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

Paper Number: 122032

122032 - Research on Effects of Surface Finishing and Mean Stress on Fatigue Lives of Notched Specimens (5) - Application of Elastic-Plastic Fe Analyses for Notched Specimens of Carbon Steel - 

The DFC3 and DFC4 subcommittees of the Japan Welding Engineering Society have investigated the effect of mean stress on fatigue properties of notched carbon steel and mean stress correction methods. In these studies, small specimens were adopted for fatigue tests, which were conducted by load control. It is well known that a complex stress triaxiality exists at the notch root due to plastic constraint. Therefore, it is very important to understand the strain or stress state at the notch root to properly evaluate the fatigue loading of notched specimens. However, the cross section of the notch root is too narrow to measure the strain directly. In addition, hardening or softening of the material and stress-strain relationship changes during repeated loading should be considered for correctly evaluating the stress-strain behavior of materials during fatigue tests.

In this study, an elasto-plastic analysis method was investigated for circular notched carbon steel specimens to determine the fatigue test conditions and evaluate the test results. The analyses were performed using cyclic stress-strain relationships derived from fully push-pull fatigue tests of smooth round bar specimens. The material was assumed to follow the Chaboche-type constitutive law, where the nonlinear transfer hardening law was taken into consideration. The code used for the analysis was Abaqus. The analytical model consist of eight-node axisymmetric elements was a two-dimensional axisymmetric model with half symmetry in the axial direction. Furthermore, based on the results of a preliminary studies, the notch root was set to a mesh size without affecting the analytical results.

To validate proposed methods by simple problems, fatigue tests with smooth round bar specimens were carried out and their elasto-plastic analysis was conducted using above mentioned analytical methods. Seven test cases which included fatigue tests with or without mean strain were subjected to the analysis. From these studies, it was confirmed that the validity of the analytical models and methods applicable to evaluation of fatigue tests of notched specimen.

The proposed analytical method was also applied to some fatigue test conditions of circular notched specimens. The equivalent strain amplitudes of the circular notched specimens calculated from the analysis were almost consistent with the S-N curves of smooth specimens. From these results, the validity of the analytical method for circular notched specimens was confirmed. Furthermore, the loading conditions for fatigue tests of circular notched specimens with mean stress were examined using this analytical method. The results indicated that fatigue tests with mean stresses of about from 5 MPa to 17 MPa and from 9 MPa to 17 MPa can be expected for the notched specimens with stress concentration factors Kt of 1.5 and 1.8, respectively.

Presenting Author: Motoki Nakane Hitachi-Ge Nuclear Energy, Ltd.

Presenting Author Biography: Motoki NAKANE received his Bachelor, Master and Dr. Eng. degrees in mechanical engineering from the Keio University, Japan, in 1992, 1994, and 2000, respectively.

He was a researcher at the Mechanical Engineering Research Laboratory and Material Research Laboratory of Hitachi, Ltd. from 2000 to 2006, and from 2006 to 2014, he was a senior researcher at the Material Research Laboratory, Hitachi, Ltd. In 2007, he was a visiting researcher at the Department of Mechanical and Aerospace Engineering, University of California San Diego. Since 2014, he has been senior technical staff engineer at Hitachi-GE Nuclear Energy, Ltd.
His current research interests include low cycle, giga cycle and multi axial fatigue, non-linear finite element analysis such as impact analysis and buckling analysis.

Authors:

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