Frequency dependence of high-cycle fatigue behavior of CVI C/SiC at room temperature

Effects of loading frequency on high-cycle fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite were investigated. Tension–tension fatigue tests were conducted at three frequencies, 4, 40 and 375 Hz. Fatigue run out was set to 107 cycles. Applied stress versus cycles to failure (S–N) relationships were developed for these three frequencies. At 4 and 40 Hz, fatigue run out was achieved at a stress level of 375 MPa. At 375 Hz, stress level for run out was 350 MPa. Frequency dependence was observed between the two lower frequencies (4 and 40 Hz) and the higher frequency (375 Hz), but not between two lower frequencies (4 and 40 Hz). This manifested as a reduction in cycles to failure at 375 Hz compared to 4 and 40 Hz at a given stress level. Specimen surface temperature increased due to internal heat generation from sliding friction between constituents of the composite under cyclic loading. This increase was directly related to frequency and/or applied cyclic stress level. There was no clear indication that frequency greatly impacted either the stress-strain response or the overall appearance of fracture surfaces. However, a closer examination of specimens cycled at the highest frequency (375 Hz) showed evidence of the localized oxidation at fiber surfaces that might have attributed to the reduction in fatigue life at this frequency.