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In Situ Tomography of Microcracking in Cross Ply Carbon Fiber Composites with Pre-existing Debonding Damage
Daniel Traudes
02/10/2012 1
Outline 1. Introduction 2. Research Objectives 3. Process
1. Equipment 2. Materials 3. Procedures
4. Results 5. Conclusions 6. Future Work
02/10/2012 2
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
02/10/2012 3
Carbon Fiber Laminates
02/10/2012 4
• Combination of two materials – Carbon fibers: High strength and stiffness
– Polymer matrix: Low strength, but binds fibers together
• Fibers are arranged in a uni-directional ply (lamina) – Good properties in one direction
• Plies are stacked to create a laminate – Good properties in many directions
0° +45° -45° 90°
90° -45° +45° 0°
[0/+45/-45/90]s
Damage in Carbon Fiber Composites
Damage is seen at loads well below failure:
02/10/2012 5
Mode I: Fiber breaks Mode III: Debonding (diffuse) Mode II: Transverse cracks
Damage depends on loading state:
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
02/10/2012 6
Equipment
02/10/2012 7
X-ray Tomograph UTM Tensile Stage Microscope
Material
T700/M21
[0/90]s 02/10/2012 8
90°
0°
[0/90]s
[±45]s
[0/90]s
Tensile Test Procedure
02/10/2012 9
Mode III Loading Mode II Loading
Diffuse damage Microcracking damage [±45]s [0/90]s
✔ Diffuse ✘ Microcracks
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 10
Mode III Results
Each load has an associated damage variable
02/10/2012 11
Diffuse damage parameter:
d = 1 −𝐸𝑛
𝐸0
Str
ess
(Mp
a)
Strain
Str
ess
(Mp
a)
Strain
Dam
age
Stress (MPa) Stress (MPa)
Dam
age
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 12
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
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
Edge Examination Process
02/10/2012 15
Grinding 1. 320 grit sandpaper 2. 500 grit sandpaper 3. 1000 grit sandpaper
Polishing 4. Al2O3 15 µm solution 5. Al2O3 5 µm solution 6. Al2O3 0.3 µm solution 7. Al2O3 0.04 µm solution
Microscopy
X-ray
Edge Examination Results
• X-ray validation
• Fiber bundle geometry known
02/10/2012 16
Optical microscopy X-ray
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 17
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
02/10/2012 18
Dye Penetrant Results
02/10/2012 19
Plain: After application: Diiodomethane application:
Dye Penetrant: Time Variance
02/10/2012 20 Diiodomethane, time after application
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
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
02/10/2012 22
Tomography Results • 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
02/10/2012 23
Iso Front Side
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
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
Acknowledgements
• Dr. Gilles Lubineau
• Dr. Aram Amassian & Dr. Aamir Farooq
• Dr. Hedi Nouri
• Ali Moussawi
• Dr. Daniel Acevedo
• Friends and family
02/10/2012 26
THANK YOU
Questions?
02/10/2012 27