A model for creep-fatigue interaction in terms of crack-tip stress relaxation
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I. INTRODUCTION
CREEP-FATIGUE interaction is an important subject that often must be addressed in the design of elevated temperature applications. It has been generally accepted that the reduction of the fatigue life under the creep-fatigue loading condition is attributed to the interaction between the fatigue damage growing from the material surface and the creep damage resulting from the cavity nucleation and growth at the grain boundaries.[1–6] On the other hand, in some works[7–10] under the creepfatigue loading condition, it has been reported that, without any cavitational damage observed, the fatigue lives were significantly reduced or crack growth rates were significantly increased compared with the results of the continuous low-cycle fatigue (LCF). According to the experimental results of James[7] on type 304 stainless steel at 810 K for saw-tooth and square waves involving the same initial load rise time, the dwell period increased the crack growth rate by a factor of 2, though the fracture surface exhibited transgranular mode. Tomkins and Wareing[8] considered it to be attributed to the stress relaxation effect during a tensile dwell period following a high rate loading, suggesting that the stress relaxation in the plastic zone effectively reduced the flow stress and induced easier local shearing deformation to increase the crack growth rate. Also, he reported a similar effect with decreasing strain rate for type 316 stainless steel at the elevated temperature of 900 K.[8] Okazaki et al.[9] have reported interesting results from the wave shape transition test for type 304 stainless steel. When one slow-fast strain cycle was introduced to the fast-fast cycling, the transgranular fracture surface on which the striation spacing was abruptly enlarged was formed at the low strain range. Wang et al.[10] have carried out a detailed study for the YONG JUN OH, Senior Researcher, and JUN HWA HONG, Director, are with the Nuclear Materials Technology Division, Korea Atomic Energy Research Institute, Taejon 305-600, Korea. SOO WOO NAM, Professor, is with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, Korea. Manuscript submitted April 22, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
creep-fatigue damage in 1Cr-1Mo-0.25V steel at 823 K. The cycling with 10 minutes hold in tension caused a significant decrease in the number of cycles to failure, and the penetration mode of cracks from the free surface into the interior was always the transgranular type, which was independent of cavity. They concluded that the reduction in fatigue life was attributed to the combination of two factors. One was a creep effect at the crack tip during hold time, which gave rise to more blunting of the crack and induced faster crack propagation. The other was an environmental effect, in spite of the fact that quite pure argon, which maintained the partial pressure of the air at about 10⫺5 torr, was used in the test chamber. Also, Ryu and Nam[11,12] have reported a
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