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Fibre reinforcements

Fibre reinforcements. John Summerscales ACMC University of Plymouth. Glossary of fibre/textile terms. Fibre/textile terms are defined at: http://www.tech.plym.ac.uk/sme/MATS324/MATS324A9%20FibreGlossary.htm. Principal fibres. basalt, boron carbon fibres glass fibres

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Fibre reinforcements

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  1. Fibre reinforcements John Summerscales ACMC University of Plymouth

  2. Glossary of fibre/textile terms • Fibre/textile terms are defined at: • http://www.tech.plym.ac.uk/sme/MATS324/MATS324A9%20FibreGlossary.htm

  3. Principal fibres • basalt, boron • carbon fibres • glass fibres • rigid-rod polymers (aramid and PBX fibres) • e.g. Kevlar, Twaron • polyethylene fibres • e.g. Dyneema, Spectra • natural fibres • flax, hemp, jute, kenaf, sisal • surface treatments on fibres

  4. 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.  • AG’s very fine fibres were strongbecause cracks in them would be very small.  • AG’s work was the key to present understanding of brittle fracture in allmaterials.  • the strength of the modern fibreglass industry is "a fitting memorial to his pioneering efforts".

  5. Glass fibres • A: high alkali grade • originally made from window glass • C: chemical resistance or corrosion grade • for acid environments • D: low dielectric • good transparency to radar:  Quartz glass • E: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 Young’s modulus and temperature resistance • significantly more expensive than E-glass

  6. Glass-forming oxides

  7. 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 > 5 μm

  8. 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

  9. Carbon fibres • natural graphite has • Young’s modulus of 910-1000 GPa in-plane • Young’s 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 > 210 GPa (E is equivalent to steel) • high-modulus (HM-) fibres • E > 350 GPa • when E>400 GPa incorrectly called “graphite fibre” in USA

  10. Carbon fibres • precursor materials are: • polyacrylonitrile (PAN) • pitch, and • rayon (regenerated cellulose) and lignin • manufacturing imposes orientation by: • spinning of polymer to fibre • stretching polymer precursor • graphitisation (pyrolysis) under tensile stress • HM fibres pyrolysed at >1650°C

  11. 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

  12. Rigid rod polymers: aramid • aramid is derived from poly aryl amide • 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)

  13. Aramid fibres

  14. 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.

  15. Rigid-rod polymer fibres • aramid (PPTA) • aramid chemical structure alternates • aromatic (aryl) benzene rings, and • the amide (CONH) group. • PBX: poly benz[x]azole O O C C N N H

  16. PBX rigid rod polymers H O N S

  17. 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?)

  18. Polyethylene fibres: beware! • fibres melt at ~150°C • fibre surface is effective release agent

  19. 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

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

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