Research On Prediction Models For Stiffness And Strength Of C/C Composites

Carbon/Carbon (C/C) composites are regarded as the most promising heat-resistant materials for long-playing structures at ultra-high temperature and have important strategic value to the national defense. However, researches on mechanical properties such as stiffness and strength of C/C composites are behindhand, because of complexities of their holes and interfaces. Such restriction has limited C/C composites for further application. Therefore, it is necessary to analyze influencing factors on mechanical properties of C/C composites and build theoretical models for predicting stiffness and strength of them.The main work done in this dissertation includes the following:(1) Unidirectional and three-dimensional and four-directional braided C/C composites are processed. The tensile strength of carbon fibers is tested. The strengthes of fiber-matrix interfaces of C/C composites are tested. The mechanical properties such as tensile, flexure, shear and fracture toughness of C/C composites are experimented, and the damage mechanisms and failure modes are analyzed. The function of poly-Weibull compound describing strength distribution of single fibe is deduced. The shear strength model of fiber-matrix interfaces with considering uneven shear stress along fibers'circle is built. The double-coattail sample for tensile test of four-directional braided composites is designed.(2) The stiffness prediction model of unidirectional composites is built, in which the influence of holes is taken into account. In this model, fibers are assumed to place in hexagonal arrays, the multi-ply nested square model proposed in this paper is adopted for predicting transverse and shear moduli of unidirectional composites. Compared with the experimental results, the stiffness prediction model is accurate enough. The influences of fiber volume fraction and porosity on stiffness of unidirectional C/C composites are analyzed.(3) Based on the stiffness prediction model of unidirectional composites, the stiffness prediction model of three-dimensional and four-directional braided composites is built, in which the influence of holes is also taken into account. The predictive results show that the prediction model is simple and accurate. It is discovered that "cutting-edge effect" consists in cutting-edge three-dimensional and four-directional braided C/C composites. The influences of fiber volume fraction, braiding angles and porosity on stiffness of three-dimensional and four-directional braided C/C composites are analyzed.(4) The random crack core theory and model for predicting the longitudinal tensile strengths of unidirectional composites is built. In this model, the evolvement probability of random crack cores is analyzed based on the perfect evolvement process; some fibers breaks simultaneously is taken into account; the influenced-length of random crack cores increasing with the number of broken fibers is considered. Compared with several classical theoretical models, the predictive results of the random crack core model are more excellent. The influences of size, interface strength, fiber volume fraction and original break density on strength of unidirectional C/C composites are analyzed.(5) Based on the theory of random crack cores and analysis of damage forms, damage analysis method and strength model of three-dimensional and four-directional braided composites is built. Compared with the experimental results, the damage analysis method and strength prediction model is reasonable. The influences of braiding angles, size, interface strength, porosity and cutting-edge effect on damage modes and strength of three-dimensional and four-directional braided C/C composites are analyzed.