Date of Award


Document Type


Degree Name

Master of Science


Department of Aeronautics and Astronautics

First Advisor

Shankar Mall, PhD


The fatigue response and life of a unidirectional SCS-6/Ti-6-4 metal-matrix composite were examined under tension-tension, load-controlled conditions at elevated temperatures and different frequencies. Composite specimens were fatigued at frequencies of 0.01, 0.1, 1 and 10 Hz, and at three stress levels. Plots of cycles to failure versus maximum stress and test frequency showed that fatigue life was more cycle-dependent at higher frequencies and more time-dependent at lower frequencies. Comparisons of tests at 427°C with their counterparts at 370°C and 538°C showed that these frequency effects were magnified as temperature increased: lower frequency tests at 538°C exhibited greater time-dependence than tests at 427°C, which was due to poorer creep and environmental resistance at the higher temperature. Macroscopic analysis of tests conducted at lower stress levels and higher frequencies revealed decreasing laminate stiffness with cycling, typical of matrix-dominated responses, while increasing strain and constant modulus histories during fatigue indicated that tests conducted at higher stress levels and lower frequencies were fiber-dominated. Microscopic observations revealed that the extent of matrix cracking increased at lower stress levels and higher frequencies.

AFIT Designator


DTIC Accession Number