Application of a threshold curve model to high-cycle fatigue behavior of small cracks induced by foreign-object damage in Ti–6Al–4V

A threshold curve method previously developed was applied to analyze the high-cycle fatigue behavior of small cracks induced by foreign-object damage (FOD) in a Ti–6Al–4V alloy for which experimental results can be obtained from recent publications. Foreign object damage represents the impact by small hard objects such as stones or pieces of metal and is a major cause of fatigue failure in gas turbine engines. Experimental results found in the literature showed that the small cracks induced by FOD in Ti–6Al–4V alloys could propagate at an applied ΔK as small as 1 MPa m1/2, i.e. well below the ‘worst-case’ ΔKthR threshold of 1.9 MPa m1/2 for long cracks in this alloy (load ratio R=0.95). According to the results obtained in the present work, the threshold for propagation of small cracks (≧20 μm) can be as small as 0.8 MPa m1/2 considering a stress ratio R=0.8, for which the threshold for long cracks is about 2.3 MPa m1/2. Further, the minimum stress ratio R at which the analyzed cracks can propagate seems to be about 0.6, for which the threshold for large cracks is about 2.7 MPa m1/2. These results show that for microstructurally small cracks, the absence of crack closure effect is not the only reason for the observed faster propagation rates and lower propagation thresholds when compared with long cracks.