Prediction of frequency effect in high temperature fatigue crack growth using damage factors

Abstract

An earlier modification of the Paris law for the growth of deep cracks in the linear elastic fracture mechanics regime is extended to include a term enabling the prediction of cyclic crack growth rates at low frequencies. The relation requires (i) a reference growth law under continuous cycling at the appropriate elevated temperature and (ii) a specified, dimensionless degradation term, defined as D_c, the creep/oxidation damage per cycle, which increases as the applied frequency decreases or as the dwell time at peak load is prolonged. The relationship is validated against data from the previous analysis on low alloy ferritic and austenitic steels in the range 538–650°C and against further published results on Ni-based alloys at temperatures up to 700°C. It appears that for the former series oxidation is the dominant damaging mode, whereas a linear creep damaging mechanism is manifest in the Ni-based alloys. Moreover, levels of cyclic damage in terms of D_c are higher in the latter, ranging between 10⁻³ and 5 × 10⁻¹ compared with 10⁻⁴ to 5 × 10⁻² for the steels. A brief comparison is made with damage factors arising from the high strain fatigue deformation mode at elevated temperatures and other models for the prediction of frequency effects are discussed.

Keywords:

• dwell
• frequency
• Paris law