Studies on crack growth rate under high temperature creep, fatigue and creep-fatigue interaction—I: On the experimental studies on crack growth rate as affected by [formula omitted] and temperature

Abstract

Many experimental studies have been reported on the measurements of crack growth rate and the observation of crack growth behaviour under high temperature creep, fatigue and creep-fatigue interaction in literatures. However, many of them have been done in air atmosphere. Furthermore, in many of them the measurements of the crack growth rate have been carried out by interrupting intermittently the running of the testing machine. In such experiments the complex effects due to the atmosphere, the interruption period and the corresponding unloading operation for the crack length measurement might have been involved. In the present paper in order to eliminate such effects, series of experimental studies on the crack growth behaviour under creep, fatigue and creep-fatigue interaction conditions on 304 stainless steel have been carried out by using high temperature microscope and observing the crack length continuously during running the test without interruption in vacuum of 10 −5mm Hg. Among the results, it was found that crack growth rates on a time basis, da/dt, under high temperature creep and creep-fatigue interaction conditions can not be described in terms of solely elastic stress intensity factor k i or only net section stress σ net, both independent of gross section stress σ g. The relation between crack growth rate and stress intensity factor under high temperature fatigue condition changes with some trend according to gross section stress at lower K I level and it can be approximately described in terms of stress intensity factor K I only, at higher K I level. The threshold stress intensity factor and the threshold net section stress under high temperature creep, fatigue and creep-fatigue interaction conditions appears to be almost independent of temperature.