The effect of combined cycle fatigue upon the fatigue performance of TI–6AL–4V fan blade material

Samples of Ti–6Al–4 V (wt.%) were tested to failure under low cycle fatigue (LCF), high cycle fatigue (HCF), and combined cycle fatigue (CCF), respectively, in order to ascertain the effects of the combined cycle situation upon the total fatigue life. The results indicate that the effect of the combined loading is variable and dependent upon the conditions during testing. It has been found that the application of CCF is detrimental to fatigue life under conditions of large vibrational amplitudes characterised by a stress ratio of 0.2, which becomes increasingly significant at the lower overall maximum stress conditions employed. Conversely an increase in total fatigue life was, however observed for constant mean stress (550 MPa) testing when the overall vibrational amplitude and total stress range approached that of the applied mean stress. The CCF loading block employed simulated two basic features of a typical gas turbine engine flight pattern. A major stress cycle represented start-stop operations, which give rise to low cycle fatigue. In-flight vibrations, which could give rise to, HCF were represented by superimposed minor cycles of high frequency upon the dwell portion of the major stress profile. This combined loading pattern was applied in a specially developed facility, consisting of two computer controlled axially aligned servo-hydraulic actuators.