Fibre reinforcements

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Fibre reinforcements

• John Summerscales
• ACMC University of Plymouth

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Glossary of fibre/textile terms

• Fibre/textile terms are defined at
• http//www.tech.plym.ac.uk/sme/MATS324/MATS324A92
  0FibreGlossary.htm

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Principal fibres

• aramid fibres
• e.g. Kevlar, Twaron
• carbon fibres
• glass fibres
• natural fibres
• flax, hemp, jute, kenaf, sisal
• polyethylene fibres
• e.g. Dyneema, Spectra
• surface treatments on fibres

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Griffith crack theory

• Alan Griffith (1920) studied strengths of glass
  rods and fibres
• fibre strength becomes markedly higheras fibre
  diameter decreases to 10 micrometres
• critical stress above which cracks of a given
  size will spontaneously propagate.
• critical stress level is higher for small
  cracks. 
• AGs very fine fibres were strongbecause cracks
  in them would be very small. 
• AGs work was the key to present understanding
  of brittle fracture in all materials. 
• the strength of the modern fibreglass industry is
  "a fitting memorial to his pioneering efforts".

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Glass fibres

• A high alkali grade
• originally made from window glass
• C chemical resistance or corrosion grade
• for acid environments
• D low dielectric
• good transparency to radar  Quartz glass
• E electrical insulation grade
• E most common reinforcement grade (E 70 GPa)
• M high modulus grade
• R reinforcement grade
• European equivalent of S-glass
• S high strength grade (a common variant is
  S2-glass)
• fibre with higher Youngs modulus and temperature
  resistance
• significantly more expensive than E-glass

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Glass-forming oxides
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Glass fibres beware!

• Handling fibres causes damage
• salts on the skin can displace bonding ions from
  the glass structural network
• oil and grease on the skin transfer to fibre and
  act as release agents
• Health and safety issues
• Commercial fibres should NOT be respirable as
  diameter is gt 5 µm

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Surface finish (known as size)

• protect fibre surfaces from damage
• lubricate fibres during mechanical handling
• impart anti-static properties
• bind fibres together for easy processing
• coupling agent promotes interfacial bond

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Carbon fibres

• natural graphite has
• Youngs modulus of 910-1000 GPa in-plane
• Youngs modulus of 30 GPa through plane
• carbon fibre
• turbostratic layered structure of contiguous
  benzene rings
• a single layer graphene
• .
• standard (high strain/high strength) fibres
• E gt 210 GPa (E is equivalent to steel)
• high-modulus (HM-) fibres
• E gt 350 GPa
• when Egt400 GPa incorrectly called graphite
  fibre in USA

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Carbon fibres

• precursor materials are
• polyacrylonitrile (PAN)
• pitch, and
• rayon
• manufacturing imposes orientation by
• spinning of polymer to fibre
• stretching polymer precursor
• graphitisation (pyrolysis) under tensile stress
• HM fibres pyrolysed at gt1650C

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Carbon fibres beware!

• as fibre modulus rises, strain to failure falls
• carbon fibres conduct electricity
• longitudinal coefficient of thermal expansion of
  carbon fibres is slightly negative
• this effect increases in magnitude with
  increasing modulus

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Aramid fibres

• aramid is derived from poly aryl amide
• chemical structure alternates
• aromatic (aryl) benzene rings, and
• the amide (CONH) group.
• commercial reinforcements fibres are
• Kevlar (DuPont) reinforcement,
• molecule is poly(para-phenylene tere-phthalamide)
  PPTA
• Twaron (Akzo) reinforcement
• Nomex (DuPont) for paper and honeycombs
• molecule is poly(meta-phenylene iso-phthalamide)

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Aramid fibres
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Aramid fibres beware!

• very low resistance to axial compression
• typically 20 of corresponding tensile strength
• poor transverse properties
• low longitudinal shear modulus
• fibres break into small fibrils (fibres within
  the fibre)
• fibrils from rod-like structure of liquid crystal
  precursor
• fibres are hygroscopic
• they absorb water
• fibre surfaces degrade in ultraviolet (UV) light.

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Polyethylene fibres

• made from UHMWPE(ultra-high molecular weight
  polyethylene)
• trade names
• Dyneema (DSM), and
• Spectra (Allied Corporation)
• excellent modulus and strength-to-weight
  properties (similar to aramid)
• lower density than aramid
• weight specific properties are superior(almost
  match those of HM carbon fibres?)

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Polyethylene fibres beware!

• fibres melt at 150C
• fibre surface is effective release agent

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Natural fibres

• reinforcement mostly uses the structural fibres
  from plant stems (bast fibres)
• the fibres most used are
• temperate zone flax, hemp
• Tropical zone jute, kenaf and sisal
• MATS324 topic dealt with in separate lecture
• MATS231 natural fibre less than ideal when wet

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Summary

• density
• aramid (1.44) lt carbon (1.6-1.8) lt glass (2.56)
• modulus of standard fibre is
• glass (70 GPa) lt aramid (140 GPa) lt carbon (210
  GPa)
• strength of synthetic reinforcement fibres
• usually 1 GPa (if not virgin fibre)
• toughness
• carbon (brittle) lt glass lt aramid (tough)
• beware! each fibre has different problems