Fatigue behavior of double-edge notched oxide/oxide ceramic matrix composite in a combustion environment

Tension–tension fatigue tests in a combustion environment were performed on double-edge notched oxide/oxide ceramic matrix composite specimens. The composite, designated as N720/A, constituted woven 0°/90° Nextel™720 fibers in alumina matrix. Monotonic tensile and cyclic loads at a frequency of 1 Hz and a stress ratio of 0.05 were applied on the specimens in a combustion environment. The maximum specimen temperature due to combustion flame impingement in the notch region was 1250 ± 50℃. A stiffness reduction of less than 10% evaluated for the run-out specimens showed the harsh combustion environment had a minimal effect on specimen degradation. The residual strength was evaluated to be ∼75%–85% the strength of non-fatigued (virgin) double-edge notch specimens in room temperature. The monotonic tensile strength and the fatigue limit for 90,000 cycles (run-out) were found to be ∼40 MPa less in the combustion environment when compared to published results for 1200℃ laboratory air environment. The damage mechanisms were also the same in the two environments. Finite element analyses showed that the reduction in strength and fatigue limit in the combustion environment (as compared to the laboratory air environment) was due to the presence of thermal gradient stresses because of non-uniform specimen temperature distribution.