| During the service process of aero-engine,the airflow channel will inevitably inhale hard foreign objects such as gravel and metal debris,which will collide with the high-speed rotating fan/compressor blades,causing foreign object damage(FOD).FOD can cause local impact damage and large residual stress on blades.During the course of take-off,cruise,and landing of aircraft,the compressor blades are subjected to high/low cycle complex fatigue loads.Notches,cracks,and large residual stresses caused by FOD are possible sources of structural fatigue,which seriously affect the service life of blades.This paper analyzed the impact damage and gravel wear damage of compressor simulation blades,including the damage morphology and residual stress distribution of the simulation blade under different impact parameters.In addition,the residual fatigue life prediction and high/low cycle complex fatigue tests were carried out on the simulation blades with prefabricated damages.These studies can provide a basis for the damage tolerance design of the compressor blade,and so as to effectively prevent flight accidents caused by foreign object damages.In this paper,numerical simulation of FOD was performed to analyze the influence of impact angle,impact velocity and impact ball diameter on the damages of simulation blades.The residual fatigue lives of the damaged specimens were predicted by using the established impact notch-equivalent crack size relationship.And the residual fatigue lives of the gravel wear specimens were predicted based on fracture mechanics.In addition,considering the effect of vibration loads,the high/low cycle complex fatigue tests were carried out on the damaged specimens,and the residual high/low cycle complex fatigue lives were predicted.The main research contents are as follows:(1)Considering the effect of vibration loads,the high/low cycle complex fatigue tests were carried out for the TC4 simulation blades with impact damages.According to the ratio of the real vibration load and the maximum stress on the blade in the operation of the compressor blade,in the fatigue tests,the high/low cycle fatigue stress ratio of 0.055 was used.The high/low cycle fatigue frequency ratio of 140 was adopted.Combining the fracture analysis with the FOD simulation results,the features of fatigue failure and the location of the fatigue sources of the simulation blades were illustrated in detail.From fracture analysis,the plastic deformation,lamellar structure,microcrack,and micro-notch of the impact notches were found,and also the loss of material(LOM)was presented at the impact notches.Combined with the FOD numerical simulation results,it was revealed that the large residual tensile stress on the surface and sub-surface of impact notches defenitely affects the fatigue crack initiations and its locations.(2)Based on the LS-D YNA software,using the Johnson-Cook constitutive model,numerical simulations of FOD were performed on the TC4 titanium alloy and the Am355 stainless steel simulation blades,respectively,and the influence of impact angle,impact velocity and impact ball diameter on simulation blade damages were analyzed.Compared with the test results,the maximum error of the simulated notch depths is 28.32%for TC4 specimens and 25.48%for Am355 specimens,respectively.From the comparison of the impact damages between the TC4 and Am355 simluation blades,the impact resistance of the Am355 simulation blades is better than that of the TC4 ones.However,without regard to the influence of the material properties of the specimens,the residual tensile/compressive stresses on the Am355 simulation blades after impact are generally higher than those on the TC4 simulation blades.(3)Based on fracture mechanics,the equivalent crack sizes of the impact notches were obtained by the backstepping method in terms of equal fatigue life.The optimal correlated parameter pair between the notches and the equivalent cracks was determined through correlation analysis.Then,the impact notch-equivalent crack size relationship was established.On account of this relationship,the residual fatigue lives of TC4 and Am355 damaged specimens were predicted,respectively.Most of the prediction results of residual fatigue life are within twice error band of the test results,which shows that this prediction method is effective for the assessment of the residual fatigue lives of structures with impact damages.(4)The influence of some random factors including the load fluctuation and the dispersion of the damages were analyzed for the high/low cycle complex fatigue tests.Based on fracture mechanics,by using the impact notch-equivalent crack size relationship,the residual life of high/low cycle complex fatigue were predicted under different load levels.Comparing the predicted results with the test results,it is detected that the high-frequency load has a significant influence on the fatigue life of the structure,and the predicted life is reduced most by 22.73%,corresponding to the maximum nominal stress of 500MPa and the high/low cycle fatigue frequency ratio of 100.(5)Based on the damage characterization of the specimens after sand washing tests,the thicknesses of the whole gravel wear specimens are thinned and the equivalent initial cracks of the specimens are set at the local micropits on the minimum net section based on fracture mechanics.After this simplification,the thicknesses of the specimens and the equivalent initial cracks are obtained.Furthermore,the residual lives of the TC4 flat damaged specimens were predicted.The predicted lives are in good agreement with the fatigue test results,and most of the prediction results are within twice error band of the test results,which indicates that this method can effectively predict the residual lives of the structures with gravel wear damages. |
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