Over the past two decades, NASA Lewis Research Center's in-house
efforts in analytical modeling for advanced composites have yielded
several computational predictive tools. These are, in general,
based on simplified micromechanics equations. During the last
3 years, our efforts have been directed primarily toward developing
prediction tools for high-temperature ceramic matrix composite
(CMC's) materials. These materials are being considered for High
Speed Research program applications, specifically for combustor
liners. In comparison to conventional materials, CMC's offer several
advantages: high specific stiffness and strength, and higher toughness
and nonbrittle failure in comparison to monolithic ceramics, as
well as environmental stability and wear resistance for both room-temperature
and elevated-temperature applications. Under the sponsorship of
the High Temperature Engine Materials Program (HITEMP), CMC analytical
modeling has resulted in the computational tool Ceramic Matrix
Composites Analyzer (CEMCAN). This code was released through COSMIC
(NASA's software distribution center) recently.
The analysis of CMC materials requires specialized modeling that
considers their unique physical and mechanical behavior. Ceramic
matrix composite materials are reinforced primarily to enhance
toughness because the matrix material is quite brittle and fails
at relatively low strain levels. The methodology incorporated
in CEMCAN is based on micromechanics models in which, customarily,
a representative volume element or unit cell is arranged in a
square array pattern. However, the present approach employs a
unique multilevel substructuring technique that allows the capture
of greater local detail. The methodology also takes into account
fracture initiation and progression, as well as nonlinear composite
behavior due to temperature.
Unlike conventional materials, CMC's exhibit a considerable amount
of scatter in the material properties. Since these materials must
sustain a reliable life of several thousand hours in High Speed
Civil Transport propulsion systems, prediction tools must be developed
to account for uncertainties in the material behavior and fabrication
parameters. Currently, work is underway to incorporate probabilistic
analysis in the micromechanics and macromechanics of CMC's. In
addition to providing more formalism to the analysis as opposed
to the conventional "safety factor approach," such procedures
will enhance interpretation of experimentally measured properties
that have a wide range of scatter. Furthermore, the procedure
will help identify the variables that influence the response most,
thereby providing guidelines for quality control during the fabrication
process of these materials.
Another area of growing interest in the composite community is
the use of woven or textile composites for a variety of structural
applications. For example, plain- and satin-weave constructions
are being considered for combustor liners in the Enabling Propulsion
Materials program, and woven polymer matrix composites are being
considered for lower temperature applications in the Advanced
Subsonic Technology program. Woven composites are constructed
by weaving two fiber tows together to form a layer. The interlacing
of fiber bundles has several advantages, such as increasing the
intralaminar and interlaminar strength, providing greater damage
tolerance, and providing a possible way to produce near-net shapes
for thick structural components. A micromechanics-based approach
for analyzing plain-weave ceramic and polymer matrix composites,
which was recently developed in-house, is being integrated into
NASA Lewis' composite mechanics computer codes.
Murthy, P.L.N.; Chamis, C.C.; and Mital, S.K.: Computational Simulation of Continuous Fiber-Reinforced Ceramic Matrix Composites Behavior. NASA TP-3602, 1996.
Mital, S.K.; Murthy, P.L.N.; and Chamis, C.C.: Simplified Micromechanics of Plain Weave Composites. NASA TM-107165, 1996.
Murthy, P.L.N.; Mital, S.K.; and Shah, A.R.: Probabilistic Micromechanics
and Macromechanics for Ceramic Matrix Composites. NASA TM-4766,
1997.
Previous articleLast updated April 30, 1997
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