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In high temperature waters that contain dissolved oxygen (DO) to certain content, the fatigue life of carbon steel is strongly affected by strain rate. A formula has been advanced to quantify this effect when the strain rate is held constant. However, the strain rate changes continuously in most of transients of actual plant operation. There is no way currently to assess the effects of strain rate when the strain rate is varied as in the actual plant transients. To find a solution to this problem, a series of strain controlled fatigue tests have been conducted with the strain rate changed stepwise or continuously. It is shown that a method, in which the product of the environmental effect and the strain increment within a unit time interval in a transient period is integrated from the minimum strain to the maximum, evaluates the environmental effect with satisfactorily high accuracy. This method is called the modified rate approach method. It is shown also that the procedure of taking the strain rate as averaged over the minimum to peak of the strain change as giving rise to more conservative evaluations than the ones the modified rate approach method produces.
fatigue life, strain vs. life (S-N) curve, environmental fatigue, light water reactor environment, strain rate dependency factor, carbon steel, nuclear power plant, fatigue degradation
Manager, Ishikawajima-Harima Heavy Industrie Co., Ltd., Yokohama,
Doctor, Professor Emeritus, University of Tokyo, Japan Power Engineering and Inspection Corporation, Tokyo,
Doctor, Professor, University of Tokyo, Tokyo,