370 G.A.Schoeppner,G.P.Tandon and K.V.Pochiraju 9.2.1 Fiber Oxidation Degradation The effects of service temperatures and oxidation on the mechanical pro- perties of high-temperature polymer composites are primarily manifested in the polymer-dominated properties,namely the transverse properties (perpendicular to the fiber direction)and the shear properties.However, the fiber-dominated properties may be affected by degradation of the fiber and deterioration of the fiber-matrix interface.This may be particularly true for composites with glass fibers [44]in which magnesium and sodium can leach out from the fiber and possibly contribute to polymer/interface degradation.It is reported that graphite fibers containing significant amounts of sodium and potassium as contaminants are less thermooxidatively stable than graphite fibers with very low alkali metal contents [39,43].Studies conducted by Bowles and Nowak [12]indicate that extreme oxidative erosion of the Celion 6000 graphite fiber occurs at elevated temperatures in the presence of the polyimide matrix. Bowles [8]has investigated the effects of different fiber reinforce- ments on thermooxidative stability of various fiber-reinforced PMR-15 composites.The ceramic Nicalon and Nextel fibers were found to drastically accelerate thermal oxidation of the corresponding composites because of the active fiber-matrix interface.Compared to polyimide matrices,rein- forcing carbon fibers [60]are usually far more stable at the elevated temperatures considered.Studies conducted by Wong et al.[126]indicate that IM6 carbon fibers are more stable than the G30-500 fibers within the first 600 h during thermal oxidation at 371C.The oxidation rate of the IM6 fibers then increases substantially,leading to complete decomposition of the fibers during the final stage.The sudden increase in the oxidation rate in the IM6 fibers implies the possibility of change in degradation mechanisms. The three PAN-based carbon fibers of interest to the present body of work are two low modulus carbon fibers T650-35 and G30-500 that are typically used in HTPMCs,and one intermediate modulus carbon fiber, IM7.Isothermal aging studies were recently conducted [105]on these three types of fibers to study their oxidation behavior.Both sized and unsized fibers were exposed to different elevated temperatures for varying time periods in an attempt to understand the influence of the fiber sizing/ coupling agent on their thermooxidative stability.Note that aging of the bare fibers may not necessarily be representative of the behavior of the in situ fibers embedded in the matrix,because the exposed surface area of the fibers in the composite is only a very small percentage of the total surface area of the fibers.Thermal degradation was quantified by the amount of weight loss measured,while degradation of mechanical properties was9.2.1 Fiber Oxidation Degradation The effects of service temperatures and oxidation on the mechanical pro￾perties of high-temperature polymer composites are primarily manifested in the polymer-dominated properties, namely the transverse properties (perpendicular to the fiber direction) and the shear properties. However, the fiber-dominated properties may be affected by degradation of the fiber and deterioration of the fiber–matrix interface. This may be particularly true for composites with glass fibers [44] in which magnesium and sodium can leach out from the fiber and possibly contribute to polymer/interface degradation. It is reported that graphite fibers containing significant amounts of sodium and potassium as contaminants are less thermooxidatively stable than graphite fibers with very low alkali metal contents [39, 43]. Studies conducted by Bowles and Nowak [12] indicate that extreme oxidative erosion of the Celion 6000 graphite fiber occurs at elevated temperatures in the presence of the polyimide matrix. Bowles [8] has investigated the effects of different fiber reinforce￾ments on thermooxidative stability of various fiber-reinforced PMR-15 composites. The ceramic Nicalon and Nextel fibers were found to drastically accelerate thermal oxidation of the corresponding composites because of the active fiber–matrix interface. Compared to polyimide matrices, rein￾forcing carbon fibers [60] are usually far more stable at the elevated temperatures considered. Studies conducted by Wong et al. [126] indicate that IM6 carbon fibers are more stable than the G30-500 fibers within the first 600 h during thermal oxidation at 371°C. The oxidation rate of the IM6 fibers then increases substantially, leading to complete decomposition of the fibers during the final stage. The sudden increase in the oxidation rate in the IM6 fibers implies the possibility of change in degradation mechanisms. The three PAN-based carbon fibers of interest to the present body of work are two low modulus carbon fibers T650-35 and G30-500 that are typically used in HTPMCs, and one intermediate modulus carbon fiber, IM7. Isothermal aging studies were recently conducted [105] on these three types of fibers to study their oxidation behavior. Both sized and unsized fibers were exposed to different elevated temperatures for varying time periods in an attempt to understand the influence of the fiber sizing/ coupling agent on their thermooxidative stability. Note that aging of the bare fibers may not necessarily be representative of the behavior of the in situ fibers embedded in the matrix, because the exposed surface area of the fibers in the composite is only a very small percentage of the total surface area of the fibers. Thermal degradation was quantified by the amount of weight loss measured, while degradation of mechanical properties was 370 G.A. Schoeppner, G.P. Tandon and K.V. Pochiraju