THERMAL FATIGUE OF COMPOSITES - ULTRASONIC AND SEM EVALUATIONS

Results are presented on the evaluation of thermal fatigue in three fi ber reinforced polymer composites, using ultrasonic techniques and sca nning electron microscopy. The composites examined were (a) continuous carbon fibers in a vinylester matrix (b) continuous aramid fibers in a vinylester matrix and (c) randomly oriented aramid fibers in a polyp henylene matrix. Specimens of these composites were subjected to therm al fatigue by thermal cycling from -25 degrees C to 75 degrees C. Chan ges in ultrasonic attenuation and velocity were monitored during therm al cycling and scanning electron microscopy was used to qualitatively evaluate any damage. It was observed that ultrasonic attenuation is se nsitive to thermal fatigue, increasing with increasing number of therm al cycles. SEM evaluations showed that the primary damage due to therm al fatigue is due to fiber-matrix debonding.