The Determination of the Fibre Volume Fraction in Natural Fibre Composites

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
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Elastic modulus of composite

• calculated by rule-of-mixtures
• neglect contribution of matrix
• Ec ?L ?O VF EF
• ?L fibre length distribution factor
• ?O 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 60C for 30 min
• cross-sectional area of the fibres
• not normally round

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Determination of Vf

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

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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

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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)

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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

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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

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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

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Density

• density of resin typically 1100 kg/m3
• density of fibre typically 1600 kg/m3
• ? resolution of 5 mg/cm3 for 1Vf
• 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 2C

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Density of jute

• dried 60C for 30 min
• weighed in air immediately
• immersed and degassed in fluid
• -990 mbar water -500 mbar acetone
• weighed in fluid
• ? 1.6690.037 in water/Ilfotol at 22.4C
• ? 1.6520.037 in acetone at 20.3C
• data from Richard Cullen
• image from Jean-Philippe le Nours

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Vf by resin burn-off

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

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Vf by TGAThermo-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

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Vf by TGA flax fibres

• primary decomposition peak
• 55-91 weight loss between 315-362C
• secondary decomposition peak
• 2-33 weight loss between 406-465C
• 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.

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Vf by TGA flax/HDPE

• Pure flax
• decomposition starts c.200-210C
• 3 wt loss by 385C
• gt400C degradation slows (ash)
• Pure HDPE
• degradation starts c.410-430C
• 9 wt loss by 490C
• T Powell et al, Engineering properties of flax
  fiber and flax fiber-reinforced thermoplastic in
  rotational moulding,ASAE/CSAE meeting,
  Saskatchewan, September 2002.

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Vf by TGA sisal/starch

• hemicellulose cellulose glycosidic links
• decomposition peak at 300C
• alpha-cellulose
• decomposition peak at 360C
• lignin
• decomposition between 200-500C
• maximum at 350C
• VA Alvarez and A Vázquez , 2004Polymer
  Degradation and Stability, 84(1), 13-21

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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 ?

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Vf by microscopy

• optical or electron microscopywith 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

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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/

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Use of ImageJ software

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

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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

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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

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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.

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To contact me ?

• Dr John Summerscales
• School of Engineering RYB 008
• University of Plymouth
• Devon PL4 8AA
• 01752.23.2650
• 01752.23.2638
• jsummerscales_at_plymouth.ac.uk
• http//www.tech.plym.ac.uk/sme/jsinfo.htm