Abstract
In order to clarify the fatigue damage of smooth specimens under complex loading, it is important to investigate the effect of stress change through the behavior of a small crack because the fatigue life of specimens is occupied mainly by the propagation life of a small crack. However, such studies made in heat-treated carbon steels are few. In this research, fatigue tests on heat-treated carbon steel smooth specimens under two-step loading were carried out, and the effect of stress change on the behavior of a small crack was investigated. Results show that the value of the cumulative cycle ratio Σ(n/Nf) is smaller than unity when the first stress levelσ1 is higher than the second oneσ2; however, Σ(n/Nf) is about unity when σ1 is lower than σ2. This difference in the value of the cumulative cycle ratio is explained by the behavior of a small crack after stress change. Moreover, by using the results of annealed and prestrained steels, the effect of material properties on the fatigue damage under two-step loading is also examined.
