Dana G. Allen

Date of Award

12-1995

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD

Second Advisor

Anthony N. Palazotto, PhD

Abstract

The purpose of this study was to investigate the combined effect of cyclic temperature and loading on the fatigue life of a ceramic matrix composite with a cross ply lay-up. The material used in this study was a potassium borosilicate glass (BSG) doped magnesium aluminosilicate (MAS) cordierite matrix reinforced with Nicalon (silicon carbide, SiC) fibers in a [0/90]4s lay-up. Thermomechanical fatigue (TMF) tests were performed with a period of 180 seconds/cycle, or 0.00556 Hz, and a triangular wave-form. The temperature was cycled between 566°C and l093°C, and the stress levels varied between 60 MPa and 145 MPa. All tests involved tension-tension cycling with a stress ratio of 0.1. These two temperatures and the stress ratio were chosen to correspond with previous studies on this material. Six tests were performed under in-phase TMF conditions, and five tests were performed out-of-phase TMF condition. Four fatigue tests were performed isothermally at 566°C, and three more at 1093°C. Monotonic tests were also performed at room temperature, 566°C and 1093°C. Load and strain data were measured during all tests. This data was then used to get histories for maximum strain, minimum strain and strain range, modulus degradation, stress-strain hysteresis loops, and fatigue life curves. Post-mortem fractographic analysis, with optical and scanning electric microscopes, was also performed on the specimen. Damage and fracture mechanisms were thus determined. These results indicate a much higher damage for thermo-mechanical fatigue than for high temperature isothermal mechanical fatigue, with the out-of-phase TMF being much higher than the in-phase TMF. By comparing with previous data, it is shown that frequency has no effect, thus the damage is a time dependent phenomenon involving environmentally assisted crack growth and fiber-matrix debonding.

AFIT Designator

AFIT-GAE-ENY-95D-01

DTIC Accession Number

ADA306410

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