Methods for Detecting Moisture Degradation in Graphite-Epoxy Composites,
ROCKWELL INTERNATIONAL THOUSAND OAKS CA SCIENCE CENTER
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Hydrothermal treatment combined high moisture and temperature of graphite fiber reinforced epoxy matrix composite produces irreversible deterioration in shear strength and modifies the Weibull distribution of survival probabilitys. Analysis of water diffusion kinetics shows that strength degradation is dominated by the matrix bulk properties. Ultrasonic 2.25 MHz velocity transverse to the fiber axis is sensitive to current moisture content while acoustic attenuation correlates with prior moisture history and strength degradation. The high glass transition temperature of the epoxy relative to maximum moisture exposure temperature restricts extensive degradation matrix and interface. Ultrasonic methods are applied for scanning the effects of positionally variable moisture content and hydrothermal damage in a composite bar. A number of NDE methods including dynamic mechanical spectroscopy 1.0-100 Hz, NMR pulse relaxation spectroscopy, and microwave spectroscopy 2.64 GHz are shown to provide highly resolved moisture effects data. The development of composite durability characterization and ultrasonics scanning coupled with direct measurement of bulk moisture content appears to offer a versatile NDE methodology for quantitative detection of hydrothermal aging effects on large composite structures.