Frequency effects on fatigue behavior of Nextel720™/alumina at room temperature

Cycling frequency effects on the fatigue behavior of an oxide/oxide ceramic matrix composite, Nextel720™/alumina, were investigated. Tension–tension fatigue tests were conducted at three frequencies: 1, 100 and 900 Hz to establish stress versus cycles to failure (S–N) relationships. Cycles to failure at a stress level increased considerably with increase of the frequency from 100 to 900 Hz, but very little from 1 to 100 Hz. Fatigue behavior of Nextel720™/alumina appeared to be a combination of cycle-dependent and time-dependent phenomena. Surface temperature of specimens tested at 900 Hz increased considerably relative to that at 1 or 100 Hz (70 °C versus 5 °C increase). Damage mechanisms showed an evidence of local fiber/matrix interfacial bonding developed during cycling due to frictional heating at the highest frequency of 900 Hz. This was not observed at the two lower frequencies. This interfacial bonding may have caused an increase in fatigue life/strength of the tested CMC system at the highest frequency of 900 Hz.