Research On Bending Fatigue Behaviors Of SPF/DB Wide-chord Hollow Blade Specimen

SPF/DB Titanium alloy wide-chord hollow fan blades with excellent performance in actual work,has become one of the key technologies of advanced aero-engines.The hollow blade stress state is complicated at high speeds operation of aero-engines.In addition,the blades frequently encounters FOD events.These unfavorable factors are a major test to the safety work of the hollow blade.Therefore,it is of great theoretical significance and engineering value to evaluate the bending fatigue behavior of hollow blades under complex working conditions.The paper studied the bending fatigue behavior of titanium alloy wide chord hollow blades.In this paper,The SPF/DB hollow specimens with sandwich structure were designed and manufactured,a reliable and convenient method for predicting the bending fatigue life is established through the combination of experiment,theoretical analysis and numerical simulation.Firstly,the three-point bending static strength test and fatigue test of hollow structure specimen were carried out,and the S-N curves of the specimen under different thicknesses and different stress ratios were established.The fatigue life prediction model considering the effect of thickness and stress ratio was established by combining with theoretical calculation.Fatigue failure process and fracture were observed.It was found that the fatigue cracks was initiated at the edge of the bottom of the loading section and extended to the other side.The effect of FOD on the bending fatigue performance of hollow structure was studied.A foreign object damage test was carried out and the morphology of impact damage was observed.The impact test not only caused surface damage to the material,but also caused additional internal damage on the weld layer.The three-point bending fatigue test after impact damage was carried out which showed that the bending fatigue life was decreased obervsely.The morphology of the fatigue fracture was analyzed which showed that the location of the fatigue source changed and it initiated at the edge of the notch as well as the weld layer.The former caused skin failure and the latter caused the inner core plate to break.Finally,based on the FEM,a new method of predicting the bending fatigue life of hollow specimen was established.The maximum bending normal stress,the maximum equivalent stress and the maximum principal stress parameter of the critical plane were taken as the damage parameters respectively,and the fatigue life of specimen under different loading levels were predicted.The results shows that the maximum principal stress method of critical plane can predict the fatigue life of the specimen effectively.The FOD process of hollow element was numerically simulated,the residual stress and strain distribution of the hollow element after impact damage were analyzed.Based on the sub-model method,the stress distubition of the impact gap is calculated.Furthermore,the fatigue life prediction method of hollow spceimen after FOD was established by equivalent normal stress correction.