The Study And Test Of The Influence Of The Angle Between Fiber Layers On The Mechanical Properties Of Carbon Fiber Composite Square Tubes

Multi-layer braided composite materials are applied to a series of engineering fields due to their good structural integrity,complex structure,adaptability to different structures,high specific strength and specific modulus.However,composite pipe fittings inevitably produce defects during the manufacturing process,such as air bubbles,cracks,and are subjected to low-speed loads and impact loads during use,causing damage to the inside of the material that cannot be observed by the naked eye.Therefore,it is more meaningful to study the mechanical properties of multi-layer braided composite pipe fittings under low speed loads.In this paper,the preparation of the double-layer braided composite square tube was carried out by the weaving process and the forming process.First of all,the properties of the carbon fiber material are analyzed,and five different inner and outer knitting angle prefabricated parts are knitted by controlling the curling speed and the rotational speed of the motor.The uniformity of the tension distribution is ensured during the weaving process to prevent yarn breakage.Next,the curing mold was designed to introduce the selection and basic characteristics of the resin and the composite molded parts obtained by the vacuum assisted molding process(VARTM).The molded parts were cut to the corresponding length according to the standard according to the two experiments of compression and bending.Next,the axial compression experiment was carried out on the test piece with a universal testing machine,and the experimental data was recorded.Three failure modes can be observed in the compression experiment: fracture,fold and split mode.The effects of braided structure on compression performance were analyzed from three aspects of compression energy absorption,compressive strength and compressive modulus.As the angle between the inner and outer layers increases,the energy absorption capacity of the test piece gradually increases;the fiber gradually changes from the axial direction to the circumferential direction,and the compressive strength and compressive modulus are gradually reduced.Simultaneously,the influence of the inner and outer braiding angles on the compression performance was studied.It was found that the outer braiding angle significantly affected the overall compression performance.Then,using a universal testing machine to perform a three-point bending test on the test piece and record the experimental data.In the bending test,it was observed that as the indenter continued to press down,the resin crack extended to the side of the tube.The bottom resin of the test piece having a large carbon fiber content began to be broken,and the fibers at the bottom of the test piece having the largest carbon fiber content in the five test pieces began to be broken.The effects of the braided structure on the bending properties were analyzed from three aspects of bending energy absorption,bending strength and flexural modulus.As the angle between the inner and outer layers increases,the energy absorption capacity,bending strength and flexural modulus of the test piece gradually decrease.At the same time,the influence of the inner and outer braiding angles on the bending properties was studied.It was found that the outer braiding angle significantly affected the overall bending performance.Finally,the double-layer braided structure model is drawn based on the intersection method of the curved surface.The model is simulated by the finite element analysis software ABAQUS under the conditions of compression and three-point bending.The simulation results are compared with the experimental data to verify the accuracy of the simulation.And explore the inner and outer stress conditions,verify the accuracy of the experimental results.