Development of A Life Prediction Scheme For the Assessment of Fatigue Performance of Composite Infra

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Development of A Life Prediction Scheme For the
Assessment ofFatigue Performance of Composite
Infrastractures

• S. W. Case1, J. J. Lesko1, and T. E. Cousins2
• 1Department of Engineering Science and Mechanics
• 2Via Department of Civil Engineering
• Virginia Polytechnic Institute and State
  University
• Blacksburg, VA 24061, USA

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Outline

• Motivation for the research
• Modeling philosophy
• Modeling predictions and comparison to data
• Conclusions
• Ongoing and future work

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Composites infrastructure

• Moisture(, n, t)
• Stress(s, n, t)
• UV(I, t)
• Temperature(T, n)

• Obstacles
• Absence of design standards
• Durability?
• New material multiple combinations processing
  methods
• Comfort, Experience Acceptance by the DOT

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Infrastructure composites durability Approach
Field Investigations
Laboratory Experimentation
Durability Prediction
Durable designs design philosophies for 50-75
year design lives
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Critical element model

• Employ a representative volume
• Define critical element so that failure of the
  critical element results in failure of the
  material
• Remainder of the material make up subcritical
  elements--control stress redistribution
• Define rate equations for each of the processes
  that govern material behavior

s
(t)
Critical
Element
Representative
Volume
Subcritical
Elements
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The use of remaining strength as a state variable

• Track remaining strength of the critical element
  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
• Can be directly validated experimentally, unlike
  Miners rule

Sult
Stress or Strength
Life Curve
t1
t2
Time
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The use of remaining strength as a state variable

• Track remaining strength of the critical element
  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
• Can 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 of the critical element
  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
• Can 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 of the critical element
  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
• Can be directly validated experimentally, unlike
  Miners rule

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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

• For step loading, introduce the concept of
  "pseudo-cycles" based on the idea of equivalent
  damage

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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 example (APC-2)
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Life prediction exampleUnidirectional S-N data
(R0.05)
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Life prediction exampleNotched quasi-isotropic
laminate (R-1)
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Life prediction exampleNotched quasi-isotropic
laminate (R-1)
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Conclusions

• A life prediction method for composites based
  upon remaining strength has been developed.
  Features of the model are
• Inclusion of sequence effects
• Ability to combine effects (such as fatigue and
  stress rupture)
• An example application has been considered

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Ongoing/Future work

• Developing and applying analysis techniques for
  composites typical of infrastructure (Phifer et
  al.)
• Using results from in-service tests to guide and
  refine the modeling efforts

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Acknowledgements

• Federal Highway Administration Priority
  Technology Program
• Virginia Transportation Research Council