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

Daniel Traudes

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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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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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Carbon Fiber Laminates

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• 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:

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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

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Equipment

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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

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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

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Mode III Results

Each load has an associated damage variable

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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

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Digital Image Correlation

• Shows surface strains

• Microcracks visible – 0° microcracks above d =

0.0986

– 90° microcracks above d = 0.1348

• Tomography can validate results

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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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Edge Examination Process

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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

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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

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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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Dye Penetrant Results

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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

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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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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

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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

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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

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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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THANK YOU

Questions?

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