Fatigue Crack Growth at Elevated Temperature in Ferritic Steels

Abstract

At elevated temperatures in air, both creep as well as oxidation have the potential for influencing the fatigue crack growth process. Both the mechanism of crack growth as well as the crack closure level can be affected by these variables. In order to evaluate the importance of creep and the environment, fatigue crack growth data and the associated crack closure levels have been obtained at room temperature and at 538°C in air and in vacuum, for a series of ferritic steels. It was found in vacuum over the range of frequencies of 03–30 Hz that the rate of crack growth was independent of frequency, an indication that the basic mechanical process of fatigue crack growth is athermal in nature for the test conditions employed. At 538°C in air however the rate of crack growth is frequency dependent, a dependency which relates to the thermally activated process of oxidation. These results as well as a discussion of the modelling process for the rate of fatigue crack growth will be presented. The importance of the chromium content on the fatigue crack growth behavior in air at 538°C will also be discussed.