High-strength carbon fibers were treated with nitric acid and periodically analyzed by several different methods to develop an understanding of overall property changes and how they relate to composite design. Fiber diameter, tensile strength, surface morphology, surface chemistry and surface energy were all evaluated as a function of treatment time and two distinct stages of change were identified; the first characterized by surface modification and the second by carbon material loss. Initially, the tensile strength, degree of surface oxidation and surface energy all increased. The surface oxidation consisted primarily of carbonyl and carboxylic acid types. Then in the second stage, both the tensile strength and surface oxidation reached stable levels and the fiber diameter began to rapidly decrease. The surface morphology and energy were the only properties that showed no obvious changes from one stage to the next. The surfaces were found to be smooth through all treatment times and the energy increased steadily throughout. It is believed that the variation of all of these properties is related to the fiber microstructure and how it varies through the cross-section of high-strength fibers. Specifically, high-strength carbon fibers are known to have better microstructural organization and alignment in the near-surface layer than within the interior.
High-strength carbon fibers were oxidized by exposure to nitric acid and single-fiber wettability predictions were compared to the actual wettability of multiple fibers in resin. Single-fiber wettability was predicted through contact-angle measurements and surface-energy calculations. Multiple-fiber wettability in resin was evaluated by immersing treated fiber bundles in catalyzed vinyl ester resin, followed by cross-sectional viewing after curing. Fiber cohesion, macro-composite void content, and transverse tensile strength were also examined as a function of fiber treatment time. Fiber surface energy increased with treatment time, suggesting improved wettability. However, fiber cohesion also increased and composite wetting was found to suffer. Increasing fiber treatment times resulted in larger unwetted areas, higher void content, and declining transverse tensile strength.
Carbon fibers exhibit exceptional properties such as high stiffness and specific strength, making them excellent reinforcements for composite materials. However, it is difficult to directly measure their tensile properties and estimates are often obtained by tensioning fiber bundles or composites. While these macro scale tests are informative for composite design, their results differ from that of direct testing of individual fibers. Furthermore, carbon filament strength also depends on other variables, including the test length, actual fiber diameter, and material flaw distribution. Single fiber tensile testing was performed on high-strength carbon fibers to determine the load and strain at failure. Scanning electron microscopy was also conducted to evaluate the fiber surface morphology and precisely measure each fiber's diameter. Fiber strength was found to depend on the test gage length and in an effort to better understand the overall expected performance of these fibers at various lengths, statistical weak link scaling was performed. In addition, the true Young's modulus was also determined by taking the system compliance into account. It was found that all properties (tensile strength, strain to failure, and Young's modulus) matched very well with the manufacturers' reported values at 20 mm gage lengths, but deviated significantly at other lengths.
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