Man-made Composites Flashcards
Lecture 18
What are composites?
Combinations of two or more materials
-> a matrix phase
-> a reinforcement phase
- particles
- short fibres
- long fibres
What is the aim of composites?
To have the best properties of each constituent material with few/none of their disadvantages
What are some examples of fibre-reinforced composites?
These materials are commonly implied by the term composite:
- Glass fibre reinforced polymers (GFRP)
- Carbon fibre reinforced polymers (CFRP)
wood is a natural fibre-reinforced composite
What are some advantages of fibre-reinforced composites?
Still, end up with lightweight but a lot more stiffer
What are particulate composites?
Concretes (aggregates, rebar), particulate composites:
- cement -> high hardness ceramic carbides in ductile material
-> e.g. WC in Co
- used in cutting tools
- WC has high hardness
- Co prevents crack propagation
- Dispersion strengthened metals -> smaller volume of particulate
What are some examples of filled polymers?
- sand
- silica flour
- glass particles
- as noted previously -> filled polymers have increased stiffness and abrasion resistance
-> may also reduce the price
What are some examples of lamellar composites?
- often wood-based
- plywood, glulam etc
What are some specific properties often of interest?
Specific stiffness = Ec/Pc
Specific strength = σc/Pc (strength to weight ratio)
Good specific strength may offest high cost in some applications
What are some composite properties?
fracture toughness (a measure of resistance to fracture) versus Young’s modulus
specific strength versus specific modulus
What are the different fibre types for polymer matrix composites (PMCs)?
Glass
-> brittle reinforcements
- E-glass
- S-glass (high strength and modulus)
Carbon fibres
-> graphitic and amorphous regions
Armid fibres
-> high strength and stiffness (molecules aligned along fibre direction, high tensile moduli and strengths, poorer performance in compression)
-> kevlar
What are some fibre composites?
Fibres can be short (chopped) or long
What are short fibres?
random fibre orientations
easier to manufacture
fibre-glass
What are long fibres?
fibres are orientated in specific directions
laminates
long fibres case is easier to analyse
- Vf = volume fraction of fibres
- Vm = volume fraction of matrix (= 1 - Vf)
- Ef = fibre modulus
- Em = matrix modulus
What is the longitudinal modulus?
Loading parallel to fibre directions
- strain in the fibres and the matrix is equal
This means:
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What is the transverse modulus?
Loading perpendicular to the fibre direction
- stress in their fibres and matric is equal
this is the lower limit for the transverse modulus
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What is K|c ?
K|c is fracture toughness - measures a material’s resistance to fracture
How can properties be made more uniform?
Properties can be made more uniform by using laminates with the fibres orientated in different directions
-> all data normalised by the maximum tensile modulus of the unidirectional case
What is the polymer matrix composite failure mechanism?
brittle fibres
polymer matrix
tensile loading along fibre directions
- multiple fibre cracking (strictly occurs over a range of values)
- final failure mode (matrix failure, fibre pullout) therefore might have a chance to do smth beforehand
transverse strength < matrix strength
- transverse piles will normally crack first
in compression, fibres may start to buckle
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What are short-fibre composites?
Fibres can be random in a plane or 3D
Lower moduli than with continuous fibres
What is glass fibre reinforced concrete (GFRC)
It uses AR (alkali-resistant) glass fibres
- contain a minimum of 16% ZrO2 (ASTM)
- chopped strand (typically 3-5mm long)
- rovings
- scrims (nets of 2 perpendicular sets of rovings)
