Investigation On The Compression Behavior Of Three-dimensional Five-directional Braided Composites In Hygrothermal Environment

The three-dimensional(3D)five-directional braided composite material is an axial yarn added on the basis of the traditional three-dimensional four-directional composite material,and its material strength and stiffness have been greatly improved.Therefore,it has been widely used in aviation,aerospace and other fields.3D braided composites are inevitably in hygrothermal environment during service,and there is a difference in hygroscopic expansion between the carbon fiber and the epoxy resin matrix in the material.The hygrothermal residual stress caused by different amounts of moisture absorption has a great impact on the compression properties of the material.In this paper,experimental and numerical simulations of compressive properties of 3D five-directional resin braided composites in typical hygrothermal environments are performed.First,the 3D five-directional carbon fiber / epoxy braided composite material was tested for moisture absorption at 40℃,60℃,and 80 ℃ to obtain the weight gain rate of the three-dimensional braided composite material.The results show that the temperature and the weaving angle have a greater effect on the moisture absorption behavior of the material.The main manifestation is that the moisture absorption rate and change rate of the specimen at 80℃ are significantly higher than the specimen at 40℃,and the braiding Angle at 15° was higher than that of the specimen at 35°.Based on the moisture absorption experiment,the axial compression test was performed on the test piece to obtain the corresponding mechanical parameters such as ultimate load,ultimate strength,and compressive modulus.The effects of temperature,aging time and weaving angle on the load carrying capacity and failure mode of the test piece were analyzed.In terms of the influence of moisture diffusion on the matrix and carbon fiber / matrix interface,the mechanism of the compressive degradation of composite materials during the hygrothermal aging process was explained.Secondly,the damage process of 3D five-directional braided composites was analyzed by acoustic emission detection technology.The acoustic emission signal characteristic parameters collected during the compression process of the test piece were used,and the signal characteristic parameter method was used,and the loaddisplacement curve of the compression process was used to study the material damage expansion process.According to the peak frequency and amplitude of the acoustic emission signal and the damage characteristics of the 3D braided composite material,the damage types of the material at different stages were analyzed.Finally,based on the meso-structure characteristics of the 3D five-directional braided composite material,a polygonal cell geometry model was established.The commercial finite element analysis software ABAQUS was used to simulate the wet-thermal-mechanical coupling behavior of the three-dimensional braided composite material.Through the moisture absorption and diffusion process of the equivalent material of the heat conduction module,not only the temperature field and stress field distribution of the material’s moisture-coupling coupling,but also the stress field distribution of the moisture-heat-mechanical coupling behavior can be obtained.The stress distributions of fiber bundles and substrates at different time points are analyzed,and the simulation calculation results are compared with the experimental data,which are in good agreement.It provides a reference for compressive strength evaluation and damage prediction of materials during use.