The Determination of the Fibre Volume

Fraction in Natural Fibre Composites

Richard Cullen and John Summerscales

Advanced Composites Manufacturing Centre

University of PlymouthUniversity of Warwick, 20 April 2004

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

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

Determination of Vf

• tow counting/areal weight• direct weighing• density gradient• Archimedes principle• resin burn-off• thermo-gravimetric

analysis• chemical digestion• microscopy

Vf by tow counting/areal weight

• tow counting – for UD composites in an open-ended

mould Grafil Test Method 302.24

• fabric areal weight– in a moulding of known thickness

CRAG method 1000-2

Calculate volume fractionfrom 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)

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 302.13

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 301.12

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 301.21 CRAG methods 800/1000-1

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

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

Vf by resin burn-off

inappropriate for natural fibre composites oven at 580-600°C until constant weight both components of the composite will burn CRAG method 1000-3c

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. http://www.tainst.com/products/tga.html

Vf by TGA: flax fibres

• primary decomposition peak – 55-91% weight loss between 315-362°C

• secondary decomposition peak– 2-33% weight loss between 406-465°C

• ash 4-9 % by weight HSS Sharma et al: Queens-Belfast/DANI

• Thermochimica Acta, 1988, 132, 101-109.• J Textile Institute, 1996, 87(2), 249-257.• J App Polym Sci, 2000, 75, 508-514.

Vf by TGA: flax/HDPE• Pure flax

• decomposition starts c.200-210°C• 3% wt loss by 385°C• >400°C degradation slows (ash)

• Pure HDPE• degradation starts c.410-430°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.

Vf by TGA: sisal/starch

hemicellulose & cellulose glycosidic links– decomposition peak at 300°C

• alpha-cellulose– decomposition peak at 360°C

• lignin– decomposition between 200-500°C– maximum at 350°C

VA Alvarez and A Vázquez , 2004Polymer Degradation and Stability, 84(1), 13-21

Vf by chemical digestion• sulphuric acid and hydrogen peroxide

Grafil Test Method 302.56 CRAG method 1000‑3a

• nitric acid CRAG method 1000-3b

• different chemicals for natural fibres ?• microwave acid digestion bomb

? Journal of Materials Science Letters ?

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 102.13

Vf by optical microscopy• coarse polish (recommended)

• gives better contrastbut manual intervention to eliminate scratches

• fine polish • removes scratches but significantly lowers

contrast

• manipulate with Photoshop or similar• to optimise contrast

• analyse with ImageJ http://rsb.info.nih.gov/ij/

Use of ImageJ software

• original optical microscopy image• optimise contrast in Photoshop• define boundaries• convert to binary data

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

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

General references Grafil Test Methods (reference 000.05), 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-88-012, February 1988.

FJ Guild and J Summerscales, Microstructural image analysis applied to fibre composite materials: a review, Composites, 1993, 24(5), 383-394.

J Summerscales (editor), Microstructural Characterisation of Fibre-Reinforced Composites, Woodhead Publishing, Cambridge, July 1998. ISBN 1-85573-240-8. CRC Press LLC, Boca Raton - Florida, July 1998. ISBN 0-8493-3882-4.

AR Clarke and CN Eberhardt, Microscopy Techniques for Materials Science, Woodhead Publishing, Cambridge, July 1998. ISBN 1-85573-587-3. CRC Press LLC, Boca Raton - Florida, July 1998. ISBN 0-8493-1552-2.

To contact me

Dr John Summerscales

School of Engineering: RYB 008University of PlymouthDevon PL4 8AA

01752.23.2650 01752.23.2638 jsummerscales@plymouth.ac.uk http://www.tech.plym.ac.uk/sme/jsinfo.htm