Research On Mechanical Properties And Progressive Damage Of 3D Surface-core Braided Composites

Three-dimensional(3D)surface-core braided composite,developed from traditional 3D braided composites,is a new kind of braided structure.Compared with conventional materials,it can effectively restrain the degradation of mechanical properties caused by surface damage,which may further broaden the applied range of braided composites,especially in some harsh environments that may harm the surface of materials,or in particular situations where the materials need to be cut.However,as a new material,related research work on its mechanical properties has not been carried out,thus its meso failure mechanism is not clear,and its performance superiority needs to be verified.This thesis focuses on the microstructure modeling,prediction of effective elastic properties,progressive damage analysis and strength prediction to deeply investigate the mechanical properties and damage mechanism of 3D surface-core braided composite.The main contents are as follows:(1)According to the movement law of yarn carriers in 3D surface-core 4-directional braiding process,the planar and spatial traces of interior,surface and corner yarns as well as their distributions are investigated.Employing three unit-cell division scheme,the spatial configurations of yarns in interior,surface and corner unit-cells are analyzed.The squeezing relationships of interior yarns are studied in detail.Considering the different squeezing situations of yarns in interior and exterior regions,corresponding cross section shapes are assumed,respectively.Then the relationships of braiding parameters and structure parameters of each unit-cell are confirmed,and the three unit-cell geometrical models of 3D surface-core braided composite are established.(2)By selecting the yarn cross section dimensions and interior braiding angle as input parameters,the parameterized solid structure models of 3D surface-core braided composite are proposed.Based on ABAQUS platform,the three unit-cell finite element models are established to predict effective elastic properties of the new material by introducing the general periodic boundary conditions and averaging method.Then the effective elastic properties of each unit-cell and overall composite are obtained,and the effects of braiding angle and fiber volume fraction on effective elastic properties are discussed.Furthermore,the mechanical responses of each unit-cell under longitudinal tensile load and in-plane shear load are minutely analyzed,respectively.(3)The periodic RVE-based micromechanical damage models are proposed to predict the damage initiation and propagation of 3D surface-core braided composite by adopting the nonlinear finite element method and employing the user-defined materials subroutines(UMAT)of the ABAQUS software.In the models,the 3D nonlinear shear stress-strain behavior is introduced to describe the constitutive relations of fiber bundles,and Hashin criteria and Von Mises criterion are selected to identify the damage occurrence of yarns and matrix,respectively.Related stiffness reduction schemes are proposed to degrade the mechanical properties of constituent materials according to corresponding damage modes judged by adopted failure criteria.By simulation,the damage initiation and propagation of the material under longitudinal tensile load and in-plane shear load are respectively investigated,and corresponding strengths are predicted according to the calculated stress-strain curves.Meanwhile,the effects of braiding angle on tensile and shear mechanical behaviors of the material are discussed.(4)Taking 3D 4-directional braided composite as a comparison object with 3D surface-core braided composite,the undamaged and pre-damaged specimens of these two materials including four configurations are prepared,then the longitudinal tensile and in-plane shear tests are conducted.Based on the experimental data,the performance changes of the two materials before and after surface cuttings are quantitatively described.By comparing the effects of surface cuttings on the mechanical properties of the two composites,whether the new material possesses superiority over traditional braided structures to effectively restrain the degradation of mechanical properties caused by surface cuttings is verified.In addition,the failure mechanism of each configuration is analyzed on the basis of the observation of failure specimens.