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A multiscale approach is used to analyze the oxidation ablation of parallel carbon fiber (the fibers are parallel to the material surface) reinforced polymer composites(CFRP). The matrix and fiber will be eroded at extreme temperatures. Because the ablation rate of matrix is bigger than fiber, the fiber will be exposed in a certain depth from surface. In the microscale, a model is developed for analyzing the number of exposed fibers and the fiber geometry at low air flow and different temperatures. The model shows that the exposed number increases with the growth of ratio of ablation rate of matrix and fiber, and the ratio doesn’t monotonously increase with the rise of temperature. Surface ablation gradually turns to volume ablation with the increase of the number of exposed fibers. In the macroscale, the effective reactivity of the material is equal to the integral over the ablation zone. Compared with perpendicular carbon fiber reinforced polymer composites, both of them reach maximum number or length of exposed fibers during the transition temperature region where "reactivity limit" turns to "diffusion limit". When the fibers are perpendicular to the surface, the macro ablation behavior agrees with “weakest link law”, in other words the matrix recession rate is the most determining parameter for the effective behavior, but it may be not reasonable for parallel fibers.
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