PREDICTION OF FREQUENCY EFFECT IN HIGH-TEMPERATURE FATIGUE-CRACK GROWTH USING DAMAGE FACTORS

An earlier modification of the Paris law for the growth of deep cracks in the linear elastic fracture mechanics regime is extended to includ e a term enabling the prediction of cyclic crack growth rates at low f requencies. The relation requires (i) a reference growth law under con tinuous cycling at the appropriate elevated temperature and (ii) a spe cified, dimensionless degradation term, defined as D(c), the creep/oxi dation damage per cycle, which increases as the applied frequency decr eases or as the dwell time at peak load in prolonged. The relationship is validated against data from the previous analysis on low alloy fer ritic and austenitic steels in the range 538-650-degrees-C and against further published results on Ni-based alloys at temperatures up to 70 0-degrees-C. It appears that for the former series oxidation is the do minant damaging mode, whereas a linear creep damaging mechanism is man ifest in the Ni-based alloys. Moreover, levels of cyclic damage in ter ms of D(c) are higher in the latter, ranging between 10(-3) and 5 x 10 (-1) compared with 10(-4) to 5 x 10(-2) for the steels. A brief compar ison is made with damage factors arising from the high strain fatigue deformation mode at elevated temperatures and other models for the pre diction of frequencey effects are discussed.