Effects of frequency on fatigue crack growth at elevated temperature

Abstract

The effects of frequency on fatigue crack growth behaviour have been studied in a prealloyed powder material, Udimet 720Li, at 650 °C. Fracture mode and fatigue crack growth behaviour were studied at frequencies ranging from 0.001 to 5 Hz using a balanced triangular waveform. Tests were carried out under constant ΔK control, with load ratio and temperature being held constant. A mechanism map was constructed where predominantly time, mixed and cycle‐dependent crack growth behaviour were identified. The results were verified by SEM analyses. Cycle‐dependent crack growth data were obtained at room temperature, while fully time‐dependent crack growth data were generated under sustained loads at 650 °C. It was found that mixed time/cycle‐dependent behaviour is of most significance for this material at the temperature and frequencies studied. Data for other nickel‐based superalloys from various sources in the literature were compiled and compared with those of U720Li alloy at a given stress intensity and temperature in the mixed regime. An analysis was developed to rationalize the observed effect of frequency on fatigue crack growth rate.

Determination of creep curves from multiple hardening-relaxation testing

“…The chemical composition of the alloy is comparable with that of U720Li alloy [2]. The alloy was solution treated followed by an oil quench and a duplex ageing treatment, as in U720Li.…”

Effects of Loading Waveform and Stress Field on High Temperature Fatigue Crack Growth of Alloy 718

“…High temperature time-dependent crack growth in nickelbase superalloys is known to be controlled by a number of variables including stress intensity, load ratio, temperature, frequency and hold or dwell times where a limited interdependence between these variables is assumed [1]. The frequency and dwell effects are often presented via so-called mechanism maps [2 -4] which indicate increased growth rates for waveforms with low frequency and/or dwell periods at high -typically max -stress levels, comp.…”

Influence of oxidation on fatigue crack initiation and propagation in turbine disc alloy N18

“…Powder metallurgy (PM) Ni-based superalloys have been widely used for aeroengine turbine disc application due to their exceptional combined mechanical properties at elevated temperatures in combination with good oxidation/corrosion resistance [1][2][3]. However, oxidation accelerated fatigue failure (shorter fatigue life or faster crack growth rate) is usually observed when assessing the fatigue performance of disc alloys at elevated temperatures, especially when a dwell period is applied at the peak load [4][5][6][7][8][9][10]. Such a phenomenon is usually associated with intergranular fracture resulting from the interaction between GB oxidation/embrittlement effects and mechanical fatigue processes [8,[11][12][13][14][15][16].…”