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The relationship between the S-N^^~ curve, where N^^~ is the median fatigue life, and empirical fatigue life distributions obtained from a fairly large number of specimens has been analyzed using two groups of data. i. e., five data for steels and seven data for aluminum alloys collected from published papers and books. The distribution shape of fatigue life is found to be related to the shape of the S-N^^~ curve in the neighborhood of the test stress level. To compare the quantitative relationship between the scatter of fatigue life and the slope of the S-N^^~ curve in various test data, the test stress levels were standardized by the median fatigue strength at 10^5 cycles in each data, and the standardized S-N^^~ curves, where "standardized" means the use of such standardized stress levels, were drawn. The standard deviation of log-life in either steels or aluminum alloys has an obvious linear relation on double logarithmic graph paper with the slope of the standardized S-N^^~ curve at the test stress level. The distribution of the median fatigue strength corresponding to each fatigue life obtained at one stress level is normal in shape and its standard deviation is approximately constant regardless of test stress levels in each data. On the basis of these results, a method for estimating fatigue life distribution was developed when available information is limited either to a simple S-N^^~ curve only or to the original fatigue life data for the construction of such an S-N^^~ curve.
