Effect of Hold Times on Fatigue Behavior of NEXTEL (trademark) 720/Alumina Ceramic Matrix Composite at 1200 deg C in Air and in Steam Environment
AIR FORCE INST OF TECH WRIGHT-PATTERSON AFB OH SCHOOL OF ENGINEERING AND MANAGEMENT
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The aerospace field requires structural materials that can maintain superior mechanical properties while subjected to high temperatures and oxidizing environments. This research investigated the effect of hold times at maximum load on fatigue performance of a Nextel 720Alumina ceramic matrix composite at 1200 C, explored the influence of environment on material response to cyclic loading with hold times at maximum load, and assessed the effects of loading history on material behavior and environmental durability. The N720A composite relies on an oxideoxide composition for inherent oxidation resistance and a porous matrix with no interphase between the fiber and matrix for damage tolerance. Mechanical testing results showed a significant decrease in material life and performance in a steam environment when compared to tests conducted in a laboratory air environment. Prior fatigue of specimens tested in the air environment resulted in an order of magnitude increase in creep life. Fracture surface observations with a Scanning Electron Microscope showed a correlation between an increase in fiber pull-out and increased time to failure. A qualitative spectral analysis indicated evidence of silicon species migration from the fiber to the matrix, especially in the steam environment. This may be the cause of the decreased creep performance of the material in the steam environment.
- Ceramics, Refractories and Glass