The influence of cyclic loading frequency on the tensile fatigue life of a woven-carbon-fiber/SiC-matrix composite was examined at room temperature. Tension-tension fatigue experiments were conducted under load control, at sinusoidal frequencies of 1, 10, and 50 Hz. Using a stress ratio (um,,,/u,,,,) of 0.1, specimens were subjected to maximum fatigue stresses of 310 to 405 MPa. There were two key findings: (1) the fatigue life and extent of modulus decay were influenced by loading frequency and (2) the postfatigue monotonic tensile strength increased after fatigue loading. For loading frequencies of 1 and 10 Hz, the fatigue limit (defined at 1 X lo6 cycles) was approximately 335 MPa, which is over 80% of the initial monotonic strength of the composite; at 50 Hz, the fatigue limit was below 310 MPa. During 1-and 10-Hz fatigue at a maximum stress of 335 MPa, the modulus exhibited an initially rapid decrease, followed by a partial recovery; at 50 Hz, and the same stress limits, the modulus continually decayed. The residual strength of the composite increased by approximately 20% after 1 X lo6 fatigue cycles at 1 or 10 Hz under a peak stress of 335 MPa. The increase in strength is attributed in part to a decrease in the stress concentrations present near the crossover points of the 0" and 90" fiber bundles.

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