Study On Shear Properties Of Three-dimensional Penta-direction Carbon/aramid Hybrid Braided Composites

Hybrid braided composites can not only improve the mechanical properties and damage tolerance of single fiber reinforced composites,but also realize the optimal allocation of resources.In this paper,three-dimensional five direction braided composites with different hybrid structures were prepared by using aramid fiber and carbon fiber as reinforcement materials,and their shear properties under transverse and longitudinal loading were analyzed.Firstly,pure carbon braided and hybrid braided composites were fabricated by four-step braiding process and RTM(resins transfer molding,RTM)composite molding process.The mechanical properties of the specimens under transverse and longitudinal loading were studied by short beam shear test.The results show that the damage modes of the specimens are matrix cracking,debonding and delamination of fiber matrix interface,fiber bundle fracture and inter fiber bundle cracking.The main causes of material transverse loading failure are fiber bundle fracture and inter fiber bundle cracking,and matrix fracture is the main cause of material longitudinal loading failure.Under transverse shear loading,the pure carbon braided composites(Ca Cb)presents brittle failure characteristics,while the carbon fiber and aramid fiber hybrid braided composites presents the ductile failure characteristic.The hybrid braided composites with aramid fiber as axial yarn and carbon fiber as braiding yarn(Ka Cb)exhibited the best shear toughness under transverse shear loading.Meanwhile,the composites with carbon fiber as axial yarn and aramid fiber as braiding yarn(Ca Kb)demonstrated the best shear toughness in the longitudinal direction.Secondly,according to the real braided structure and geometric characteristics of3 D five direction braided composites,a full-scale meso structure finite element model is established.The finite element results are in good agreement with the experimental results,which verifies the validity of the finite element model of the full-size mesoscopic structure.The results show that under transverse loading,the stress propagation mode of pure carbon braided specimens is consistent with that of hybrid braided specimens in the initial loading stage,and the stress distribution and damage are significantly different in the plastic stage and the failure stage.The matrix of Ca Cb sample first suffers from shear damage,and the initial damage expands rapidly,resulting in shear fracture of braided yarn and axial yarn.The braided yarn carbon fiber of the Ca Kb sample formed a shear gap,which accelerated the failure process.The bending deformation of Ka Cb axial yarn made the longitudinal stress propagation hindered,the damage area increased and the damage propagation speed slowed down.Under the longitudinal loading,the transverse stress propagation of the sample is weakened,and the axial yarn slips to both sides of the load.The braided yarn and matrix are the main load-bearing parts.The matrix of Ca Cb and Ca Kb produced shear damage,and the carbon fiber of the braided yarn was broken under shearing.The stress localization in the loading area of the Ca Kb specimen is slowed down due to the bending deformation of the braided aramid yarn.Finally,in order to further analyze the hybrid effect of the braided structure,the amount of removed elements and the damage energy displacement curve of the specimen at the failure time are extracted,and the stress distribution and damage evolution of the characteristic node and special time are selected for analysis.It is found that the upper and lower braided yarns of pure carbon and hybrid braided samples have higher shear stress firstly,and the braided yarn and axial yarn of Ca Cb samples undergo sudden shear fracture,while the hybrid braided sample shows progressive damage.When aramid fiber is used as axial yarn and braided yarn,the number of failure units of axial yarn and braided yarn is greatly reduced,and the corresponding number of failure removal units of carbon fiber braided yarn and carbon fiber axial yarn will also be reduced.Hybrid braided structure shows better structural synergy effect.