1. The Determination of the Fibre Volume Fraction in Natural Fibre Composites
Richard Cullen and John Summerscales
Advanced Composites Manufacturing Centre
University of Plymouth
University of Warwick, 20 April 2004

2. Elastic modulus of composite
calculated by rule-of-mixtures
neglect contribution of matrix
Ec = ~ ηL ηO VF EF
ηL fibre length distribution factor
ηO fibre orientation distribution factor
VF fibre volume fraction
EF elastic modulus of fibre

3. Determining Vf is problematic
fibres “float” in water
fibres are hygroscopic
weight is function of moisture content
4.6% loss in jute yarn dried 60°C for 30 min
cross-sectional area of the fibres
not normally round

4. Determination of Vf tow counting/areal weight direct weighing
density gradient
Archimedes principle
resin burn-off
thermo-gravimetric analysis
chemical digestion
microscopy

5. Vf by tow counting/areal weight
for UD composites in an open-ended mould
Grafil Test Method
fabric areal weight
in a moulding of known thickness
CRAG method

6. Calculate volume fraction from fabric areal weight
Vf = j AF / f t
j number of layers of fabric
AF areal weight of fabric (kg/m2)
f density of fibre (kg/m3)
t thickness of laminate (m)

7. Vf by direct weighing a closed mould is used
no fibre is lost in the moulding flash
mass fraction after fabrication = mass of fibre/mass of the composite.
accurate densities needed to convert the mass fraction to a fibre volume fraction.
Grafil Test Method

8. Vf by density gradient
observe the level to which the test specimen sinks in a column of liquid when the density of liquid changes uniformly with height.
absorption of liquid may complicate the analysis when natural fibres are under test
Grafil Test Method

9. Vf by Archimedes principle
weight measurements
in air and in water
absorption of liquid may complicate the analysis when natural fibres are under test.
Grafil Test Method
CRAG methods 800/1000-1

10. Density density of resin typically 1100 kg/m3
density of fibre typically 1600 kg/m3
 resolution of 5 mg/cm3 for 1%Vf
CRAG method 800 for density of FRP
1g or more weighed in air and in fluid
accuracy ±0.2% desirable for Vf and Vv
immersion fluid at 23 ± 2°C

11. Density of jute dried 60°C for 30 min weighed in air immediately
immersed and degassed in fluid
-990 mbar water : -500 mbar acetone
weighed in fluid
ρ = 1.669±0.037 in water/Ilfotol at 22.4°C
ρ = 1.652±0.037 in acetone at 20.3°C
data from Richard Cullen
image from Jean-Philippe le Nours

12. Vf by resin burn-off inappropriate for natural fibre composites
oven at °C until constant weight
both components of the composite will burn
CRAG method c

13. Vf by TGA: Thermo-Gravimetric Analysis
TGA measures weight changes in a material as a function of temperature (or time) under a controlled atmosphere
principal uses include measurement of material thermal stability & composition.

14. Vf by TGA: flax fibres primary decomposition peak
55-91% weight loss between °C
secondary decomposition peak
2-33% weight loss between °C
ash 4-9 % by weight
HSS Sharma et al: Queens-Belfast/DANI
Thermochimica Acta, 1988, 132,
J Textile Institute, 1996, 87(2),
J App Polym Sci, 2000, 75,

15. Vf by TGA: flax/HDPE Pure flax Pure HDPE
decomposition starts c °C
3% wt loss by 385°C
>400°C degradation slows (ash)
Pure HDPE
degradation starts c °C
9% wt loss by 490°C
T Powell et al, Engineering properties of flax fiber and flax fiber-reinforced thermoplastic in rotational moulding, ASAE/CSAE meeting, Saskatchewan, September 2002.

16. Vf by TGA: sisal/starch
hemicellulose & cellulose glycosidic links
decomposition peak at 300°C
alpha-cellulose
decomposition peak at 360°C
lignin
decomposition between °C
maximum at 350°C
VA Alvarez and A Vázquez , 2004 Polymer Degradation and Stability, 84(1), 13-21

17. Vf by chemical digestion
sulphuric acid and hydrogen peroxide
Grafil Test Method
CRAG method 1000‑3a
nitric acid
CRAG method b
different chemicals for natural fibres ?
microwave acid digestion bomb
? Journal of Materials Science Letters ?

18. Vf by microscopy
optical or electron microscopy with computer image analysis
for natural fibres enhance the contrast
polarising filters or staining techniques
image splitting eyepiece
to determine individual fibre “diameters”
Grafil Test Method

19. Vf by optical microscopy
coarse polish (recommended)
gives better contrast but manual intervention to eliminate scratches
fine polish
removes scratches but significantly lowers contrast
manipulate with Photoshop or similar
to optimise contrast
analyse with ImageJ

20. Use of ImageJ software original optical microscopy image
optimise contrast in Photoshop
define boundaries
convert to binary data

21. Vf by optical microscopy
SEM images have low contrast
similar chemistry of fibre and matrix
TEM preparation difficult
sample area too small for sensible statistical significance

22. Pro and con for microscopy
data on fibre cross section size and shape
data on fibre clustering
can be quantified by e.g. fractal dimension
long preparation times
need to distinguish at multiple scales

23. General references
Grafil Test Methods (reference ), Courtaulds Limited, Coventry, March 1980.
PT Curtis, CRAG Test Methods for the Measurement of the Engineering Properties of Fibre Reinforced Plastics, Royal Aircraft Establishment Technical Report RAE-TR , February 1988.
FJ Guild and J Summerscales, Microstructural image analysis applied to fibre composite materials: a review, Composites, 1993, 24(5),
J Summerscales (editor), Microstructural Characterisation of Fibre-Reinforced Composites, Woodhead Publishing, Cambridge, July ISBN CRC Press LLC, Boca Raton - Florida, July ISBN
AR Clarke and CN Eberhardt, Microscopy Techniques for Materials Science, Woodhead Publishing, Cambridge, July ISBN CRC Press LLC, Boca Raton - Florida, July ISBN

24. To contact me  Dr John Summerscales 01752.23.2650 01752.23.2638
School of Engineering: RYB 008
University of Plymouth
Devon PL4 8AA