Elevated Temperature Fatigue Behavior of a Nextel 610 Composite

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Elevated Temperature Fatigue Behavior of a Nextel
610 Composite
Howard G. Halverson Scott W. Case Materials
Response Group Virginia Tech January 25, 2001
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Life Prediction Techniques

• Micromechanical Models
• Simulation
• Analysis
• Phenomenological Models
• Residual Strength
• Stiffness
• Structural modeling (FEA)

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Advantages and Disadvantages

• Simulations
• Very accurate representation of damage mechanisms
• Computationally intensive
• Analysis
• Much quicker than simulations
• Difficult to evaluate for more complicated
  scenarios
• Residual Strength
• Loses information about damage mechanisms
• Requires calibration with experimental results

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Question

• Can we create a methodology which combines the
  advantageous elements of both micromechanical and
  phenomenological models?

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Demonstration of Methodology

• Begin with fatigue tests at room temperature and
  stress-rupture tests at 1093C on a Nextel 610
  reinforced alumina-yttria composite
• Represent the changes in remaining strength due
  to these mechanisms with a residual-strength
  based model
• Create predictions based on the summation of
  damage due to the action of both mechanisms
• Verify predictions with fatigue tests at 1093C

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Damage Accumulation

• Track remaining strength during the fatigue
  process
• Define a scalar failure function based upon
  tensor strength and stresses use this failure
  function for calculations
• May include the effects of changing loading
  conditions
• May be directly validated experimentally, unlike
  Miners rule

Residual Strength
Sult
Stress or Strength
Life Curve
N2
Cycles
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Damage Evolution Model
The remaining strength of the composite varies
according to the current strength, sr, the
initial strength, su, the applied stress, sa, the
lifetime t, and an exponent a
Thus, over time dt, the change in remaining
strength is given by
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Sequence Effects

• Track remaining strength during the fatigue
  process
• Define a scalar failure function based upon
  tensor strength and stresses use this failure
  function for calculations
• May include the effects of changing loading
  conditions
• May be directly validated experimentally, unlike
  Miners rule

Residual Strength
Sult
Stress or Strength
Life Curve
N1
N2
Cycles
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Material System

• Woven Nextel 610 Fiber - 35 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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Basic Inputs
-5 MPa / decade
-35 MPa / decade
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Fatigue Testing

• An increase in hysteresis loop area -
  consistent with degradation of interface
  frictional stress
• A decrease in composite stiffness - associated
  with composite delamination

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Rupture Testing

• In stress-rupture tests there is little
  evidence of modulus decrease
• Strength reduction is accomplished by the
  degradation of the Nextel fibers

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Elevated Temperature Fatigue
Sum the changes in remaining strength due to each
mechanism acting independently
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All Lifetime Data
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Conclusions and Issues

• The residual-strength based method for combining
  the stress-rupture and fatigue mechanisms results
  in accurate predictions for lifetime

Issues

• Are the mechanisms actually independent?
• Will this work at lower (more practical use)
  temperatures?
• Is the shape of the residual strength curve
  accurate?