The effect of microcracking in the phenolic matrix of a three-dimensional woven thermal protection system (TPS) and resulting material stiffness reduction was studied via a comparison of finite element results from linear and iterative linear analyses. A dual-layer continuous dry weave material with a low-density phenolic resin matrix has been developed for use in extreme environments. Because of high stresses in the through-the-thickness direction, microcracks may form in the matrix. The matrix does not have structural load transfer requirements, and testing has shown that microcracked phenolic resin satisfies thermal requirements. Microcracks in the matrix would result in a reduction of stiffness, which could alter the structural performance. A study was conducted to determine if reduction in material stiffness would change the load paths or structural margins. A linear finite element analysis that did not account for microcracking and an iterative linear finite element analysis that accounted for propagation microcracks were compared. Four subcases were analyzed with results indicating that the assumed propagation strength for the microcracking is the critical parameter for determining the extent of microcracking. Phenolic microcracking does not appear to have an adverse effect on the structural response and is not a critical failure for the modeled TPS.

通过比较线性和迭代线性分析的有限元结果，研究了三维编织热防护系统（TPS）的酚醛基体中微裂纹的影响以及材料刚度的降低。具有低密度酚醛树脂基体的双层连续干法编织材料已被开发用于极端环境。由于沿厚度方向的高应力，可能会在基体中形成微裂纹。该基质不具有结构载荷传递要求，并且测试表明微裂纹酚醛树脂满足热要求。基体中的微裂纹将导致刚度降低，这可能会改变结构性能。进行了一项研究以确定材料刚度的降低是否会改变载荷路径或结构裕度。比较了不考虑微裂纹的线性有限元分析和考虑了传播微裂纹的迭代线性有限元分析。对四个子案例进行了分析，结果表明，假定的微裂纹扩展强度是确定微裂纹程度的关键参数。酚醛微裂纹似乎对结构响应没有不利影响，对于模型化的TPS而言并不是关键性的失效。对四个子案例进行了分析，结果表明，假定的微裂纹扩展强度是确定微裂纹程度的关键参数。酚醛微裂纹似乎对结构响应没有不利影响，对于模型化的TPS而言并不是关键性的失效。对四个子案例进行了分析，结果表明，假定的微裂纹扩展强度是确定微裂纹程度的关键参数。酚醛微裂纹似乎对结构响应没有不利影响，对于模型化的TPS而言并不是关键性的失效。