Effect of strain wave shape on low-cycle fatigue crack propagation of SUS 304 stainless steel at elevated temperatures
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I.
INTRODUCTION
COMPONENTS of machine and structure operating at elevated temperatures are subjected to thermal stress cycling resulting from start-up, shut-down, and load change.~'2"3 In this case, various thermal stress wave shapes are repeated by various operating procedures, for example, cold start-up, hot start-up, weekend shut-down, overnight shut-down, and emergency shut-down. To cope with this problem, many studies on the effect of strain wave shape on fatigue lives at elevated temperatures have been performed. 4's'6 As a result, it has been emphasized that it is very important to investigate the fatigue crack propagation behavior for design and assessment of high temperature power plants. 7-~3 Recently it has become apparent that the rate of cycledependent and time-dependent fatigue crack propagation can be correlated with the range of cyclic J-integral, AJi and that of creep J-integral, AJc, respectively. 9-~ However, most of these tests were carried out under the condition of load-control. Therefore, in order to verify the generality of the above correlations, it is necessary to assure that these parameters can be applied under various loading conditions, for example, under strain-control. In this work, the effect of strain wave shape on straincontrolled low-cycle fatigue crack propagation of SUS 304 stainless steel was investigated at 600 and 700 ~ The rate of crack propagation was correlated with the range of cyclic J-integral, creep J-integral, and total J-integral. Based on the results thus obtained, a creep-fatigue interaction damage rule4 proposed semi-empirically was interpreted with regard to crack propagation. Furthermore, fatigue crack initiation mechanism in slow-fast strain wave shape was studied.
MASAKAZU OKAZAKI, Research Assistant, ICHIRO HATTOR1, Professor, and FUJIO SHIRAIWA, Graduate Student, are all with the Department of Mechanical Engineering, Technological University of Nagaoka, 1603-1, Tomioka, Nagaoka, Japan. TAKASH! KOIZUMI is Professor, Department of Mechanical Engineering, Tokyo Institute of Technology, 2-12-I, Ouokayama, Meguro-ku, Tokyo, Japan. Manuscript submitted October 25, 1982.
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II.
EXPERIMENTAL PROCEDURE
The material tested is SUS 304 stainless steel. Its chemical composition, heat treatment, and mechanical properties at elevated temperatures are given in Table I and Table II, respectively. To obtain a uniform temperature distribution in the radial direction, a thin-walled cylindrical specimen shown in Figure 1, whose outside diameter was 13 mm and inside one was 10 mm, was used. To investigate the crack propagation behavior, a notch was introduced by drilling at the center of the specimen gauge section. The length of the crack propagating from this notch and the range of crack opening displacement at the center of the crack were measured by means of a traveling microscope. Strain-controlled low-cycle fatigue tests were carried out in air by means of a servo-electro hydraulic fatigue testing machine. Axial strain was measured and control
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