The size effect on fatigue strength of structural steel materials in high-humidity environment

Abstract

Fatigue tests under rotating bending fatigue testing machine has been carried out to estimate the size effect on fatigue strength of structural steel materials in high-humidity environment. The size and geometry of the specimens were in accordance with specifications of ASTM E-466. The specimen contains a shallow notch to observe crack initiation. The specimen surface and shallow notch were ground with grade 1200 emery paper and then were buff finished. The tests were carried out a variation of diameter size specimens from 8 mm, 2 mm, and 1 mm in humidity environment with 60 pct Relative Humidity, 70 pct Relative Humidity, 80 pct Relative Humidity, and 90 pct Relative Humidity. The results show that the fatigue strength increases with decreasing the specimen size to diameter of 2 mm and 1 mm of SS400 and S45C steels in high-humidity environment. In ion-exchanged water remarkably reduces the fatigue strength of these steels. The initiation and propagation behaviors of micro cracks were investigated on the specimen surface. Cracks were observed by a scanning electron microscopy (SEM).