Development of design margin decision system considering fatigue limit

The experimental results of fatigue at low stress levels around the fatigue limit have large scatter. For the design of structures and machines, it is important to evaluate the scatter and to decide the design margin. In this paper, the statistical method to decide the fatigue limit is proposed. In the proposed method, the fatigue limit is decided with considering the distribution of fatigue strength. Because each fatigue data is transformed into normalized fatigue strength, this method has the advantage point that the distribution of fatigue strength and the fatigue limit can be found by relatively small samples. Using the proposed method, the computer system to decide the design margin for fatigue is developed. In this system, the S-N curve with fatigue limit and the distribution of fatigue strength are obtained at first. Then the design margin with a given probability of failure and a confidence level is calculated. The developed system is applied to some experimental results for fatigue test. As a result, the S-N curve with the design margin has obtained easily. It is recognized that the obtained S-N curve is decided after consideration of scatter.