INFLUENCE OF LOADING FREQUENCY ON THE ROOM-TEMPERATURE FATIGUE OF A CARBON-FIBER SIC-MATRIX COMPOSITE

The influence of cyclic loading frequency on the tensile fatigue life of a woven-carbon-fiber/SiC-matrix composite was examined at room temp erature. Tension-tension fatigue experiments were conducted under load control, at sinusoidal frequencies of 1, 10, and 50 Hz. Using a stres s ratio (sigma(min)/sigma(max)) of 0.1, specimens were subjected to ma ximum fatigue stresses of 310 to 405 MPa. There were two key findings: (1) the fatigue life and extent of modulus decay were influenced by l oading frequency and (2) the post-fatigue monotonic tensile strength i ncreased after fatigue loading. For loading frequencies of 1 and 10 Hz , the fatigue limit (defined at 1 x 10(6) cycles) was approximately 33 5 MPa, which is over 80% of the initial monotonic strength of the comp osite; 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 i nitially 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 10 (6) fatigue cycles at 1 or 10 Hz under a peak stress of 335 MPa. The i ncrease in strength is attributed in part to a decrease in the stress concentrations present near the crossover points of the 0-degrees and 90-degrees fiber bundles.