Research On The Preparation Process And Shear Properties Of Unidirectional Braided Symmetric Laminated Composites

In this paper,the preparation process and interlaminar shear properties of unidirectional braided symmetric laminated composites and traditional 2D braided symmetric laminated composites were experimentally studied.On the basis of analyzing the advantages and disadvantages of the traditional unidirectional laminated composites and 2D braided laminated composites,the unidirectional braided symmetric laminated composites are taken as the main research objects and the structure and partial preparation process of braided composites were improved and designed.The experimental analysis method was used to study the structural regularity during braiding and the failure effect of unidirectional braided symmetric laminated composites under the three-point bending shear mode.The new structure was analyzed from the experimental results of interlaminar shear properties.At present,this new structure still lacks basic research at home and abroad,and it has hardly been reported.The in-plane mechanical properties of the composites are improved with the decrease of the fiber buckling degree,but the trend of the influence on the interlaminar shear properties remains to be explored.Because laminated composites are prone to delamination failure and lead to premature failure under external forces,it is essential to study the interlaminar shear properties.Compared with the traditional laminated composites,the 2D braided laminated composites show stronger integrity,which greatly improves its in-plane and interlaminar mechanical properties,and the process is simple and quantifiable,and is widely used.But there is buckling of the fibers produced by the interleaving in the 2D braided laminated composites.According to theoretical and empirical analysis,the in-plane mechanical properties of the material are reduced when the material is subjected to external force.The elongation and bending of fibers in the structure have a great influence on the in-plane mechanical properties of the composites.The paper aims to measure and evaluate the structure and interlaminar shear properties ofunidirectional braided symmetric laminated composites.A new unidirectional structure was designed for the defect of buckling of interlaced fiber in 2D braided laminated composites.This structure uses one direction of carbon fiber,while the other direction is a finer nylon thermal fuse for weaving,and the buckling of the fabric is significantly reduced.In the preparation process of unidirectional braided symmetric laminated structure,many important improvements had been made to the braiding process.The structure was designed as a symmetrical layer.By adjusting the size of mould and the speed of braiding,the yarn width and fiber gap of each layer were consistent,and the braiding angle increased with the increase of diameter;In order to straighten the fiber in the fabric,gaskets was installed on the spindle to increase the tension of the fiber;An effective transfer method was designed to solve the problem that the fabric structure was unstable and easily destroyed when the fabric was removed from the mold.Through the optimization of braiding process,the best unidirectional braided symmetrical laminated structure is obtained,which is helpful to improve the mechanical properties of the material.In order to study the interlaminar shear properties of unidirectional braided symmetric laminated composites,the paper selects three-point bending short beam method as the shear test method,and compares with two-dimensional braided symmetric laminated composites.According to the load-displacement curve of each specimen,the paper calculated the interlaminar shear strength and modulus of the material.The paper used Origin software to process the experimental data and comparing the shear strength,modulus and failure displacement of the two structures.In order to determine the failure mechanism and regularity of the unidirectional braided symmetric laminated composites,the paper used stereo microscope to analyse the crack distribution and regularity of the shear failure cross section of specimen.Compared the shear strength and modulus of the two structures,the results show that the 2D braided symmetric laminated composites have better interlaminar shear properties.The reason is that in the 2D braided structure,the interlaminar movement is not easy to occur because of the friction between layers from interwoven of the fiber.The straightness of the fiber is disadvantageous to the transverse force of the specimen,and the interlaminar shear properties of the material is reduced.The results shows that there are two kinds of failure modes,internal delamination and delamination cracking,both of which have the phenomenon of fracture and delamination of fiber and resin,and there are more extensional cracks parallelling to the laying direction.By analyzing the damage on the side of the specimen,it was found that the nylon has good fusion with the resin and has no effect on the material.Observed the characteristics of cracks on the side and found that the shear neutral plane is below the geometric middle plane.And by observing the failure process,it was found that,in most cases,the specimen is stratified first from the shear neutral plane,with obvious cracks,and then extends to produce new delamination,which becomes the most serious site of damage.Although unidirectional braided symmetric laminated composites showed poor interlaminar shear properties,the difference in interlaminar shear properties between the two structures is very small.It is believed that the interlaminar shear properties of unidirectional braided symmetric laminated composites will be better if the braiding process is improved.