As part of a comprehensive materials characterization program
at the NASA Lewis Research Center, tensile creep-rupture tests
were performed on a SiC-fiber-reinforced SiC-matrix composite.
The results of these tests and subsequent analysis revealed an
oxidation embrittlement phenomena that occurs readily at a discreet
temperature range below the maximum use temperature. The graph
shows rupture lives for a creep stress of 83 MPa as a function
of temperature. Note that the rupture time is constant at an intermediate
temperature range of 700 to 982 °C. This graph also shows the failure
location, as measured from the center of the specimen. Whereas
for temperatures of 500 to 700 °C, failure occurred in the specimen
gage section; at higher temperatures, the failure location migrated
toward the cooled grip ends. Although the results initially suggested
that the test procedure was influencing the measured creep rupture
lives and driving the failure location out of the gage section,
subsequent experiments and thermal stress analyses verified the
robustness of the test method employed.
Metallurgical examination of failed specimens revealed an environmentally
assisted material degradation operating in the 700 to 800 °C range.
The photomicrographs show the distribution of damage in two specimens
tested at the same stress but different gage section temperatures.
In the views shown, the sections are polished in the thickness
direction, showing the broad specimen face. The loading direction
is horizontal. In the 700 °C specimen, creep damage is distributed
throughout the uniformly heated test section. However, in the
982 °C specimen, damage is concentrated only at the point outside
the gage section where the temperature was found to be 700 to
800 °C; less damage occurred within the hotter gage section. Both
specimens failed because of an aggressive environmental attack
of the SiC fibers at location where the temperature was in the
700 to 800 °C range.
SiC/SiC composites are candidates for a combustor liner application in the engine of the High Speed Civil Transport vehicle. During its service cycle, the combustor liner will experience a thermal gradient, being cooled near attachment regions and exposed to combustor gases on the inner wall. The existence of a minimum in creep rupture behavior over a discreet temperature range indicates that the kinetics of this process are unconventional. Thus, material properties must be well characterized over the temperature range of expected operation. Also, this behavior must be accounted for in designing and determining the life of components.
Verrilli, M.J.: Calomino, A.M.; and Brewer, D.N.: Creep Rupture
Behavior of a SiC/SiC Composite. Thermal and Mechanical Test Methods
and Behavior of Continuous Fiber Ceramic Composites, ASTM STP
l309, Michael G. Jenkins, Stephen T. Gonczy, Edgar Lara-Curzio,
Noel E. Ashbaugh, and Larry P. Zawada, Ed., American Society for
Testing and Materials, in press.
Lewis contact: Michael J. Verrilli, (216) 433-3337, SMVeril@popserve.grc.nasa.gov
Author: Michael J. Verrilli
Headquarters program office: OA
Previous articleLast updated April 30, 1997
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