Research On Key Fundamental Mechanical Properties Of 3D Woven C/C Composites

Because of the high specific stiffness, high specific strength, good impact resistance, and excellent high temperature performance, the three-dimensional (3D) woven C/C composites are important aviation and aerospace materials. It is an interesting and important work to investigate their mechanical properties. However, the present contact measuring methods, which have been widely used in the experiments of homogeneous material, are difficult to be applied in the 3D woven C/C composites, due to the small deformation, anisotropy and material heterogeneity. In order to obtain the accurate displacement and strain distribution of specimens, the non-contact measurement digital image correlation (DIC) method was proposed. Based on the DIC technique, the tensile behavior, shear behavior, and notch properties of 3D woven C/C composites were investigated. On the basis of the experimental results, the improved progressive damage analysis model was proposed to analyze the damage behavior of 3D woven C/C composites. The main points in this dissertation are organized as follows:1 The non-contact displacement measuring system based on digital image correlationAlthough contact displacement measuring techniques can obtain satisfactory results in traditional experiments, they are not the ideal methods to measure the deformation of 3D woven C/C composites, due to the fact of their shortcomings, for examples, that only little displacement information can be measured in one test; the interaction between the device and the specimen will reduce the reliability of measurement results since the displacement measuring devices are in contact with the specimen. In order to remedy the shortcomings of contact measuring methods, the DIC based displacement measuring system was investigated and set up. The experimental results showed that the accuracy and reliability could be improved by using non-contact measurement method.2 Tensile and shear behavior of 3D woven C/C compositesThe in-plane tensile behavior, in-plane shear behavior, and through-the-thickness shear behavior of 3D woven C/C composites were investigated by using DIC method. The experimental results indicated that the tensile stress-strain curve was linear, and the brittle fracture was observed when the specimen failed. Meanwhile, the shear stress-strain curves showed a strong nonlinear, which was similar to the plastic flow of metal materials. From the full-field strain mapping, the phenomenon that material heterogeneity of 3D woven C/C composites leaded to periodic strain distribution was quantitatively revealed. Furthermore, the relationship between the macroscopic mechanical properties and the mesoscopic fiber bundles structures was analyzed. Besides, the size of specimen/gauge was discussed in order to obtain accurate results. This study provided a new insight into the accurate measurement of the mechanical properties of 3D woven C/C composites.3 The notch sensitivity of 3D woven C/C compositesSix kinds of specimens with different notch patterns including one central hole, multi-holes, and double edge notches, were examined to analyze notch fracture properties of 3D woven C/C composites composites. Using the full-field measurement, the strain contour was obtained in the vicinity of notches, and then the corresponding relation between strain distribution and fiber bundle structures was analyzed. For the specimens with central hole, the results showed that the fiber structures in the vicinity of hole had great influence on the strain distribution, stress concentration, and fracture position, but little influence on the strength of specimens. All the results indicated that the fracture stress on the net cross section was constant and it was independent on the material heterogeneity, notch patterns, and notch depth. Full-field shear mapping revealed that the mechanism of the notch insensitivity was the shear inelastic deformation in the vicinity of notch.4 Three-dimensional progressive damage analysis modelFor the analysis of the strength of the structures of 3D woven C/C composites, the progressive damage model which is used to describe material damage behavior was established. In this dissertation, the expression of the damage factor was proposed, the Jacobian matrix used in the numerical calculation was deduced, and the progressive damage model was built in UMAT. Then, the simulation of material damage behavior was carried out based on finite element analysis software ABAQUS 6.10. Finally, the effectiveness of the progressive damage model was verified by the load-displacement curve of simulation and experiment data. The results showed that the progressive damage model could be used in damage analysis and strength analysis of 3D woven C/C composites structures.In this dissertation, the key fundamental mechanical properties of 3D woven C/C composites were investigated under the support of several projects. Based on digital image correlation method and finite element analysis method, the tensile behavior, shear behavior, notch properties and damage behavior of 3D woven C/C composites were systematically investigated. The results help to further understand the mechanism of deformation, damage and failure of 3D woven C/C composites.