Multi-scale Mechanical Properties And Damage Analysis Of 3D Four-directional Braided Composites

Three-dimensional four-directional braided composites are fiber reinforced composites and structures.Due to the interwoven distribution of three-dimensional four-directional braided material fibers in space along multiple directions,it has excellent mechanical properties.It can not only meet the requirements of aerospace,medical equipment and other fields for high performance materials,but also greatly reduce the structural weight.However,the internal structure of this kind of material is complex and the test cost is high.At present,finite element numerical simulation is used to study its mechanical properties.In this paper,the multi-scale method is used to study the stiffness performance and low-velocity impact performance of three-dimensional four-directional braided materials,considering the internal domain,plane domain and angular domain structures.Based on the periodic unit cell idea,the strength performance of the internal domain material is used to characterize the strength performance of the overall material.The main contents are as follows :(1)Based on the four-step weaving process,the yarn trajectory in the surface and angular domains was improved,and the cross-sectional rotation angle and deformation factor were introduced to characterize the yarn extrusion state.The mathematical relationship between the improved cell geometry and the weaving parameters was derived.The three-cell solid model is established by using TexGen parameterization,and the feasibility of the three-cell model for stiffness,strength and low-velocityimpact performance is proved by comparing the array inter cells,surface cells with test slices.(2)Based on the multi-scale three-cell finite element model,the macro elastic constants of three-dimensional four-directional braided composites were predicted,and the errors were 5.4% and 0.68%,respectively,which were smaller than the prediction error only considering the internal cells.The stress distribution and deformation state of micro cell and micro three cell under six independent loads are analyzed.The effects of braiding angle and fiber content on the macroscopic elastic properties were investigated.The braiding angle had the greatest influence on the axial tensile modulus.When the braiding angle increased from 15° to 35°,the axial tensile modulus decreased by nearly 60%.The pore unit was introduced.It was found that there were some differences in the effects of pores on the stiffness of three types of cells.The effects of pore type and porosity on the elastic modulus and macroscopic elastic modulus of three types of cells were discussed.(3)Based on the periodic unit cell analysis method,the micro and meso unit cell damage models were established for the internal structure of the material.The longitudinal tensile progressive damage process of the specimens with large and small braided angles was simulated,and the damage failure mechanism of the specimens with large and small braided angles was analyzed.The specimens with small braided angle showed brittle characteristics,and the specimens with large braided angle showed plastic characteristics.The effects of porosity,braiding angle and fiber content on the strength properties of three-dimensional four-directional braided composites were investigated.When the braiding angle increased from 15°to 35°,the longitudinal tensile strength decreased by nearly 80%.(4)A multi-scale low-velocity impact model considering surface cells and corner cells is established.The mechanical response of three-dimensional four-directional braided materials is analyzed when the impact velocity is 1 m/s,2 m/s and 3 m/s.The maximum impact load error is 9.9% and the total energy absorption error is less than8%.The main damage forms of three-dimensional four-directional braided composites under low velocity impact were explored.