OC AIAA 2014 SoCal ASAT Conference
OC AIAA 2014 SoCal ASAT Conference
Moisture Absorption
of
Composite Sandwich Structures
Todd Coburn
California State Polytechnic University Pomona
[email protected]
US Technical
[email protected]
May 3, 2014
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Overview
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Composite Structures Absorb Moisture
Moisture has detrimental effect on Composite Strength
Strength Effect on Solid Laminate generally understood
Strength Effect on Sandwich Structures Important as well
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Five Step Process was developed to assess
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Compression Strength
Facesheet Wrinkling
Shear Crimping
Core Stresses
Diffusion Constant Development
Moisture Level Predictions
Allowable Determination
Structural Analysis
Modification of Design
Process successfully implemented on Delta IV launch vehicle
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Review: Moisture Absorption Theory
Moisture Diffusion Model
• Moisture Diffusion is governed by Fick’s Law
c  
c 
  Dz 
t z 
z 
– c is the moisture concentration
– Dz is the Diffusion Coefficient of the material
x
– Which becomes
c
 2c
 Dz 2
t
z
– For constant Dz
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Review: Moisture Absorption Theory
Structural Idealization
FTCS Approximation at time n for slice j
C jn  1 - C jn
t
n  2C n  C n
C
j
j - 1 
 j  1
 Dz
z 2
BTCS Approximation at time n for slice j
1
2
3
Cj
n 1


- Cj
C j1  2  C j  C j-1
 D6z 7
4
5
t
8x 2 9 10
n
n
n
n
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C j n  1 - C jn
t
Crank Nicholson Approximation @ (t,n)
C n  1  2  C n  1  C n  1 
j
j -1
 j 1

 Dz 
z 2
Note:   Dx t 
2
z 
 
   n1   
 n
n 1
n 1
n
n
  C j1  1   C j    C j-1   C j1  1   C j   C j-1
 2
 2
2
2
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Review: Moisture Absorption Theory
Boundary Conditions
CSurface  M EQ1
CSurface  M EQ15
CSurface  M EQ1
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Conservation of Mass Flux @ Interfaces: Ci  ρi  Ci 1  ρi 1
ρ 
Ci  Ci 1   i 1 
 ρi 
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Review: Moisture Absorption Theory
System of Equations
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 1: Diffusion Constant Development
Step 1a: Determine Equilibrium Moisture
Steps:
• Dry Samples
•Condition in Humidity Chamber
•Weigh Samples Periodically
•Plot Moisture vs. Time
• Find Equilibrium values @ each RH
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 1: Diffusion Constant Development
Step 1b: Determine Equilibrium Constants
Steps:
•Plot Equilibrium Moisture vs. RH
•Fit Curve to Determine a & b
M EQ
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
 RH 
a

 100 
b
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OC AIAA 2014 SoCal ASAT Conference
Step 1: Diffusion Constant Development
Step 1c: Determine Diffusivity
Steps:
• Plot Moisture vs. Time
• Determine initial slope @ each T
• Use Slope to determine Dz vs. T
Dz @ T 
π  K  t 2 ( slope) 2
16  M EQ
where the slope is given by
slope 
M 2  M1
t2  t1
and the correction for edge diffusion
is given by
K
1
t t 

1   
 L w
2
Ref. Springer, Environmental Effects on Composite Materials,
Technomic Publishing Co., 1981
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 1: Diffusion Constant Development
Step 1d: Determine Diffusion Constant
Steps:
• Plot Dz versus 1/T
• Fit Curve to Determine Do & Eo
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
R=1.98586 cal/mol-K
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OC AIAA 2014 SoCal ASAT Conference
Step 2: Moisture Level Predictions
Step 2: Determine Moisture Levels
.056” Gr/Ep
1.0” Core
.056” Gr/Ep
Typical Composite Sandwich
(90F @ 86% RH for 3 months)
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 2: Moisture Level Predictions
Step 2b: Verify Moisture Levels / Methods
.056” Gr/Ep
1.0” Core
.056” Gr/Ep
Typical Composite Sandwich
(160F @ 100% RH for 30 days)
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 2: Moisture Level Predictions
Step 2c: Control Moisture Levels
0.056” Gr/Ep
1.0” Core
0.10” Gr/Ep
Typical Composite Sandwich
(90F @ 86% RH for 3 months)
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 3: Allowable Determination
Material Properties Affected
• Facesheet Materials
– Significant Reduction in Strength
• Compression
• Tension
• Shear
– Small Reduction in Modulus
• Foam Core Materials
– Significant Reduction in Strength
• Compression
• Tension
• Shear
– Significant Reduction in Modulus
– May Cause Significant Swelling at Elevated Moisture & Temperature
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 3: Allowable Determination
Primary Sandwich Failure Modes
• Compression
Affected by Facesheet Moisture
• Euler Buckling
Minimal Moisture Affect
• Facesheet Wrinkling
Affected by Facesheet & Core Moisture
• Sandwich Shear Crimping
Affected by Core Moisture
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 3: Allowable Determination
Determine Effects on Strength
Facesheet Compression
Core Tension, Compression, Shear
Steps:
• Prepare Sandwich Panels
•Condition Panels
•Cut Samples
•Test at Temperature
Facesheet Wrinkling
Influences:
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Facesheet Stiffness
Core Stiffness
Core Strength
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
Shear Crimping
• Sandwich Local Instability
• Dominated By Shear Stiffness
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OC AIAA 2014 SoCal ASAT Conference
Step 4: Structural Analysis
Basic Approach
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Understand Structure
Determine Loads
Determine Environment
Predict Moisture
Determine Allowables
– Including Moisture Effects
• Develop Finite Element Models
• Perform Analysis
• Evaluate Structural Margins
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Step 5: Design Modification
Moisture Issue Resolution
• Insulation can be added to reduce temperatures
• Coating Can be Added to Block Moisture
• Facesheet material can be added
– Reduces Transient Temperature
– Reduces Stresses
– Improved Facesheet Wrinkling Allowable
• Environment can be controlled
– Environmental control can be used in storage facilities
– Structures can be bagged and desiccated
– Structures can be tented
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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OC AIAA 2014 SoCal ASAT Conference
Conclusions
• Composite Structures Absorb Moisture
• Moisture has detrimental effect on Composite Sandwich Strength
• Five Step Process was Developed to Evaluate Moisture
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Diffusion Constant Development
Moisture Level Predictions
Allowable Determination
Structural Analysis
Modification of Design
Moisture Absorption of Composite Sandwich
Todd Coburn, May 3, 2014
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