Analysis Of Low-velocity Impact And The Damage Mechanism For 3D Braided Composites

Three-dimensional(3D)braided composites exhibit improved impact damage resistance,impact damage tolerance and designability compared with traditional two dimensional(2D)laminated composites.Moreover,the excellent integrated mechanical properties of 3D braided composites highly meet the requirements of aerospace structural materials for weight reduction and high load-bearing capacity.Numerous studies on the mechanical properties of3 D braided composites have been made,including the prediction of stiffness and strength properties,and the investigation of damage and failure mechanisms under various loadings,However,due to the complicated meso-structure,the problem of low-velocity impact damage analysis has not yet been effectively solved,and the impact damage mechanism is still unclear.Therefore,the study of low-velocity impact damage mechanism is of great theoretical significance and engineering application value to improve the utilization and efficiency of 3D braided composites in order to better serve the national economic construction.In order to give a deep insight into the low-velocity impact damage mechanism of 3D braided composites,firstly,the relevant experimental research is carried out,and the damage mechanism is revealed through the experimental phenomenon.Secondly,taking 3D four directional(3D4d)braided composites as the baseline architecture,the meso-structure analysis,elastic property prediction and progressive damage analysis for quasi-static loading condition,as well as low-velocity impact are carried out step by step.The main contents and innovation work are summarized as follows:(1)The quasi-static and low-velocity impact tests of 8 kinds of 3D braided composites and a laminated composite,which were manufactured by the same kind of fiber and matrix materials,are carried out.The difference of mechanical properties between 3D braided composite and laminated composite,and the influence of braiding parameters on the mechanical properties and damage modes of 3D braided composite are compared and analyzed.The low-velocity impact test results show that there is a ‘knee point phenomenon’in the impact resistance of 3D braided composite,and the effect of different braiding parameters on the low-velocity impact damage resistance and damage tolerance are compared and analyzed.The experimental results also provide input parameters for further theoretical analysis.(2)Based on the meso-scale ‘positive axis model’ of 3D 4-directional(3D4d)braided composite,the improved model takes the cross-sectional shape of the outer yarn and the extrusion effect,and a hybrid model for predicting mechanical properties is proposed based on the iso-strain and the iso-stress model.The results show that the hybrid model can effectively predict the elastic engineering constants of 3D4 d braided composites.At the same time,based on the fabric architecture of braiding yarn,a parametric modeling method at meso-scale level for 3D4 d braided composite is also presented.(3)The meso-scale finite element models(FEM)for inner and surface unit cells of 3D4 d braided composite are established based on ABAQUS/Standard and UMAT user-defined subroutine.Different damage criterions are adopted for fiber bundle and matrix respectively,and the appropriate degradation methods of material properties are implemented.Through the FEM results,the tensile,compressive and shear stress-strain curves along with the strength properties of two kinds of unit cells are obtained,and the damage initiation,propagation mode and final failure mode are analyzed in detail.The results show that the mechanical properties of the inner and surface unit cells are significantly different.(4)Based on the finite element analysis software ABAQUS and VUMAT user-defined subroutine,an integrated FEM of low-velocity impact damage and post-impact compression of 3D braided composite materials is established.In this model,the areas of inner and surface unit cells are homogenized,and the elastic and strength properties of inner and surface unit cells are given respectively.Combined with corresponding damage criteria and degradation methods,the damage mechanism of 3D4 d braided composite under low-velocity impact is analyzed in depth.The results show that the model can effectively capture the low velocity impact damage area,and the relative errors of characteristic parameters during the impact process between FEM and test results are less than 10%.Furthermore,the prediction of the compression after impact strength is also in good agreement with the experimental results.