Fibre reinforcements

John SummerscalesACMC University of Plymouth

Glossary of fibre/textile terms

• Fibre/textile terms are defined at:o http://www.tech.plym.ac.uk/sme/MATS324/

MATS324A9%20FibreGlossary.htm

Principal fibres

• basalt, boron• carbon fibres • glass fibres • rigid-rod polymers (aramid and PBX fibres)

o e.g. Kevlar, Twaron• polyethylene fibres

o e.g. Dyneema, Spectra• natural fibres

o flax, hemp, jute, kenaf, sisal• surface treatments on fibres

Griffith crack theory• Alan Griffith (1920) studied strengths of glass rods and

fibres• fibre strength becomes markedly higher

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.  • AG’s very fine fibres were strong

because cracks in them would be very small. • AG’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".

Glass fibres• A: high alkali grade

o originally made from window glass

• C: chemical resistance or corrosion gradeo for acid environments

• D: low dielectric o good transparency to radar:  Quartz glass

• E: electrical insulation gradeo E = most common reinforcement grade (E ~70 GPa)

• L: high lead content for radiation absorption• M: high modulus grade • R: reinforcement grade

o European equivalent of S-glass

• S: high strength grade (a common variant is S2-glass)o fibre with higher Young’s modulus and temperature resistanceo significantly more expensive than E-glass

Glass-forming oxidesOxid

e% in E-glass

% in S-glass

Effect on Fibre Properties

SiO2 54 65 very low thermal expansion

Na2O trace trace high thermal expansion, moisture sensitivity

K2O - - high thermal expansion, moisture sensitivity

Li2O - - high thermal expansion, moisture sensitivity

CaO 17.5 trace resistance to water, acids and alkalis

MgO 4.5 10 resistance to water, acids and alkalis

B2O3 8.0 trace low thermal expansion

Al2O3 14 25 improved chemical durability

Fe2O

3

trace trace green colouration

ZnO - - chemical durability

PbO - - increased density and brilliance (light transmission) and high thermal expansion

BaO - - high density and improved chemical durability

TiO2 improved chemical durability especially for alkali

F2 trace

Glass fibres: beware!beware!

• Handling fibres causes damageo salts on the skin can displace bonding

ions from the glass structural networko oil and grease on the skin

transfer to fibre and act as release agents

• Health and safety issueso Commercial fibres should NOT be

respirable as diameter is > 5 μm

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

Carbon fibres• natural graphite has

o Young’s modulus of 910-1000 GPa in-planeo Young’s modulus of 30 GPa through plane

• carbon fibreo turbostratic layered structure of contiguous benzene

ringso a single layer of graphite = graphene

.• standard (high strain/high strength) fibres

o E > 210 GPa (E is equivalent to steel)

• high-modulus (HM-) fibreso E > 350 GPao when E>400 GPa incorrectly called “graphite fibre” in

USA

Carbon fibres

• precursor materials are:o polyacrylonitrile (PAN) o pitch, and o rayon (regenerated cellulose) and lignin

• manufacturing imposes orientation by:o spinning of polymer to fibre o stretching polymer precursor o graphitisation (pyrolysis) under tensile

stresso HM fibres pyrolysed at >1650°C

Carbon fibres: beware!beware!

• as fibre modulus rises, strain to failure falls

• carbon fibres conduct electricity• longitudinal coefficient of thermal

expansion of carbon fibres is slightly negativeo this effect increases in magnitude

with increasing modulus

Rigid rod polymers: aramid

• aramid is derived from poly aryl amide

• commercial reinforcements fibres are:o Kevlar (DuPont) reinforcement,

molecule is poly(para-phenylene tere-phthalamide) [PPTA]

o Twaron (Akzo) reinforcemento Nomex (DuPont) for paper and honeycombs

molecule is poly(meta-phenylene iso-phthalamide)

Aramid fibres

Fibre Character E (GPa

)

σ' (GPa

)

ε' (%)

Kevlar 29 high-toughness,high-strength, intermediate modulusfor tire cord

83 3.6 4.0

Kevlar 49 high modulus,high-strengthfor composite reinforcement

131 3.6 2.8

Kevlar 149

ultra-high modulusrecently introduced

186 3.4 2.0

Aramid fibres: beware!beware! • very low resistance to axial compression

o typically ~20% of corresponding tensile strengtho poor transverse propertieso low longitudinal shear modulus

• fibres break into small fibrils (fibres within the fibre)o fibrils from rod-like structure of liquid crystal precursor

• fibres are hygroscopico they absorb water

• fibre surfaces degrade in ultraviolet (UV) light.

Rigid-rod polymer fibres• aramid (PPTA)

• aramid chemical structure alternateso aromatic (aryl) benzene rings, ando the amide (CONH) group.

• PBX: poly benz[x]azole

CO

N NC

O

H

PBI

PBO

PBT

PBX rigid rod polymers

S

N

H

O

Polyethylene fibres• made from UHMWPE

(ultra-high molecular weight polyethylene)• trade names

o Dyneema (DSM), and o Spectra (Allied Corporation)

• excellent modulus and strength-to-weight properties (similar to aramid)

• lower density than aramido weight specific properties are superior

(almost match those of HM carbon fibres?)

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

Natural fibres

• reinforcement mostly uses the structural fibres from plant stems (bast fibres)

• the fibres most used areo temperate zone: flax, hempo Tropical zone: jute, kenaf and sisal

• MATS324: topic dealt with in separate lecture• MATS231: natural fibre less than ideal when wet

Summary• density

o aramid (1.44) < carbon (1.6-1.8) < glass (2.56)

• modulus of standard fibre iso glass (70 GPa) < aramid (140 GPa) < carbon (210 GPa)

• strength of synthetic reinforcement fibreso usually ~ 1 GPa (if not virgin fibre)

• toughnesso carbon (brittle) < glass < aramid (tough)

• beware!: beware!: each fibre has different problems