Comparative Study of Multiple Small Cracks in Some Steels and Fatigue Life Prediction through Maximum Surface Crack Length Distributions

Abstract

This paper investigates the statistical distributions of surface length of multiple small cracks and their maximum length appearing in rotating bending fatigue of three different types of unnotched steels in air i.e., S45C steel, a degraded stainless steel with 10wt% σ phase in it, and a nondegraded stainless steel. The characteristics of these distributions were compared for these three types of steels, and the maximum surface length distributions were analyzed on the basis of the statistics of extremes in order to examine the possibility of the fatigue life prediction. The main results of the present investigation were as follows:
(1) A great number of multiple small surface cracks were initiated in all three types of steels; i.e., approximately 400 cracks per cm² in S45C steel, while over 1000 cracks per cm² in the stainless steels.
(2) Not only surface length but maximum surface length in the three types of steels followed the three-parametric Weibull distribution. In every case, the logarithms of the parameters varied linearly with load cycle ratio, N/N_f, and the variation of the maximum surface length distribution was greater than that of the surface length distribution.
(3) Fatigue lives were predicted by the use of the return periods obtained from the maximum surface length distribution. They agreed well with the experimental results for all three types of steels.