Fibre reinforcements John Summerscales University of Plymouth Glossary
Fibre reinforcements John Summerscales University of Plymouth
Glossary of fibre/textile terms • Fibre/textile terms are defined at: • MATS 347 extranet page > right-hand menu > Definitions >Glossary of fibre/textile terms http: //www. fose 1. plymouth. ac. uk/sme/MATS 347 A 9%20 Fibre. Glossary. htm
Griffith crack theory • Alan Griffith (1920) studied strength of glass rods and fibres • fibre strength increases markedly as 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. • very fine fibres were strong because cracks in them would be very small. • Griffith’s 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".
Principal fibres • glass fibres • size: surface treatments on fibres • carbon fibres • rigid-rod polymers (aramid/PBX fibres) • e. g. Kevlar®, Twaron • polyolefin fibres • • e. g. Dyneema®, Spectra UHMWPE e. g. Innegra. TM high-modulus polypropylene • natural fibres flax, hemp, jute, kenaf, sisal
Glass fibres • A: high alkali grade • originally made from window glass • C: chemical resistance or corrosion grade • for acid environments • D: low dielectric • good transparency to radar: Quartz glass • E: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) • L: high lead content for radiation absorption • M: high modulus grade • R: reinforcement grade • European equivalent of S-glass • S: high strength grade (a common variant is S 2 -glass) fibre with higher Young’s modulus and temperature resistance • significantly more expensive than E-glass •
Glass-forming oxides Oxide % in E-glass % in S-glass Si. O 2 54 65 Na 2 O trace high thermal expansion, moisture sensitivity K 2 O - - high thermal expansion, moisture sensitivity Li 2 O - - high thermal expansion, moisture sensitivity Ca. O 17. 5 trace resistance to water, acids and alkalis Mg. O 4. 5 10 resistance to water, acids and alkalis B 2 O 3 8. 0 trace Al 2 O 3 14 25 Fe 2 O 3 trace green colouration Zn. O - - chemical durability Pb. O - - increased density and brilliance (light transmission) and high thermal expansion Ba. O - - high density and improved chemical durability Ti. O 2 F 2 Effect on Fibre Properties very low thermal expansion improved chemical durability especially for alkali trace
Glass fibres: beware! • handling fibres causes damage salts on skin displaces bonding ions from the glass structural network • oil and grease on skin transfer to fibre and act as release agents • • health and safety issues • commercial fibres should not be respirable as diameter is > 5 μm
Surface finish/coating on fibre (known as “size”) • textile processing aid protect fibre surface from damage • lubricate fibres during mechanical handling • anti-static properties • bind fibres for easy processing • • for high performance, above removed then • coupling agent to promote interfacial bonding
Carbon fibres • carbon fibre • • turbostratic layered structure of contiguous benzene rings a single layer of graphite = graphene • natural graphite has • • Young’s modulus of 910 -1000 GPa in-plane Young’s modulus of 30 GPa through plane • standard (high strain/high strength) fibres • E > 210 GPa (E is equivalent to steel) • high-modulus (HM-) fibres • • HM = E > 350 GPa E>400 GPa incorrectly called “graphite fibre” in USA
Carbon fibres • precursor materials are: • polyacrylonitrile (PAN) • algal glycerine + ammonia > PAN > solar pyrolysis (DOI) pitch, and • rayon (regenerated cellulose) • • lignin being researched • manufacturing imposes orientation by: spinning of polymer to fibre • stretching polymer precursor • graphitisation (pyrolysis) under tensile stress • HM fibres pyrolysed at >1650°C •
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
Rigid rod polymers: aramid • aramid is derived from poly aryl amide • commercial reinforcements fibres are: • Kevlar® (Du. Pont) reinforcement, • molecule is poly(para-phenylene tere-phthalamide) [PPTA] Twaron (Akzo) reinforcement • Nomex ® (Du. Pont) for paper and honeycombs • • molecule is poly(meta-phenylene iso-phthalamide)
Aramid fibres Fibre Character Kevlar 29 high-toughness, high-strength, intermediate modulus for tire cord Kevlar 49 high modulus, high-strength for composite reinforcement Kevlar 149 ultra-high modulus (no longer available? ) E σ' ε' (GPa) (%) 83 3. 6 4. 0 131 3. 6 2. 8 186 3. 4 2. 0
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.
Rigid-rod polymer fibres • aramid (reinforcement) C O O N N H C • aramid chemical structure alternates aromatic (aryl) benzene rings, and • the amide (CONH) group. • • aramid (paper) • bonds on benzene ring not directly opposite • PBX: poly benz[x]azole
PBX rigid rod polymers PBI H PBO PBT O N S
Polyethylene fibres • made from UHMWPE (ultra-high molecular weight polyethylene) • trade names Dyneema® (DSM), and • Spectra® (Allied Corporation) • • excellent specific (property/density) modulus/strength properties • lower density than aramid • weight specific properties are superior (almost match those of HM carbon fibres? )
Polyethylene fibres: beware! • fibres melt at ~150°C • fibre surface is effective release agent
Polypropylene fibres • high modulus polypropylene fibre • trade name • Innegra™ (Innegra Technologies LLC/Sigmatex sigma. IF) • lightest commercial fiber available bulk density 0. 84, specific gravity 0. 91 • M % <0. 1% water absorption • • hybridise with CF or GF to add • impact resistance, durablity, or vibration damping
Natural fibres • animal • • silk mineral • • • MATS 347 extranet: fibres. php asbestos basalt vegetable • • fruit (coir, cotton, kapok) grass leaf (banana, pineapple, sisal) root seed stem (bast: flax, hemp, jute, etc) wood
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 • • MATS 347: topic dealt with in lecture A 9 • MATS 232: natural fibre less than ideal when wet
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 after processing (better if virgin fibre) • E-glass: 3. 4 GPa, PAN carbon: 3. 4 GPa, Kevlar 49: 3. 6 GPa • • toughness • carbon (brittle) < glass < aramid (tough) • cost • glass < aramid < carbon • beware!: each fibre has different problems
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