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