Crack propagation of steels during low cycle thermal-mechanical and isothermal fatigue at elevated temperatures
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I.
INTRODUCTION
COMPONENTS of machines and structures operating at elevated temperatures are subjected to thermal stress cycling resulting from start-up, shut-down, and load change. ~2"3 To cope with this problem, many studies on low cycle isothermal or thermal-mechanical fatigue life at elevated temperatures have been performed. 4'5'6 As a result, it has been found to be necessary to investigate the fatigue crack propagation behavior for the design and assessment of high temperature power plants. 7s Recently it has been reported that it is effective to correlate the rate of cycle-dependent fatigue crack propagation with the range of cyclic J-integral, mJf. 9-16 For example, Dowling has investigated the fatigue crack growth of A533B steel and has shown that there is a linear relationship between the rate of crack propagation and AJ/ under the conditions of linear elastic, elastic-plastic, and general yielding.9'~~However, most of the tests have been carried out on one material or crack configuration. To prove the effectiveness of AJ/, it is necessary to investigate several materials, crack configurations, and test conditions. In this work the surface and through crack propagation of 12Cr-Mo-V-W steel, cast low-alloy ferritic steel, and Type SUS 304 stainless steel, which have been used for the components of high temperature power plants, has been studied in thermal-mechanical and isothermal fatigue at elevated temperatures. The rate of crack propagation obtained is then correlated with the range of cyclic d-integral. Furthermore, fatigue life is predicted by integrating the equation of crack propagation thus obtained, and it is compared with the experimental results.
sizes, and mechanical properties at elevated temperatures are given in Table 1 and Table II, respectively. To obtain a uniform temperature distribution in the radial direction, the thin-walled cylindrical specimen shown in Figure 1, whose outside diameter was 13 mm and inside one was 10 mm, was used. At the center of the specimen gauge section, a through notch (Figure l(b)) or a surface notch (Figure l(c)) was artificially introduced by punch or drilling. The length of the crack propagating from this notch was 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, which can control axial strain and temperature independently. The strain wave form was a symmetrical triangular one of push-pull type and it was added cyclically to the specimen axial direction. Axial strain was measured and controlled by a strain detector attached to the specimen surface. The detector consisted of a linear differential transformer and springs supporting two quartz rods at intervals of 10 mm on the specimen ridge. In thermal-mechanical fatigue tests, the strain range was compensated for a thermal free expansion strain range, A eCe, by subtracting it from an apparent strain range, Aeop (Figure 2). The specimen was heated by a high frequency induction heatin
