Fatigue crack growth from foreign object damage under combined low and high cycle loading. Part I: Experimental studies

The study aims to understand small crack growth behaviour in Ti–6Al–4V plate specimens after foreign object damage (FOD), under combined low and high cycle fatigue (LCF/HCF) loading conditions. The loading block studied represents one of the simplest loading sequences experienced by aero-engines and each block includes a single LCF cycle at a load ratio (R) of 0.01 with 1000 HCF cycles at R of 0.7 superimposed at peak load. In Part I of this paper, the experimental studies are described. FOD was simulated by firing a 3 mm hardened steel cube onto targeted specimens with an impact velocity of 200 m/s, leading to a blunt “V” notch in a flat surface. Microstructural features associated with the FOD damage, e.g. microcracks, were characterized by optical and scanning electronic microscopy. The microcracks were found to be preferred sites for subsequent crack propagation. The growths of these FOD-initiated small cracks under combined LCF/HCF loading were drastically different from the long-crack results obtained from corner-cracked specimens. Stress relief annealing was conducted on selected specimens to remove the residual stresses due to FOD. The results indicated that residual stresses significantly affected both the development of crack front shape and the fatigue life.