Research On The Process Theory And Structure Of Closed Loop Two-dimensional Braided Composite Prefor

Closed annular axis carbon fiber composites are still processed by lamination and molding at present.The processing efficiency and performance of its preform will be improved by using two-dimensional braiding technology.However,only the linear mandrel can be used on the conventional two-dimensional braiding machine,while the closed annular axis preforms can not be braided on it.Therefore,a two-dimensional braiding machine using annular axis mandrel is designed and manufactured,and four groups of process parameters are used to braid continuously on the closed annular axis mandrel with irregular cross section.Then the analytical yarn path model based on kinematics and the numerical yarn path model based on finite element simulation are established,to predict the yarn path according to the rotation speed of yarn carrier and mandrel.According to the analytical yarn path model,the braiding angle based on kinematics and the braiding angle based on yarn friction are derived.According to the numerical yarn path model,the braiding angle and yarn spacing are predicted by a method of projection and surface flattening.Meanwhile,the simulation model of annular axis braiding structure is composed of virtual fibers,and the coverage coefficient is predicted.By studying the influence of process parameters on the annular axis braiding structure,it is found that the braiding angle increases by 20.9% and the yarn spacing decreases by 19.8% when the rotation speed of yarn carrier is doubled,and the braiding angle decreases by 31.3% and the yarn spacing increases by 28.6% when the rotation speed of mandrel is doubled,which indicates that the rotation speed of mandrel has a greater influence than that of yarn carrier.Due to the difference between the inner diameter and the outer diameter of the mandrel,the braiding angle and yarn spacing of the preform outside the mandrel are larger,and its coverage coefficient is smaller.By analyzing the yarn friction in the braiding process,it is found that the yarn friction leads to the decrease of braiding angle,in which the friction between yarn and braiding ring leads to the decrease of braiding angle by3.2%,and the friction between yarns leads to the decrease of braiding angle by 2.1%.Through the finite element simulation of the braiding process,it is found that the difference of braiding angle at different positions of the preform is more than 10°.Compared with the actual braiding angle,the prediction error of braiding angle based on kinematics is 9.9%,that based on yarn friction is 4.6%,and that based on finite element simulation is 1.8%.The two-dimensional braiding machine using annular axis mandrel realizes continuous braiding on the closed annular axis mandrel,which can be applied to the processing of carbon fiber composite bicycle rim preform or other annular axis braided preforms.Compared with the analytical yarn path model based on kinematics,the numerical yarn path model based on finite element simulation has higher prediction accuracy.By using the projection and surface flattening method,the yarn path can be presented intuitively in two-dimensional diagram,and the braiding angle of each position including the turning point of the preform can also be predicted.According to the numerical yarn path model based on finite element simulation and the simulation model of braiding structure composed of virtual fibers,the theoretical prediction and experimental verification of three structural parameters,including braiding angle,yarn spacing and coverage coefficient,are realized,which provides the theoretical basis for the processing of annular axis braided preform and the prediction of its composite properties.