Date of Award

3-12-2004

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD

Abstract

The main objective of this research effort was to examine the impact that cyclic loading frequency has on the life of a C/SiC composite at an elevated temperature of 550°C. Cyclic loading of C/SiC was investigated at frequencies of 375 Hz, 10 Hz, 1 Hz, and 0.1 Hz. Creep-Rupture tests and tests that were combinations of creep-rupture and fatigue were also accomplished. A monotonic tensile test was performed at 550°C and compared to a room temperature monotonic test. This study showed that an elevated temperature of 550°C has very little effect on the Ultimate Tensile Strength (UTS) of C/SiC. The UTS of C/SiC at 550°C was 487 MPa, while the room temperature UTS is 493 MPa. The three creep-rupture tests in this study performed at 350 MPa, 175 MPa and 105 MPa had lives of less than 11 hours despite the fact that the UTS of C/SiC is 487 MPa at 550°C. The short life of the specimens is due to the oxidation of the carbon fibers within the C/SiC composite. S-N curves developed from the fatigue tests indicate that there is an increase in cycles to failure as the frequency is increased. Another important discovery in this study was the fact that oxidation of the carbon fibers within C/SiC is reduced when frequency of fatigue is increased. At high frequency fatigue (10Hz to 375 Hz), C/SiC composites have longer cycle lives and time lives than at low cycle fatigue. Microscopic and SEM analysis verified that oxidation of carbon within C/SiC is slowed as frequency of fatigue is increased.

AFIT Designator

AFIT-GMS-ENY-04-M01

DTIC Accession Number

ADA424138

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