Influence of laminate lay-up on oxidation and damage growth: Isothermal aging

The thermo-oxidative behavior of a composite is significantly different from that of the constituents as the composite microstructure, including the fiber/matrix interphase/interface, architecture and ply lay-up introduce anisotropy in the diffusion and oxidation behavior. In this work, light microscopy and scanning electron microscopy techniques are used to characterize the oxidative process in laminated carbon fiber-reinforced polyimide composites. Four different composites are considered, namely, unidirectional [0]16T, quasi-isotropic [0/±45/90]2S, cross-ply [0/90]4S, and angle-ply [±45]2S laminates. The observed anisotropy in composite oxidation is explained by carefully monitoring the development and growth of damage through the use of fluorescence imaging using dye impregnation. It is shown that the oxidation behavior of a laminate is strongly dependent on the ply stacking sequence, while alternative pathways for transport of oxygen into the interior of the composite are fiber–matrix debonds and matrix cracks that propagate with the oxidation front.