The Research On Dynamic Compressive Mechanical Properties Of Two-step Three-dimensional Braided Composites

The three-dimensional braided composites is a kind of seamless material made of fiber reinforced structure distributed in space and matrix by 3D braiding technology,which has changeable shapes.Compared with the traditional composites,it has the advantages of designable,high strength and high specific modulus,it also has the characteristics of high damage tolerance and fracture toughness as well as excellent impact resistance,less delamination,high crack resistance and fatigue resistance,so they are widely used in aerospace,transportation and other fileds.In the process of production and application,this kind of material is often subjected to the impact of external loads,resulting in damage or even failure.Therefore,it is necessary to research the mechanical response of three-dimensional two-step braided composite and epoxy resin under impact loading.In this paper,the dynamic compression mechanical properties of two-step braiding composites with different braiding angles are studied by combining experimental and numerical methods.On the basis of research of braided fiber and epoxy resin under high velocity impact,the numerical model of the response of epoxy resin,which is matrix material of composites,is established by using Material Point Method(MPM).comparing the response results and test results,the regularity of the response are analyzed and summarized.The main work is as follows:First of all,analyzing the braiding process of 3D braided two-step composite,braided yarn and axial yarns on the cross section shape are simplified expressed.The calculation formulas of outline size,braiding angle and fiber volume fraction are put forward,and the results get by those formulas are compared with the measured results.Secondly,the experimental study on the responses of 3D braided materials with different braiding angles and pitches under different impact loads is carried out.By using the Split Hopkinson Pressure Bar(SHPB),dynamic compression experiments between three different braiding angles composite specimens are conducted,considering the influences of strain rates,braiding angles and the pitch to anti impact property of composite materials.The result shows that this material is sensitive to strain rate,and the braiding angle has a certain influence to the impact resistance of materials,but the quality of the effects is affected by the strain rate also.Finally,micromechanical model of matrix epoxy resin is found for the simulation of its mechanical response under impact load.The numerical results are in good agreement with the experimental results,which proves that the reliability of this numerical model,and the epoxy resin is a sensitive material to strain rate,which largely contribute to the conclusion that 3D two-step braided composite is sensitive to strain rate.