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MODE II FRACTURE TOUGHNESS
OF WOVEN LAMINATES:
UNIT CELL SIZE EFFECT
© COPYRIGHT IKERLAN 2015
M. Olave, I. Vara, H. Husabiaga, L. Aretxabaleta, S.V. Lomov, D. Vandepitte
OUTLINE
1)
2)
3)
4)
5)
INTRODUCTION
MATERIAL
TEST SET-UP: End-notch flexure (ENF) test
RESULTS: 3K vs. 12K
CONCLUSIONS
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1. INTRODUCTION
Carbon fibre reinforced woven
laminates
Lightness
Composites
Delamination
Internal structure
Mode II fracture toughness measurements
for different woven materials
UNIT CELL SIZE EFFECT
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OUTLINE
1) INTRODUCTION
2) MATERIAL
3) TEST SET-UP: End-notch flexure (ENF) test
4) RESULTS: 3K vs. 12K
5) CONCLUSIONS
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2. MATERIAL

Structure: 2/2 Twill M10.1 prepegs (Carbon fibre T700)
 14 layers for 3K (thickness ~ 3.26mm)
 10 layers for 12K (thickness~ 3.32mm).

Autoclave: 5 bar, 120°C for 1 hour
Same structure, different tow size
3K
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Similar Vf%
12K
3.000 filaments per yarn
12.000 filaments per yarn
Laminate: 14 layers
Laminate: 10 layers
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OUTLINE
1) INTRODUCTION
2) MATERIAL
3) TEST SET-UP:
End-notch flexure (ENF) test
4) RESULTS: 3K vs. 12K
5) CONCLUSIONS
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3. TEST SET-UP:
End-notch flexure (ENF) test
Note:
ASTM D7905(*) was recently developed for UD → the test for this research were performed previously
ENF test:
• Simplicity of the test set up
• Friction effect is lower
The general rule for a stable
propagation → the initial crack length
must be at least 70% of the span .
Problems with this material
In the tested material: initial crack length
50% of the span (avoids slipping)
* ASTM D7905-14 Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional FiberReinforced Polymer Matrix Composites. 2014.
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3. TEST SET-UP:
End-notch flexure (ENF) test
Complexity of acquiring the crack tip position on woven material during the test: analytical prediction is done.
Corrected Beam Theory with effective crack length method (CBTE)
This method does not require an initial test measurement for the apparent flexural modulus (
Effective crack length (
) is calculated
DATA
POST-PROCESSING
Beam Theory including Bending rotations effects method (BTBE)
The method includes shear, local deformations and bending rotations
© COPYRIGHT IKERLAN 2015
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OUTLINE
1) INTRODUCTION
2) MATERIAL
3) TEST SET-UP: End-notch flexure (ENF) test
4) RESULTS: 3K vs. 12K
5) CONCLUSIONS
© COPYRIGHT IKERLAN 2015
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4. RESULTS: 3K vs. 12K
Method comparison
Test set-up influence:
supporting cylinders have 20mm of diameter
The assumption in method CBTE that the compliance is
constant and the flexural modulus based on the initial
compliance slope before crack propagation is not
fulfilled by our samples
Comparison CBTE and BTBE method using
Digital Image Correlation:
Measurement of the delamination length
BTBE method closer to DIC results
(for this set-up configuration)
30
Delamination length ∆a [m]
Delamination length DIC
CBTE Method
25
BTBE Method
20
15
10
5
0
8
9
10
11
Test displacement [mm]
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4. RESULTS: 3K vs. 12K
3K: Stable zone 15-25mm away from the initial crack position
12K: Sudden jumps → Fracture toughness =averaging fracture energy values before unstable crack propagation
4000
3500
GII (J/m2)
3000
2500
3K
2000
12K
1500
1000
0
5
10
15
20
25
30
Crack advance (mm)
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4. RESULTS: 3K vs. 12K
15 Samples /material type
Unit cell
Average (J/m2)
Standard deviation (J/m2)
Standard deviation [%]
Median (J/m2)
3K
3526
227
6%
3493
12K
3062
405
13%
3006
17% ↓ 12K than 3K
The matrix distribution on the delaminated surface may be the reason
3K: Matrix pockets on the 38% of the delaminated length. (every 2mm): higher amount of matrix → higher GII
12K: Matrix pockets on the 24% of the delaminated length. (every 5mm):lower amount of matrix → lower GII
3K
12K
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Stable propagation
Jumpy propagation
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OUTLINE
1) INTRODUCTION
2) MATERIAL
3) TEST SET-UP: End-notch flexure (ENF) test
4) RESULTS: 3K vs. 12K
5) CONCLUSIONS
© COPYRIGHT IKERLAN 2015
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5. CONCLUSIONS
 POST-PROCESSING METHODS:
- The CBTE method does not work properly with the material and test set-up
defined in this research (supporting cylinders = ∅20mm; textile)
- The BTBE method that takes into account bending effects seems to be more
suitable when non-linearities appear during ENF tests.
 SIZE EFFECT ON THE MODE II FRACTURE TOUGHNESS:
3K
12K
• Smaller unit cell size shows better matrix distribution • Bigger unit cell size shows spread matrix distribution
• Stable propagation
• Jumpy propagation
• Higher fracture toughness
• Lower fracture toughness
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Eskerrik asko!
Thank you!
Muchas Gracias!
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