The effect of one slow-fast strain cycle on the fatigue crack growth behavior of SUS 304 stainless steel at elevated tem
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I.
INTRODUCTION
COMPONENTS of machine and structure operating at elevated temperatures are subjected to thermal stress cycling which results from start-up, shut-down, and load change. L2,3 Various thermal stress wave shapes occur due to various operating procedures, such as cold start-up, hot start-up, weekend shut-down, overnight shut-down, and emergency shut-down. To cope with this problem of predicting fatigue life of these components, many studies on the effect of strain wave shape on fatigue lives at elevated temperatures have been performed. 4'5'6 It is now established that the fatigue lives under slow-fast and slow-slow strain wave shapes are shorter than the ones under fast-fast and fast-slow strain wave shapes. Several investigators have also studied the crack growth behavior at elevated temperatures. 7-~3 Especially, several fracture mechanics studies for characterizing the crack growth rate behavior by a J-integral have been carried out after Dowling ~4 reported a good correlation between the crack growth rate and the cyclic J-integral. However, in an actual plant, it is rare that the same shape of strain wave (for example, fast-fast, fast-slow, slow-fast, and slow-slow) is repeated or the various shapes of strain wave are intricately repeated in a fixed sequence. Therefore, it is important to investigate the effect of strain wave shape variation on the fatigue crack growth behavior at elevated temperatures, However, few investigators have studied such an effect. ~5 In this work, as the first step to investigate the effect of variation in strain wave shape on crack growth behavior at elevated temperatures, the effect of introduction of one slow-fast strain cycle on fatigue crack growth rate in fastfast strain cycles was investigated.
MASAKAZU OKAZAKI, Research Assistant, and ICHROH HATTORI, Professor, are with the Department of Mechanical Engineering, Technological University of Nagaoka, 1603-1, Tomioka, Nagaoka, Japan. TAKASHI KOIZUMI ~s a Professor with the Department of Mechanical Engineering, Tokyo Institute of Technology, Tokyo, Japan. Manuscript submitted July 14, 1983. METALLURGICALTRANSACTIONSA
II.
EXPERIMENTAL PROCEDURE
The material tested is SUS 304 stainless steel which has the following chemical composition in weight percent: 0.06C, 0.62Si, 1.19Mn, 0.03P, 8.39Ni, 18.17Cr, 0.11Mo, 0.09Cu, 0.05N, and bal. Fe. The material was solutionannealed at 1050 ~ for 1 hour and water cooled. Its mechanical properties are given in Table I. 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 growth, a notch was introduced at the center of the specimen gauge section by means of an electric spark machine. The length of the crack propagating from this notch was measured by means of a traveling microscope whose magnification was 30• Strain-controlled low-cycle fatigue tests were carried out in air by means of a servo-electro hydraulic fatigue
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