| When the aero-engine is in operation,the fan and compressor blade under the action of high-frequency vibration stress are easy to be impacted by the hard particles coming with the airflow,forming foreign object damage.Under the repeated action of alternating load,fatigue crack is easy to generate and expand rapidly,leading to blade fracture failure and serious flight accident.In this paper,FOD simulation test of plate specimens and simulated blades of titanium alloy for blade was carried out by a gas gun with different diameters of steel ball(?1??2??3 and ?4 mm)in different impact velocities(200?250?300 and 350 m/s).The dents and microstructure changes of titanium alloy materials impacted by steel balls in high-speed were studied,especially the formation of adiabatic shear band.And the high cycle bending vibration fatigue test was carried out on the damaged simulated blade by using the step-loading test method.The fatigue strength,fatigue crack initiation and growth in different notches and its relationship with adiabatic shear band were studied,which provided theoretical basis and date support for the formulation of blade damage evaluation criteria.There was an important engineering significance on the design of blades against foreign object damage and the repair of damaged blades.The results showed that:(1)The TC4 titanium alloy plate specimens were impacted by steel ball with high speed,which caused dents in the surface.With the diameter of steel ball and impact speed increases,the size D and depth H of the dents in TC4 titanium alloy plate specimen approximately showed the tendency of increase.The characteristics of damage area included abrasion,material rheology,slip trace,shear slip,loss of material and even microcracks.(2)Because of uneven deformation,high speed impact could lead to adiabatic shear band in titanium alloy materials,causing damage to its microstructure.Adiabatic shear band were mainly distributed in the damage on the edge of the dent,rather than the bottom of the dent,forming a 45° angle with the impact direction of the steel ball.Along with the rising of the diameter of steel ball and impact speed,the number,width and bifurcate tendency of adiabatic shear band also increased.The existence of adiabatic shear band was related to the depth H of the damaged pit.When the depth H of the damaged dent was greater than about 180 ?m,adiabatic shear band generally exists in the microstructure.(3)There were notches in the TC17 titanium alloy simulated blade under high-speed impact of steel ball.With the increase of steel ball diameter and impact speed,the width L1 and depth L2 of the notch in TC17 titanium alloy simulation blade increased.The characteristics of the edge of notches were material folding,material rheology,loss of material and microcrack.(4)Foreign object damage had an important impact on the fatigue strength of blades.Compared with impact velocity,diameter of the steel ball had a greater influence on fatigue strength of the sample.The damage samples impacted by ? 3 mm steel ball at different speeds obtained the fatigue strength of about 200 MPa,while the damage samples impacted by ? 4 mm steel ball at different speeds obtained the fatigue strength of about 150 MPa.(5)Failure mode of damaged blade under high cycle bending vibration load was brittle cleavage fracture,and adiabatic shear band caused by impact test was an important factor of blade fatigue failure.Adiabatic shear band was obviously visible at the crack source of partial fatigue fracture.In the crack growth zone,there were macroscopic characteristics such as river pattern and fatigue curve,and microscopic characteristics such as cleavage step,brittle fatigue strip and secondary crack. |
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