Date of Award

9-2006

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD

Abstract

Advanced aerospace structures and components require materials that exhibit high strength at high operating temperatures. Nextel ™ 720/Alumina (N720/A) is an oxide/oxide, porous-matrix ceramic-matrix composite developed for load bearing applications at elevated temperatures. The current research investigates the rate dependence of tensile properties of the N720/A CMC at 1200°C as well as the creep behavior of this composite under applied stresses in the 1-25 MPa range. In addition, influence of prior loading rate on the creep response of N720/A under applied stresses above 50 MPa was evaluated at 1200°C. Tensile tests conducted at loading rate of 0.0025 and 25 MPa/s revealed a strong influence of rate on the ultimate tensile strength, elastic modulus and failure strain. Several samples subjected to additional heat-treatments were tested to ascertain whether the stress-strain behavior observed in the 0-30 MPa stress range in tests conducted at 0.0025 MPa/s was an artifact of incomplete processing of fibers in the as-received material. Additional heat treatment had no effect on stress-strain behavior at 0.0025 MPa/s. Negative creep (i. e. decrease in strain under constant stress) was observed in creep tests conducted with the applied stresses < 30 MPa. Microstructural investigation using scanning electron microscope together with the results of mercury porosimetry suggest that a decrease in matrix porosity and matrix densification may be taking place in the N720/A CMC exposed to loads <30 MPa for prolonged periods of time. Prior loading rate was found to have no appreciable effect on creep behavior at stresses >50 MPa.

AFIT Designator

AFIT-GAE-ENY-06-S09

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