Characterization of the Isothermal Elevated Temperature High Frequency FatigueBehavior of SCS-6/TIMETAL(R)21S 0/90s

摘要

An evaluation of the high frequency fatigue behavior of a SCS-6/Timetal(registered)21S (0/90)s composite at elevated temperature was conducted. Experimental isothermal fatigue tests were performed on a unique high frequency test system capable of operating at frequencies in excess of 200 Hz. The effects of frequency, maximum stress, stress ratio, and elevated temperature on fatigue life were investigated. The results indicate that fatigue life of the cross-ply composite is primarily cycle dependent at 650 deg C, and both cycle and time dependent at 815 deg C. A frequency effect was noted, and fatigue results suggest that an endurance limit exists for this material at 650 deg C. Fractography and metallography were performed to characterize the failure mechanisms and progression of damage for specimens tested at 650 deg C and 815 deg C. Evidence of internal crack initiation from the off-axis fibers leading to fully fiber bridged through cracks was observed at both temperatures. Direct current electric potential was used to monitor the evolution of fatigue damage during the tests. A linear summation model was adapted to predict the fatigue life as a function of frequency, stress range, and stress ratio based on fractions of life under high frequency fatigue and creep. Model predictions were compared to experimental data at frequencies ranging from 0.01 Hz to 200 Hz.