Investigation On Characterization Of Micro And Meso Structures And Their Influence On Effective Properties Of Carbon/carbon Composites

Multi directional braided Carbon/carbon (C/C) composites are considered as the most important structural materials for high temperature application in Aviation and Aerospace. However,the microstructure characteristics such as micro-crack, hole,.interface debonding inevitably formed in composite manufacturing, processing and utilizing have significant effect on service performance of materials. Due to the nonhomogeneity and randomness of Multi directional braided Carbon/carbon (C/C) composites microstructure characteristics, the mechanical properties behave random distribution; however, for long period it is difficult to acquire In Situ /Nondestructive microstructure characteristics, the research of mechanical properties discreteness and its mechanism for Multi directional braided Carbon/carbon (C/C) composites at its beginning stage. So in this paper, the micro and meso structures of Multi directional braided Carbon/carbon (C/C) are observed and characterized. Based on the results of these characterizations, the mechanical properties of the component materials and the interfaces in Multi directional braided Carbon/carbon (C/C) are investigated. The effective properties of the C/C composites are analysed in this paper, and the relationship between the micro and meso structures and the effective properties are obtained also. The main research contents are as follows:The matrix and the interfacial layer are supposed for the multi-phase non-homogeneous materials, The holes and delamination as a constituent phase in homogeneous medium, Derive the formula for calculating the effective elastic properties, by using Micro-structural mechanics, and micro-mechanics and the derived its strength calculating formula and its influencing factors by using the porous media strength theory. Suppose fiber bundle for the transversely isotropic material, by Means of mean-field approximation and self-consistent Approximation, analyzing the effective elastic properties, and analyzing the intensity and distribution of fiber bundle through micromechanics strength theory, results showed that the strength of fiber bundles fitted the normal distribution.Macro-meso-micro structure characteristics of Multi directional braided Carbon/carbon (C/C) composites were studied. First, researched the Periodical structure characteristics, based on its braiding ways combines observational analysis gained by stereomicroscope and polarizing microscope, the materials are classified into periodical unit-cell and sub unit-cell, because of obvious interface layer between matrix and fiber bundle, witch is founded in observations, particularly propose interface as composition unit in sub unit-cell. Then focuses on the internal micro-structure, using the latest high-resolution Mirco-CT observation systems, conducted systematic observation and analysis for micro-structure of 3D-Carbon/carbon (C/C) composites. Obtained non-destructive, situ micro-structural information, Founded in Multi directional braided Carbon/ carbon(C/C) composites, within the matrix exists in a large number of holes, in the interface layer exists in a large number of delamination, its diameter, concentrated on between 20 microns to 150 microns. In addition, using the mercury intrusion method, the pores of the material were tested; results showed the pore size distribution were consistent with the Micro-CT observations.Studied the the mechanical properties and its distribution of component materials (matrix and fiber bundles). Nanoindentation tests used to obtain the theory elastic properties of pure carbon matrix to obtain the elastic modulus. Statistical analysis of t microstructural characteristics found that within the matrix pore size distribution fitted the log-normal distribution, calculated on this basis, the matrix containing the hole analyzed the effective elastic properties of fiber bundles, the fiber bundle strength and its distribution were calculated.The interface layer micro-structure information for statistical analysis and found that the interfacial layer delamination exists to meet the scale of the distribution of the same log-normal distribution, The statistical analysis for interface layer micro-structure information found that the delamination interface layer size distribution fitted the log-normal distribution, as a basis,the mechanical properties of the interfacial layer was calculated and analyzed. Through the fiber bundle push-out test , the mechanical properties calculation results of the interfacial layer are verified and found that the shear strength test was lower than the calculated values. On the fiber bundle push-out to establish the finite element model of push-out test analysis of the stress distribution, explaining the fiber bundle push-out reasons for the low results. FEM analysis of fiber bundle push-out test, analyzed the stress distribution, explaining the low results of the fiber bundle push-out.Finite element model for Fine Weave Pierced Carbon/Carbon Composites is founded. Conducted FEM Simulation analysis of carbon / carbon composite mechanical behavior,then Comparative analysed With the experimental results. Simulate the mechanical behavior of sub-unit cell by means of seting the mechanical properties of component materials and interfacial layer as a sub-unit cell model input parameters,obtained its Effective Elastic Properties,on this basis,molding the unit-cell of 3D-Carbon/carbon (C/C) composites,simulated and analyzed the macroscopic mechanical properties of materials. The influences on effective properties by component materials are investigated also. The results showed that Z-load The materials is under a uniform load, so the strength bears little dispersibility.; that the X or Y -load within overlap in the carbon cloth the bending part of fiber produce stress concentration, generated the amplification effect to the strength of the fiber bundle dispersion, causing the material to the intensity of XY greater dispersion; and that the influences on effective properties by fiber bundles are more notable than by matrixes and interfaces. Testing Fine Weave Pierced Carbon/carbon (C/C) composites mechanical properties, the Carbon/Carbon Composites taked as specimen measured in different directions gained the tensile strength and tensile modulus, found that Z-dirction test a small discrete pieces to the XorY–directions a great dispersion of the specimen. Comparing the calculation results with the experimental results, it is found that these two results are accordant...