Effects of stress ratio and temperature on fatigue crack growth in a Ti-6Al-4V alloy

Fatigue thresholds and fatigue crack growth (FCG) rates in corner notched specimens of a forged Ti-6Al-4V aero-engine disk material were investigated at room temperature and 350 °C. The threshold stress intensity range, ΔKth, was determined by a method involving a step change in stress ratio (the 'jump in' method). It was found that for three high stress ratios (R=0.7-0.9), where crack closure effects are widely accepted to be negligible, there were similar ΔKth values at room temperature and 350 °C under the same R. For a given temperature, ΔKth was observed to decrease from 3.1 to 2.1 MPam with R increasing from 0.7 to 0.9. The fatigue crack growth rate was influenced by increasing temperature. For high stress ratios, FCG rate at 350 °C was higher than that at room temperature under the same ΔK. For a low stress ratio (R=0.01), higher temperature led to higher FCG rates in the near-threshold regime, but showed almost no effect at higher ΔK. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a Kmax effect and the implication of this effect, or related mechanisms, are discussed. In light of this, an equation incorporating the effects of the Kmax and fatigue threshold, is proposed to describe FCG rates in the near-threshold and Paris regimes for both temperatures. The predictions compare favourably with experimental data.