Session: DA-03-01 Fatigue 1-Research on Effects of Surface Finishing and Mean Stress on Fatigue Lives of Notched Specimens

Paper Number: 121365

121365 - Research on Effects of Surface Finishing and Mean Stress on Fatigue Lives of Notched Specimens - Surface Finishing Effect for Notched Specimens of Carbon Steel - 

The DFC and DFC2 subcommittees of the Japan Welding Engineering Society (JWES) have developed a new approach to fatigue analysis. A part of approach has been revised and incorporated into the JSME Environmental Fatigue Evaluation Method for Nuclear Power Plants 2022 Edition (JSME S NF1-2022), which also includes a new approach to surface finish effect evaluation.

The succeeding subcommittees, DFC3 and DFC4, mainly studied the effect of surface finish on fatigue strength and the mean stress correction methods for notched materials. This paper reports a series of studies on the effect of surface finish on fatigue strength of carbon steel JIS STPT370 (equivalent to ASME SA-106 Gr. A).

First, smooth round bar specimens were prepared by polishing the parallel portions and by machining them to the three kinds of target surface roughness, Rz = 6.3, 25 and 50 μm, respectively. Strain-controlled fatigue tests were conducted on these specimens, and fatigue strength was determined from 10⁴ to 10⁶ cycles. The fatigue strength of Rz = 6.3 μm was almost equal to that of the polished material. However, the fatigue strength decreased with increasing surface roughness. Therefore, the fatigue strength ratio of the polished material to that of the machined material was calculated as a factor for the effect of surface finish, Ksf. The maximum value of Ksf was about 1.15, which became larger in the high cycle region.

Since the fatigue strength in an actual component is concerned at structural discontinuities, the next fatigue tests were conducted under a load control on round notched specimens with the notch root polished and machined to Rz = 25μm. Ksf was calculated by the same method as for smooth specimens, and the effect of surface finish was examined. The Ksf for shortest life, 4.4 × 10⁴ cycles, was about 1.42. In the higher cycle region, Ksf decreased gradually as the life increased. In other words, the life dependence of Ksf for the notched specimen was opposite to that for the smooth specimen.

The observation near the fracture area showed that multiple cracks were initiated in the same machined groove at the notch root in the short-life notched specimen, and it was confirmed that the cracks coalesced quickly and grew to a circumferential crack. As a result, the fatigue strength was significantly reduced, and the machined notched specimens were estimated to have higher Ksf at the low cycle region.

In the large actual components, however, the phenomenon of cracks aligning into a single groove hardly occurs, because the machining marks spread widely at the structural discontinuities. Therefore, it is unlikely in large actual components that a circumferential crack propagates as in the case of the small specimen. From the above discussion, it was confirmed that the test result with high Ksf is a phenomenon specific to the specimen and that, except for this one data, the surface finish effect can be expressed by the Ksf by JSME, even for structural discontinuities.

Presenting Author: Masahiro Takanashi IHI Corporation

Presenting Author Biography: Work for IHI for 30 years as a research engineer.
Main research fields are metal fatigue, SCC, reliability design and failure analysis

Authors:

Masahiro Takanashi IHI Corporation
Yun Wang Hitachi, Ltd.
Akihiko Hirano Hitachi-GE Nuclear Energy, Ltd
Yoshihide Kitamura The Kansai Electric Power Co., Inc.