In Situ Tomography of Microcracking in Cross Ply Carbon Fiber Composites with Pre-existing Debonding Damage

1. In Situ Tomography of Microcracking
in Cross Ply Carbon Fiber Composites
with Pre-existing Debonding Damage
Daniel Traudes
02/10/2012 1

2. Outline
1. Introduction
2. Research Objectives
3. Process
1. Equipment
2. Materials
3. Procedures
4. Results
5. Conclusions
6. Future Work
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3. Why Composites?
• Metals are strong and stiff
• But heavy
• Weight is critical in aerospace
applications
• Fiber based composites are strong
and stiff
• And light
• But risks must be understood
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4. Carbon Fiber Laminates
• Combination of two materials
– Carbon fibers: High strength and stiffness
– Polymer matrix: Low strength, but binds fibers 0°
together +45°
-45°
• Fibers are arranged in a uni-directional ply 90°
(lamina) 90°
-45°
– Good properties in one direction +45°
• Plies are stacked to create a laminate 0°
– Good properties in many directions [0/+45/-45/90]s
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5. Damage in Carbon Fiber Composites
Damage is seen at loads well below failure:
Damage depends on loading state:
Mode I: Fiber breaks Mode II: Transverse cracks Mode III: Debonding (diffuse)
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6. Research Objectives
Determine connection between diffuse and microcracking
damage
1. Mode III Testing: create diffuse damage
1. Determination of diffuse damage parameter
2. Digital image correlation
2. Preparatory Studies for Mode II testing
1. Laminate edge examination
2. Observe microcracks
1. Dye penetrants
2. Tomography
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7. Equipment
UTM X-ray Tomograph Tensile Stage Microscope
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8. Material [0/90]s
[0/90]s
90°
T700/M21
[0/90]s
02/10/2012 0° [±45]s 8

9. Tensile Test Procedure
Mode III Loading Mode II Loading
[±45]s Diffuse damage [0/90]s Microcracking damage
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10. Research Objectives
Determine connection between diffuse and microcracking
damage
1. Mode III Testing: create diffuse damage
1. Determination of diffuse damage parameter
2. Digital image correlation
2. Preparatory Studies
1. Laminate edge examination
2. Observe microcracks
1. Dye penetrants
2. Tomography
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11. Mode III Results
Stress (Mpa)
Diffuse damage parameter:
Stress (Mpa)
𝐸𝑛
d=1−
𝐸0
Strain Strain
Damage
Each load has an
associated
Damage
damage variable
Stress (MPa) Stress (MPa)
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12. Research Objectives
Determine connection between diffuse and microcracking
damage
1. Mode III Testing: create diffuse damage
1. Determination of diffuse damage parameter
2. Digital image correlation
2. Preparatory Studies
1. Laminate edge examination
2. Observe microcracks
1. Dye penetrants
2. Tomography
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13. Digital Image Correlation
• Shows surface strains
• Microcracks visible
– 0° microcracks above d =
0.0986
– 90° microcracks above d =
0.1348
• Tomography can
validate results
02/10/2012 13

14. Research Objectives
Determine connection between diffuse and microcracking
damage
1. Mode III Testing: create diffuse damage
1. Determination of diffuse damage parameter
2. Digital image correlation
2. Preparatory Studies
1. Laminate edge examination
2. Observe microcracks
1. Dye penetrants
2. Tomography
02/10/2012 14

15. Edge Examination Process
Microscopy
Grinding Polishing
1. 320 grit sandpaper 4. Al2O3 15 µm solution
2. 500 grit sandpaper 5. Al2O3 5 µm solution
3. 1000 grit sandpaper 6. Al2O3 0.3 µm solution X-ray
7. Al2O3 0.04 µm solution
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16. Edge Examination Results
Optical microscopy X-ray
• X-ray validation
• Fiber bundle
geometry known
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17. Research Objectives
Determine connection between diffuse and microcracking
damage
1. Mode III Testing: create diffuse damage
1. Determination of diffuse damage parameter
2. Digital image correlation
2. Preparatory Studies
1. Laminate edge examination
2. Observe microcracks
1. Dye penetrants
2. Tomography
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18. Dye Penetrants
• Microdamage is difficult to spot in
x-rays
• Dye penetrants absorbed into
damage areas
• Due to high density, appears dark
on x-rays
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19. Dye Penetrant Results
Plain: Diiodomethane After application:
application:
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20. Dye Penetrant: Time Variance
02/10/2012 Diiodomethane, time after application 20

21. Research Objectives
Determine connection between diffuse and microcracking
damage
1. Mode III Testing: create diffuse damage
1. Determination of diffuse damage parameter
2. Digital image correlation
2. Preparatory Studies
1. Laminate edge examination
2. Observe microcracks
1. Dye penetrants
2. Tomography
02/10/2012 21

22. Computerized Tomography (CT)
• Radial array of 2D x-ray
projections → 3D volume
• Data is based on material
densities
Advantages:
• Internal imaging
• Micr0-scale
• Non-destructive
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23. Tomography Results
Iso Front Side
• 40 kV, 18 W, 360
projections
• 12.1 µm voxels
• Unloaded
• No dye penetrant
• Filtering to reduce noise
• Major transverse cracks
clearly visible
• Data is not clean
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24. Conclusions
• Diffuse damage parameter developed
• Microcracks in digital image correlation
• Diffuse damage samples prepared for Mode II testing
• Edge inspection: X-ray results correlate with microscopy
observations
• Microcrack observation:
– Dye penetrant is effective
– Tomography is effective
• In situ testing possible
• Clearer results expected during loading
02/10/2012 24

25. Future Work
Mode II Campaign
• Samples with diffuse damage:
d = 0, 0.05, 0.10, 0.15, 0.20, and 0.25
• Tensile tests to failure
• Crack identification either with
– Dye penetrant
– Tomography
• Plot of crack density vs. strain
• Microcracking fracture toughness (Gc) from
plot
02/10/2012 25

26. Acknowledgements
• Dr. Gilles Lubineau
• Dr. Aram Amassian & Dr. Aamir Farooq
• Dr. Hedi Nouri
• Ali Moussawi
• Dr. Daniel Acevedo
• Friends and family
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27. THANK YOU
Questions?
02/10/2012 27