Changes in microstructure and mechanical properties are investigated as a function of epoxy-amine stoichiometry. The epoxy-amine system studied exhibits a two-phase structure consisting of a hard microgel phase and a dispersed phase of soft, unreacted and/or partially reacted material. The size distribution of the microgel regions tends to increase with increasing amine content. Concurrently, the connectivity of the softer phase increases dramatically. This two-phase structure is inherently fractal, exhibiting a single glass transition temperature, T g . The T g and elevatedtemperature properties of the epoxy are directly correlated with crosslink density and the percentage of microgel phase observed in microstructure studies. The fracture toughness at room temperature increases with increasing amine content, most likely due to the increased presence of the soft phase, which absorbs more energy during crack growth. Changes in modulus values at 30°C with stoichiometry are explained by considering the effective aspect ratio of the polymer structure in the determination of sample rigidity. Relationships between microgel sizes and the sizes of interphase regions that form in composite and adhesive systems are also discussed in terms of interphase properties.

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Translation of tensile properties from high-performance fibers to end-use fabrics is sensitive to weaving-induced filament defects and environmental exposure. In this effort, isolated and combined effects of hygrothermal exposure and curvature-induced kink bands on tensile strength of Vectran TM HT (polyarylate liquid crystal polyester fiber) and Kevlar Õ KM2 are studied. Hygrothermal conditioning was conducted at temperatures ranging from 40 C to 100 C in water for 30 days. Curvature-induced defects were created by wrapping tows around stainless steel rods of different diameters (0.25 mm to 5 mm) to create kink bands. Combined effects were evaluated by conditioning tows with kink bands at 100 C for 30 days. All conditioned samples were dried and tested at room temperature. Hygrothermal aging showed that tensile properties for Vectran fibers were not appreciably affected below 100 C ($12% reduction), while KM2 fibers dropped continuously with increasing temperatures ($48% at 100 C). The influence of curvature on kink band density was established for each fiber type. The isolated effect of kink band density on residual strength was approximately 15% for both Vectran 1670/600 and KM2-600. Combined effects of curvature-induced kink bands followed by hygrothermal exposure showed significant reductions in tenacity up to $96% for KM2 and 60% for Vectran HT1670/600. Inspection of the microstructure within the kink bands reveals extensive micro-cracking and fibril failure due to accelerated moisture ingress.

A novel susceptor concept for metal mesh susceptors, designed to achieve uniform in-plane temperatures during induction heating, is documented. The process involves redirecting eddy current flow patterns in the resistive mesh susceptor by specifically designed cut patterns in the mesh. A theoretical model was developed to predict heat generation in metal mesh susceptors with any prescribed network pattern. Initial results for meshes with cut patterns show significant changes in heating compared to an uncut mesh. Cut patterns can be optimized to reduce temperature gradients in the susceptor to within the processing window of the composite. Experimental results are presented for qualitative comparisons.