Composites Flashcards
What are the two components of a composite?
Composites are made of two component, these are a fibre/filament reinforcement and a matrix.
What is the role of the matrix in the composite?
-Transfer load to the reinforcement
-Binds reinforcement together
-Protects the reinforcement
What is the definition of a composite?
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.
List the properties that can be improved by replacing monolithic materials with composites
Strength, stiffness, weight, fatigue life, corrosion resistance, wear resistance, temperature dependent behaviour and thermal insulation
What is the classification of composites by the type of matrix?
Polymer matrix composites (PMC) - Metal matrix composites (MMC) - Ceramic matrix composites (CMC)
What is the classification of composites by the type of reinforcement?
Particulate - Fibrous → Short fibres → Continuous fibres → Unidirectional- Woven/braided - Laminate
Why do we use fibre reinforced composites (FRC’s)
Continuous fibres are inherently much stiffer and stronger than the same material in bulk form
Why is fibre diameter important?
Smaller diameter fibres provide higher fibre surface areas, spreading the fibre/matrix interfacial loads but come at a greater price
What are the ranges of FVF we expect in a composite?
Square packing: Vf=0.785
Hexagonal packing: Vf=0.907
In reality FVF is typically around 0.6 (lower due to manufacturing)
In what directions are fibres designed to be loaded?
Fibres are designed to be loaded along their length, and not across their width
Are FRC’s isotropic?
No, FRS’s are anisotropic materials. The properties of FRC’s are highly direction dependent on the direction within the material
How are carbon and graphite fibres produced?
They are produces by the controlled oxidation and carbonisation of carbon rich organic precursors (such as PAN polyacrylonitrile)
What are the important characteristics of carbon fibres?
-Bend easily and can be woven into fabric
-Have very low coefficients of thermal expansion
-Are anisotropic
What occurs during the graphitisation process of carbon fibre production?
The variation of the graphitisation process produces either high strength or high modulus fibres.
What are the groupings for carbon and graphite fibres?
-High strength (HS)
-intermediate modulus (IM)
-High modulus (HM)
-Ultra high modulus (UHM)
Are glass fibres isotropic?
Yes, glass fibres are isotropic
What are the main characteristics for E-glass?
-Good strength, stiffness and electrical properties
-Resistant to water leaching
What are the main characteristics for S-glass?
-It has higher strength, stiffness and temperature resistance
Why are glass fibres treated with sizing?
Fibres are treated with a SIZE to minimise damage during manufacturing
What is SIZE?
Size is a coating which both helps protect the glass filaments for processing as well as ensure proper bonging to the resin matrix.
What are aramid fibres?
Aramid is aromatic polyamide, this is most commonly known as kevlar.
How are aramid fibres produced?
They are produced by spinning a solid fibre from a liquid blend
What are the main characteristics for aramid fibres?
-Have high strength, low density, good resistance to impact
How are silicon carbide (SIC) fibres produced?
They are produced by the chemical vapour deposition of silicon and carbon core which turns them into titanium matrix composites (TMC’s)
How are boron fibres produced?
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.
What are the main properties of thermoplastic?
-They can be repeatedly softened by heating and hardened by cooling
-Can be repaired and recycled
What are the main properties of thermosets?
-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
What are the main properties of metal matrices (Al, Ti, Mg, Cu)?
-Usually ductile
-Very reactive
What are the main properties of ceramic matrices?
-Offer much higher use temperature (2200C)
-Very brittle, reinforcement is used to improve fracture toughness
What are the two types of ceramic matrices?
-Glass ceramics
-Technical ceramics
What are the main properties of glass ceramics and how are they produced?
-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
How are technical ceramics reinforced?
They are reinforced with whiskers and particles
What is prepreg?
Prepreg is a matrix preimpregnated unidirectional tape
What are the fabrication procedures for PMC’s?
-Hand lay up
-Vacuum bagging
-Resin transfer moulding (RTM)
-Filament winding
-Pultrusion
Explain the hand lay up procedure
Fibres in the form of fabric are impregnated with resin by hang using brushes and rollers, the resultant composite is cured at atmospheric pressure
What materials can be used for hand lay up?
-Resins: any
-Fibres: any, but heavy aramid fibres may be difficult to wet out
What are the main advantages of hand lay up?
-Wide choice of suppliers
-Simple to teach
-Low cost tooling
-High FVF’s
What are the main disadvantages of hand lay up?
-Skills dependent
-High void content
-Health and safety risk posed by resins
What are the typical applications of hand lay up?
-Wind turbine blades
-Boats
-Architectural mouldings
What is vacuum bagging?
This is an extension of the hand lay up method where pressure is applied to improve consolidation
What materials can be used for vacuum bagging?
-Resins: primarily epoxy
-Fibres: A variety of heavy fabrics can be used
What are the main advantages of vacuum bagging?
-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)
What are the main disadvantages of vacuum bagging?
-Extra labour and material costs
-Skills dependent control of mixing and resin content
What are the typical applications of vacuum bagging?
-Large cruising boats
-Race car components
What is resin transfer moulding (RMT)?
Fabric is pre pressed to the mould shape and held together by a binder, after clamping resin is injected into the cavity.
What materials can be used for RMT?
-Resins: thermoset
-Fibres: any
What are the main advantages of RMT?
-High FVF’s
-Good health and safety
-Both sides have a moulded surface
What are the main disadvantages of RMT?
-Matched tooling is expensive and heavy in order to withstand pressure
-Unimpregnated areas can occur
-Generally limited to smaller components
What are typical applications of RMT?
Small but complex components
What is filament winding?
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
What are the winding patterns of filament winding?
-Helical
-Circumferential
-Polar
What materials can be used for filament winding?
-Resins: any
-Fibres: any
What are the main advantages for filament winding?
-Very fast and economic
-Good control of resin content
-Fibre cost is minimised
-Very good structural properties
What are the main disadvantages for filament winding?
-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
What are the typical applications for filament winding?
Primarily hollow, generally circular or oval sectioned components (tanks, pipes etc)
What is the pultrusion process?
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.
What materials are suitable for pultrusion?
-Resins: generally epoxy, polyester, vinylester
-Fibres: any
What are the main advantages for pultrusion?
-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
What are the main disadvantages for pultrusion?
-High heated die costs
-Constant cross section components only
What are the typical applications for pultrusion?
-Beams
-Columns
-Platforms
-Decking
-Girders
