Progressive Damage And Failure Analysis Of 3D Four-step Carbon/epoxy Braided Composites

In order to meet the requirement of the structural components in aeronautic andastronautic fields, three dimensional (3D) braided composites have been designed anddeveloped, which can resist the multi-directional stress and thermal stress and have highstiffness and strength at low density. With the development of braid processes and mold-ing techniques, it has predominant advantage to form the 3D braided composites withlarge sizes in an automatic braid process and an integral braid manner. In addition, comb-ing the excellent resistance impact, high damage tolerance and material structure redesigncharacteristics, 3D braided composites have been become the third generation fiber re-inforced composites. It is the key to study the thermal mechanical properties, such asstrength, damage tolerance and reliability analysis, for 3D braided composites which canbe widely used in structural components. However, the mechanical properties of the 3Dbraided composites are determined the complicated mesoscopic structures and materialbehavior of constituents. In this dissertation, the macroscopic and microscopic experi-ments for the 3D braided composites are conducted. Based on the periodic braid structureof the 3D braided composites, the representative volume cell (RVC) is established. Themacroscopic mechanical properties, damage and strength of the braided composites areanalyzed by using mesoscopic damage theory.The domestic and foreign researching development of the 3D braided composites,including the geometrical structure, mechanical models, experimental studies and dam-age models for investigating the nonlinear properties of composites have been discussedand analyzed. The general routine to evaluate the mechanical properties of 3D four-stepbraided composites has been provided. 3D braided composites which can be divided intothree levels: macroscopic, mesocopic and microscopic, which can be regarded as multi-scale problems.Based on the micro-braid structural characteristics of the 3D four-step braided com-posites (internal and surface of the braided composites with different braid structures),the different thickness and diameter of the specimens are chosen to conduct the tensileand compressive tests in order to minimize the effect of surface braid structure on themechanical properties of the braided composites. The macro and micro failure modes of the braided composites can be obtained by observing the fractographs of the failurecross-section, which is the experimental basis to establish damage model.Owing to the squeezing of braid yarn each other within the 3D four-step braidedcomposites, the surface of these braid yarns exhibits the undulation and twisted phe-nomenon. In the present dissertation, the surface of each braid yarn within an RVC isdivided into six regions. The effective properties of each region are obtained by stochas-tic function theory. The RVC is applied the periodic boundary conditions by using finiteelement method to calculate the effect of the surface twisted fiber of braid yarn on themechanical properties of the braided composites. Meanwhile, the fiber undulation withinRVC is considered. The effect of fiber undulation on the macroscopic mechanical prop-erties has been studied by using volume homogenization method.In order to investigate the progressive damage and failure process, the damage modelof composites has been established, in which some mescoscopic damage modes, such aslongitudinal and transverse tensile-shear, compressive-shear modes of braid yarn and ten-sile, compressive modes of matrix, are considered. The damage evolution model has beenbuilt, which are depended on the fracture energy of the constituents, characteristic lengthof element and local strain. The Murakami-Ohno damage model has been used to com-plete the stiffness reduction of the constituents of the braided composites. Therefore, theprogressive damage analysis for the braided composites under different loading are con-ducted. In order to improve the numerical convergence, an artificial Duvaut-Lions viscousmodel is used in the damage model. In this dissertation, when the braided composites aresubjected to the uniaxial tensile loading, the interface damage between yarn/yarn andyarn/matrix are considered in the damage model further. The effect of interfacial strengthand viscous parameters on the macroscopic stress-strain curves of the braided compositesis discussed. When the braided composites are under only uniaxial compressive loading,the damage of matrix is neglected but regarded as perfect plastic material. The effect offiber misalignment angle of surface fiber of braid yarn on the mechanical properties of thebraided composites is considered. In addition, the longitudinal shear nonlinear of braidyarn in the numerical model is included. The effect of braid angle (30°and 45°) on themacroscopic stress-strain curves and damage development of the constituents is analyzedfor the braided composites under the longitudinal tensile and compressive loading. Thedamage mechanisms of the constituents are also discussed. Compared with experimental results, the reliability of the numerical results is verified.By applying complicated boundary conditions on the RVC of the braided compos-ites, the strength of the braided composites under different boundary conditions has beenobtained. The macroscopic failure criterion of the braided composites is constructed,which can provide a theoretical criterion to check the strength of structural componentsmade by the braided composites and can be used to the structural optimization design.The researching method can also fit for other multi-directional braided composites, andprovide a theoretical basis for the braided composites widely used in engineering appli-cation.