13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- purpose of C/C thermal protective materials based on the ablation research and simulation In recent study, Aspa et al.[5] carried out a one-dimensional diffusion model to describe the effective surface ablation recession. Their analysis focused on the ablation of diffuse-controlled reaction. Vignoles et al.[6,7] investigated surface roughness of C/C composites by SEM. They discovered that fibers, matrices and interphase have different ablation resistance, which usually leads to typical surface roughness feature. In their work, a three-dimensional reaction-diffusion local model was set up to simulate the formation of the typical needle shape of the carbon fibers during ablation. Laborde et al.[8] established a damage model for carbon-carbon orthotropic composite materials with a special attention to the thermo-mechanical effects. They performed a numerical solution using an implicit incremental scheme implemented in ABAQUS. The origanization of the paper is as follows. In the section 2, a brief introduction is given of ablation mechanism of C/C composites. Section 3 is devoted to classify the C/C ablation recession into three types according to the ablation mechanism. In section 4, a mesoscopic analytical model of C/C composite with high temperature heat conduction was established. The analysis used a heat-transfer finite element in ABAQUS to simulate the ablation morphology of C/C composites at ultra high temperature. The last section is the summary of the main conclusion resulted from the present work. 2. Ablation mechanism of C/C composites There are mainly two types of carbon-oxygen (C/O) reaction for C/C composites in oxidation atmosphere: C+O2=CO2 and2C+O2=2CO. In the ablation process, the surrounding oxygen is transferred to the surface of the C/C composites. Ablation reaction primarily occurred on the surface of those materials, which resulted in mass loss with melting, sublimation and particle erosion. Then with the inward diffusion of oxygen in the surface microcracks and pores of C/C structure, further oxidation occurred. During the reaction process, fiber and matrix are oxidized simultaneously. Whereas the reaction rate of fibers is lower than the matrix. That means, along with the aggravation of ablation, the first melted interface layers separate the fibers and matrix. Ultimately, the fibers are burned into needle shape, while the around matrix are eroded into cylindrical shell. Fig.1 shows the ablation process of a C/C composite. Figure 1. The ablation of the C/C composites 3. Ablation reaction rate of C/C composites The oxidation reaction of C/C composites without high-temperature oxidation resistant coating is time-independent, and is mainly controlled by gas/solid interface reaction rate. In this case, the mass loss rate of C/C composites is linear with response time, as shown in Equation 1. l m k t A Δ = (1) where m A Δ is the mass loss of C/C composites per unit area; lk denotes coefficient of linearity
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