Life Prediction Philosophy for Composite Materials

1
Life Prediction Philosophy for Composite Materials

• Scott Case
• Materials Response Group
• Department of Engineering Science and Mechanics
• Virginia Polytechnic Institute and State
  University
• Blacksburg, VA 24061

2
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

3
Outline

• Life prediction based upon residual strength
• Application of strength and lifetime prediction
  techniques to three classes of composite
  materials
• Flowtite pressure pipe (Owens Corning product)
• Woven graphite epoxy composite (for jet engine
  applications)
• Ceramic matrix composite (for gas turbine
  applications)
• Comparison with micromechanics models

4
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

5
Key Features

• Strength (and, as a result, life) of composite
  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 (often on the
  fiber/matrix scale) controls failure

6
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
N1
Cycles
7
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
N2
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
N1
N2
Cycles
9
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
Miners rule
N1
N2
Cycles
10
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

11
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
12
Application to Composite Materials

• Flowtite pressure pipe
• Woven graphite/epoxy composites
• Ceramic matrix composites

13
Failure Criteria Employed

• Fiber direction failure (in hoop direction of
  skin)
• maximum stress failure criterion
• predicts burst failure
• Off-axis failure (in all plies)
• Tsai-Wu failure criterion
• predicts cracking for leaking (weeping) failure

14
Stress Analysis

• Axisymmetric linear elasticity solution
• internal/external pressure
• axial load (or constrained ends)
• axial torque
• Approximate transverse loading solution based
  upon Castiglianos theorem
• Use superposition for combined loading cases

15
Axisymmetric Stress Analysis

• Ply displacements
• Boundary and continuity conditions

2 N 2 equations 2 N 2 unknowns
16
Transverse Loading Solution

• Apply Castiglianos theorem of least work to
  calculate deformed geometry
• Approximation Use lamination theory bending
  stiffness for EI
• Have developed a solution that includes
  non-linear geometry (but not constitutive
  behavior)

17
Implementation PipeLife code
18
Models Included in Analysis

• Viscoelastic creep--linear TTSP based upon
  constant applied stress
• Moisture diffusion solution (finite difference
  solution for Fickian diffusion non-Fickian
  diffusion readily handled)
• Creep rupture--degradation of strength
• Moisture concentration dependent strength and
  stiffness values
• Rate equations for degradation processes
• Life and remaining strength calculations for each
  of the plies

19
Analysis Results Internal Pressure Loading (HDB)
20
Analysis Results Transverse Loading (Ring
Bending)
21
Application to Composite Materials

• Flowtite pressure pipe
• Woven graphite/epoxy composites
• Ceramic matrix composites

22
Engine Mission and Aging Profile
23
Combined Hygro-thermal Aging/Fatigue Effects
24
S-N Data for Unaged Material
25
Hygro-thermal Aging Effects
26
Comparison of Measured Residual Strength to Model
Values
27
Comparison of Measured Residual Strength to Model
Values
28
Application to Composite Materials

• Flowtite pressure pipe
• Woven graphite/epoxy composites
• Ceramic matrix composites

29
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

30
Implementation CCLife Program

• Begin with matrix stiffness reduction as a
  function of time and stress level
• Use a simple stress model (2-D, laminate level)
  to calculate failure function as a function of
  time, stress, and temperature
• Fit stress rupture data at 1800 F
• Shift fit to match rupture data from 925 F to
  2000 F
• Use incremental approach previously presented to
  sum influence of changing stresses (rupture
  influence)
• Adaptively refine increments until residual
  strength converges to some prescribed tolerance
• Account for cyclical loading by counting
  reversals and reducing remaining strength

31
Stiffness Reduction Data for Nicalon/E-SiC 2-D
Woven Composite 0/902s
32
Stress Rupture Data for Nicalon/E-SiC 2-D Woven
Composite 0/902s
33
Stress Rupture Data for Nicalon/E-SiC 2-D Woven
Composite 0/902s
34
Fatigue Data for Nicalon/E-SiC 2-D Woven
Composite 0/902s
35
Residual Strength Data for Nicalon/E-SiC 2-D
Woven Composite 0/902s
36
Residual Strength Data for Nicalon/E-SiC 2-D
Woven Composite 0/902s
37
Residual Strength Data for Nicalon/E-SiC 2-D
Woven Composite 0/902s
38
Validation Mission loading profile
39
Validation Mission loading profile
40
Validation results Trapezoidal loading profile
41
Validation results Spike loading
42
Results for Nicalon/E-SiC 2-D Woven Composite
0/902s
43
Micromechanics of Combined Degradation Mechanisms

• Two particular damage mechanisms
• Slow crack growth
• Interfacial creep
• Analytic solution has not been develop ? use
  simulation approach

44
Stress-Rupture Lifetime Prediction
Assume that crack growth is the mechanism for
fiber failure at elevated temperatures.
Crack growth is dictated by the Paris Law
So with time the strength of an individual fiber
is
Iyengar Curtin (1997)
45
Fiber Rupture Behavior
Obtain the fiber stress rupture parameters from
single fiber testing
Yun DiCarlo, 1993
46
Micromechanics of Combined Degradation Mechanisms

• Two particular damage mechanisms
• Slow crack growth
• Interfacial creep
• Analytic solution has not been develop ? use
  simulation approach

47
Micromechanics of Combined Degradation
Mechanisms Both Mechanisms
48
Test Case Results from Micromechanical
Simulation
49
Sponsors of Durability Work

• NASA Langley - life prediction for HSR (HSCT)
• Pratt and Whitney - high-T PMCs
• Wellstream - life prediction for flexible pipes
• Goodyear - truck tire durability
• McDermott Technologies - hot gas filters radiant
  burners
• Martin Marietta - CFCCs, 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
• Schlumberger Technology - performance of
  high-temperature polymer composites in down-hole
  environments

50
Summary

• Presented the philosophy for a life prediction
  method for composites based upon residual
  strength
• Applied the method to three composite systems
• Flowtite pressure pipe (Owens Corning product)
• Woven graphite epoxy composite (for jet engine
  applications)
• Ceramic matrix composite (for gas turbine
  applications)
• Discussed making the connection between
  micromechanics and residual strength approach

51
Acknowledgements

• Jean Matthieu Bodin, Tozer Bandorawalla, Sneha
  Patel, Nirmal Iyengar, Mike Pastor, Mehran Elahi
• Sponsors
• Owens Corning
• Pratt and Whitney
• National Science Foundation
• General Electric
• ORNL