The effects of isothermal aging on the interfacial and shear properties of composites

The research for this thesis was divided into two segments. The first segment examines the effects of isothermal aging on the shear properties of a G40/BMI composite. The composite was aged at 150°C for periods ranging from 0 to 10,000 hours. Tests were conducted after the composite was aged in order to determine weight loss, change in glass transition temperature, inplane shear strength and shear modulus. After 10,000 hours of aging, the composite specimens had a weight loss of 0.3 percent and a shift in glass transition temperature from 215°C to 265°C. Using the losipescu test method, the shear properties were measured, and it was determined that no change occurred in either the inplane shear strength or shear modulus after 10,000 hours of aging. Thus, long-term exposure at 150°C for 10,000 hours appears to have no effect on the shear properties of this composite.

The second segment examines the effects of isothermal aging on the interfacial properties of a IM7/epoxy composite. Single fiber coupons were manufactured and aged at 100°C for periods ranging from 0 to 2000 hours. Tests were conducted after the coupons were aged in order to determine changes in glass transition temperature and changes in the fiber-matrix interface. After 2000 hours of aging, no change in glass transition temperature was detected. Using the single fiber fragmentation method, the fiber-matrix interfaces of the composite coupons were examined. After 2000 hours of aging, interfacial shear strength decreased, fiber-matrix debonding increased, and fiber slippage increased. Also, SEM examination revealed aging-induced cracking between the fiber and the matrix along the interface. Thus, exposure to elevated temperatures can severely degrade a composite's interfacial properties.