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NCM 1 > Composites > Flashcards

Composites Flashcards

1
Q

What 3 things make up a fibre reinforced polymer matrix

A

Matrix - substrate
Interface - coupling agent that binds fibres & matrix together
Reinforcement - fibre

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2
Q

What will happen to a polymer if you add more epoxy groups to it?

A

Create a highly cross-linked system (thermoset)

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3
Q

Why does carbon fibre have strong tension resistance but bad compression resistance?

A

The layers are covalently Bonded together so are strong in tension, but layers are joined together with van der waals forces so aren’t strong in compression (when the forces act on them)

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4
Q

How do you increase the strength of carbon fibres with processing?

A

Higher processing temp = increased chain motion = increased Xc = greater strength

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5
Q

Describe and explain Kevlars properties

A

Strong in tension - covalently bonded fibres
Weak in compression - layers bonded with weak hydrogen bonds
Extremely strong ballistic resistance - short and stiff aligned chains, with lots of failure modes that require a lot of energy to be broken
When Kevlar extruded through spinneret high degree of molecular orientation, forming rigid sheets

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6
Q

Name 4 factors that affect he tensile properties of a composite material

A

Fibre volume fraction (proportion of fibres to matrix)
Fibres tensile properties
Fibre alignment compared to tensile axis
Void content (high void = low strength)

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7
Q

Name 4 factors that affect compressive properties of composite materials

A

Matrix properties affect this more as they stop fibres buckling.

  • strength, stiffness & bonding of matrix
  • fibre length:diameter ratio (small prevents buckling)
  • specimen quality (already fatigued?)
  • fibre alignment to compressive axis
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8
Q

How does the production of fibres change dependant on fibre type?

A

Organic fibres use spinnerets

Non-organic fibres use bushings

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9
Q 
Define:
Strand
Yarn 
Roving 
Tex
A

Strand - intentionally twisted fibres
Yarn - large number of filaments twisted together
Roving - untwisted filaments
Tex - weight (g) of 1000 metres of yarn

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10
Q

Name the 6 requirements of reinforcing fibres

A
  • high aspect ratio (length:diameter)
  • good mechanical properties (high failure strain)
  • high Tm & Tg
  • constant diffusion of moisture
  • Mw distribution
  • functional group polarity, chain symmetry and stiffness
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11
Q

Why does Kevlar have a high affinity to water?

A

It contains polar bonds. As water is polar they attract.

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12
Q

Describe the process of melt spinning

A

Polymers pellets are heated above Tg, filtered and then forced through a spinneret (or bushing). Then fibres cooled and spun onto a spool.

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13
Q

What’s the difference between wet, dry and melt spinning?

A

Melt spinning - heat polymer above Tg so flows
Wet spinning - polymer dissolved in solvent, filtered, through spinneret in coagulation bath (solvent diffused out), polymer dried and spun.
Dry spinning - same as wet except not in coagulation bath

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14
Q

How are E-glass fibres produced?

A

Materials mixed with a solvent, then heated (roughly 1400°c), filtered and put through bushing, then quenched to form glass, then spun

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15
Q

What does the coupling agent do?

A

Combined the fibre and matrix. Strong bonds create a good transfer of load, weak bonds = poor load transfer.

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16
Q

What happens during an epoxy resin and amine hardener reaction?

A

Cyclic bond opens up and reacts, causes increased density and viscosity, shrinkage and highly crosslinked system

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17
Q

What precautions should be taken when working with thermoset resins?

A

Usually produce toxic gas, so should be one in a ventilated atmosphere with safety equipment worn

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18
Q

What are the equations for:

  • weight of the composite
  • volume of the composite
  • volume fraction of the fibre
  • volume fraction of the matrix
A
  • Wc = Wf + Wm (fibre weight + matrix weight)
  • Vc = Vf + Vm (volume)
  • øf = vf/vc (volume fraction of fibre)
  • øm = vm/Vc
    Øm + øf = 1
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19
Q

What is the equation for ROM?

A

Ec = EfØf + Em(1-øf)

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20
Q

What are the 5 stages of deriving the ROM equation?

A

1- fibre, matrix & composite all elongate at same rate
2- modulus of fibre&raquo_space; modulus of matrix
3 - load applied depends on volume fractions of f and m
4 - no defects in the material, Vc = Ac
5- eliminate everything, using equations above, to get ROM

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21
Q

What are the assumptions for the ROM derivation?

A
  • Perfect bonding between matrix and fibre
  • linear elastic behaviour between fibre and matrix
  • fibres and continuous and uni directional
  • no voids or residual stresses
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22
Q

Derive the ROM equation

A

For answer use sheet on wall/in images

23
Q

What happens to the composite when a fibre or matrix fails? (in terms of stress)

A

Stress is applied to the surrounding fibres/matrix

24
Q

What is the failure mechanism when Strength matrix > strength fibre?

A

Fractures in fibre will occur first, with load transferred to surrounding fibres when one fractures - increasing fracture rate - until fractures reach a critical point - then no more fractures can occur.

25
How does fibre volume fraction the composites strength? (M stronger than F)
Low Øf = matrix continues to carry Laos when all fibres fractured High Øf = matrix can’t carry load, so failure of fractures = failure of composite To enable effective reinforcement of matrix Øf must be above a critical value (or it decreases strength)
26
How does fibre volume fraction the composites strength? (F stronger than M)
Matrix will fracture first, spreading load to surrounding fibres, fracture continues until critical value, then load applied to fibres
27
What is meant but the critical fibre length?
Fibre length > critical length to be able to carry any load. (Hence maximum fracture value) Stress is applied across length of fibre which causes deformation at ends of fibre - due to sheer and no fibre support as matrix has deformed away
28
How do you manufacture thermoset composites?
- dry/thaw material - add resin & hardener to obtain homogeneous mixture - degas mixture - impregnate preform & layer preforms - initiate cross linking (eg temp/uv) - cool and then quality control
29
What is meant by composite impregnation and how is it done?
Impregnation - covering the fibres in the matrix | Matrix can be applied as liquid, film, powder etc
30
Name 5 factors that affect impregnation
Surface tension, surface energy, viscosity, temperature, thickness of fibre
31
Describe wet-filament winding
Fibre creels unwind through a fibre guide, drum impregnator applies resin to fibres (doctor eye removes excess), transverse carriage puts fibres on rotating mandrel, heat applied to initiate cross linking.
32
What are the two ways of extracting fibres from creels?
Centre pull - no core required and simple tensioner required = cheap, but twists can end up in final product (sometimes wanted) Outside draw - rotating mandrel required to unravel and more complex tensioner required = expensive, no twists introduced.
33
Explain the pultrusion technique
Continuous process where: Fibre extracted from creel through guide, into resin bath, pulled through pre-die profilers to reduce size and then pulled through final die, heated to initiate cross linking and then cut to size
34
Compare the benefits of pultrusion vs wet filament winding
Pultrusion - control of Øf and low void content, good mechanical properties but limited cross-sections available, heated dies are expensive, large amounts of solvent needed WFW - low cost production (mandrel expensive), each part needs to be cleaned thoroughly, poor surface finish
35
Explain the hand layup technique
Apply lubricant to clean mould, apply resin & hardener, place fabric on mould apply more resin & hardener to impregnate fibres, consolidate (removes air between layers) and add them add more layers
36
What are the positives and negatives of the hand layup technique?
+Ve - simple process and good surface finish, wide range of resin & hardeners, low investment cost, complex shapes can be made -Ve - human error in production, safety issues due to resins, low viscosity resins required
37
describe the spray lay up technique with advantages and disadvantages
Lubricate clean mould, spray mixture of chopped fibres and harderner and resin onto mould, impregnate layers and consolidate - then more layers Low cost and simple but mechanical properties reduced due to chopping fibres, product quality suffers, safety concerns (airborne resins etc)
38
What are the Advantages of optical fibres?
``` Data travels at speed of light Large bandwidth Immune to EM Interference Secure data transmission (as any tapping results in lost light) Low data loss (low light attenuation) ```
39
What are the requirements for optical fibres?
Circular cross section Uniform cladding Cladding RI < Core RI Large amounts need to be able to be produced Must be very pure (defects scatter light)
40
Define the refractive index
Speed of light in vacuum/speed of light in medium
41
What factors affect the refractive index of a material?
High polarisability = high RI High density = high RI Low atomic number = low RI Low valence electrons = low RI
42
What is meant by polarisability of materials?
Ease at which electron cloud can be distorted by electric field - high atomic number = large electron clouds = high polarisability - low oxidation number = high polarisability - non bridging O increase polarisability
43
What are multi mode fibres?
Several rays sent down one tube - causes interference and loss of data over long distances Single mode = no interference
44
Does blue or red light travel fast?
Red light as it has a longer wavelength
45
What are the two methods for making smarter materials?
Embedded sensors | Reinforcement as sensor (better as doesn’t add inclusions)
46
What is Bragg’s law
Na sin øa = Nb sinøb | If sin b = 0, sin a =0 but some light is always reflected at an interface
47
When can TIR happen?
When Øa > critical angle | When Na > Nb (core RI > cladding RI)
48
What is the critical angle?
``` Angle at which light is reflected along the interface, above this light is reflected back into material Inverse sin(Nb/Na) ```
49
What are the 3 causes of attenuation?
Absorption - intrinsic or extrinsic Scattering - linear or non linear Bending - micro/macro
50
Explain absorption attenuation
Intrinsic - energy absorbed by electrons and bonds | Extrinsic - contaminants (inclusions/defects) absorb energy
51
Explain scattering attenuation
Linear - Rayleigh (density fluctuation <10%, causes by density flux) and Mie scattering (density fluctuations >10%, causes by defects in core/cladding interface) Non linear - increase input power increases scattering
52
What is bending attenuation
Micro - imperfections along core/cladding interface (processing) Evanescence’s field - standing EM wave set up at core/cladding interface that travels through cladding
53
What is meant by the acceptance angle?
Maximum angle (relative to fibre axis) that light can be transferred into the core

NCM 1 (3 decks)

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