Using Optical Microscopy to Monitor Anisotropic Oxidation Growth in High-Temperature Polymer Matrix Composites

Abstract

Durability and degradation mechanisms in composites are fundamentally influenced by the fiber, matrix, and interphase regions that constitute the composite domain. In this work, unidirectional G30-500/PMR-15 composite specimens are aged at elevated temperature in air resulting in oxidation propagation parallel and perpendicular to the fibers. The observed anisotropy in composite oxidation is explained by carefully monitoring the development and growth of damage in composite specimens through the use of fluorescence imaging using dye impregnation in conjunction with optical microscopy. Optical micrographs are taken on polished internal sections and viewed in the dark-field mode to measure the degree, depth, and distribution of thermal oxidation development from external surfaces perpendicular and parallel to the fibers. It is shown that alternative pathways for transport of oxygen into the interior of the composite are fiber—matrix debonds and transverse matrix cracks that propagate with the oxidation front for which the oxidation front consistently precedes the debond crack front.