Fatigue Crack Propagation Behaviors of 18-8 Austenitic Steel under Double Repeated Stresses

Abstract

Rotating bending fatigue tests under constant stress and two stress levels were carried out on notched 18-8 austenitic steel at room and elevated temperatures, and fatigue crack propagation behaviors during the tests were investigated. The cumulative cycle ratios were discussed from the experimental results.The temperature dependence of fatigue fracture mechanism was studied by means of electron fractographic observations.The main results obtained are as follows:(1) On High-Low tests at room temperature the delay time in continued cracking was observed after stress change, while at 600°C this phenomenon was not observed.(2) Cumulative cycle ratios were almost unaffected by stress sequence. The fatigue life under double repeated stresses should be discussed not only from the delay time, but also from the differences in crack growth behaviors at low and high stress levels.(3) The regular striations peculiar to fatigue fracture were detected on the fracture surface tested at elevated temperature, while at room temperature these were not observed. This fact might be explained by the presence of the strain induced martensite detected at room temperature test.