Prediction of fatigue interaction from static creep and high frequency fatigue crack growth data

Many engineering components operating at elevated temperatures are subjected to combined steady and cyclic loading which can give rise to creep, fatigue and combined creep/fatigue failure. In many cases only static creep and high frequency data is available. In this paper the relationship between crack growth rates under static load and cracking rate under slow cyclic rates (<0.1Hz) has been examined. Crack growth behaviour at elevated temperatures under combinations of creep, creep fatigue and fatigue are investigated for an aluminium alloy RR58 tested at 150 degrees C and an 11Cr Martensitic steel FV448 tested at 550 degrees C. Attention is focused on the intermediate (steady state) region of cracking. It is shown that for static and low frequency tests (<0.1Hz) good correlation are obtained in general for crack growth rate versus the C* line integral. At high frequencies where transgranular fatigue processes control, it is demonstrated that the crack growth per cycle can be described by the Paris Law. At intermediate frequencies correlation can be obtained using static creep and pure fatigue data if a linear cumulative damage law is assumed.