StressRupture of OxideOxide Composites

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Stress-Rupture of Oxide/Oxide Composites
Howard G. Halverson Materials Response
Group Virginia Tech Bill Curtin Division of
Engineering Brown University January 27, 2000
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Objectives

• Determine quasi-static and stress-rupture
  properties of an oxide/oxide ceramic composite at
  23C, 950C, and 1050C.
• Develop a micromechanical model for
  deformation and stress-rupture lifetime

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Material System

• Nextel 610 Fiber - 50 fiber volume fraction
• Alumina-Yttria Matrix - 20 porosity
• Fugitive Carbon Interface - 80-100 nm
  thickness
• Produced via sol-gel infiltration by
  McDermott Technologies, Inc. (Lynchburg, VA)

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Quasi-Static Properties
Slight decrease in tensile strength and modulus
with increasing temperature
40 µ crack spacing which remains unchanged after
testing
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Steady-State Strain Rate
Steady-state strain rates are approximately 40
of that expected from the fiber data alone.
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Fiber Stress Profile
The fiber stress profile away from a matrix crack
will affect the rate of slow crack growth
Hysteresis and push-out tests indicate t is 30
MPa. Post-test microscopy indicates that the
crack spacing is unchanged at 40µ
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Micromechanical Model

• Degradation in the fibers due to slow crack growth

Crack growth is dictated by the Paris Law
So with time the strength of an individual fiber
is
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Micromechanical Model

• Accounts for
• Fiber statistical strength distribution
• Fiber stress profile away from the matrix crack
• Global load sharing from broken fibers to intact
  fibers
• Fiber pullout from other matrix crack planes

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Stress-Rupture Modeling

• b - slow crack growth exponent
• C - slow crack growth coefficient
• m- Weibull modulus
• sc - characteristic strength
• t - interface frictional stress
• x - matrix crack spacing

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Fiber Rupture Behavior
Obtain the fiber stress rupture parameters from
single fiber testing
Yun DiCarlo, 1993
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Steady-State Strain Rate
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Effect of Crack Spacing
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Unidirectional Lifetimes
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Fitted Lifetimes
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Residual Strength Testing
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Conclusions

• Model explains the creep deformation of the
  Nextel 610/alumina-yttria composite fairly well
• However, composite stress-rupture lifetime is
  underpredicted by two orders of magnitude
• Perhaps in situ behavior is different from
  reported ex situ data, or fibers have improved
  stress-rupture properties

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Acknowledgements

• McDermott Technologies, Inc.
• NASA-Glenn Research Center