L1: Intro to Components and Properties Flashcards
What must a composite’s constituents have?
- Reasonable presence
- Different properties
- Chemically distinct phases on a microscopic scale, separated by a distinct interface
What is a composite?
A composition of two or more materials (reinforcement and matrix phase) to achieve enhanced properties
What are the properties of a composite a function of?
The properties of constituents, their relative amount, and the geometry of the dispersed phase
Give an overview of the matrix phase (4 key points)
- Continuous
- Often presents in more quantity
- Usually lower stiffness/strength
- Ceramic, metallic or polymeric
Give an overview of the dispersed phase (3 key points)
- Usually harder, stronger and stiffer than matrix
- Small in at least one direction
- Fibres, particles or structural
What are cermets?
Ceramic-metal composites
What are cermets used for? Why?
Cutting tools - hard and resistant against heat (metal would fracture if not used with matrix)
Give the components of concrete and the reason they are used
Portland cement + sand + gravel + water
Different particle sizes allows for dense packing
Why is concrete reinforced (with steel rods etc)?
It is weak in tension
What gives the upper and lower limit of a composite’s elastic modulus?
Upper: rule of mixtures
Lower: inverse rule of mixtures
Why are fibre composites the most technologically important?
They achieve high specific strength and stiffness
How do the stiffness and strength of carbon fibre vary directionally?
Very good along fibre direction
Poor in transverse direction
What are the three key fibre parameters?
- Fibre volume fraction
- Length
- Orientation
How is longitudinal loading modelled?
How is strain distributed?
Parallel springs
Same strain in fibres and matrix
Give the formula for composite longitudinal modulus
E(cl) = E(f)v(f) + E(m)v(m)
How is transverse loading modelled?
How is stress distributed?
Series springs
Stress acts equally on fibre and matrix
Give the formula for composite transverse modulus
E(ct) = E(f)E(m) / ( E(m)v(f) + E(f)*v(m) )
How does fibre length relate to composite mechanical properties?
Shorter fibres contribute less in sharing load, so composites with shorter fibres have lower properties
Describe stress variation across a fibre
Stress is zero at the ends
Gradually increases towards centre
Becomes equal to global extension in the specimen
What effect does the stress variation across a fibre have on fibre breakage?
Fibres never break at the ends as stress is higher in the middle
What is critical fibre length?
Min fibre length to achieve a max stress equal to fibre’s strength
What happens if fibre length is lower than critical fibre length?
It is not possible to break fibres
Load increase leads to fibre pull-out
Give the equation for critical fibre length
l(c) = sigma(f)*d / 2*tau Where sigma(f) is the fibre strength, d is its diameter, and tau is the matrix shear strength
Describe the effectiveness of fibre length
As fibre length increases the fibre becomes more effective as average stress taken by the fibres increases
How long must fibres be to classify as continuous and discontinuous fibres?
Continuous: l»_space; 15 l(c)
Discontinuous: l = l(c)
Why are fibres orthotropic?
Their stiffness is orientation dependant - aligned fibres provide higher stiffness than randomly oriented ones
Why are composite laminates used?
Single layers of uni-directional fibres are weak in transverse direction -> layers with different fibre angles stacked and bonded give strength/stiffness in all directions
Describe sandwich panels
Two stiff and strong outer faces bonded to lightweight core - core separates faces from neutral axis
Gives lightweight beams/panels with high flexural stiffness and strength
