The Effect of Temperature and Moisture Content on the Flexural Response of Kevlar/Epoxy Laminates: Part I. [0/90] Filament Orientation*

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Abstract
This study examines the effects of moisture and temperature on the flexural response of [0/90] Kevlar 49 fabric-reinforced epoxy laminates. Two high temperature (190°C) cure commercially available epoxies, Narmco 5208 and Ferro CE-9000, were used as matrix materials. Results indicate that the mechanical properties of Kevlar 49/epoxy laminates are temperature dependent over the range -55°C to 150°C. The addition of moisture magnifies temperature sensitivity. At 150°C and near saturation moisture content (5 weight percent), a strength loss of 60-70 percent and an effective stiffness loss of 40 percent from dry, room temperature values are measured. At 21°C, saturated specimens are found to be 35-40 per cent weaker than dry material. At low temperature (-55°C), strengths increase with the addition of moisture. Over the range of conditions tested, failure was not catastrophic, but was characterized by a loss in load carry ing capacity. Microscopic examination of failure modes indicates that flexural strength is determined by the ease with which filaments compres sively buckle. These results indicate that care must be taken when design ing with Kevlar-reinforced composites for use at elevated temperatures after exposure to moist environments. An accurate prediction of the ex pected composite moisture content and service temperature is important to the design process.