Date of Award

9-1997

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD

Abstract

The purpose of this study was to determine the effects of fiber volume fraction on the fatigue behavior of Silicon Carbide fiber-reinforced Titanium alloy, SCS-6/Ti-15-3. Three fiber volume fractions were investigated; 15%, 25%, and 42%. The tests were performed under fully-reversed, strain-controlled conditions at 427 °C. The primary objectives of this study were to develop a fatigue life diagram and to document the damage and failure mechanisms. Compressive loads on the slender specimens were kept from buckling the specimens through the use of a buckling guide. This device allows unrestricted axial movement of the composite, while preventing any out-of-plane motion. No buckling damage due to compression was found in any of the specimens. Modulus behavior and stress versus strain curves were recorded during cycling for each test. An applied strain range between 0.5% and 1.1% was used for the majority of the tests. This resulted in fatigue lives between approximately 10,000 and 100,000 cycles. The resulting fatigue life diagram showed similar life at strain ranges at and above 0.008 mm/mm for all fiber volume fractions. At strain levels below 0.008 mm/mm, there was an increasing fatigue life with increasing fiber volume fraction. The 15% and 42% Vf material was consolidated with a molybdenum cross-weave to hold the fibers in alignment. The 25% Vf material had a titanium-niobium cross-weave for the same purpose. The Mo cross-weave was present on every fracture surface of the 15% and 42% Vf material. This indicated that it was detrimental to the fatigue life of the composite. No cross-weave material was found on the fracture surfaces of the 25% Vf. Major cause of specimen failure was the initiation and propagation of fatigue cracks in the matrix that were perpendicular to the applied load.

AFIT Designator

AFIT-GAE-ENY-97S-01

DTIC Accession Number

ADA329447

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