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 degrees C. The threshold stress intensity range, Delta 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 Delta K-th values at room temperature and 350 degrees C under the same R. For a given temperature, Delta 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 degrees C was higher than that at room temperature under the same Delta 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 Delta K. The influence of stress ratio and temperature on threshold and FCG rates was analysed in terms of a Km 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. (c) 2005 Elsevier Ltd. All rights reserved.