composites Flashcards
What are composites?
Materials with more than 1 phase present, each of which
are chemically dissimilar and separated by a distinct interface.
What are the phases of most composite materials?
The matrix and the dispersed or reinforcing phase
What are the phases of concrete?
- Cement
- Aggregate
What are the phases of reinforced concrete?
- Cement + Aggregate
- Rebar
What are the phases for eutectoid steel?
- Ferrite
- Cementite
What are the phases for wood?
- Cellulose fibers
- Lignin and hemicellulose
What are the applications of composite materials?
- Aerospace
- Underwater
- Transportation
What are the beneficial material properties of composites?
- Low density
- High stiffness
- Small coefficient of thermal expansion
- Fatigue and impact resistant
- Corrosion resistance
- High modulus-to-weight ratio
- High strength-to-weight ratio
What can the matrix phase consist of?
- metals (Al, Ti)
- ceramics (Al2O3, ZrO2)
- polymers (epoxy, polyester, phenolic)
What types of geometries can be in the reinforcing phase?
- fibres
- particulates
- whiskers (single crystal with nearly zero defect)
What are the most widely used and studied composite structure?
Fibre & polymer matrix composites
What are fiber-based composites?
Usually combinations of fibres made
from ceramic, polymer, or glass
embedded in a polymer matrix
What is the typical range of % volume of fiber in fiber-based composites?
Typically 40
-60 % fiber by volume
What are the common patterns for fibre-based composites?
Characteristic ”weave” pattern (e.g.
carbon fibre), or less organized fibre
patterns with randomized cut fibres
(e.g. fiberglass
-based chairs)
What are the advantages of fiber-based composites?
-good combination of
stiffness, density and fracture
toughness.
What are the disadvantages of fiber-based composites?
-often expensive
-May be difficult to process
-Often difficult to detect damage (cracks).
-Ideally suited to withstand tension,
not compression.
What do all fiber materials have in common?
All fiber materials are high performance (high strength and stiffness)
What are common matrix materials in composites?
Polyesters, vinyl esters, polyimides
What is the role of the matrix regarding the separation of fibres and crack propagation of fibres?
It separates the fibres and stops cracks from propagating directly
from fibre to fibre
Why must the matrix bind to fibres very well?
To protect them from external damage
What does the matrix transmit to the fibres in composites?
Transmits external loads to the fibers, while carrying only a small
fraction of the load itself
Describe the density and ductility of matrix in composites.
Typically low density, relatively ductile materials
What are the most common fibre materials?:
- glass (fiberglass)
- carbon
- Spectra (Polyethylene)
- Kevlar (aramid, a type of polymer)
How is the high strength of fibres achieved?
Due to low probability of flaws in
individual fibres
What are the properties specific to carbon fibers?
high specific modulus and specific strength
compared to all fibre materials.
What type of version are carbon fibres compared to carbon?
A semi-crystalline version of
carbon.
What does anisotropic mean?
mechanical behavior is
direction-dependent
What are the two cases to determine the upper and lower bounds of fibre-based composites?
Isostrain vs. isostress
What does isostrain represent?
-The ideal arrangement as the composite structure
has been optimized for loading direction
-Upper bound for overall performance
What does isostrain mean in terms of how the fibres are oriented?
The fibers oriented parallel to force.
How is the stiffness of the overall material determined in isostrain analysis?
By the stiffness and
volume fractions of the fibre and matrix.
What does isostress mean in relation to the orientation of fibres?
All of the fibers are oriented
perpendicular to the applied forces
How is the isostress analysis modelled to make it easier to analyze?
Instead of considering
individual fibres, we represent this with
layers of matrix and fibre-type material
interspersed with one another
Which of the following values are equivalent to each other in isostrain conditions?
The strain of the composite = fibre strain = matrix strain
Which of the following values are equivalent to each other in isostress conditions?
Composite stress = matrix stress = fibre stress
How can the fiber performance be maximized?
It is desirable to have all
fibers as long as possible, and oriented in the direction of
greatest tension.
Why are fibres either “discontinous and aligned” or “randomly oriented”
To allow easier
fabrication of the composite.
How can fibers be more isotropic?
Randomly oriented fibers
also makes the material more isotropic.
Compare the ductility of fibre and matrix in a fibre composite
-The fibre is to be totally
brittle
-The matrix is to be reasonably
ductile
What type of stress response will an aligned fiber composite exhibit?
A uniaxial stress-strain response
What happens in Stage I of the uniaxial stress-strain response of an aligned fiber composite?
Stage I - both fibres and matrix deform
elastically
What happens in Stage II of the uniaxial stress-strain response of an aligned fiber composite?
Stage II - the matrix starts to yield as the
fibres continue to deform elastically
What happens to the load from Stage 1 to Stage II in the stress-strain behaviour of an aligned fiber composite?
The proportion of load carried by the fibres
increases
What happens to the fractured fibres during the stress-strain response of an aligned fibre composite?
- These fractured fibres, now shorter, are still
embedded within the matrix and are
capable of sustaining a smaller load as the
matrix is plastically deformed.
At what condition is the tensile strength of a fibre determined
predominantly by the fibre tensile
strength?
When fibres are oriented in the
direction of the tensile force (0°)
When is the weakest link the strength of the matrix for a composite sample?
When the sample is oriented with the
fibres perpendicular to tensile axis
(90°)
Where do all of the other orientations fall based on the strength of fibre composites?
All other orientations fall between
these two extremes
What stops cracks from propagating in the composite?
The matrix.
What is the primary load carrier in composite?
Fibers
What are the mechanical properties of fiber-reinforced composites influenced by?
- Properties of constituents
- Fiber orientation
- Fiber length and volume fraction
When does the onset of composite failure begin?
When fibres start to fracture.
Compare the Young’s Modulus of the aligned composite to the matrix and fiber during a uniaxial stress-strain response.
The Young’s modulus of the composite is higher than the matrix and lower than fiber.
Compare the ductility of the aligned composite to the matrix and fiber during a uniaxial stress-strain response.
The ductility of the composite is lower than the matrix and higher than fiber.
Compare the fracture strength of the aligned composite to the matrix and fiber during a uniaxial stress-strain response.
The fracture strength of the composite is higher than the matrix and lower than fiber
Compare the Young’s Modulus of parallel carbon-fiber composites to perpendicular carbon-fiber composites at the same density.
Parallel carbon-fibre composites have a greater Young’s Modulus than perpendicular carbon-fibre composites.
Which type of composite analysis represents the best-case scenario?
Isostrain analysis
Which type of composite analysis represents the worst-case scenario?
Isostress analysis
What type of composite analysis does a longitudinal modulus of elasticity refer to?
Isostrain analysis (fibres oriented parallel to force)
What type of composite analysis does a transverse modulus of elasticity refer to?
Isostress analysis (fibers oriented perpendicular to force)
