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engineering materials > Composites > Flashcards

Composites Flashcards

1
Q

What are the two components of a composite?

A

Composites are made of two component, these are a fibre/filament reinforcement and a matrix.

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2
Q

What is the role of the matrix in the composite?

A

-Transfer load to the reinforcement
-Binds reinforcement together
-Protects the reinforcement

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3
Q

What is the definition of a composite?

A

Composites are heterogeneous material systems in which individual constituents retain their characteristics but are incorporated into a single structure to give a new material possessing superior properties to the constituent materials acting independently.

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4
Q

List the properties that can be improved by replacing monolithic materials with composites

A

Strength, stiffness, weight, fatigue life, corrosion resistance, wear resistance, temperature dependent behaviour and thermal insulation

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5
Q

What is the classification of composites by the type of matrix?

A

Polymer matrix composites (PMC) - Metal matrix composites (MMC) - Ceramic matrix composites (CMC)

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6
Q

What is the classification of composites by the type of reinforcement?

A

Particulate - Fibrous → Short fibres → Continuous fibres → Unidirectional- Woven/braided - Laminate

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7
Q

Why do we use fibre reinforced composites (FRC’s)

A

Continuous fibres are inherently much stiffer and stronger than the same material in bulk form

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8
Q

Why is fibre diameter important?

A

Smaller diameter fibres provide higher fibre surface areas, spreading the fibre/matrix interfacial loads but come at a greater price

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9
Q

What are the ranges of FVF we expect in a composite?

A

Square packing: Vf=0.785
Hexagonal packing: Vf=0.907
In reality FVF is typically around 0.6 (lower due to manufacturing)

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10
Q

In what directions are fibres designed to be loaded?

A

Fibres are designed to be loaded along their length, and not across their width

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11
Q

Are FRC’s isotropic?

A

No, FRS’s are anisotropic materials. The properties of FRC’s are highly direction dependent on the direction within the material

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12
Q

How are carbon and graphite fibres produced?

A

They are produces by the controlled oxidation and carbonisation of carbon rich organic precursors (such as PAN polyacrylonitrile)

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13
Q

What are the important characteristics of carbon fibres?

A

-Bend easily and can be woven into fabric
-Have very low coefficients of thermal expansion
-Are anisotropic

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14
Q

What occurs during the graphitisation process of carbon fibre production?

A

The variation of the graphitisation process produces either high strength or high modulus fibres.

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15
Q

What are the groupings for carbon and graphite fibres?

A

-High strength (HS)
-intermediate modulus (IM)
-High modulus (HM)
-Ultra high modulus (UHM)

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16
Q

Are glass fibres isotropic?

A

Yes, glass fibres are isotropic

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17
Q

What are the main characteristics for E-glass?

A

-Good strength, stiffness and electrical properties
-Resistant to water leaching

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18
Q

What are the main characteristics for S-glass?

A

-It has higher strength, stiffness and temperature resistance

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19
Q

Why are glass fibres treated with sizing?

A

Fibres are treated with a SIZE to minimise damage during manufacturing

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20
Q

What is SIZE?

A

Size is a coating which both helps protect the glass filaments for processing as well as ensure proper bonging to the resin matrix.

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21
Q

What are aramid fibres?

A

Aramid is aromatic polyamide, this is most commonly known as kevlar.

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22
Q

How are aramid fibres produced?

A

They are produced by spinning a solid fibre from a liquid blend

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23
Q

What are the main characteristics for aramid fibres?

A

-Have high strength, low density, good resistance to impact

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24
Q

How are silicon carbide (SIC) fibres produced?

A

They are produced by the chemical vapour deposition of silicon and carbon core which turns them into titanium matrix composites (TMC’s)

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25
Q

How are boron fibres produced?

A

Produced by the vapour deposition of boron on a tungsten wire and coating the boron with a thin layer of born carbide. They cannot be bent or woven into fabric.

26
Q

What are the main properties of thermoplastic?

A

-They can be repeatedly softened by heating and hardened by cooling
-Can be repaired and recycled

27
Q

What are the main properties of thermosets?

A

-When heated, they form molecular networks
-They degrade when heated but do not melt
-Cannot be changed in form once they are set during curing
-Cannot be reshaped by heating

28
Q

What are the main properties of metal matrices (Al, Ti, Mg, Cu)?

A

-Usually ductile
-Very reactive

29
Q

What are the main properties of ceramic matrices?

A

-Offer much higher use temperature (2200C)
-Very brittle, reinforcement is used to improve fracture toughness

30
Q

What are the two types of ceramic matrices?

A

-Glass ceramics
-Technical ceramics

31
Q

What are the main properties of glass ceramics and how are they produced?

A

-Formed when glass is treated by heat and undergoes controlled crystallisation
-It is a composite itself, made of glass and crystalline ceramics
-Has superior mechanical properties to the parent glass and has easy fabrication

32
Q

How are technical ceramics reinforced?

A

They are reinforced with whiskers and particles

33
Q

What is prepreg?

A

Prepreg is a matrix preimpregnated unidirectional tape

34
Q

What are the fabrication procedures for PMC’s?

A

-Hand lay up
-Vacuum bagging
-Resin transfer moulding (RTM)
-Filament winding
-Pultrusion

35
Q

Explain the hand lay up procedure

A

Fibres in the form of fabric are impregnated with resin by hang using brushes and rollers, the resultant composite is cured at atmospheric pressure

36
Q

What materials can be used for hand lay up?

A

-Resins: any
-Fibres: any, but heavy aramid fibres may be difficult to wet out

37
Q

What are the main advantages of hand lay up?

A

-Wide choice of suppliers
-Simple to teach
-Low cost tooling
-High FVF’s

38
Q

What are the main disadvantages of hand lay up?

A

-Skills dependent
-High void content
-Health and safety risk posed by resins

39
Q

What are the typical applications of hand lay up?

A

-Wind turbine blades
-Boats
-Architectural mouldings

40
Q

What is vacuum bagging?

A

This is an extension of the hand lay up method where pressure is applied to improve consolidation

41
Q

What materials can be used for vacuum bagging?

A

-Resins: primarily epoxy
-Fibres: A variety of heavy fabrics can be used

42
Q

What are the main advantages of vacuum bagging?

A

-Higher FVFs (than with hand lay-up)
-Lower void content
-Better fibre wet-out (due to pressure)
-Better health and safety (the vacuum bag reduces the amount of hazardous emissions)

43
Q

What are the main disadvantages of vacuum bagging?

A

-Extra labour and material costs
-Skills dependent control of mixing and resin content

44
Q

What are the typical applications of vacuum bagging?

A

-Large cruising boats
-Race car components

45
Q

What is resin transfer moulding (RMT)?

A

Fabric is pre pressed to the mould shape and held together by a binder, after clamping resin is injected into the cavity.

46
Q

What materials can be used for RMT?

A

-Resins: thermoset
-Fibres: any

47
Q

What are the main advantages of RMT?

A

-High FVF’s
-Good health and safety
-Both sides have a moulded surface

48
Q

What are the main disadvantages of RMT?

A

-Matched tooling is expensive and heavy in order to withstand pressure
-Unimpregnated areas can occur
-Generally limited to smaller components

49
Q

What are typical applications of RMT?

A

Small but complex components

50
Q

What is filament winding?

A

Fibre tows are passed through a resin bath before being wound onto a mandrel. Fibre orientation is controlled by the fibre feeding mechanism and rate of rotation of the mandrel

51
Q

What are the winding patterns of filament winding?

A

-Helical
-Circumferential
-Polar

52
Q

What materials can be used for filament winding?

A

-Resins: any
-Fibres: any

53
Q

What are the main advantages for filament winding?

A

-Very fast and economic
-Good control of resin content
-Fibre cost is minimised
-Very good structural properties

54
Q

What are the main disadvantages for filament winding?

A

-High mandrel costs for large components
-Difficult to lay fibres along the length of a component
-Convex shaped components only
-Resins of low viscosity must be used

55
Q

What are the typical applications for filament winding?

A

Primarily hollow, generally circular or oval sectioned components (tanks, pipes etc)

56
Q

What is the pultrusion process?

A

Liquid resin mixture and reinforcing fibres are pulled through a heated forming die using a continuous pulling device. The hardening of the resin in initiated by the heat from the die.

57
Q

What materials are suitable for pultrusion?

A

-Resins: generally epoxy, polyester, vinylester
-Fibres: any

58
Q

What are the main advantages for pultrusion?

A

-Very fast and economic
-Very good control of resin
-Fibre cost is minimised (taken from a creel)
-Very good structural properties
-Limited emissions due to enclosed resin impregnation area

59
Q

What are the main disadvantages for pultrusion?

A

-High heated die costs
-Constant cross section components only

60
Q

What are the typical applications for pultrusion?

A

-Beams
-Columns
-Platforms
-Decking
-Girders

engineering materials (8 decks)

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