Composites Flashcards
What are the three most common forms of fibreglass? What are their properties?
E-Glass (electrical): This has good strength and stiffness aswell as good insulation and weathering properties.
C-Glass (chemical): This has very good all round chemical resistance. It is more expensive than E-glass and has lower strength.
S-Glass (strength): Again, more expensive than E-glass but higher strength and stiffness.
Describe the glass manufacturing process. What temperatures are applied? What is the purpose of water mist?
The manufacturing process involves the heating of a materials feedstock to approx. 1370°C, followed by successive cooling (1340°C - 1260°C) until it is extracted due to gravity towards a winding drum.
A water mist is applied to the extracted glass to improve is cooling rate and ensure solidification.
What is the diameter of the produced glass fibres and what controls this?
Diameter is typically 6-15μm and is largely controlled by:
- The head of the glass (pressure).
- Viscosity (temperature/composition).
- Winding speed.
Within carbon fibre, what is the structure of graphitic carbon, describing the two types of layers?
Within a plane, the atoms are held together by strong covalent bonds, but between the planes are weak Van der Waals bonds.
This makes the material highly ANISOTROPIC.
In a single crystal graphite structure, the atoms are in hexagonal arrays and are stacked in ABAB planes, meaning layer B is slighty misaligned with layer A.
Describe the other form of carbon and how its crystalline structure is different from graphitic.
A very common form is TURBOSTATIC carbon which structure has a large number of very small crystals (crystallites).
Turbostatic carbon is similar to graphite but there is no regular stacking (no ABAB) structure.
The average interlayer spacing in single crystal graphite is 0.335nm and in turbostatic graphite it is 0.344nm.
During fabrication, the sheet becomes entangled, randomly folded, tilted and crumpled.
What process can be used in the manufacturing of carbon fibres?
The pitch, the PAN (polyacrylonitrile) and the pre-preg method can be applied in their manufacture.
What is the trade name of aromatic polyamide fibres? What is the composition of these fibres and what contributes to their strength?
The best known trade name is Kevlar. They are based on the high strength and stiffness that is possible in fully aligned polymers.
Chain alignment (crystallinity) and extension can occur during drawing, stretching and spinning.
Consists of long molecular chains from 5 to 1 million Mers bonded together. 85% is of amide linkages (-CO-NH-) attached directly to 2 aromatic rings.
What is a matrix material fibre and what are some of its properties?
This is a fibre structure encased by a matrix material.
It binds the fibres and transmits loads to the fibre matrix.
The matrix should be more ductile, the fibre should have a higher Young’s modulus.
The matrix protects the fibres from surface damage (cracks). It prevents cracks from propagating from one fibre to the next which could cause catastrophic failure.
What are the properties of thermoset resins and thermoplastic resins?
Thermoset: Relative to polyester, epoxy has/is;
higher strength and elastic properties;
lower shirkage on curing;
lower thermal expansion coefficient;
matrix-fibre interface bond strength;
higher pre-cure viscosity;
more expensive.
Thermoplastic: A wide range of properties are possible depending on the chemistry, molecular weight and processing history. Thermoplastics can usually undergo large deformations before fracture but are susceptible to creep.
Unlike thermosets, thermoplastics are not cross-linked.
Strength and stiffness are largely delivered from:
the properties of the monomer units;
molecular entanglements;
the degree of crystallinity (order and alignment).
May lead to anisotropy (different properties in different directions).
Describe what a continuous fibre system is.
The greatest improvement in strength and stiffness is achieved with unidirectional, continuous fibre.
The strength improvement is only in the direction of fibre alignment.
Under unidirectional loading, what is the relationship between the forces and the strains within the matrix and the fibres?
If the composite is loaded but in equilibrium, equations will be:
Fcl = Ff + Fm
ε = εf = εm
Under traverse loading, what is the relationship between stress and the strains within the matrix and the fibre?
σcl = σm = σf = σ
Vm is the volume fraction of the matrix
εct = εmVm = εfVf
What are short fibre composites and how does fibre influence their properties?
They are composite materials which contain broken up, shorter fibres within their composition.
The maximum strength the fibre can undergo is influenced by its length. Up to a critical length the stress applied increases however after a critical size the strain cannot increase anymore.
