Life Prediction of Composite Materials and Structures

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Life Prediction of Composite Materials and
Structures

• Scott Case
• Materials Response Group
• Virginia Tech
• Blacksburg, VA 24061-0219

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Life Prediction Definitions

• Life
• time/cycles/history to failure of component
• failure - of suitability for service, based on
  measurement of stiffness, strength, properties,
  appearance, ...
• component - may be structure, element, joint,
  bond, sub-element, ...

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

• To certify structures for service
• Lack of life prediction techniques is currently
  viewed as the single biggest limitation to the
  use of composite in civil infrastructure
• 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 Issues

• 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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Life Prediction Problem Elements

• Elements of the problem
• Physical behavior damage and failure modes
• Modeling discrete events, multiple processes
• Measurements independent observables as inputs
  to the models
• Life prediction extensions, generalizations,
  accelerations of laboratory experience

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Objectives

• To develop a life-prediction method based on
  remaining strength that may be applied to
  composite structures
• To validate the method by comparing with existing
  experimental evidence

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Basic Assumptions

• Strength (and, as a result, life) of material
  systems is controlled by
• Statistical accumulation of flaws subsequent
  interaction of flaws
• Large changes in material states and stress
  states
• Highly local level behavior (on the fiber/matrix
  scale)

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Damage in Composite LaminatesIM7/K3B laminates
(room temperature 65UTS)
10000 cycles
20000 cycles
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Stiffness Changes IM7/K3B laminates (room
temperature 65UTS)
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Stiffness Changes IM7/K3B laminates (177C
65UTS)
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The use of remaining strength as a state variable

• Track remaining strength during 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
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The use of remaining strength as a state variable

• Track remaining strength during 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
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The use of remaining strength as a state variable

• 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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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)
• From kinetics we have the 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

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Life Prediction Interactive Effects

• Modeling combined, interactive effects
• MRLife

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Models Employed

• Modeling currently included in analysis
• MRLife

• Viscoelastic creep--linear TTSP based upon
  constant applied stress
• Stiffness reduction due to microcracking
  including continuum damage mechanics (Talreja)
• Creep rupture--degradation of strength
• Delamination growth based on strain energy
  release rate (OBrien)
• Anisotropic elasticity solution for a plate
  containing a hole (Lekhnitskii) with stress
  redistribution due to damage
• Life equations for the critical element
• Rate equations for degradation processes
• Moisture diffusion solution
• Remaining strength equation

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Characterize Fatigue Effect(Unidirectional
AS-4/PPS Composites)

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

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Characterize Temperature Effect

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

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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

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Sponsors of Durability Activities

• NASA Langley - life prediction for high
  temperature polymer composites
• Pratt and Whitney - high-T PMCs
• Halliburton (Wellstream) - life prediction for
  flexible risers
• Goodyear - truck tire durability
• McDermott Technologies - hot gas filters radiant
  burners development of new materials (CMCs)
• Martin Marietta - continuous fiber CMCs, time
  dependence
• Taylor Made Golf - composite golf shafts
• Boise Cascade - building product (using recylced
  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
• Johnson Johnson - bioprosthetic devices

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Ongoing/Future work

• Refine analysis to eliminate discrepancies
  between model/experiments
• Incorporate with finite element analysis to
  better model progressive failure as well as
  statistical strength distributions
• Apply analysis to out-of-plane failures
  (particularly important for infrastructure
  applications)

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Delamination Modeling

• Pultruded fiber reinforced polymer (FRP)
• Hybrid Layup
• Glass
• Carbon
• Non-symmetric flanges

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Failure Mode Delamination
Observed delamination
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Four-Point Bend FRP Beam Analysis
Moment
Ply Stresses sx, sy, txy
kxo
CLT
EIeff
Interfacial Stresses sz, txz
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Stress Distribution in Top Flange
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Bending Fatigue Life Prediction
New Flange Modulus
Reduce Beam Modulus
NO
YES
NO
Evaluate Stresses
Delamination?
Failure?
YES
Beam Failed
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Bending Fatigue Life Prediction