Durability of Woven Polymer Composites in Aerospace Applications

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Durability of Woven Polymer Composites in
Aerospace Applications

• MIT - Virginia Tech Symposium, May 24-26, 1999
• S.R. Patel
• Advisor S.W. Case
• Materials Response Group
• Virginia Tech

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• Durability of Woven Composite Materials in
  Aerospace Applications
• Sneha R. Patel (snpatel_at_vt.edu)
• Advisor Scott W. Case
• Materials Response Group
• Department of Engineering Science and Mechanics
• Virginia Polytechnic Institute and State
  University
• Abstract
• Woven graphite/epoxy composites are being
  increasingly considered for use in aircraft
  components. Though woven composites may have
  somewhat decreased strength and stiffness
  properties relative to corresponding angle-ply
  laminates, they are generally easier to
  manufacture than angle-ply laminates and offer
  the additional advantages of increased
  drapeability and impact resistance. To
  confidently implement these materials in aircraft
  design, however, the fatigue behavior of the
  material under service conditions must be better
  understood. Such an understanding can aid in the
  development of life predictions schemes which
  will lower the time and expense associated with
  experimental testing in the material selection
  process. The objective of the present work is to
  evaluate the effects of moisture, temperature,
  and aging on the durability (strength and life)
  of a five harness satin graphite/epoxy composite
  material system. Experimental testing showed
  that the life and tensile properties (strength
  and stiffness) of the unaged material specimens
  in the loading direction were virtually
  unaffected by typical aircraft engine service
  environments. Typical service conditions in the
  engine will include a maximum temperature of
  250F during flight and a storage condition of
  85 relative humidity at 85F. Similar testing
  on material aged for 12,000 hours under a
  hygrothermal profile representative of a mission
  flight cycle also exhibited very little
  degradation of tensile properties. However,
  differences in the progression of events leading
  up to failure of the material were observed, and
  insight about the fatigue processes in woven
  materials was obtained. These events may affect
  other important material properties, the
  evaluation of which were beyond the scope of this
  project, but which should be investigated in the
  future.

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Objectives

• Study of damage, property degradation, and
  residual strength for a graphite reinforced
  polymer 0/902ws five harness satin weave
  under specified service conditions
• To develop a life-prediction method based on
  remaining strength that may be applied to the
  specified material system

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Composites in Aircraft Engines
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Mission Profile
250F
90 min
In Flight
Storage
85F, 85 RH
24 hours
Time
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Motivation

• Durability/fatigue life studies on composite
  materials provides the base of knowledge needed
  to
• develop accurate methods for in service
  assessment of component integrity
• develop life prediction models which cut time and
  expense in component design
• determine effects of changing operating
  conditions (loading and environment)
• Environmental conditions may affect fatigue life,
  e.g.
• Elevated temperature and moisture levels may
  result in redistribution of stresses at
  fiber/matrix interface
• Moisture may plasticize the matrix, leading to a
  lower Tg and increased ductility

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Approach
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S-N Data
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Residual Properties
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Dynamic Stiffness Loss
Stiffness measured from stroke signal
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Delamination
High cycle fatigue at 72F
High cycle, fatigue at 250F
Low cycle fatigue at 72F
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Transverse Cracking
Debond
2.2 mm
Crack
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Aged Delamination
Unaged, Fatigued at 72F
Aged 12000 hours, Fatigued at 72F
Aged 12000 hours, Fatigued at 250F
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Aged Residual Strength
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Aged Dynamic Stiffness Loss
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Microcracking
Unaged
Aged 6000 hours
Aged 12000 hours
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Aged Crack Density
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Moisture Uptake
Fickian Fit
Aged 12000 hours
Unaged
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Summary/Conclusions

• The fatigue life and residual strength of the
  material system (aged and unaged) studied was
  largely unaffected by environmental conditions
  and/or fatigue cycling.
• Good for industry
• Bad for modeling
• The rate of damage progression was affected by
  temperature and by aging.
• Needs more attention
• Hygrothermal aging affects the integrity of the
  material
• microcracking
• possible interfacial damage

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Future Work

• Further investigation into the effects of
  hygrothermal aging
• Further investigation into the effects of
  temperature on delamination
• Completion of testing of material aged for 6000
  hours and 12000 hours
• Modeling of moisture uptake curves for damaged
  material

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Acknowledgments

• NASA Glenn Research Center and Pratt and Whitney
  for sponsoring this project
• Virginia Space Grant Consortium for providing
  additional funding