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

Abstract

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.

Tensile and fatigue behavior of oxide/oxide ceramic matrix composite with simulated foreign object damage in combustion environment

“…This first and foremost shows that the harsh combustion environment had negligible effect on the material degradation. A similar conclusion was reached in an earlier study by Singh et al where it was shown that thermal gradient stresses rather than environmental degradation due to the harsh combustion environment influenced the strength and life of CMCs in combustion environment. In the present study, the stiffness reduction being directly proportional to the applied maximum stress supports micro level damage due to mechanical loadings being the prime cause of material degradation and subsequent catastrophic failure.…”

“…An earlier work by Singh et al uses finite element method to investigate the effects of thermal gradient stresses on the strength and life of notched Oxide/Oxide CMCs in essentially the same combustion environment. Those evaluations were deemed essential in that particular research, so that the strength and fatigue life comparisons could be made between combustion and elevated temperature “isothermal” laboratory air environment. It was shown that the presence of thermal gradient stresses resulted in significantly lower tensile strength and fatigue life in combustion environment in comparison to isothermal laboratory air environment .…”

“…Those evaluations were deemed essential in that particular research, so that the strength and fatigue life comparisons could be made between combustion and elevated temperature “isothermal” laboratory air environment. It was shown that the presence of thermal gradient stresses resulted in significantly lower tensile strength and fatigue life in combustion environment in comparison to isothermal laboratory air environment . Since there are no available data for making comparisons in isothermal conditions for test coupons made of the same material and with similar level of FOD, a computational approach for evaluating the thermal gradient stresses is not pursued in this research.…”

“…One major drawback in the previous FOD studies was the lack of an environmental chamber that more closely simulated the gas turbine engine combustion environment. This has been a challenge traditionally and has led to the development of a unique test facility where mechanical testing of CMC test coupons can be performed simultaneously in a combustion environment that closely simulates an aircraft gas turbine . This unique test facility uses a High‐Velocity Oxygen Fuel (HVOF) combustion gun that continually heats one side of the test coupons to the desired temperature while mechanical loads are applied simultaneously using a servohydraulic mechanical testing system .…”

“…In addition, it also has the capability to adjust and control the program for thermal and mechanical loading conditions, which enables testing a wide spectrum of thermal loading scenarios of the candidate materials including severe temperature excursions experienced and anticipated during the operation of an advanced aircraft engine. A number of tests have been successfully performed in this test facility on both oxide/oxide and carbide/carbide CMC systems under monotonic, cyclic, and sustained loads …”