Effects of Strain Holding and Continuously Changing Strain Rate on Fatigue Life Reduction of Structural Materials in Simulated LWR Water

Abstract

The fatigue life reduces remarkably with reduction in strain rate in simulated light water reactor (LWR) water but the effects of strain wave form on this reduction are still not clear. This paper provides fatigue life data obtained from stepwise strain rate change tests, sine wave tests and strain holding tests. The effects of varying strain rate on fatigue life reduction can be estimated very well by the modified rate approach (MRA) method in the case of the step wise strain rate changing as shown in authors’ previous papers [1, 2, 3, 4, 5]. In the case of sine wave, however, the fatigue life reduction is much less compared to that predicted by the MRA method. The mechanism of such difference is not clear and the quantitative assessment of the fatigue life reduction caused by irregular strain wave form in actual transient seems impossible. The current MRA method gives always conservative assessment for sine wave straining and thus it is judged that this method need not be revised any more. The fatigue life reduction caused by strain holding at the peak of straining cycle in simulated BWR water had been reported in the previous paper [6]. In actual thermal transients, however, strain is not usually held at the peak of straining cycle but at the point somewhat reduced from the peak after the stabilization of temperature. In considering this phenomenon, additional fatigue tests in which the strain was held at the point somewhat reduced from the peak were carried out. In such conditions, the fatigue life reduction caused by strain holding disappeared. The similar fatigue tests with peak strain holding were also carried out in simulated PWR water and no fatigue life reduction can be observed. Considering the effects of strain holding on fatigue, the model for evaluating fatigue life reduction in LWR water was revised.