Matrix Microcracking Effect on the Structural Response of a Thermal Protection System

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. Due to 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.