2: Constituent Materials Flashcards
List some fibre material requirements for a suitable composite
-High strength and modulus
-Low density
-low cost (and readily available)
-Consistency/repeatable
-Environmental/Chemical resistance
-Compatible with commonly used matrices and manufacturing processes
List some synthetic fibres used in industry
-Glass
-Carbon
-Aramid
-Boron
-HDPE
List some natural fibres used in industry
-Hemp
-Jute
-Flax
-Sisal
-Silk
Describe and explain the Weibull theory for fibres
Weakest link theory - as fibre diameter increases the strength deceases, as the likelihood of a critical flaw within the element increases with volume
What is the role of the matrix within a composite?
To transfer the stress between fibres
List some material properties of industrial carbon fibres
Density: 1.78 g/cm^3
Modulus: 240 GPa
UTS: 4,300 MPa
Describe the synthesis process of glass fibre production
-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)
How does E-glass vary from S-glass?
E-glass: provides electrical resistance, thermal insulation and good mechanical properties
S-glass: provides high strength but also has high costs
Glass fibres are described as isotropic, what does this mean?
Material properties are the same in both the transverse and longitudinal directions, due to the crystalline glass structure
What are some graphitised precursors used for carbon fibre production?
PAN (polyacrylonitrile), Pitch (derived from petroleum/coal) and Lignin (plant based, Ef=40-50GPa)
Carbon fibres are described as anisotropic, What does this mean and why is this behaviour observed?
-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.
How does chosen precursor material affect cost of carbon fibres?
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
Describe the PAN process outline
-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
How do material properties change with varying graphitisation temperature of PAN?
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
What is an estimated modulus of Pitch-based fibres?
~960GPa (4x standard industrial PAN carbon)
What is the estimated strain to failure of PAN?
1.8%
What is the estimated strain to failure of Pitch?
0.5%
What are the thermal expansion properties of Pitch-based fibres?
It possesses a low negative coefficient, and therefore is good for space applications (very expensive ~£1000/kg)
What is the rough size of carbon fibres?
5-8 micrometres
How does cost vary with tow size (K)
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
What affect does tow size have on mechanical properties?
Smaller bundles lead to better mechanical properties, as the laminate structure is more compact
List some properties of aramid fibres (kevlar29 & kevlar49)
-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)
Describe the aramid production process
-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
Common composite fibres include: glass, carbon and aramid. Rank the density of these from highest to lowest
-Glass
-Carbon
-Aramid
Common composite fibres include: glass, carbon and aramid. Rank the tensile modulus and strength of these from highest to lowest
-Carbon
-Aramid
-Glass
Common composite fibres include: glass, carbon and aramid. How does compression and tensile strength vary when loaded axially?
-Glass (compression ~ tensile)
-Carbon (compression < tensile)
-Aramid (compression «_space;tensile)
Common composite fibres include: glass, carbon and aramid. What are the fracture/failure types for the following?
-Carbon (brittle)
-Glass (brittle)
-Aramid (ductile)
Common composite fibres include: glass, carbon and aramid. What are the maximum temperature the fibre (only) can withstand?
-Carbon < 2500°C
-Glass < 250°C
-Aramid < 250°C
List some matrix material requirements for a suitable composite
-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
What are some advantages of thermoset matrices?
-Dimensionally stable (low shrinkage)
-Cost effective
-Low viscosity (easy to wet fibres, better interface)
What are some disadvantages of thermoset matrices?
-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
What are some advantages of thermoplastic matrices?
-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
What are some disadvantages of thermoplastic matrices?
-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
Explain industrial properties of unsaturated polyester as a matrix
-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
Describe the polyester synthesis reaction
-Requires initiator to produce free-radicals
-free radicals provide cross-linking
-Usually organic peroxide catalyst
-Exothermic reaction (temperature dependant rate)
Explain industrial properties of epoxy as a matrix
-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)
Describe the 2-part epoxy synthesis reaction, what are the property differences to 1-part epoxy?
-Low molecular weight
-Liquid
-Brittle
-Trapped bubbles in mix
-Lower cost
-Room temperature storage
Describe the 1-part epoxy synthesis reaction, what are the property differences to 2-part epoxy?
-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)
Explain industrial properties of vinylester as a matrix
-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))
Describe the polyvinyl synthesis reaction
List some properties of amorphous thermoplastics
-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
List some properties of crystalline thermoplastics
-Highly ordered molecular structure
-Sharp melting point (rapid change to low viscosity fluid)
-High toughness
-Anisotropic flow (heat shrinkage transverse to flow direction)
-Opaque
What are common thermoplastic matrix properties and materials used in commodity items?
-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
What are common thermoplastic matrix properties and materials used in engineering items?
-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
What are common thermoplastic matrix properties and materials used in high performance items?
-Semicrystalline
-High purity
-Chemically resistant
-Excellent mechanical and physical properties
-High processing temperatures
-Very expensive (~£200/kg)
-Fire retardant
-Usually PEEK, PEKK and PPS
How can thermoplastic fibres be used to achieve a lower void content?
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
How would the interface layer be described?
An infinitesimally small region between the matrix and fibre
Explain how can the interface layer be reinforced?
-Through the addition of a coupling agent between the fibre and matrix
-Leading to: chemical compatibility, better load transfer and increased environmental resistance
Name and describe the purpose of the following sizing agents: PVA and Silane
PVA - Film former (protects fibres and provides cohesion)
Silane - Coupling agent (increases chemical adhesion)
What is the purpose of an ideal interface layer?
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
How does aspect ratio affect fibre axial stress along fibre position?
-Aspect ratio=fibre length/fibre diameter
-The higher the aspect ratio the greater the axial stress at the midpoint of the fibre
How does aspect ratio affect interfacial shear stress along fibre position?
-Zero shear stress is observed at the centre of the fibre
-Max shear stress is observed at the ends of the fibre
What determines whether the composite fails by tension or pull-out?
-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
The microdroplet test is a commonly used method to determine the interfacial shear stress characteristics. Explain the test procedure
-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
What are some advantages of the microdroplet test?
-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
What are some disadvantages of the microdroplet test?
-The droplet shape influences the precision of the results (must be spherical)
-Large data scatter
-No standardised equipment
What are some by-products of failure from a poor interface layer?
-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
What are some by-products of a strong interface failure?
-Matrix coated fibres at the fracture site
-Mixture of fibre failure and matrix failure
What is the purpose of binder additives and what are some of their properties?
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
What are some properties of non-reactive binder additives?
-Acts like a thermoplastic (reheated and shaped)
-May soften during resin injection (causing washing)
What are some properties of reactive binder additives?
-Fully cross linked at high temperatures (120°C)
-The binder softens at lower temperatures (80°C), allowing preforming
What is a common low density filler applied to composites?
-Glass microspheres
What are some common shrinkage control additives applied to composites?
-PVA
-Polystyrene
-Polyethylene
What are some common fire retardant additives applied to composites?
-Aluminium trihydrate (Al2O3H2O)
-Halogens
What are some common thermal property additives applied to composites?
-Aluminium powder
-Copper powder
What are some common pigment additives applied to composites?
-Cadmium salts
-Carbon black
-Iron oxide
-Titanium dioxide (UV blocker)
-Kaolin
-Organic dyes
What are some common (cost reduction) bulk additives applied to composites?
-Mineral fillers (eg. CaCO3)
-Wood/flour
-Reground scrap
What are some common toughening agent additives applied to composites?
-Rubber particles
-Nano particles
-Graphene
List some material properties of high strength carbon fibres
Density: 1.78 g/cm^3
Modulus: 245 GPa
UTS: 5,100 MPa
List some material properties of high modulus carbon fibres
Density: 1.86 g/cm^3
Modulus: 380 GPa
UTS: 2,700 MPa
List some material properties of A-glass fibres
Density: 2.7 g/cm^3
Modulus: 75 GPa
UTS: 1,700 MPa
List some material properties of E-glass fibres
Density: 2.54 g/cm^3
Modulus: 70 GPa
UTS: 2,200 MPa
List some material properties of S-glass fibres
Density: 2.49 g/cm^3
Modulus: 80 GPa
UTS: 2,600 MPa
List some material properties of Kevlar29 fibres
Density: 1.44 g/cm^3
Modulus: 70.5 GPa
UTS: 3,600 MPa
Failure strain: 3.6%
List some material properties of Kevlar49 fibres
Density: 1.44 g/cm^3
Modulus: 112 GPa
UTS: 3,600 MPa
Failure strain: 2.4%
List some material properties of mild steel
Density: 7.8 g/cm^3
Modulus: 207 GPa
UTS: 650 MPa
List some material properties of Polypropylene
Density: 0.91 g/cm^3
Modulus: 1.5 GPa
Melt temp: 160 degrees
Service Temp: 80 degrees
List some material properties of HDPE
Density: 0.95 g/cm^3
Modulus: 0.8 GPa
Melt temp: 140 degrees
Service Temp: 120 degrees
List some material properties of LDPE
Density: 0.92 g/cm^3
Modulus: 0.2 GPa
Melt temp: 110 degrees
Service Temp: 80 degrees
List some material properties of PA6 (nylon)
Density: 1.14 g/cm^3
Modulus: 2 GPa
UTS: 66 MPa
Melt temp: 220 degrees
Service Temp: 120 degrees
List some material properties of PA12 (nylon)
Density: 1.02 g/cm^3
Modulus: 1.4 GPa
UTS: 55 MPa
Melt temp: 250 degrees
Service Temp: 130 degrees
List some material properties of PA66 (nylon)
Density: 1.17 g/cm^3
Modulus: 3 GPa
UTS: 65 MPa
Melt temp: 265 degrees
Service Temp: 140 degrees
List some material properties of PEI
Density: 1.26 g/cm^3
Modulus: 3.1 GPa
UTS: 101 MPa
Melt temp: 350 degrees
Service Temp: 180 degrees
List some material properties of PEEK
Density: 1.33 g/cm^3
Modulus: 4.5 GPa
UTS: 110 MPa
Melt temp: 343 degrees
Service Temp: 260 degrees
List some material properties of PEKK
Density: 1.28 g/cm^3
Modulus: 3 GPa
UTS: 105 MPa
Melt temp: 350 degrees
Service Temp: 204 degrees
List some material properties of PPS
Density: 1.43 g/cm^3
Modulus: 3.6 GPa
UTS: 87 MPa
Melt temp: 280 degrees
Service Temp: 160 degrees
