Effect of FOD notch-type damage on fatigue limit strength of TC4

To deal with foreign object damage (FOD) notch-type damages which the leading edge of TC4 titanium alloy fan/compressor blades suffered, test study on high-speed ballistic impacts, and analysis of damage characteristics and stress concentrations were carried out under different impact angles, while high cycle fatigue tests and fatigue limit strength prediction of as-impacted and residual stress relief annealed specimens were conducted. Conclusions were made below: with the increase of impact angles, damage sizes at incident side and stress concentrating coefficients remained constant, and the notched damage depth and length decreased. The range of the damage depth was 0.6－1.5mm, and the range of stress concentrating coefficients was 2.6－3.4. The fatigue limit strength of notch-type damaged specimens reduced to 27%－35% of smooth specimens, and was not inverse to stress concentrating coefficients. The high cycle fatigue (HCF） behavior of annealed specimens declined slightly or remained basically unchanged, indicating that the effect of the residual stress was slight. Influence of the residual stress on the fatigue limit strength was less than 10% of the smooth specimen. The HCF behavior of notch-type damaged specimens had no obvious correlation with the damage root radius, but declined with the increase of the maximum damage depth and damage length, indicating that special consideration must be given when preparing the maintenance manual. Prediction accuracy of HCF behavior by Peterson formula was unsatisfactory, of which the maximum error was 45%. It required to develop high precision prediction method for HCF behavior of FOD notch-type damaged components.