Date of Award

3-26-2015

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Marina B. Ruggles-Wrenn, PhD.

Abstract

Tension-compression fatigue behavior of an oxide-oxide ceramic matrix composite was investigated at 1200 deg C in air and steam. The composite is comprised of an alumina matrix reinforced with Nextel 720 fibers woven in an eight harness satin weave. The composite relies on a porous matrix for damage tolerance. Compression and tension tests to failure were conducted to characterize basic mechanical properties. Tension-compression fatigue tests were performed at 1 Hz frequency with a ratio of minimum to maximum stress of -1. Maximum stresses ranged from 60-120 MPa. Fatigue run-out (defined as 105 cycles) was achieved in air at 80 MPa and in steam at 70 MPa. Specimens subjected to 105 fatigue cycles in air retained 100% tensile strength. Steam reduced fatigue lives one order of magnitude and tensile strength 17-38%. Tension-compression fatigue lowered fatigue lives three orders of magnitude from published tension-tension fatigue lives. In air, tension-compression fatigue was shown to be more damaging than creep. The post-test composites microstructure was examined. Failure mechanisms were identified and fracture surface morphologies were characterized. The dominant failure mode under tension-compression fatigue was fiber micro-buckling. Shortened composite lives were associated with planar fractures and coordinated fiber failure due to increased fiber-matrix bonding and matrix densification.

AFIT Designator

AFIT-ENY-MS-15-M-222

DTIC Accession Number

ADA616269

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