Effect Of Foreign Object Damage To The High Cycle Fatigue Strength Of Fan Blade And Prediction

Foreigh object damage(FOD) is a kind of mechanic damage that fan blades are impacted by small hard debris during the usage of aero-engine resulting in dents or nicks. Fan blades experience high cycle fatigue(HCF) loading due to vibrations and resonant airflow dynamics. Under such cyclic loads, the FOD damage can lead to unexpected HCF failures of fan blades, which seriously affect the safety of aircraft. FOD events seen on real fan baldes were simulated under laboratory conditions to analyze the damage characteristics, and the effect of FOD damage on the HCF strength of fan blades and prediction was studied which has important significance for improving the tolerance to FOD damage. In this pater, the TC4 titanium alloy is used to process the leading edge specimen and flat plate specimen which are designed according to the geometry of real fan blades. FOD damage characteristics and the effect of FOD damage on the HCF strength of the damaged blades were systematically studied. The main work and conclusions are as follows:(1)The effect of FOD damage on the HCF strength of the fan blades was studied and summarized in this paper. The methods of physical and numerical simulations of FOD are introduced, and the way to study the effect of FOD damage on HCF strength of fan blades is also explained. The results show that gas gun is the best way to reproduce the service FOD experience. In the domestic FOD damage is still lack of study, which needs more systematic research.(2)Leading edge specimen and flat plate specimen were ballistically shot with a 3 mm diameter steel ball at a velocity of approximately 300m/s using gas gun to physical simulations of FOD, and the features of FOD damage were studied. The results indicate that the stress concentration factor of FOD damage increases with the increase of impact angle, and the stress concentration factor of impact dent is about 2 and the stress concentration factor of impact nick is about 3. The residual stresses around the impact dent are larger than that of the impact nick. Adiabatic shear damage around the FOD damage site due to high speed impact is observed.(3)TC4 smooth sample, TC4 notch sample, leading edge and flat plate smples subjected to FOD damage(a portion of leading edge samples and flat plate samples subjected to FOD are stress relief annealed in order to examine the effect of residual stresses) were tested using step-loading method to determine HCF strength. The fatigue notch factor of post-FOD damage sample was predicted by the traditional fatigue notch formula(Peterson), and the fracture of FOD damage smples was analyzed by SEM. The results show that the HCF strength of TC4 at stress ratio of 0.1and 0.5 are lower than those given in the manual, the fatigue notch factor of the notched specimen decreases with the increase of the stress ratio. The scatter involved in the HCF testing of post-FOD specimens and impact damage lead to severe reductions in the HCF strength of specimen. The HCF strength of impact nick samples decrease more severe than that of impact dent. The dangerous angle for impat dent is 60 degrees and for impact notch is 75 degrees. The HCF strength for impact dent can be reduced by as much as 57% and for impact nick can be 73%. The impact velocity change has little effect on the HCF strength of the specimen with post-FOD damage, and the HCF strength of the FOD damage specimen decrease slightly with the increase of impact velocity. The residual stress is more important for HCF strength of impact dent than that of impact nick, the HCF strength of impact dent is increasing while the HCF strength of impact nick is decreasing after residual stress relief was annealed. With the increase of load stress ratio, R, the residual HCF strength of the FOD damaged smples increase. The Peterson’s fatigue notch factor formula can not predict the FOD damaged samples’ fatigue notch factor accurately. The surface or sub surface of FOD site is where the fatigue cracking failure initiated. The micro-cracks and voids formed by the impact will be beneficial to crack initiation and propagation.