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

Abstract

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, Δ K th, 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 Δ K th values at room temperature and 350 °C under the same R. For a given temperature, Δ K th 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 Δ K. 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 Δ K. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a K max effect and the implication of this effect, or related mechanisms, are discussed. In light of this, an equation incorporating the effects of the K max 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.