Ablation behavior and damage mechanisms of carbon/boron-modified phenolic 2.5D woven composite

2.5D woven structures are of interest in new thermal protection systems for their excellent interlayer properties and structural stability. This paper mainly presents the influence of the reinforcement structure and matrix microstructure on the ablation mechanism and tensile progressive failure of carbon/phenolic 2.5D woven composites. Two types of carbon/boron-modified phenolic 2.5D woven composites, named CDP (1.46 g/cm3) and CLP (1.23 g/cm3), were prepared by adjusting the resin concentration. The specimens were subjected to oxyacetylene flame ablation tests with different ablation times (30 s and 60 s), and the internal features were obtained by X-ray micro-computed tomography (Micro-CT). Tensile mechanical properties of 2.5D woven composites before and after ablation were characterized in combination with acoustic emission (AE). The results indicated that the microstructure of the matrix has an important influence on the ablative properties of the specimens. Specifically, CDP exhibited a better linear ablation rate (0.029 mm/s), while CLP indicated a lower mass ablation rate (0.019 g/s). Furthermore, the strength and modulus of the specimens were reduced after ablation, in which CLP retained higher post-ablation residual properties, 38.82% of the original strength and 68.31% of the original modulus, than CDP. More importantly, the final failure of CDP was controlled by mode I and mode II cracks, whereas that of CLP was only dominated by mode I crack.