Foreign Object Damage And Its Effect On High Cycle Fatigue Strength Of Titanium Alloy Engine Blades

During the service life of modern aircraft gas turbine engines, fan and compressor blades are susceptible to Foreign Object Damage (FOD) events caused by small hard particles ingested into the flow path. Geometric stress concentration in FOD notch, residual stress and microstructural damage from FOD event severely degrade the High Cycle Fatigue (HCF) capability of the blade material. Premature airfoil HCF failure caused by high frequency vibrations may occurr prior to design service life or inspection interval. Developing evaluation methods for FOD damage, accurately evaluating and predicting the effect of FOD on HCF strength of blades, and designing fan/compressor blades according to the degraded material allowables, will be of engineering significance to reduce malfunction and catastrophic mishaps due to FOD events, reduce FOD-related maintenance costs, and enhance engine readiness.Study in this thesis is focused on FOD numerical simulation methods and the effect of FOD on HCF strength of TC4 titanium alloy engine blades. Five aspects of research have been accomplished as follows:Essential characteristic of FOD tolerance design is analyzed. The development status of FOD tolerance design technology is reviewed. It is concluded that vibratory stress design and control of blades according to reduced material allowable due to FOD is the fundamental feature of FOD tolerance design. FOD tolerance design is part of HCF design process of blades. Probabilistic design for FOD tolerance based on statistical FOD distributions, realistic component impact tests supplemented by computational modeling of the impact event is the development direction.Uniaxial deformation behavior of TC4 alloy under quasi-static and dynamic loading is studied experimentally at 20℃, 200℃, and 400℃over a strain rate range from 10-4s-1 to 104s-1. TC4 notch tension experiments are conducted under quasi-static loading at 20℃. Bammann material model and modified Zerilli-Armstrong material model in LS-DYNA are evaluated for their ability to model deformation and failure behavior of TC4 alloy. The Result indicates that plastic flow behavior of TC4 is sensitive to strain rate and temperature. Stress triaxiality has significant effect on the equivalent plastic strain to fracture of TC4. Bammann model correlates TC4 deformation data best, and predicts the failure behavior of notch samples under quasi-static loading well. Modified Zerilli-Armstrong model correlate the test data at high strain rates poor. The constant equivalent plastic strain failure criteria can't represent the failure behavior of TC4 at different stress triaxiality. The effect of seven important finite element parameters on FOD simulation results is investigated using LS-DYNA code. These parameters include: geometry size of finite element model, system damping, finite element mesh density, hourglass control method, scale factor for the interface stiffness in contact-impact algorithm, interface coefficient of friction, artificial bulk viscosity, etc. The result shows that most of the parameters have no significant effect on the simulated morphology and size of FOD. Finite element model size, system damping and finite element mesh density have remarkable effect on the computational time and residual stresses.Numerical simulation methods for FOD events to TC4 blades based on LS-DYNA code and Bammann material model are developed, and verified by impact tests. The result indicates impact damage from hard body to TC4 blades can be numerically simulated with reasonable accuracy by the methods developed. The effect of Equation of State on FOD numerical simulation results is of insignificance.A ballistic impact experimental apparatus is set up. FOD is simulated by impact tests on TC4 plate specimens with steel balls of diameter of 4mm. TC4 specimens with and without impact damage are fatigue tested axially using the step-loading test method to establish the fatigue strength corresponding to a life of 106 cycles at a stress ratio of R=0.1. It is found that adiabatic shear plugging is the primary failure mode in impact tests of TC4 specimens. Impact damage has significant effect on HCF strength of TC4 alloy. Most of the simulated FOD notches have an elastic stress concentration factors Kt not exceeding 3.0, but fatigue notch factor Kf is greater than 3.0.