Analysis Of Flexural Properties Of Three-dimensional Surface-core Braided Composites

Three dimensional braided composites overcome the shortcomings of low delamination resistance of traditional laminated composites.They have been used in numerous fields such as aerospace,military,transportation industries because of the characteristics of high specific strength,high specific stiffness,excellent damage tolerance,fatigue resistance and designability.As a new structure developed by 3D braided composites,three dimensional surface-core braided composites not only have the performance characteristics of 3D braided composites,but also the special fiber network configuration produced by its unique braiding process may have the effect of resisting the degradation of mechanical properties caused by cutting damage.In this study,the flexural behaviors and the influence of cutting damage on the mechanical properties of three-dimensional surface-core braided composites were investigated systematically,and the surface deformation fields information of three-dimensional surface-core braided composites were monitored by 3D Digital Image Correlation(3D-DIC)to study the damage evolution behaviors.The main work of this thesis includes the following:Three dimensional surface-core braided fabric with different braiding angles and sizes were woven according to the braiding principle,and prepares a three dimensional surface-core braided composite through a vacuum assisted resin infusion molding process.The bending experiments of three-dimensional surface-core braided composites with different braiding angles and cut were carried out by universal testing machine.The load displacement curve,maximum bending load,strength and modulus were obtained.The failure modes and damage evolution process of three-dimensional surface-core braided composites were analyzed by 3D-DIC technology.Through the experimental analysis,the flexural properties of 3D surface-core braided composites with different braiding angles are different.The maximum load,strength and modulus of the composites decrease with the increase of braiding angle.The curves change linearly in the early stage of loading,after reaching the limit load,the bending curves of different braiding angles show different characteristics: the curves of composites with larger braiding angle decrease slowly,and the curves with smaller braiding angle decreases in stages;Cutting damage will affect the flexural performance of three-dimensional surface-core braided composites.Cutting destroys part of the fiber,which makes the braided structure loose and reduces the mechanical properties of composites.The undamaged fibers parallel to the cutting surface maintain the integrity of local braided structure,and maintain the carrying capacity as much as possible,reflecting the bearing superiority of surface-core braided structure in the case of damage;The ultimate failure of three-dimensional surface-core braided composites under flexural load is due to a large number of cracks and fiber fracture.Combined with the damage appearance of the composites,the fibers on the compressive side are broken by shearing and buckling,while the fibers on the tensile side are obviously pulled out under tensile loading.Different load-bearing modes on the cutting side lead to differences in the final failure morphology of the composites.Through the strain image analysis of 3D-DIC,the failure process of three-dimensional surface-core braided composites mainly has the following two stages: according to the high strain area displayed in the strain diagram,the location of micro crack can be judged.In the process of bending loading,the strain concentration first appears at the indenter of the composites compression side,that is,the matrix crack first occurs and propagates;With the continuous loading,a new strain concentration area appears on the tensile side of the composites,the matrix of the outermost braided structure cracks and the fiber to bear the main load,the fiber at the high strain concentration will break,and the damage extend from the upper and lower surfaces of the composites to the neutral surface.3D-DIC technology can predict the location of material failure and provide a certain reference for composites damage detection.