HYGROTHERMAL EFFECTS ON THE THERMAL AND MECHANICAL PROPERTIES OF IM7/5250-4 COMPOSITES

摘要

The objectives of this study are (1) to understand moisture absorption during hygrothermal exposure and desorption in heating up and (2) to find out hygrothermal degradation effects on thermal and mechanical properties of IM7/5250-4 composites. Two types of environmental conditioning were applied to the composites. The first condition consisted of accelerated isothermal aging in moist environment at elevated temperature in pipe bomb. The second was hygrothermal cycling between 98 percent relative humidity/room temperature and elevated temperature. It was found that (1) the saturated absorption of the composites under room temperature and 98 percent relative humidity is about 1.56 percent; (2) the moisture diffusivity is 3.26X10~(-7) mm~2/hr; (3) certain amount of moisture is retained in the composite even at temperature much above 100 deg C over a period of time; (4) the glass transition temperature of the composite exposed to isothermal hydrolytic condition generally increased with exposure temperature and time whereas composite undergoing hygrothermal cycling showed significant increase during initial several cycles following by a quick drop with further increase of cycles; (5) the reduction in in-plane shear strength during hygrothermal cycling was more than the reduction within the same high temperature exposure time in isothermal holding. In addition, a correlation was developed that predicts the long term cycling time, t_l, that produces the same reduction in shear strength as that obtained from a given accelerated thermal conditioning time, t_s.