| Foreign object damage(FOD)is a common problem of aeroengines in outfield,which is mainly represented as stress concentrations by geometric discontinuousness such as dents and notches,accompanied by microscopic damages and local residual stresses.Foreign object damage can easily cause early initiation of fatigue cracks of aeroengine blades during design service life or interval between inspection and the risk of high cycle fatigue(HCF)failure.On the one hand,it’s necessary to enhance the FOD tolerance capability of blade,on the other hand,it’s necessary to determine the damage degree of the generated FOD and develop maintenance standards.Although there is extensive research in the field of FOD/HCF,there is little research on the effect of centrifugal force on the performance of blade FOD and HCF,and it still relies on engineering experience in determining the serviceable limit of blades,and there is a lack of scientific methods of the determination.In this paper,engineering research,numerical calculation and experimental research are carried out on the effect of centrifugal filed on HCF of FOD titanium alloy blade and method of determining serviceable limits.The main work and conclusions are as follows:Investigation of the serviceable and repairable limits of FOD for different engine fan/compressor blades was conducted.It found that the inspection and repair of blade serviceable and repairable is partitioned,serviceable limits are related to types of damage,repairable limits are not related to the damage type but related to repair methods,the mainly limit size of the two limits is the maximum damage depth and has a “Allowable Time” requirement.In order to reveal the effect of centrifugal forces on the macro and micro characteristics of FOD in TC4 titanium alloy,the experimental device of simulating centrifugal loads is designed and constructed for the light gas gun.This high-speed impact test simulation method was used to study the influence of different centrifugal forces on the macro and micro characteristics and stress concentration of dent-type and notch-type FOD of TC4 titanium alloy.A simplified geometric modeling method of notched damage was proposed.The results show that the centrifugal tensile stress has a great influence on the number of adiabatic shear bands and material losses along the shear plane at the dent-type and notch-type damage bottom,and has little effect on macroscopic morphology,size,stress concentration and distribution depth of adiabatic shear bands of the damages.The micro-voids and micro-cracks are located on the innermost adiabatic shear band,and the distribution depth of adiabatic shear bands is0.030 ～ 0.163 mm.In order to reveal the effect of centrifugal forces on the residual stress field of FOD in TC4 titanium alloy,the implicit/explicit sequential numerical simulation method was used to study the effect of different centrifugal forces on the residual stress field around FOD damage.The results show that the centrifugal tensile stress has a small effect on the residual stress distribution near the bottom of the notched damage,but has a large impact on the value of the residual stress.The degree of residual compressive stress increases with the increase of centrifugal tensile stress.The damage depth has a significant effect on the residual stress distribution at the bottom of the notched damage,but for all different depth damages the surface of the damage bottom has a layer of residual compressive stress,and the sub-surface at the bottom has a peak residual tensile stress.In order to reveal the influence of centrifugal forces on the high cycle fatigue performance of FOD in TC4 titanium alloy,research of HCF strength test of TC4 titanium alloy smooth samples,HCF strength test and failure mechanism research of dent-type and notch-type FOD under different centrifugal forces,and effect of residual stress annealing on HCF strength and failure mechanism of notch-type damage under centrifugal force were conducted.The results show that the HCF strength of the dent-type damage increases with the increase of the centrifugal tensile stress,and the centrifugal force has a greater effect on the fracture surface morphology of the dent-type damage;the HCF strength of the notch-type damage increases significantly with the increase of the centrifugal tensile stress.The centrifugal force has a greater effect on the initiation location of HCF cracks of the notch-type damage.the HCF strength of residual stress annealed notched damages with centrifugal tensile stress is significantly lower than that of unannealed.Residual stress is the decisive factor for the influence of centrifugal tensile stress on the HCF performance of notched damage and is a favorable factor.A method for determining the serviceable limit of blades based on the fracture mechanics FOD exemption criterion was proposed which included that two kinds of FOD exemption criteria were proposed,the key parameters of the serviceable limit determination method based on the fracture mechanics FOD extinction criterion were evaluated and determined,and the serviceable limit determination process was established,the fracture mechanics model and method were evaluated,and serviceable limits of a certain type fan blade was determined using the proposed FOD serviceable limit determination method.The results show that the all estimated values of the fracture mechanics method are less than the test results,and the minimum safety factor can reach 2.28.Serviceable limits of the notched FOD of a fan blade determined by this method is close to the value in the maintenance manual. |
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