Striation’s width is useful to estimate stress intensity factor range. To estimate maximum and minimum stress intensity factor, the relationship between stress ratio and aspect ratio of striation obtained by Furukawa, is used. However, accuracy of that method depends on the observer’s skill. In this paper, the parameter (P.S.sum), which relates to the striation height, is proposed and method for estimation of stress ratio, which is independent of observer’s skill, is also proposed. In order to calculate P.S.sum, the frequency analysis is applied to the three-dimensional geometry of striation obtained by laser microscope. The wavelength and the power spectrum are considered to indicate the striation width and height respectively. To consider the variation of striation, P.S.sum is calculated by the integration of power spectrum. The method is applied to the fatigue fracture surface of aluminum (A2024), steel (0.25% carbon) and high-tension steels. The condition of fatigue test was ΔK constant to keep the crack growth rate constant, in other words, to make the striations of which width were same. Applying the method, almost linear relationship between stress ratio and P.S.sum was obtained. Therefore, stress ratio can be quantitatively estimated by P.S.sum.

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