Effects of frequency on fatigue behavior of CVI C/SiC at elevated temperature

Effects of frequency on fatigue behavior of a chemical vapor infiltrated carbon fiber reinforced silicon carbide composite (C/SiC) were investigated at an elevated temperature of 550 °C. Tension–tension fatigue tests were conducted at three frequencies: 0.1, 10 and 375 Hz to establish stress versus cycles to failure (S–N) relationships. There was an increase in cycles to failure at a given stress level as frequency increased from 0.1 to 375 Hz at elevated temperature. This trend was different at room temperature where cycles to failure decreased when frequency changed from 40 to 375 Hz but remained almost same below 40 Hz. There was a reduction in cycles to failure at frequencies less than 40 Hz but cycles to failure remained same at a higher frequency of 375 Hz when test environment changed from room temperature to 550 °C. Analysis of damage mechanisms showed that the oxidation of carbon fibers was the major difference between the room and elevated temperatures, which caused a reduction in cycles to failure with lower frequencies at elevated temperature in comparison to that at room temperature. However, oxidation of carbon fibers was almost absent or negligible at higher frequency at elevated temperature, which caused practically no reduction in cycles to failure at elevated temperature in comparison to their counterparts at room temperature.