Investigation On The Mechanical Properties And Size Effect Of Three-dimensionial Braided Composites

With rapid development of aeronautic and astronautic fields, three dimensional (3D) braided composites, combining high stiffness and strength at low density, resistant high temperature ability and structural designability characteristics, are used in fields of aeronautics and space predominately. It will become mature material with the improvement of manufacture process and constituent materials. The mechanical properties of the braided materials are required to be recognized correctly because they determine the application foreground of the braided composites. However, the mechanical properties of 3D braided composites are influenced by several factors, such as the complicated fibers configuration, braid process parameters, geometrical parameters, squeezing deformation of braided preforms, mechanical properties of constituents (fibers and matrix), structural integral, fiber volume fraction etc. Because great discrete characteristic of the braided composites comparison with unidirectional material, especially the discrete behavior of material strength, the geometrical size effect, stress-strain relation and strength of 3D braided composites are being in exploratory phase, they are still required to be studied by theories and majority of experiments. In this dissertation, theories and experiments are conducted greatly to study the size effect and mechanical properties of fine woven punctured carbon/carbon composites and 3D four-step braided carbon/epoxy composites. The stiffness and strength of the braided composites are evaluated by numerical and theoretical methods. The detail study contents and conclusions are listed as follows:First, the tensile and compressive experiments of fine woven punctured carbon/carbon composites are conducted. The stress-strain relation and fracture toughness of the material are obtained. And the damage mechanisms and failure modes are analyzed. The study shows that when specimen fails under tensile loading in the Z direction, fractograph normal to Z direction is flat. Some superficial fibers are breakage and pulled out. When specimen fails subjected to tensile loading in the XY direction, the fractograph exhibits step shape. The fibers in the Z direction are failure due to the combining tensile and shear stresses of carbon fabrics. When specimen are applied the compressive loading, the failure modes of specimens under Z directional loading is similar to that of specimens under XY directional loading. The normal of fracture surface is 45°with respect to the loading direction. The failure modes of XY carbon fabric and Z directional punctured fibers are compression-shear failure, respectively, when the specimen is subjected the compressive loading in the XY direction and Z direction. The stress-strain relation of specimen under Z directional loading depends on the geometrical size of specimen. When the area of compressive surface is small, the material of specimen near the edge fails earlier than the failure of specimen. It can be found that the edge effect and fiber effective length have great effect on the mechanical properties of material.Second, the geometry model of fine woven punctured composites is established. The elastic properties of the braided composites are studied by finite element method. The effect of material properties and braid geometrical parameters on the macroscopic mechanical properties of the braided composites is considered. It is found that the variation of Poisson's ratio and elastic modulus in the X direction is converse when the thickness and width of fibers in X and Y direction of carbon fabric. The variation of fiber volume fraction of carbon fabric do not effect he variation of the in-plane stiffness of carbon fabric. Based on the stress analysis, the braided composites can be simplified a unidirectional fiber composites when the braided composites is subjected the loading in the fiber punctured direction. The deformation and inner stress states of the composites under different loading conditions are analyzed to obtain the inner stress distribution of the composites.Third, the tensile, compressive and bending experiments of 3D four-step braided composites are conducted. The size effect of 3D four-step braided composites is discussed. In addition, the effect of braid angles and geometrical sizes on the strength of the braided composites are analyzed. The mechanical properties of the braided composites dependant on the number of inner cell in the thickness and height direction are obtained. The study shows that the fractographs of braided composites with 30°and 45°braid angles under tensile loading are all flat. It is belonged to brittleness material. The tensile stress-strain curves all exhibit linear. The constitutive relations do not change with the variation of braid angle and thickness of specimen in the tensile testing. The bulk specimens of the braided composites with 30°and 45°braid angles under compressive loading appear compressive-shear failure. The cracks of the braided composites with 30°braid angle are extended in the width direction, while the cracks of the braided composites with 45°braid angle are extended in two 45°with respect to the loading direction. In the compressive testing, the epoxy is squeezed out to form matrix strip. And the morphology of matrix strip is different with the variation of the geometrical size of specimen.Fourth, the stiffness of 3D four-step braided composites with different thickness is predicted by stiffness average theory and three cell model. The elastic properties of the braided composites appear size effect with increase of thickness of specimens. The longitudinal elastic modulus decreases with increase of the number of inner cell of specimens to approach a constant value which is the longitudinal elastic modulus of material all comprised by inner cell geometry. The Poisson's ratio increases with increase of the number inner cell of specimens to approach a constant value as well. The geometrical model and finite element model of 3D four-step braided composites are established. The stiffness of the braided composites is obtained by homogenization combining with finite element method. The predicted results of the two methods are in agreement with the experimental data.Finally, because the strength of fiber satisfies Weibull distribution, the strength of fibers should be a statistic magnitude as well. In the process of computation of strength of 3D four-step braided composites, the strength of fibers as a discrete magnitude is introduced to predict the statistical strength of 3D braided composites. Damage analysis of braid fibers and matrix with Hashin 3D stress criteria and maximum stress criteria, respectively, is conducted to obtain the statistical strength of 3D braided composites by using finite element method. The statistical results show that the strength of the braided composites with lower braid angle is determined by the strength of the inner fibers.