Study on Variations of Spatial Distributions of Cracks During Fatigue Process of an Unnotched Carbon Steel Specimen

In order to investigate variations of spatial distributions of cracks observed on the unnotched specimen surface during the fatigue process, plane bending fatigue tests were performed on carbon steel JIS, SS 41 in laboratory air and a sodium chloride aqueous solution. Two methods, developed by Morishita and Ono in the field of mathematical ecology, were used in analyzing the spatial distributions of Cracks. The following results were obtained. The spatial distributions of cracks are influenced by both stress amplitude and stress cycling. In the high stress amplitude region, the distributions are nearly random, while in the low stress amplitude region, the distributions have clumps indicating nonrandom distribution. This result was common to both fatigue in air and corrosion fatigue. With increasing stress cycling, the spatial distributions of cracks approach random distributions in the high stress amplitude region, while in the low stress amplitude region, the distributions show a strong tendency to have two kinds of clumps whose sizes are 1/4 mm2 and 8 mm2.