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

摘要

The fatigue response and life of a unidirectional SCS-6/Ti-6-4 metal-matrixcomposite were examined under tension-tension, load-controlled conditions at elevated temperatures and different frequencies. Composite specimens were fatigued at frequencies of 0.01, 0.1, 1 and 10 Hz, and at three stress levels. Plots of cycles to failure versus maximum stress and test frequency showed that fatigue life was more cycle-dependent at higher frequencies and more time-dependent at lower frequencies. Comparisons of tests at 427 deg C with their counterparts at 370 deg C and 538 deg C showed that these frequency effects were magnified as temperature increased: lower frequency tests at 538 deg C exhibited greater time-dependence than tests at 427 deg C, which was due to poorer creep and environmental resistance at the higher temperature. Macroscopic analysis of tests conducted at lower stress levels and higher frequencies revealed decreasing laminate stiffness with cycling, typical of matrix-dominated responses, while increasing strain and constant modulus histories during fatigue indicated that tests conducted at higher stress levels and lower frequencies were fiber-dominated. Microscopic observations revealed that the extent of matrix cracking increased at lower stress levels and higher frequencies. jg p147.