppage test is carried out. However, the crack depth at the finish of the fatigue test is not constant because it depends on the test conditions such as stress amplitude and extensometer position, and the definition of fatigue life differs from that of the strain control test. Therefore, the fatigue life is defined as the estimated number of cycles with a crack length of 2 mm, which was considered in another study.
In addition, elasto-plastic FEA simulating a notched specimen was carried out in another research to confirm the behavior of the notch bottom when the stress concentration due to the notch and the mean stress act simultaneously, and the applicability of the SWT approach was confirmed using FEA results.
The regression curve of the notched specimen was below that of the smooth specimen, and the effect of the notch was confirmed. Furthermore, the nominal stress amplitude Sa of the regression curve at Kt=1.8 was smaller than Kt=1.5, and the higher Kt, the shorter the fatigue life.
In the fatigue test of the notched specimen, it was confirmed that the effect of the presence or absence 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 examined as SWT approach. The Cases 1 to 3 is evaluated by setting the stress and strain values of the notch bottom obtained by FEA, and the case 4 is simply evaluated by multiplying the nominal stress by the stress concentration factor. In the Cases 1 to 3 based on elasto-plastic analysis, the overall variation was smaller for Kt=1.5 than for Kt=1.8. In the Case 4, although simple, was excessively evaluated by the maintenance side. From these results, it was confirmed that the evaluation of the maintenance side was possible by the SWT approach.
 

Presenting Author: Yuichiro Nomura Mitsubishi Heavy Industries, Ltd

Presenting Author Biography: I belong to the strength research institute of Mitsubishi Heavy Industries. In the past, it has mainly carried out research on the intensity relation of the atomic energy relation.
Specifically, environmental fatigue evaluation in the PWR primary system environment, monitoring tests to determine irradiation embrittlement of reactor pressure vessels, and SCC by cold working of stainless steel piping are the main research topics. In recent years, the strength evaluation in hydrogen environment has also been carried out.

Authors:

Yuichiro Nomura Mitsubishi Heavy Industries, Ltd
Daiki Takagoshi Mitsubishi Heavy Industries, Ltd
Seiji Asada Mitsubishi Heavy Industries, Ltd
Atsushi Sugeta Hiroshima University
Yoshihide Kitamura The Kansai Electric Power Co