Mixed-mode creep fatigue interactions in SRR99

Tucker, Paul Henry (1998) Mixed-mode creep fatigue interactions in SRR99. University of Southampton, Doctoral Thesis.

Abstract

The temperature at which a jet engine can operate is limited by the temperatures that its component materials can withstand, and so there is an ongoing requirement for the development of candidate jet-engine materials which can operate at increasingly higher temperatures. Nickel superalloys have been established as disc and blade materials as they meet the material requirements for such components. The combination of high temperatures and high stresses in blade alloys makes them susceptible to creep, and the interactions of creep and fatigue can seriously affect component life. In addition, mixed-mode loading produces stress-states which may be closer to those actually applied in service than those applied using simpler loading techniques.

This thesis looks at the effect of stress-state, temperature and environment on fatigue crack growth of SRR99, an established single crystal blade alloy, with a view to studying the creep-fatigue interactions of this alloy. The data generated are compared with equivalent data from a single crystal version of Udimet 720 (U720-SX), a candidate disc alloy studied in another project at Southampton University.

It is found that Stage I fatigue crack growth in SRR99 is promoted: by a single-crystal orientation where the active slip-planes are "out-of-plane" to the nominal crack growth direction; by vacuum due to enhanced slip reversibility; by low temperatures due to less operative dislocation climb and cross-slip mechanisms; by mixed-mode (I + II) loading at low temperatures due to increased applied shear; and by high γ volume fractions due to a greater concentration of highly ordered material. It is found that fatigue crack growth rates are increased: by a single crystal orientation such that the active slip-planes set up a symmetrical slip system in-plane with the nominal crack growth direction; by high temperatures due to decreased mechanical properties leading to high crack tip opening displacements; by mixed-mode (I + II) loading at high temperatures due to synergistic damage and crack opening mechanisms; by high R-ratios due to less influence of roughness induced closure.

High temperature fatigue crack growth is along the direction of nominally greatest pure mode I opening. A dwell superimposed onto the fatigue cycle at 850°C manifests itself in pure mode I loading with accelerated crack growth, but growth rates are greatest at 0.25Hz, implying that longer dwells lead to crack tip blunting. Under mixed-mode loading conditions, no dwell effect is seen at 0.25Hz, but an acceleration is seen at 1-30-1-1 due to increased constraint around deflected (normally mixed-mode) crack tips reducing crack tip blunting.

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