Residual StrengthBased Life Prediction of Composite Materials

1
Residual Strength-Based Life Prediction of
Composite Materials

• Scott Case
• Materials Response Group
• Virginia Tech
• Blacksburg, VA 24061

2
Outline

• Overview of residual strength approach to life
  prediction
• Example applications
• Ongoing activities
• Summary

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Necessity for life prediction (or Why go to all
this trouble?)

• To certify structures for service
• To reduce the need for experimental testing
• To design new components or structures (what if
  studies)
• To warranty existing or new products

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Life Prediction of Composite Materials

• Some observations
• changes in composite properties may not follow
  changes in matrix properties
• property evolution may be substantial
• mechanical / thermal / chemical coupling may be
  significant

5
Life Prediction of Composite Materials

• Basic Issues
• Understanding physical degradation processes at
  the basic level
• Modeling physical rate processes
• Establishing independent physical observables
  that track the processes
• Modeling the effect of combined processes
• Validation of models on real structures

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Remaining Strength Predictions

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

Residual Strength
Sult
Stress or Strength
Life Curve
N1
Cycles
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Remaining Strength Predictions

• 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

Sult
Stress or Strength
Cycles
8
Remaining Strength Predictions

• 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
Cycles
Implication n1 cycles at Sa1 is equivalent to
n20 cycles at Sa2
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Remaining Strength Predictions

• 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
Failure
Miners rule
Cycles
Failure occurs when residual strength equals
applied load
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Mathematical Representation

• Define a failure criterion, Fa, and a remaining
  strength in terms of that failure criterion, Fr
• Define a generalized time (for example n/N or
  t/trupture)
• Postuate change in remaining strength over the
  interval
• Fa is constant over
  
• For the special case in which is equal to zero

• Some possible choices for failure criteria
• Maximum stress/strain
• Tsai-Hill/Tsai-Wu

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Mathematical Representation

• Calculate change in remaining strength over the
  interval
• Calculate number of cycles required for failure

High-low n2 gt Miners rule Low-high n2 lt
Miners rule
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All Phenomena Are Reduced to Changes in Stiffness
or Strength

• Property Change
• Stiffness
• Strength

• Characterization/Phenomenon
• DE vs. damage (determined by X-ray, SEM, AU,
  AE)
• Viscoelastic behavior
• DX vs. damage (determined by X-ray, SEM, AU,
  AE)
• DX vs. environmental exposure

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Life Prediction MRLife

• Modeling combined, interactive effects

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Approach for Variable Loading with Rupture and
Fatigue Acting

• Divide each step of loading into time increments
• Treat each increment as a stress rupture problem
  (constant applied stress and temperature)
• Reduce residual strength due to time dependent
  damage accumulation
• Refine number of intervals until residual
  strength converges
• Input next load level
• Check for load reversal. If load reversal,
  increment by 1/2 cycle and reduce residual
  strength due to fatigue damage accumulation

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Application Flexible Pipe for Offshore Oil
Industry
(Wellstream Inc.)

• Advantages of Polymer Composite material in
  flexible pipes
• 30 weight reduction - greater depths, lower deck
  loads
• Corrosion resistance - longer life, more fluid
  options

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Typical Loading Environment

• Mechanical loads
• Tensile loads due to hanging weight
• Cyclic Bending loads due to wave-motion
• Positive and negative internal pressures
• Environmental loads
• Exposure to elevated temperatures
• Exposure to aggressive chemicals

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Material System

• Carbon Fiber/Polyphenylene Sulfide (PPS)
• Manufactured by Baycomp, Ontario Canada

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Characterize Fatigue Effect

• Fatigue Tests at 25C
• R 0.1
• frequency 10 Hz
• Fit data with S-N Curve

Reference James S. Loverich, Blair E. Russell,
Scott W. Case and Kenneth L. Reifsnider, Life
Prediction of PPS Composites Subjected to Cyclic
Loading at Elevated Temperatures, Time Dependent
and Nonlinear Effects in Polymers and Composites,
ASTM STP 1357, R. A. Schapery and C. T. Sun,
Eds., ASTM, 2000, pp. 310-317.
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Characterize Rupture Effect

• Tensile rupture tests at 90 C
• Fit data with Kachanov-type curve

Reference James S. Loverich, Blair E. Russell,
Scott W. Case and Kenneth L. Reifsnider, Life
Prediction of PPS Composites Subjected to Cyclic
Loading at Elevated Temperatures, Time Dependent
and Nonlinear Effects in Polymers and Composites,
ASTM STP 1357, R. A. Schapery and C. T. Sun,
Eds., ASTM, 2000, pp. 310-317.
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Predict Elevated Temperature Fatigue Behavior

• Fatigue behavior accurately predicted at 90C, R
  0.1
• Validates the life prediction technique for this
  case

Reference James S. Loverich, Blair E. Russell,
Scott W. Case and Kenneth L. Reifsnider, Life
Prediction of PPS Composites Subjected to Cyclic
Loading at Elevated Temperatures, Time Dependent
and Nonlinear Effects in Polymers and Composites,
ASTM STP 1357, R. A. Schapery and C. T. Sun,
Eds., ASTM, 2000, pp. 310-317.
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Residual Strength Prediction

• Residual strength prediction for
  45/0/-45/902s IM7-K3B laminates (25C)
• Combines matrix cracking, delamination, fatigue

Residual Strength Predictions
Laboratory Observations
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Residual Strength Prediction

• Residual strength prediction for
  45/0/-45/902s IM7-K3B laminates
  (177C)
• Combines matrix cracking, delamination, stress
  rupture, viscoelasticity, fatigue

Laboratory Observations
Residual Strength Predictions
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Summary

• A life prediction method for composites based
  upon remaining strength has been developed. The
  general approach is
• Conduct characterization tests and model behavior
  (under a single condition)
• Combine effects using life prediction analysis
• Validate life prediction using coupon level tests
• Apply validated analysis to a composite structure
• Validate structural analysis with limited testing

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Current and Recent Sponsors of Durability Efforts

• Air Force Office of Scientific Research - high
  temperature PMCs
• Pratt and Whitney - environmental effects on
  PMCs
• Wellstream - life prediction for flexible pipes
• Goodyear - truck tire durability
• McDermott Technologies - hot gas filters radiant
  burners fuel cells
• Taylor Made Golf - composite golf shafts
• Boise Cascade - building product (using recycled
  materials)
• Owens Corning - shingles, pipe, tension members
• Strongwell - infrastructure applications (bridge
  and bridge deck)
• Federal Highway Administration - bridge and
  bridge deck
• National Science Foundation - durability of
  composites for infrastructure applications
• Schlumberger Technology - performance of
  high-temperature polymer composites in down-hole
  environments
• BMW - lifetime prediction of polymer composites
  for specialty automotive applications
• ABB - lifetime prediction of composite motor
  bandages