Research On Macro-meso-mechanical Properties And High Velocity Impact Damage Of3D Multi-directional Braided Composites

The yarns of3D multi-directional braided composites were intertwined to form the spatialnear-net-shape structure, which can fundamentally overcome the fatal shortcomings of lowinter-laminar strength and low delamination resistance of conventional laminated composites.Meanwhile, the excellent intergrated mechanical properties of3D braided composites can highly meetthe requirements of aerospace structure materials for weight reduction and high load-bearing capacity.Thus,3D multi-directional braided composites are believed to have broad potential applications inaeronautics and astronautics industries. However, the microstructure of3D braided composites isextremely complicated and shows intensive inhomogeneity and anisotropy, which results inconsiderable difficulties in the researches of macro-meso-mechanical properties and damagemechanicsms. This thesis is focused on the establishment of structure model, prediction of stiffnessand strength properties and analysis of high velocity impact damage of3D braided composites byfinite element method. The main contents are as follow:(1) According to the four-step1×1braiding process and the moving mechanisms of carriers onthe machine bed, the in-plane and spatial movement traces of yarns in the interior, surface and cornerregions were studied for3D four-directional and five-directional braided composites, and the topologygeometrical model was established by analyzing the spatial configuration of yarns and microstructurecharacteristics of different unit-cells in these three regions. Based on the specific cross-sectional shapeassumption of yarns and the consideration of yarn jamming conditions in different regions, the threeunit-cells structure model was founded through fitting the spatial yarn path by a series of positionalnodes and spline interpolation function method. By building the relationships of braiding processingparameters and micromechanical structure parameters, these structure models were parameterizedwith input parameters of yarn cross-section dimensions and interior braiding angle. The surfacemulti-cells structure model and the spatial structure of interior unit-cell revealed by virtual cuttingwere compared with the actual specimen structures, which verified the validity and effectiveness ofthe three unit-cells models.(2) Based on the three unit-cells structure model, a three unit-cells finite element model wasestablished for predicting the elastic properties of3D multi-directional braided composites. The finiteelement model was implemented by introducing appropriate boundary conditions and averagingmethod on the platform of ABAQUS/Standard. The differences of mechanical characteristics of the three unit-cells were analyzed. Moreover, the deformations and stress distributions of the threeunit-cell model were presented and analyzed in various loading cases. The effects of the braidingangle and fiber volume fraction on the elastic constants were finally investigated in detail.(3) In order to resolve the conflict between quick mesh generation of unit-cell and successfulapplication of periodic boundary conditions, the mathematical expression for general periodicboundary conditions was developed in the micromechanical analysis of textile composites withaperiodic mesh. The application of general periodic boundary conditions in finite element softwarewas realized by enforcing muti-point constraints (MPC) to the corresponding nodes on paired faces,edges and corners of the object unit-cell. The deformation, stress distribution and the predicted elasticconstants of3D four-directional braided composites were compared between the unit-cell models withperiodic and aperiodic mesh, thus verifying the effectiveness and applicability of the proposed generalperiodic boundary conditions.(4) The periodic unit-cell-based micromechanical damage model by using nonlinear finiteelement method was presented to simulate the damage and failure of3D multi-directional braidedcomposites under unidirectional tension on the basis of user material subroutine UMAT in the finiteelement software ABAQUS/Standard. Different constitutive models, damage initiation criteria andcorresponding material degradation schemes were adopted for yarns, matrix and interface tosimulate different damage modes in constituent materials. The whole process of damage initiation,propagation and catastrophic failure for multi-directional braided composites with typical braidedangles were simulated in detail. The damage mechanisms were revealed in the simulation process andthe strength of braided composites was predicted from the calculated stress-strain curve.(5) A finite element model for simulating the high velocity impact damage in3Dmulti-directional braided composites was proposed by describing the structures and materials of thethree unit-cells through user material subroutine VUMAT in the finite element softwareABAQUS/Explicit.3D rate-dependent constitutive models were used to determine the constituentbehaviors in the three unit-cells, and Tsai-Wu failure criterion with various damage modes and Misescriterion were considered for predicting damage initiation and progression of yarns and matrix,respectively. The damage propagation process and ballistic resistance of3D braided composites werestudied detailedly. In addition, discussions were carried out to uncover the influences of strikevelocity on the ballistic performance and energy absorption characteristics of the braided compositestructures.