fibre reinforcements john summerscales acmc university of plymouth
TRANSCRIPT
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o originally made from window glass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