The challenges of natural fibres as an engineering composite reinforcement

(1)

INDUSTRY LEADER INNOVATION GLOBAL VISION SOLUTIONS PARTNERSHIP

Jim Thomason,

Fiona Gentles, Jamie Carruthers

The Challenges of Natural Fibres

as Engineering Composite

Reinforcements

19

th

Annual BEPS Meeting

September 28-30

th

2011

Vienna, Austria

(2)



Introduction



Composite fibre content



Thermo-mechanical anisotropy of natural fibres



Natural fibre non-circular cross section



Conclusions

(3)

INDUSTRY LEADER INNOVATION GLOBAL VISION SOLUTIONS

PARTNERSHIP



Fibre natural variability



Fibre anisotropy



Fibre non-circular



Composite fibre content measurement



Moisture sensitivity



Fibre-matrix interaction

Natural Fibre Composites - Challenges

(4)

INDUSTRY LEADER INNOVATION GLOBAL VISION SOLUTIONS

PARTNERSHIP

Sisal

Jute

Flax

Glass

Modulus (GPa)

17-28

20-45

27-70

75 Strength (GPa)

0.1-0.8

0.2-0.9

0.3-0.9

>1.5

Density

1.3

1.5

2.6 Specific

Modulus

13-21

15-35

18-47

29 m

m

f

L

C

=

V

E

V

E



₀





Some typical fibre properties are shown in the Table below

Why Natural Fibre Composites ?

(5)

Comparison Predicted Composite Modulus

0

2

4

6

8

10

0

10

20

30

40 Fibre Content (% weight)

M

o

dul

us

(GP

a

)

Glass Fibre

NF 20 GPa(Sisal)

NF 40 GPa (Jute)

NF 60 GPa (Flax)

For injection moulded long fibre polypropylene

(6)

Actual Modulus Injection Moulded Jute-PP

0

2

4

6

8

10

0

10

20

30

40 Fibre Content (% weight)

M

o

dul

us

(GP

a

)

ASTM GFPP

ASTM Bar

Plaque Flow

Plaque Cross Flow

Glass

(7)

Thermoelastic Anisotropy of Flax

and Sisal Fibres

• Goal

– Quantify anisotropy of Flax & Sisal fibres

– Full thermoelastic characterisation

• Measure

– UD fibre-epoxy laminates E(q,T), G

₁₂

,n

₁₂

, n

₂₁

,a(q,T)

– Epoxy matrix E

_m

(T),n

_m

, a

_m

(T)

– Laminate fibre volume fraction ?

– Flax & Sisal fibre E

_1f

(fibre cross section ?)

• Calculate

(8)

Water Absorption for Fibre Content

0

4

8

12

16

0

1

2

3

4 Sqrt Days

Wa

te

r

A

b

so

r

p

ti

o

n

(

%w

t)

Epoxy Matrix,

D

m

_m

=

D

M

_m

/M

_m

Sisal Fibres,

D

m

_m

=

D

M

_m

/M

_m

Sisal-Epoxy Composites

(9)

NF Composite Fibre Volume Fraction

• Sisal composite W

_f

= 0.46, V

_f

= 0.4

• Flax composite W

_f

= 0.36, V

_f

= 0.31

m

f

m

c

f

Δm

W





1 f

f

m

f

W

)

W

(1

ρ

1 V















_





• Sisal fibre density

r

_f

= 1400 kg/m

3

• Flax fibre density

r

_f

= 1400 kg/m

3

(10)

Composite DMA Results

7.5

8.0

8.5

9.0

9.5

10.0

10.5 -50

-25

0

25

50

75

100

125

150

(11)

Anisotropy of Fibre Modulus

0.1

1

10

100 -60

-40

-20

0

20

40

60 Temperature (°C)

M

o

d

u

lu

s

(G

P

a

)

Flax E1

Flax E2

Sisal E1

Sisal E2

(12)

Composite Thermal Strain

-5

0

5

10

15

20

25 -50

-25

0

25

50

75

100

125

150 Temperature (°C)

T

h

e

r

m

a

l

S

tr

a

in

(

m

/m

)

Epoxy

_{Flax 90}

(13)

Fibre Expansion Coefficients

-20

0

20

40

60

80 -60

-40

-20

0

20

40

60 Temperature (°C)

C

L

T

E

m

m/

m

°C

_{Sisal Transverse}

Flax Transverse

Sisal Axial

Flax Axial

(14)

Summary Thermo-Mechanical Properties NF

Glass

Flax

Sisal

Longitudinal Modulus (GPa)

75

61.5

24.9 Transverse Modulus (GPa)

75

1.2

1.6 Shear Modulus (GPa)

30

1.7

1.1 Axial LCTE

(

m

m/m.

o

_C)

₅

_-7.3

_-2.7

(15)

Single Fibre Testing

Fibre Stress = Load/Area = P/A

_f

(= 4P/

p

D

_f

2 ???)

(16)



A

_f

in single fibre testing is almost universally

evaluated from D

_f

using a transverse image of

fibre and assumption of circular cross-section



Is this acceptable for Natural Fibres ??

(17)

Single Fibre Measurements

• Single flax and sisal fibres

mounted on test card windows

• Fibre “diameter” determined

by averaging 4 transverse

measurements

• Fibre tensile testing

(10,15,20,25,30 mm gauge)

• Residual fibre ends glued to

card tab sectioned in 2 places

and “true” cross sectional

area determined

1. Single fibre “diameter”

determined by averaging 4

transverse measurements

2. Fibres embedded, cut and

polished

3. “true” cross sectional area

determined

4. Sample ground down 2mm

and polished

(18)

(19)

Single Flax Fibre CSA Variability

0

0.01

0.02

0.03

0

5

10

15

20 Measurement Position Along Fibre (mm)

Fibre Cross

Section (mm

2 )

F1

F2

F3

F4

F5

F6

F7

F8

F9

F10

(20)

Single Sisal Fibre CSA Variability

0

0.01

0.02

0.03

0.04

0.05

0

5

10

15

20 Measurement Position Along Fibre (mm)

Fibre Cross

Section (mm

2 )

S1

S2

S3

S4

S5

S6

S7

S8

S9

S10

(21)

Average

CSA (mm

2 )

% standard deviation of the

average CSA

Intra-fibre

Inter-fibre

Sisal

0.0272

7.3%

30.3%

Flax

0.0125

9.2%

42.0%

Variability in CSA Determination

Better to focus on measuring many different fibres

rather than many measurement along the same fibre

CSA variability

Flax

>

Sisal

(22)

Natural Fibre CSA Evaluation

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.00

0.01

0.02

0.03

0.04

0.05 Measured CSA (mm

2 )

“D

iam

et

er

”

C

SA

(

m

2 )

Flax

_Sisal

Y=X

y=1.97x

y=2.55x

(23)

Single Fibre Modulus

0

10

20

30

40

50

60

70

0

1

2

3

4 Fibre CSA/Gauge Length (

m

m)

1/

M

odu

lus

(

10

6 /G

P

a)

Sisal

Flax

Flax, 1000/14.1=71 GPa

y=10.7x+33.0

(24)

Natural Fibre CSA Evaluation

0

1

2

3

4

5

0.0

0.1

0.2

0.3

0.4 Average “Diameter” (mm)

“D

iam

et

er

”

C

SA

/T

rue

C

SA

(25)

Natural Fibre CSA Evaluation



“Diameter” method significantly

overestimates CSA



Underestimates single fibre modulus and

strength

(26)

CSA method

Diameter Actual

Flax Strength (MPa)

293

688 Sisal Strength (MPa)

255

530 Flax Modulus (GPa)

36

71 Sisal Modulus (GPa)

20

30

(27)

Effect of “Diameter” CSA on

Apparent NF Modulus

0

10

20

30

40

50

60

0.0

0.1

0.2

0.3

0.4 Average “Diameter” (mm)

A

pp

ar

ent

M

odu

lus

(

G

P

a)

Flax, E1f=71.0 GPa

Sisal, E1f=30.5 GPa

(28)

20mm

10mm

Flax

Simple Model of NF CSA“Diameter” Errors

NF non-circular –

simplest model is

oval X-section

(29)

Simple Model of NF CSA“Diameter” Errors

Due to NF natural twist the oval cross section will be

viewed differently at different positions along the fibre

(30)

Parameteric Ellipse Analysis

fibre “diameter” D

B

A

f

X(t),Y(t)

t

x

y

X

_max

Can solve for X

_max

for any

f

and then average over

f

=0-90° for different A:B ratios

AB

25 .

0 CSA

True



p

2 D

25 .

0 CSA

Diameter"

"



p

(31)

CSA Ratio from Ellipse Analysis

0

1

2

3

4

5

0

20

40

60

80 CS

A

Ratio (

D

2 /AB)

Ellipse Major Axis Orientation Angle

A/B 5

A/B 3

A/B 2

A/B 1

Av CSA

Ratio

2.6

1.7

1.3

(32)

Natural Fibre CSA Evaluation

0

1

2

3

4

0

0.1

0.2

0.3

0.4 'Diam

et

er"CSA/T

rue

CS

A

Average Fibre "Diameter" (mm)

(33)

Natural Fibre CSA Evaluation

0

1

2

3

4

0

0.1

0.2

0.3

0.4 'Diam

et

er"CSA/T

rue

CS

A

Average Fibre "Diameter" (mm)

Flax measured

Sisal measured

Flax thin

Flax average

Flax thick

Sisal thin

Sisal average

Sisal thick

Lines of fixed CSA

and varying ellipse

(34)

Other Fibres

(35)

Other Fibres Ellipse A:B

Abaca

Coir

Kenaf

Jute

1.23

1.86

1.42

2.62

1.43

2.41

1.38

1.15

(36)

Summary Thermo-Mechanical Properties NF

Glass

Flax

Sisal

Longitudinal Modulus (GPa)

75

61.5 (71.0)

24.9 (30.5)

Transverse Modulus (GPa)

75

1.2

1.6 Shear Modulus (GPa)

30

1.7

1.1 Axial LCTE

(

m

m/m.

o

_C)

₅

_-7.3

_-2.7

(37)

What does this anisotropy mean for the

reinforcement performance of natural fibres ?

m

f

L

C

=

V

E

V

E



₀





• Comparison NF and GF often “assumes” isotropic fibre

• Hence simple Krenchel analysis for



₀

)

(

cos

4

0 

q



• NF is more like an orthotropic composite material

(38)

Engineering Stiffness, Off-axis

Orthotropic Lamina

x

ε

σ

E



0

33

32

31

23

22

21

13

12

11 xy

y

x







































_x

S

s





S

1 E

,

all

for

and

11 x



q

xy

S

σ

ε



set

s

_xy



{

s

_x

0

0 }

q



q



q



4

22

2

33

12

4

11

11 S

cos

(2S

S

)sin

cos

S

sin

S

The terms S

₁₁

, etc., are found from S=















12 22 11 12 22 21 11

G

1

0

0 E

1 E

ν

0 E

ν

E

1 x

11 x

S

(39)

Offaxis Stiffness Contribution of

Anisotropic Fibre

0

10

20

30

40

50

60

0

10

20

30

40

50

60

70

80

90 Off-axis Angle (°)

F

ib

re

M

od

u

lu

s C

on

tr

ib

u

tion

Flax Krenchel

(40)

Conclusions (1)

• Estimation of natural fibre cross section area via the „diameter‟

method leads to significant overestimation of CSA.

–

results in significant underestimation of mechanical

properties obtained by single fibre testing.

–

also contributes significantly to the variability observed in

the measurement of natural fibres properties.

–

since the magnitude of the CSA error is “diameter”

dependent – single fibre properties will appear to be

diameter dependent.

• Comparison of the CSA of single Flax and Sisal fibre along

their lengths indicated that –

(41)

Conclusions (2)

• A value for the fibre content of NFCs can be obtained from

study of their moisture absorption characteristics.

• Flax and Sisal fibres exhibit very high levels of mechanical and

thermomechanical anisotropy.

• Ignoring natural fibre anisotropy and using only the axial

modulus of natural fibres in estimating their composite