Research On Mechanical Behavior Of Two Directional Carbon/Carbon Composites

Because of its superior performance, Two directional Carbon/carbon（C/C）composites are widely used in aviation, aerospace and other high-tech fields. Thematerial defects such as holes and micro cracks, inevitably formed in the process ofthe preparation, have obvious effects on effective properties of component materials,which seriously affect the effective performance material. So it is crucial toaccurately obtain micro structure of the material. In the paper, the micro and mesostructures of Two directional Carbon/carbon （C/C） composite material arecomprehensively and systematically observed and characterized in using a variety ofobservational methods. Based on the characterizations, the mechanical properties ofcomponent materials are analyzed. Based on observed periodic representativevolume element, the mechanical properties and damage of the material are analyzedin the finite element method.First of all, the micro and meso structures are observed and characterized inusing a variety of observational methods. Using traditional observation methods,Such as the metallographic microscope and laser confocal microscope and scanningelectron microscopy, observed the micro and meso structures of the material.According to the observation, the fiber bundle section morphology, size anddirection, the structure of representative volume unit and size are determined.Through observation found that matrix are mainly distributed between the layers ofcarbon cloth, pores are randomly distributed inside the fiber bundle and matrix. Themicro structures are observed through the high resolution micro computedtomography. According to the observation, found that the pores which areapproximately spherical are randomly distributed inside the matrix and satisfylognormal distribution. Combined with the observation results, mercury injectionexperiments are performed in order to analyze the distribution of pores, theexperimental measured results were consistent with the observations.Secondly, the mechanical property of component materials are analyzed basedon the micro structure observation information. It is assumed that the matrixis isotropic material, the effective elastic properties of the porous matrix arepredicted by using Eshelby equivalent inclusion theory and self-consistent theory;the effective strength of porous matrix is predicted by the porous media strengththeory. The Fiber bundle is supposed for transverse isotropic material,monofilament strength obeys Weibull distribution, by using the empirical formulaof NASA predicting the effective elastic properties, and using the mesoscopic mechanical strength theory to predict effective strength of the fiber bundle.Thirdly, the RVE model is set up based on the micro structure observation，withthe finite element method to analyze the stiffness and strength properties anddamage process, and through the macroscopic experimental verification. Uniaxialtension compression and biaxial tensile test are performed on macroscopic specimen,analyzing its failure mechanism. Periodic boundary conditions are imposed on unitcell to calculate the effective stiffness matrix of the material, verified through themacro experiment. Establish initial damage criterion of component materials and thedamage evolution equation of damage analysis was carried out in order to analyzethe damage process. Result shows that when stretched, the damage of matrixextended along the Z direction before the X direction, the damage of the X directionfiber bundle in the bending area extended along the Z direction, the damage in the Zdirection extended along the X direction. The damage of transverse direction wastoo much to cause a brittle fracture of the fiber bundle, characterized by brittletensile fracture; when compressed, the damage of matrix extended along alldirections, the matrix surface of RVE appeared a large area of damage, becausethere is no reinforced fiber material in the third direction, the material appearedstratification, the transition area between line segment and curve segment in fiberbundle was badly damaged leading to shear failure happening, characterized bybrittle tensile fracture; Based on the uniaxial test, biaxial tensile test was analyzed,the damage of matrix extended along the X direction and Z direction, the damage oftransition area between line segment and curve segment in two direction fiberbundle extended along transverse direction, characterized by brittle tensile fracture.