Impact Compression Characteristics And Failure Mechanism Of 3D Five-directional Braided Composites

Three-dimensional(3D)braided composite material is near-net-shape manufactured by multi-direction yarns with high integrity and excellent impact resistance.It has become an indispensable new material in many high-tech fields.At present,there are abundant research achievements on mechanical properties and failure mechanism of 3D braided composites,which provide an important theoretical basis for accurate evaluation of reliability and safety in structural design and engineering application.However,few reports have been done on the mechanical properties and failure characteristics of 3D five-directional carbon/epoxy braided composites under high strain rates.In this dissertation,the compressive mechanical properties and failure mechanism of 3D five-directional carbon/epoxy braided composites under high stain rates were studied systematically by using experimental research,theoretical analysis and numerical simulation.So the main achievements are as follows:(1)Based on quasi-static compression test,the compression behaviors and failure characteristics of 3D five-directional carbon/epoxy braided composites under two loading modes(longitudinal compression and transverse compression)were obtained.And the effects of braiding angle on compression behavior and failure characteristics of 3D five-directional carbon/epoxy braided composites was presented.(2)Considering the orientation of the braided yarns arrangement,longitudinal impact compression and transverse impact compression tests(SHPB)were carried out on the specimens of 3D five-directional carbon/epoxy braided composites.The variations of mechanical properties,macroscopic failure characteristics and energy dissipation with strain rate were obtained.And the sensitivity differences of strain rate on compression characteristics was compared under two loading modes.(3)The effects of braiding angle on compression behaviors and failure characteristics of 3D five-directional carbon/epoxy braided composites under high strain rate were studied systematically.The variations of mechanical properties,macroscopic failure characteristics and energy dissipation with braiding angle were obtained.And the sensitivity differences of braiding angle on compression characteristics was given under two loading modes.(4)By means of SEM analysis system,the micro fracture morphologies of the specimens were obtained.The differential variations of breaking patterns and mechanisms of the matrix and yarns in 3D five-directional carbon/epoxy braided composites were analyzed with strain rate,braiding angle and two loading modes,which revealed the macroscopic deformation and failure mechanisms of specimens under high strain rates.By means of the high-speed photographic analysis system,the whole process video information of dynamic deformation and failure characteristics of specimens was obtained.The corresponding relationship between the dynamic macroscopic failure process and the deformation and failure characteristics in the stressstrain curve was analyzed,which interpreted the macroscopic deformation and failure mechanism of specimens effectively.(5)Based on damage mechanics theory and Marzars damage mechanics model,the damage evolution equation and constitutive model of 3D five-directional braided composites under longitudinal and transverse impact compression were established.The stress-strain relationships of 3D five-directional braided composites considering the influence of strain rate,braiding angle and loading direction were given,which was in good agreement with the experimental results.(6)The improved meso-scale three cell model was established by the variable section parameterization method,which could reflect the bending and torsion characteristics of braided yarns at 90° and 180° in the surface cell and corner cell.By means of ABAQUS/Explicit software platform,the deformation and failure characteristics of 3D five-directional braided composites under two kinds of impact loads(longitudinal and transverse)were simulated by building the multi-scale geometric structure model and macro homogenization mechanics model.The simulation results included the variations of the total stress-strain curve,dynamic peak stress and strain,dynamic elastic mode,and dynamic damage characteristics with strain rate and braiding angle,which was in good agreement with the test results.(7)Based on the braiding process of 3D five-directional braided composites and MATLAB R2016 a software modeling and simulation platform,the space motion trajectory of braiding yarns and axial yarns were described respectively.And the virtual braiding macro-scale simulation model was realized by the way of “point-line-body”modeling.So the macroscopic simulation model was established on this basis,which could reflect the actual braided structure of 3D five-directional braided composites.With ABAQUS/Explict software platform,the deformation and failure characteristics of matrix and fiber bundles of composite materials under impact loading were simulated.The results showed that the matrix fractures first and then the fiber bundles bends and buckles under longitudinal impact compression,but the matrix fracture and the fiber bundles shear failure occurs simultaneously under transverse impact compression.And the influence mechanisms of strain rate,braiding angle and loading direction on deformation and failure characteristics were also compared.The research results can provide an important reference for the dynamic mechanical response analysis and the structural design and application of 3D fivedirectional braided composites.There are 170 figures,33 tables and 191 references in this dissertation.