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APSC 278 (Engineering Materials) > composites > Flashcards

composites Flashcards

1
Q

What are composites?

A

Materials with more than 1 phase present, each of which
are chemically dissimilar and separated by a distinct interface.

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2
Q

What are the phases of most composite materials?

A

The matrix and the dispersed or reinforcing phase

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3
Q

What are the phases of concrete?

A
  1. Cement
  2. Aggregate
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4
Q

What are the phases of reinforced concrete?

A
  1. Cement + Aggregate
  2. Rebar
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5
Q

What are the phases for eutectoid steel?

A
  1. Ferrite
  2. Cementite
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6
Q

What are the phases for wood?

A
  1. Cellulose fibers
  2. Lignin and hemicellulose
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7
Q

What are the applications of composite materials?

A
  1. Aerospace
  2. Underwater
  3. Transportation
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8
Q

What are the beneficial material properties of composites?

A
  1. Low density
  2. High stiffness
  3. Small coefficient of thermal expansion
  4. Fatigue and impact resistant
  5. Corrosion resistance
  6. High modulus-to-weight ratio
  7. High strength-to-weight ratio
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9
Q

What can the matrix phase consist of?

A
  • metals (Al, Ti)
  • ceramics (Al2O3, ZrO2)
  • polymers (epoxy, polyester, phenolic)
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10
Q

What types of geometries can be in the reinforcing phase?

A
  • fibres
  • particulates
  • whiskers (single crystal with nearly zero defect)
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11
Q

What are the most widely used and studied composite structure?

A

Fibre & polymer matrix composites

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12
Q

What are fiber-based composites?

A

Usually combinations of fibres made
from ceramic, polymer, or glass
embedded in a polymer matrix

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13
Q

What is the typical range of % volume of fiber in fiber-based composites?

A

Typically 40
-60 % fiber by volume

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14
Q

What are the common patterns for fibre-based composites?

A

Characteristic ”weave” pattern (e.g.
carbon fibre), or less organized fibre
patterns with randomized cut fibres
(e.g. fiberglass
-based chairs)

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15
Q

What are the advantages of fiber-based composites?

A

-good combination of
stiffness, density and fracture
toughness.

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16
Q

What are the disadvantages of fiber-based composites?

A

-often expensive
-May be difficult to process
-Often difficult to detect damage (cracks).
-Ideally suited to withstand tension,
not compression.

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17
Q

What do all fiber materials have in common?

A

All fiber materials are high performance (high strength and stiffness)

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18
Q

What are common matrix materials in composites?

A

Polyesters, vinyl esters, polyimides

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19
Q

What is the role of the matrix regarding the separation of fibres and crack propagation of fibres?

A

It separates the fibres and stops cracks from propagating directly
from fibre to fibre

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20
Q

Why must the matrix bind to fibres very well?

A

To protect them from external damage

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21
Q

What does the matrix transmit to the fibres in composites?

A

Transmits external loads to the fibers, while carrying only a small
fraction of the load itself

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22
Q

Describe the density and ductility of matrix in composites.

A

Typically low density, relatively ductile materials

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23
Q

What are the most common fibre materials?:

A
  • glass (fiberglass)
  • carbon
  • Spectra (Polyethylene)
  • Kevlar (aramid, a type of polymer)
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24
Q

How is the high strength of fibres achieved?

A

Due to low probability of flaws in
individual fibres

25
Q

What are the properties specific to carbon fibers?

A

high specific modulus and specific strength
compared to all fibre materials.

26
Q

What type of version are carbon fibres compared to carbon?

A

A semi-crystalline version of
carbon.

27
Q

What does anisotropic mean?

A

mechanical behavior is
direction-dependent

28
Q

What are the two cases to determine the upper and lower bounds of fibre-based composites?

A

Isostrain vs. isostress

29
Q

What does isostrain represent?

A

-The ideal arrangement as the composite structure
has been optimized for loading direction

-Upper bound for overall performance

30
Q

What does isostrain mean in terms of how the fibres are oriented?

A

The fibers oriented parallel to force.

31
Q

How is the stiffness of the overall material determined in isostrain analysis?

A

By the stiffness and
volume fractions of the fibre and matrix.

32
Q

What does isostress mean in relation to the orientation of fibres?

A

All of the fibers are oriented
perpendicular to the applied forces

33
Q

How is the isostress analysis modelled to make it easier to analyze?

A

Instead of considering
individual fibres, we represent this with
layers of matrix and fibre-type material
interspersed with one another

34
Q

Which of the following values are equivalent to each other in isostrain conditions?

A

The strain of the composite = fibre strain = matrix strain

35
Q

Which of the following values are equivalent to each other in isostress conditions?

A

Composite stress = matrix stress = fibre stress

36
Q

How can the fiber performance be maximized?

A

It is desirable to have all
fibers as long as possible, and oriented in the direction of
greatest tension.

37
Q

Why are fibres either “discontinous and aligned” or “randomly oriented”

A

To allow easier
fabrication of the composite.

38
Q

How can fibers be more isotropic?

A

Randomly oriented fibers
also makes the material more isotropic.

39
Q

Compare the ductility of fibre and matrix in a fibre composite

A

-The fibre is to be totally
brittle

-The matrix is to be reasonably
ductile

40
Q

What type of stress response will an aligned fiber composite exhibit?

A

A uniaxial stress-strain response

41
Q

What happens in Stage I of the uniaxial stress-strain response of an aligned fiber composite?

A

Stage I - both fibres and matrix deform
elastically

42
Q

What happens in Stage II of the uniaxial stress-strain response of an aligned fiber composite?

A

Stage II - the matrix starts to yield as the
fibres continue to deform elastically

43
Q

What happens to the load from Stage 1 to Stage II in the stress-strain behaviour of an aligned fiber composite?

A

The proportion of load carried by the fibres
increases

44
Q

What happens to the fractured fibres during the stress-strain response of an aligned fibre composite?

A
  • 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.
45
Q

At what condition is the tensile strength of a fibre determined
predominantly by the fibre tensile
strength?

A

When fibres are oriented in the
direction of the tensile force (0°)

46
Q

When is the weakest link the strength of the matrix for a composite sample?

A

When the sample is oriented with the
fibres perpendicular to tensile axis
(90°)

47
Q

Where do all of the other orientations fall based on the strength of fibre composites?

A

All other orientations fall between
these two extremes

48
Q

What stops cracks from propagating in the composite?

A

The matrix.

49
Q

What is the primary load carrier in composite?

A

Fibers

49
Q

What are the mechanical properties of fiber-reinforced composites influenced by?

A
  • Properties of constituents
  • Fiber orientation
  • Fiber length and volume fraction
50
Q

When does the onset of composite failure begin?

A

When fibres start to fracture.

51
Q

Compare the Young’s Modulus of the aligned composite to the matrix and fiber during a uniaxial stress-strain response.

A

The Young’s modulus of the composite is higher than the matrix and lower than fiber.

52
Q

Compare the ductility of the aligned composite to the matrix and fiber during a uniaxial stress-strain response.

A

The ductility of the composite is lower than the matrix and higher than fiber.

53
Q

Compare the fracture strength of the aligned composite to the matrix and fiber during a uniaxial stress-strain response.

A

The fracture strength of the composite is higher than the matrix and lower than fiber

54
Q

Compare the Young’s Modulus of parallel carbon-fiber composites to perpendicular carbon-fiber composites at the same density.

A

Parallel carbon-fibre composites have a greater Young’s Modulus than perpendicular carbon-fibre composites.

55
Q

Which type of composite analysis represents the best-case scenario?

A

Isostrain analysis

56
Q

Which type of composite analysis represents the worst-case scenario?

A

Isostress analysis

57
Q

What type of composite analysis does a longitudinal modulus of elasticity refer to?

A

Isostrain analysis (fibres oriented parallel to force)

58
Q

What type of composite analysis does a transverse modulus of elasticity refer to?

A

Isostress analysis (fibers oriented perpendicular to force)

APSC 278 (Engineering Materials) (30 decks)

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