Frequency Effects on Fatigue Behavior of a Unidirectional Metal Matrix Compsite at Elevated Temperature

摘要

Metal Matrix Composites (MMCs) have great potential for a variety of aerspace applications, but they must be better understood before they can be incorporated into the advanced aerospace ocmponents. Despite the many studies that have contributed to the understanding of fatigue response of titanium alloy MMCs, a few have studied the effects of cyclic load frequency. This research examined the fatigue response and life of unidirectional SCS-6/Ti-6-4 under tension-tesnion, load controlled conditon at different temperatures and frequencies. Specimens were cycled using the load-controlled mode at frequencies of 0.01 Hz, 0.1 Hz, 1 Hz and 10 Hz under isothermal conditions (23 deg C, 370 deg C, 427 deg C and 538 deg C). The macroscopic response (i.e., trends in the mean strain and modulus) revealed that specimens cycled at lower stress levels and higher frequencies experimenced decreasing laminate stiffness with cycling, typical of a response dominated by matrix damage. On the other hand, specimens cycled at higher stress levels and/or slower frequencies showed increasing strain and constant modulus histories during fatigue, which is indicative of a response domainated by fiber damage. Plost of test frequency versus cycles to failure showe dthat fatigue life was more cycle-dependent at higher frequencies and more time-dependent at lower frequencies. Comparisons of tests under different isothermal temperatures showed that these frequency effects were magnified as temperature increased.