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Fibre Composite Reinforcement > 2: Constituent Materials > Flashcards

2: Constituent Materials Flashcards

1
Q

List some fibre material requirements for a suitable composite

A

-High strength and modulus
-Low density
-low cost (and readily available)
-Consistency/repeatable
-Environmental/Chemical resistance
-Compatible with commonly used matrices and manufacturing processes

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2
Q

List some synthetic fibres used in industry

A

-Glass
-Carbon
-Aramid
-Boron
-HDPE

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3
Q

List some natural fibres used in industry

A

-Hemp
-Jute
-Flax
-Sisal
-Silk

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4
Q

Describe and explain the Weibull theory for fibres

A

Weakest link theory - as fibre diameter increases the strength deceases, as the likelihood of a critical flaw within the element increases with volume

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5
Q

What is the role of the matrix within a composite?

A

To transfer the stress between fibres

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6
Q

List some material properties of industrial carbon fibres

A

Density: 1.78 g/cm^3
Modulus: 240 GPa
UTS: 4,300 MPa

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7
Q

Describe the synthesis process of glass fibre production

A

-Feedstock (sand, cullet or marbles) are fed into an initial furnace
-Which is then fed into a secondary refiner furnace at 1340°C
-A forehearth unit at 1260°C distributes the molten glass to be extruded
-Molten glass is extruded through Bushing’s (platinum or ceramic, each has 204 holes), producing 8-25 micrometre filaments
-The filament are cooled with a water spray, given a protective coat (liquidsizing) and drawn onto a roller for later downstream processes (twisting into a yarn, weaving or chopping)

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8
Q

How does E-glass vary from S-glass?

A

E-glass: provides electrical resistance, thermal insulation and good mechanical properties

S-glass: provides high strength but also has high costs

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9
Q

Glass fibres are described as isotropic, what does this mean?

A

Material properties are the same in both the transverse and longitudinal directions, due to the crystalline glass structure

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10
Q

What are some graphitised precursors used for carbon fibre production?

A

PAN (polyacrylonitrile), Pitch (derived from petroleum/coal) and Lignin (plant based, Ef=40-50GPa)

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11
Q

Carbon fibres are described as anisotropic, What does this mean and why is this behaviour observed?

A

-Different material properties depending on the load direction
-High longitudinal modulus (240GPa), covalent bond failure
-Low transverse modulus (8GPa), Van Der Waals failure
-The internal structure of the carbon fibres is composed of lamellae aligned axially.

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12
Q

How does chosen precursor material affect cost of carbon fibres?

A

Majority of costs are due to the energy/heat requirement for graphitisation of the precursor material. Therefore the material chosen will greatly impact cost of production

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13
Q

Describe the PAN process outline

A

-Initial reel of PAN fibre (60-70% carbon)
-Polymer atoms are chemically and mechanically aligned through stretching
-Further stretching takes place on rollers in hot water at 350°C
-Carbonisation takes place in an oven at 900°C in inert conditions
-Graphitisation takes place in an oven at 2000°C in inert conditions

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14
Q

How do material properties change with varying graphitisation temperature of PAN?

A

Tensile modulus increases with graphitisation temperature (INSERT WHY)

An increase in tensile strength is seen up to 2000°C, after which a reduction in UTS is observed as increasing temperature increases pore size within the microstructure

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15
Q

What is an estimated modulus of Pitch-based fibres?

A

~960GPa (4x standard industrial PAN carbon)

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16
Q

What is the estimated strain to failure of PAN?

A

1.8%

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17
Q

What is the estimated strain to failure of Pitch?

A

0.5%

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18
Q

What are the thermal expansion properties of Pitch-based fibres?

A

It possesses a low negative coefficient, and therefore is good for space applications (very expensive ~£1000/kg)

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19
Q

What is the rough size of carbon fibres?

A

5-8 micrometres

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20
Q

How does cost vary with tow size (K)

A

Cost decreases as tow size (number of fibres in a bundle) increases, as manufacture time is constant across all tow sizes. Therefore the yield decreases for smaller tows/bundles

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21
Q

What affect does tow size have on mechanical properties?

A

Smaller bundles lead to better mechanical properties, as the laminate structure is more compact

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22
Q

List some properties of aramid fibres (kevlar29 & kevlar49)

A

-Anisotropic (due to pleated lamellar sheets)
-poor compressive strength
-UV degradation and moisture uptake (therefore, usually sub-surface plies or a protective coat is applied)
-Non-conductive & high melting point (>500°C)

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23
Q

Describe the aramid production process

A

-The polymer powder is mixed with sulphuric acid in a solvent solution at 80°C
-Molecular alignment occurs through an extrusion die at 200°C
-The fibres are quenched and the solvent is evaporated
-The fibres are then stretch dried, to be drum wound as the finished aramid fibre

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24
Q

Common composite fibres include: glass, carbon and aramid. Rank the density of these from highest to lowest

A

-Glass
-Carbon
-Aramid

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25
Q

Common composite fibres include: glass, carbon and aramid. Rank the tensile modulus and strength of these from highest to lowest

A

-Carbon
-Aramid
-Glass

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26
Q

Common composite fibres include: glass, carbon and aramid. How does compression and tensile strength vary when loaded axially?

A

-Glass (compression ~ tensile)
-Carbon (compression < tensile)
-Aramid (compression &laquo_space;tensile)

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27
Q

Common composite fibres include: glass, carbon and aramid. What are the fracture/failure types for the following?

A

-Carbon (brittle)
-Glass (brittle)
-Aramid (ductile)

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28
Q

Common composite fibres include: glass, carbon and aramid. What are the maximum temperature the fibre (only) can withstand?

A

-Carbon < 2500°C
-Glass < 250°C
-Aramid < 250°C

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29
Q

List some matrix material requirements for a suitable composite

A

-Inexpensive and readily available
-Non-toxic/allergenic
-Good mechanical properties
-Good thermal properties (resistance)
-Compatible with fibres (strong interface)
-Easy to process
-Stable properties
-Corrosion protection

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30
Q

What are some advantages of thermoset matrices?

A

-Dimensionally stable (low shrinkage)
-Cost effective
-Low viscosity (easy to wet fibres, better interface)

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31
Q

What are some disadvantages of thermoset matrices?

A

-Long cycle times (due to cross-linking)
-Generally brittle (once set)
-Cannot be remodelled or reshaped
-Difficult to recycle
-Poor fire, smoke and toxicity rating

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32
Q

What are some advantages of thermoplastic matrices?

A

-Short cycle times
-Recyclable
-High in-service temperatures
-Chemically resistant
-High ductility (therefore good impact resistance)
-Can be welded (with applied heat)
-Commonly used without reinforcement or with very short fibres
-Higher impact resistance than thermosets (more ductile)
-Unlimited shelf-life due to no cross-linking
-Lower material cost

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33
Q

What are some disadvantages of thermoplastic matrices?

A

-High processing temperatures (Higher processing costs)
-Very high viscosity (~1000x higher in comparison to thermosets)
-High cost
-Joining to metals can lead to high levels of creep
-Poor surface finish (poor paint adhesion)
-Poor fibre/matrix adhesion

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34
Q

Explain industrial properties of unsaturated polyester as a matrix

A

-Thermoset
-Most common used with glass fibres
-Environmentally hazardous
-2 part mixture: Catalyst (initialises cross-linking) and Accelerator (speeds up reaction)
-Low viscosity (due to added styrene, therefore can be moulded at room temperature)
-Can take additives (pigments, fillers, fire retardants)
-Short pot-life (problems with storage and processing)
-Prone to air bubbles (2 part) which greatly reduces mechanical properties

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35
Q

Describe the polyester synthesis reaction

A

-Requires initiator to produce free-radicals
-free radicals provide cross-linking
-Usually organic peroxide catalyst
-Exothermic reaction (temperature dependant rate)

36
Q

Explain industrial properties of epoxy as a matrix

A

-Thermoset
-High performance
-Expensive
-Slow process time
-Usually used with carbon fibres
-Cross-linking from liquid to solid with use of a hardener (usually amine)
-Low volumetric shrinkage ~1% vol, which leads to lower residual stresses, but can make demoulding more difficult)

37
Q

Describe the 2-part epoxy synthesis reaction, what are the property differences to 1-part epoxy?

A

-Low molecular weight
-Liquid
-Brittle
-Trapped bubbles in mix
-Lower cost
-Room temperature storage

38
Q

Describe the 1-part epoxy synthesis reaction, what are the property differences to 2-part epoxy?

A

-High molecular weight (prepreg)
-Solid at room temperature
-High toughness
-Lower void content
-Higher cost
-Store in freezer to increase shelf-life (as self curing is sensitive to temperature)

39
Q

Explain industrial properties of vinylester as a matrix

A

-Thermoset
-Processing ease similar to polyester with epoxy like performance
-High degree of crosslinking and high molecular weight
-Higher toughness compared to polyester
-Less susceptible to water degradation (due to fewer ester groups)
-Chemically resistant
-Intermediate cost
-Lower density than polyester and epoxy (thinned by styrene)
-Requires and initiator (highly explosive -Methyl-Ethyl-Ketone-Peroxide (MEKP))

40
Q

Describe the polyvinyl synthesis reaction

A
41
Q

List some properties of amorphous thermoplastics

A

-Randomly ordered molecular structure
-Gradual melting point (gradual softening before melting)
-Sensitive to stress cracking (due to presence of hydrocarbons)
-Isotropic flow (dimensionally stable)
-Translucent

42
Q

List some properties of crystalline thermoplastics

A

-Highly ordered molecular structure
-Sharp melting point (rapid change to low viscosity fluid)
-High toughness
-Anisotropic flow (heat shrinkage transverse to flow direction)
-Opaque

43
Q

What are common thermoplastic matrix properties and materials used in commodity items?

A

-Low cost (<£2/kg)
-Tough
-Chemically resistant
-Often unreinforced or reinforced with short/chopped glass fibres
-Mass produced pellet feedstock injection moulded components, accounting for 80% of thermoplastic composite usage
-Sufficient stiffness and strength without extreme properties
-Usually Polypropylene, HDPE and LDPE

44
Q

What are common thermoplastic matrix properties and materials used in engineering items?

A

-Most commonly from the nylon family
-Continuous and discontinuous fibre reinforcements
-Typically glass or carbon
-Tendency to absorb water
-Amorphous PEI (polyetherimide) becoming more popular, possesses high mechanical properties and high processing temperatures
-Usually from the nylon family

45
Q

What are common thermoplastic matrix properties and materials used in high performance items?

A

-Semicrystalline
-High purity
-Chemically resistant
-Excellent mechanical and physical properties
-High processing temperatures
-Very expensive (~£200/kg)
-Fire retardant
-Usually PEEK, PEKK and PPS

46
Q

How can thermoplastic fibres be used to achieve a lower void content?

A

Extrude the matrix as a filament and interweave with fibres to produce yarns/fabrics. The thermoplastic fibres can later be activated and formed by applying heat

47
Q

How would the interface layer be described?

A

An infinitesimally small region between the matrix and fibre

48
Q

Explain how can the interface layer be reinforced?

A

-Through the addition of a coupling agent between the fibre and matrix
-Leading to: chemical compatibility, better load transfer and increased environmental resistance

49
Q

Name and describe the purpose of the following sizing agents: PVA and Silane

A

PVA - Film former (protects fibres and provides cohesion)
Silane - Coupling agent (increases chemical adhesion)

50
Q

What is the purpose of an ideal interface layer?

A

To transfer the load between fibres to provide optical strength within the composite. For discontinuous fibres, shear stress/strains permit the transfer of load between the fibres

51
Q

How does aspect ratio affect fibre axial stress along fibre position?

A

-Aspect ratio=fibre length/fibre diameter
-The higher the aspect ratio the greater the axial stress at the midpoint of the fibre

52
Q

How does aspect ratio affect interfacial shear stress along fibre position?

A

-Zero shear stress is observed at the centre of the fibre
-Max shear stress is observed at the ends of the fibre

53
Q

What determines whether the composite fails by tension or pull-out?

A

-Relative strengths of the fibre and interface
-Length of the fibre (critical length is a key parameter).
-If L<Lc (pull out)_sub-critical length fibres
-If L>Lc (failure in tension)_super-critical length fibres

54
Q

The microdroplet test is a commonly used method to determine the interfacial shear stress characteristics. Explain the test procedure

A

-Heat the matrix material and extrude into a polymer fibre
-Knot the matrix fibre around the fibre and apply heat, the matrix material melts and forms a sphere due to surface tension
-A “knife-edge” test rig is used to measure the load and slide point at which the droplet shears off

55
Q

What are some advantages of the microdroplet test?

A

-Simple specimen preparation
-Can be used for multiple fibres and polymers
-Embedded length is easy to measure
-Stress concentrations at the fibre ends are avoided

56
Q

What are some disadvantages of the microdroplet test?

A

-The droplet shape influences the precision of the results (must be spherical)
-Large data scatter
-No standardised equipment

57
Q

What are some by-products of failure from a poor interface layer?

A

-Fracture site is dominated by fibre pull-out
-Pulled out fibres are “clean” (minimal matrix residue)
-Fibre-sized holes left at the fracture site in the matrix

58
Q

What are some by-products of a strong interface failure?

A

-Matrix coated fibres at the fracture site
-Mixture of fibre failure and matrix failure

59
Q

What is the purpose of binder additives and what are some of their properties?

A

To assemble multiple layers of fibre mats/fabrics

-Can be a powder or liquid (acts as an adhesive)
-The bond strength is a function of particle size and distribution
-The applied binder must be compatible with the matrix material

60
Q

What are some properties of non-reactive binder additives?

A

-Acts like a thermoplastic (reheated and shaped)
-May soften during resin injection (causing washing)

61
Q

What are some properties of reactive binder additives?

A

-Fully cross linked at high temperatures (120°C)
-The binder softens at lower temperatures (80°C), allowing preforming

62
Q

What is a common low density filler applied to composites?

A

-Glass microspheres

63
Q

What are some common shrinkage control additives applied to composites?

A

-PVA
-Polystyrene
-Polyethylene

64
Q

What are some common fire retardant additives applied to composites?

A

-Aluminium trihydrate (Al2O3H2O)
-Halogens

65
Q

What are some common thermal property additives applied to composites?

A

-Aluminium powder
-Copper powder

66
Q

What are some common pigment additives applied to composites?

A

-Cadmium salts
-Carbon black
-Iron oxide
-Titanium dioxide (UV blocker)
-Kaolin
-Organic dyes

67
Q

What are some common (cost reduction) bulk additives applied to composites?

A

-Mineral fillers (eg. CaCO3)
-Wood/flour
-Reground scrap

68
Q

What are some common toughening agent additives applied to composites?

A

-Rubber particles
-Nano particles
-Graphene

69
Q

List some material properties of high strength carbon fibres

A

Density: 1.78 g/cm^3
Modulus: 245 GPa
UTS: 5,100 MPa

70
Q

List some material properties of high modulus carbon fibres

A

Density: 1.86 g/cm^3
Modulus: 380 GPa
UTS: 2,700 MPa

71
Q

List some material properties of A-glass fibres

A

Density: 2.7 g/cm^3
Modulus: 75 GPa
UTS: 1,700 MPa

72
Q

List some material properties of E-glass fibres

A

Density: 2.54 g/cm^3
Modulus: 70 GPa
UTS: 2,200 MPa

73
Q

List some material properties of S-glass fibres

A

Density: 2.49 g/cm^3
Modulus: 80 GPa
UTS: 2,600 MPa

74
Q

List some material properties of Kevlar29 fibres

A

Density: 1.44 g/cm^3
Modulus: 70.5 GPa
UTS: 3,600 MPa
Failure strain: 3.6%

75
Q

List some material properties of Kevlar49 fibres

A

Density: 1.44 g/cm^3
Modulus: 112 GPa
UTS: 3,600 MPa
Failure strain: 2.4%

76
Q

List some material properties of mild steel

A

Density: 7.8 g/cm^3
Modulus: 207 GPa
UTS: 650 MPa

77
Q

List some material properties of Polypropylene

A

Density: 0.91 g/cm^3
Modulus: 1.5 GPa
Melt temp: 160 degrees
Service Temp: 80 degrees

78
Q

List some material properties of HDPE

A

Density: 0.95 g/cm^3
Modulus: 0.8 GPa
Melt temp: 140 degrees
Service Temp: 120 degrees

79
Q

List some material properties of LDPE

A

Density: 0.92 g/cm^3
Modulus: 0.2 GPa
Melt temp: 110 degrees
Service Temp: 80 degrees

80
Q

List some material properties of PA6 (nylon)

A

Density: 1.14 g/cm^3
Modulus: 2 GPa
UTS: 66 MPa
Melt temp: 220 degrees
Service Temp: 120 degrees

81
Q

List some material properties of PA12 (nylon)

A

Density: 1.02 g/cm^3
Modulus: 1.4 GPa
UTS: 55 MPa
Melt temp: 250 degrees
Service Temp: 130 degrees

82
Q

List some material properties of PA66 (nylon)

A

Density: 1.17 g/cm^3
Modulus: 3 GPa
UTS: 65 MPa
Melt temp: 265 degrees
Service Temp: 140 degrees

83
Q

List some material properties of PEI

A

Density: 1.26 g/cm^3
Modulus: 3.1 GPa
UTS: 101 MPa
Melt temp: 350 degrees
Service Temp: 180 degrees

84
Q

List some material properties of PEEK

A

Density: 1.33 g/cm^3
Modulus: 4.5 GPa
UTS: 110 MPa
Melt temp: 343 degrees
Service Temp: 260 degrees

85
Q

List some material properties of PEKK

A

Density: 1.28 g/cm^3
Modulus: 3 GPa
UTS: 105 MPa
Melt temp: 350 degrees
Service Temp: 204 degrees

86
Q

List some material properties of PPS

A

Density: 1.43 g/cm^3
Modulus: 3.6 GPa
UTS: 87 MPa
Melt temp: 280 degrees
Service Temp: 160 degrees

Fibre Composite Reinforcement (10 decks)

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