The Study On Dynamic Mechanical Properties And Bird-striking Characteristics Of 3D Four-directional Braided Composites

The development trend of aircraft engine is using bigger and lighter fan blades.One of the ways to achieve both high bypass ratio and less weight is to utilize composite fan blades.The yarn is spatially interlocked because 3D four-direction braided composites have interwoven fiber bundles in thickness direction,therefore the material possesses high interlaminar shear strength and good fracture toughness,having a wide range of applications in the field of aero-engine fan blades.Owing to the direct impact of fan blades' bird-striking resistance to aero-engine's work,it is essential to investigate 3D four-direction braided composites' mechanical properties in dynamic load and damage mechanism in bird strike.This thesis carries out research into dynamic mechanical properties and bird-striking characteristics of 3D four-direction braided composites,and has important reference value for the two qualities.The research includes the following three aspects:(1)The dynamic compressive experiments on the material were conducted using split Hopkinson pressure bar,and the process of experiments was recorded by high-speed camera.The influence of strain rate,compressive direction and braid angle on dynamic mechanical properties and failure mode is discussed.The results show that this kind of composite is sensitive to strain rate in all three compressive directions;the mechanical properties are enhanced in longitudinal and transverse directions and are weakened in thickness direction as strain rate rises;the failure mode of the composite changes in different compressive directions.(2)The microstructure model of the material was established,and the numerical simulation of the test in longitudinal direction was conducted using MATLAB and HyperMesh.The stress-strain curve and strain rate obtained in the simulation is consistent with that obtained in the test,proving the microstructure model is valid.(3)The gelatin bird strike experiments were carried out using air gun system,and the plates were scanned by computed tomography.The influence of impact velocity,impact location and braid angle on the results is discussed.The results show that the gelatin bird demonstrates distinct fluid properties during impact;impact velocity has great influence on the composites' damage;this composite with braided angle of 45 degrees has the best impact resistance;the bird strike changes the internal structure of the impact area,and the debonding of fiber bundles and matrix occurs.This thesis has great practical significance for this material's application to aero-engine fan blades.