S.V. Lomov CompTest-2011 Lausanne 1

Inherent carbon fibre stiffening

as seen

in textile reinforced composites

Stepan V. Lomov1, Alexander E. Bogdanovich2,

Ichiro Taketa1,3, Jian Xu1, Ignaas Verpoest1

1Depertment MTM, Katholieke Universiteit Leuven, Belgium

23Tex Inc, Cary, NC, USA

3Toray Industries, Japan

S.V. Lomov CompTest-2011 Lausanne 2

Contents

1. Introduction: The carbon fibre stiffening phenomenon and previously observed stiffening effects in carbon fibre textile composites

2. Strain measurements during tensile test

3. Stiffening in 3D woven non-crimp carbon/epoxy composites

4. Conclusions

S.V. Lomov CompTest-2011 Lausanne 3

1. Introduction: • The carbon fibre stiffening phenomenon

• Previously observed stiffening effects in carbon fibre textile composites

2. Strain measurements during tensile test

3. Stiffening in 3D woven non-crimp carbon/epoxy composites

4. Conclusions

S.V. Lomov CompTest-2011 Lausanne 4

Inherent stiffening of carbon fibres

First observed:

Curtis, G. J., J. M. Milne and W. N. Reynolds (1968). "Non-Hookean Behaviour of Strong Carbon Fibres." Nature 220(5171): 1024-1025

strain 1%

20% increase E

figure from:

Shioya, M., E. Hayakawa and A. Takaku (1996). "Non-hookean stress-strain response and changes in crystallite orientation of carbon fibres." Journal of Materials Science 31(17): 4521-4532

S.V. Lomov CompTest-2011 Lausanne 5

Literature on carbon fibre stiffening

1. Curtis, G.J., J.M. Milne and W.N. Reynolds, Non-Hookean behaviour of strong carbon fibres, Nature, 1968, 220(5171): 1024-1025.

2. van Dreumel, W.H.M. and J.L.M. Kamp, Non Hookean behaviour in fibre direction of carbon-fibre composites and the influence of fibre waviness on the tensile properties, Journal of Composite Materials, 1977, 11(Oct): 461-469.

3. Morley, H., A simple strand test for routine fibre strength and modulus evaluation, Composites, 1982, 13(1): 21-23.

4. Beetz, C.P., Jr., Strain-induced stiffening of carbon fibres, Fibre Science and Technology, 1982, 16: 219-229.

5. Beetz, C.P., Jr. and G.W. Budd, Strain modulation measurements of stiffening effects in carbon fibers, Review of Scientific Instruments, 1983, 54(9): 1222-1226.

6. Vangerko, H. and A.J. Barker, The stiffness of unidirectionally reinforced CFRP as a function of strain rate, strain magnitude and temperature, Composites, 1985, 16(1): 19-22.

7. Ishikawa, T., M. Matsushima and Y. Hayashi, Hardening non-linear behaviour in longitudinal tension of unidirectional carbon composites, Journal of Materials Science, 1985, 20: 4075-4083.

8. Hughes, J.D.H., Strength and modulus of current carbon fibres, Carbon, 1986, 24(5): 551-556.

9. Stecenko, T.B. and M.M. Stevanovic, Variation of elastic moduli with strain in carbon/epoxy laminates, Journal of Composite Materials, 1990, 24: 1152-1158.

10. Northolt, M.G., L.H. Veldhuizen and H. Jansen, Tensile deformation of carbon fibers and the relationship with the modulus for shear between the basal planes, Carbon, 1991, 29(8): 1267-1279.

11. Shioya, M and A. Takaku, Rotation and extension of crystallites in carbon fibers by tensile stress, Carbon, 1994, 32(4): 615-619.

12. Shioya, M., E. Hayakawa and A. Takaku, Non-hookean stress-strain response and changes in crystallite orientation of carbon fibres, Journal of Materials Science, 1996, 31(17): 4521-4532.

13. Toyama, N. and J. Takatsubo, An investigation of non-linear elastic behavior of CFRP laminates and strain measurement using Lamb waves, Composites Science and Technology, 2004, 64: 2509–2516

1 1

5

4

1

1960s 1970s 1980s 1990s 2000s

S.V. Lomov CompTest-2011 Lausanne 6

Cross-ply carbon laminates: stiffening vs damage softening

Toyama, N. and J. Takatsubo (2004). "An investigation of non-linear elastic behavior of CFRP laminates and strain measurement using Lamb waves." Composites Science and Technology 64: 2509–2516.

S.V. Lomov CompTest-2011 Lausanne 7

Textile composites

Ishikawa, T., M. Matsushima and Y. Hayashi, Hardening non-linear behaviour in longitudinal tension of unidirectional carbon composites, Journal of Materials Science, 1985, 20: 4075-4083

Stiffening effect is noted for 8-harness satin – more pronounced effect for straight fibres

Truong, T. C. . The mechanical performance and damage of multi-axial milti-ply carbon fabric reinforced composites. PhD thesis, Department MTM. Leuven, Katholieke Universiteit Leuven, 2005

S.V. Lomov CompTest-2011 Lausanne 8

Carbon/PP woven composites

+20%

spectacular increase of stiffness …

… may be caused by decrimping as well …

S.V. Lomov CompTest-2011 Lausanne 9

Carbon/epoxy twill woven composite

stress, MPa

E, GPa

strain, %

0.4 0.6

60

700.2

0.2 0.4 0.6

S.V. Lomov CompTest-2011 Lausanne 10

1. Introduction: The carbon fibre stiffening phenomenon and previously observed stiffening effects in carbon fibre textile composites

2. Strain measurements during tensile test

3. Stiffening in 3D woven non-crimp carbon/epoxy composites

4. Conclusions

S.V. Lomov CompTest-2011 Lausanne 11

Optical extensometry

y = -4.9477x2 + 0.6852x

R2 = 0.9996

y = 0.1848x + 0.0059

R2 = 0.9994

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.000 0.010 0.020 0.030 0.040

eps grips

eps_

X

0

200

400

600

800

1000

0 0.01 0.02 0.03 0.04 0.05 0.06

strain

sig,

MP

a

grips

true: LIMESS

Instron

LIMESS

1. Precise position of zero of LIMESS curves

2. Two regions on the curves

3. Choice of the fitting

S.V. Lomov CompTest-2011 Lausanne 12

1. Introduction: The carbon fibre stiffening phenomenon and previously observed stiffening effects in carbon fibre textile composites

2. Strain measurements during tensile test

3. Stiffening in 3D woven non-crimp carbon/epoxy composites

4. Conclusions

S.V. Lomov CompTest-2011 Lausanne 13

The 3D woven non-crimp carbon/epoxy composite

Warp/fill yarnsToho Tenax 12K, 800

texAreal density, g/m2 2499

Z yarns Toho Tenax 1K, 66 tex Ends/picks, per inch in layer 12 / 10

Fiber diameter, µm 7* Fibre Young modulus, GPa 238*

Fibre strength,MPa 3950* Fibre ultimate elongation 1.7%*

Composite thickness, mm

2.760.028Fiber volume fraction in the

composite51.1%0.5%

Waviness, warp 0.03%Fibre volume fraction, fibres in

warp:fill:vertical directions24.2%:26.2%:0.7%

Waviness, fill, outer/inner layer

0.08% / 0.02%Fibre volume fraction inside

warp and fill yarns61…73%

* datasheet of Toho Tenax

S.V. Lomov CompTest-2011 Lausanne 14

Expected values of Young’s modulus

Fibre Young modulus, GPa 238*

Fiber volume fraction in the composite 51.1%0.5%

Fibre volume fraction, fibres in warp:fill:vertical directions

24.2%:26.2%:0.7%

Waviness, warp 0.03%

Waviness, fill, outer/inner layer 0.08% / 0.02%

GPaGPaE

GPaGPaE

fill

warp

65 4.62262.0*238

60 7.57242.0*238

* datasheet of Toho Tenax

S.V. Lomov CompTest-2011 Lausanne 15

Change of Young’s modulus

0

100

200

300

400

500

600

700

800

900

1000

0 0.005 0.01 0.015

strain

stre

ss,

MP

a

0

10

20

30

40

50

60

70

80

90

100

E,

GP

a

warp

fillexpected

GPaGPaE

GPaGPaE

fill

warp

65 4.62262.0*238

60 7.57242.0*238

expected:

S.V. Lomov CompTest-2011 Lausanne 16

0

100

200

300

400

500

600

700

800

900

1000

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

Change of Young modulus and damage

0

100

200

300

400

500

600

700

800

900

1000

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

strain

AE

AE

stress, MPa

E*10, GPa

20% increase of E

S.V. Lomov CompTest-2011 Lausanne 17

Impregnated carbon yarns: 6K and 12K

stress – strain:

mixed data for 3K and 12 K

6K

12K

fabric, warp and fill

Young’s modulus, normalised

VF = 70%

S.V. Lomov CompTest-2011 Lausanne 18

Stiffening and fatigue: S-N curve

0

100

200

300

400

500

600

700

800

900

1000

1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

fatigue cycles

max

imum

fat

igue

str

ess,

MP

a

WARP staticWARPWARP no breakFILL staticFILLFILL no breakLog. (FILL)Log. (WARP)

I

II

III

warp

fill

tim e

stre

ss

m ax

m in

S.V. Lomov CompTest-2011 Lausanne 19

Post-fatigue tensile test: Fatigue @ 450 MPa, 1,000,000 cycles

0

100

200

300

400

500

600

700

800

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02

0

10

20

30

40

50

60

70

80

0

100

200

300

400

500

600

700

800

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016

0

10

20

30

40

50

60

70

80

warp fill

static @450 MPastatic @450 MPa

50

55

60

65

70

0 200 400 600 800 1000

stress, MPa

E, G

Pa

quasi-static evolution of Young’s modulus

stress E

strain strain

tim e

stre

ss

m ax

m in

initial static

S.V. Lomov CompTest-2011 Lausanne 20

1. Introduction: The carbon fibre stiffening phenomenon and previously observed stiffening effects in carbon fibre textile composites

2. Strain measurements during tensile test

3. Stiffening in 3D woven non-crimp carbon/epoxy composites

4. Conclusions

S.V. Lomov CompTest-2011 Lausanne 21

Conclusions: Evolution of Young’s modulus of carbon/epoxy textile composites

Quasi-static tension:

• inherent stiffening of carbon fibres

• softening of the composite due to damage

Fatigue and post-fatigue:

• change of Young modulus in the first cycles: possible high modulus

• softening due to damageE, warp

55

60

65

0 200 400 600 800 1000

stress, MPa

E, G

Pa

tim e

stre

ssm ax

m in