Study On The Mechanical Properties Of Fiber-bar Composites Reinforced By Three-dimensional Weaving

Three-dimensional weaving (3D) composites have been widely used in the engineering fields due to the excellent mechanical performances, which has drawn much concern from the material researchers and engineers. There are complicate spatial structures fabricated by the weaving yarns inner the 3D weaving composites. The mechanical properties of 3D weaving composites are dependent on not only the components, but also the weaving structures. Fiber-bar composites reinforced by three dimensional weaving (FBCR3DW) has new structure which is formed by the combination of 2D laminate structure and 3D weaving structure. The yarns are intertwined with the fiber-bars alternately, and the directions of yarns varied as the different weaving laminates. The microstructure of FBCR3DW is complicate and shows intensive inhomogeneity and anisotropy, which results in considerable of difficulties in researching of the mechanical properties. The thesis is focused on the calculation of the elastic constants, the analysis of the deformation of the preform, the analysis of the stiffness properties and strength properties and the analysis of the low velocity impact damage of FBCR3DW. The main contents are as follows:In chapter 2, we first went back to the development of 3D weaving composite, the methods of study of the mechanical properties including the experimental studies, the theory analysis model, the numerical simulation of stiffness properties and strength properties. Then the routine of evaluating the mechanical properties of FBCR3DW was provided. The study of the mechanical properties of FBCR3DW was carried out from the meso-structure and the meso-mechanical properties to the macro-mechanical properties.According to the characteristics of the structure of FBCR3DW, a model of stiffness calculating layer-by-layer was proposed in Chapter 3. Based on the Eshelby-Mori-Tanaka equivalent inclusion theory, the elastic constants of unidirectional laminate composites were calculated. The effects of array distance of fiber-bars on the stiffness properties of unidirectional laminate composites were investigated and discussed. Subsequently, with the elastic constants of unidirectional laminate composites obtained by the model of stiffness calculating layer-by-layer, the elastic constants of composites with 0/90,45/135 and 0/90/45/135 weaving schemes of FBCR3DW were calculated based on volume average method. The effects of array distance of fiber-bars on the stiffness properties of were also studied. The cross weaving yarn inner the composites subjected complex stresses when the composite was loaded. Thus, the mechanical analytical model for cross weaving yarn was built and the stresses were calculated. The preform of FBCR3DW was deformed in the manufacturing process due to the unbalanced tensile loading on the fiber-bars. Thus, the deflection of the fiber-bars were calculated and then the effects of diameter, the array distance of fiber-bars and the yarn tensile force on the deformation of fiber-bar as well as preform were studied respectively.In Chapter 4, the finite element models of FBCR3DW were establised in the form of representative volume elements (RVEs) according to the weaving geometrical structures. By applying periodic boundary condition, the stiffness properties of FBCR3DW with different weaving schemes were predicted by finite element method. The effects of yarn packing factor on the elastic constants of FBCR3DW with three kind of weaving schemes were analyzed in detail. The numerical values agree well with experimental results. The stress distribution on the yarns, fiber-bars and matrix were presented and analyzed when the RVEs were under transverse and longitudinal loading case. Resulting from the winding with the array of fiber-bars alternately, the fibers in the circumference of cross weaving yarns are distorted. The geometrical model which can reflect the distortion characteristics of cross weaving yarn was created. The stochastic function theory was introduced and adopted to calculate the flexure matrix of the distorted yarn. The distortion characteristics of cross weaving yarn were implemented in the finite element model of FBCR3DW. By numerical simulation, the effects of yarn distortion on the stiffness properties of independent yarn and FBCR3DW were investigated and discussed in detail. Moreover, the effects of yarn distortion on the tensile properties of independent yarn and FBCR3DW were studied as well.In chapter 5, the progressive damage analysis model of FBCR3DW under unidirectional tension was established by introducing Hashin damage initiation criteria and Murakami-Ohno damage evolution theory. The damage evolution and failure of FBCR3DW with three weaving schemes were simulated by finite element method and the tensile stress-strain curves were obtained. The multiple damage modes happened on yarns, fiber-bars and matrix were discussed. The whole process of damage initiation, propagation and final failure of FBCR3DW under unidirectional tension were investigated and studied in detail. Therefore, the failure mechanisms of FBCR3DW with different weaving schemes under unidirectional tension were obtained. The numerical results agree well with the experimental results. In order to investigate the effect of strength of interface between matrix and fiber-bars, a cohesive element was introduced into the damage analysis model. The effect of interfacial strength on the tensile stress-strain curves and strength properties of FBCR3DW were studied and discussed. The damage model with Puck damage initiation criteria and Blackkette stiffness reduction scheme were proposed to investigate the damage and failure of FBCR3DW under unidirectional compression. The process of damage and failure were obtained and studied by numerical simulation. Based on the material parameters obtained by the laminated model of stiffness calculation in Chapter 3, a macro finite element model of low velocity impact was built. The Chang-Chang failure criteria were adopted to judge the damage of FBCR3DW under low velocity impact in the FEM. By finite element method, the damage modes and damage propagation for each weaving laminated composites in different weaving structures were investigated and discussed detailedly. In addiation, the meso damage evolvement of independent yarn composites under impact was investigated.In Chapter 6, some specimens of FBCR3DW with different weaving structures and different procedure parameters were manufactured and conducted on tensile and compressive tests. The elastic constants of specimens were measured and the tensile and compressive behaviors were obtained. The macro and micro failure modes of specimens were gotten by observing the fracture morphology of the fracture cross-section. The experimental results verified the reasonability of the stiffness calculating model in Chapter 3, finite element model in Chapter 4 and progressive damage model in Chapter 5 effectively.