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Materials 121 > Chapter 10 - Composites > Flashcards

Chapter 10 - Composites Flashcards

1
Q

What are the three main types of composites?

A
  1. Particle Reinforced Composites
  2. Fibre Reinforced Composites
  3. Structural Laminates and Sandwich Panels
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2
Q

Define “composite”.

A

In general a composite is a combination of materials that exhibits a significant proportion of the properties of both constituent phases, such that a better combination of properties is achieved.

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3
Q

What are the two main components of many composites?

A
  1. The dispersed componed

2. The matrix phase.

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4
Q

What is the dispersed component of a composite?

A

This is usually strong stiff particles or fibres that convey increased strength, stiffness and creep resistance.

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5
Q

What is the matrix phase of a composite?

A

A continuous phase which surrounds the other component in the composite. Typically a polymer or metal. Transmits stress to the dispersed component, protects the dispersed particles/fibres from abrasion and corrosion and finally provides toughness and prevents brittle crack propagation.

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6
Q

What is the modulus of a composite, with oriented continuous fibres? (Isostrain stiffness in the direction of the fibres).

A 
Ec|| = VAEA + VBEB
Ec|| = VAEA + (1 - VA)EB
Ec|| = Young's modulus of the composite parallel to fibres.
VA = Volume fraction of component A.
VB = Volume fraction of component B.
EA = Young's modulus of component A.
EB =Young's modulus of component B.
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7
Q

What is the modulus of a composite, with oriented continuous fibres? (Isostress stiffness in direction perpendicular to the fibres).

A 
1/Ec = VA/EA + VB/EB
1/Ec = VA/EA + (1 - VA)/EB
Ec|| = Young's modulus of the composite perpendicular to fibres.
VA = Volume fraction of component A.
VB = Volume fraction of component B.
EA = Young's modulus of component A.
EB =Young's modulus of component B.
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8
Q

How do you calculate the density of a composite?

A 
Pc = VAPA + VBPB
Pc = Density of the composite.
VA = Volume fraction of component A.
VB = Volume fraction of component B.
EA = Density of component A.
EB = Density of component B.
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9
Q

What is the specific modulus of a composite?

A

An indication of the stiffness of a composite relative to its weight.

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10
Q

How is the specific modulus calculated?

A

Ec/Pc
Ec = Youngs Modulus of composite.
Pc = Density of composite.

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11
Q

What are fibre-reinforced composites?

A

Fibre reinforced composites consist of strong, high modulus fibres embedded in a more ductile lower strength matrix so as to obtain optimum stiffness strength and toughness.

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12
Q

Properties of fibre-reinforced composites are dependant on the fibre _______?

A

Direction.

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13
Q

In many applications it is extremely difficult to make a composite with continuous fibres, how can this be overcome?

A

Discontinuous fibres can still provide a high level of reinforcement provided they exceed the critical length (Lc) and are much easier to fabricate.

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14
Q

How is a load applied to a composite transferred to the discontinuous fibres within it?

A

Through shear forces in the bond between the fibre surface and the matrix. Therefore the strength of this interface bond is extremely important.

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15
Q

How do we achieve optimum fibre reinforcement in a composite?

A
  1. High volume fraction of Fibres.

2. Fibres should be loaded as close as possible to their fracture stress, to make use of their strength.

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16
Q

How do you improve the amount of stress that is transferred to the fibre?

A

Increase the strength of the bonds between the matrix and the fibre.

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17
Q

True/False: For a given interface bond strength we can increase the length of the fibre until the stress in the fibre reaches the fracture stress.

A

True. The weaker the bond is the longer the fibre needs to be and vice versa.

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18
Q

What is the critical fibre length?

A

The critical fibre length is the length of the fibre at which, with a given interface bond strength, the stress in the fibre reaches the fracture stress of the fibre.

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19
Q

What happens if the fibre is longer than the critical fibre length?

A

The stress in the fibre becomes constant in the central region due to the isostrain condition (the strain in the fibre = the strain in the matrix, so there is no shear stress).

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20
Q

True/False: In order to make the best use of the fibres the fibre length should be larger than 10x it’s critical length.

A

True. Because the stress does not increase beyond the critical stress the longer the fibre is the better.

21
Q

Why do we use very small diameter fibres in composites?

A
  1. Square Cube Law -> More Interface Area -> More bond strength.
  2. It is possible to make small fibres that are defect free, and therefore have extremely high strength.
  3. A design rule states that the length:diameter ratio should be at least 50:1
22
Q

How is the shear bond strength calculated?

A

Force = Tau x Interface Area
A fibre is embedded in a matrix.
Force = The force required to pull the fibre from the matrix.
Interface Area = The area of the interface between the fibre and matrix.
Tau = The interface bond strength.

23
Q

What are the three regions of a composite deformation graph?

A

Region 1 - Both Phases (Fibre and Matrix) deform elastically, composite response is linear.
Region 2 - Matrix plastically deforms, fibres continue to stretch elastically. Composite response is still linear but lower slope.
Region 3 - Composite failure begins when fibres begin to fracture at critical strain.

24
Q

True/False: Composite fracture is catastrophic.

A

False. Not all the fibres fracture at the same time, the matrix is still intact and is actually supported by the remaining fibre fragments.

25
Q

True/False: Stress needed for failure in tension is generally greater than for failure in compression for composites.

A

True.

26
Q

Define Toughness.

A

Toughness is the energy absorbed before fracture occurs.

27
Q

How can toughness be measured?

A

Toughness can be measured by the amount of energy required to produce a unit area of crack.

28
Q

Why do composites have such a significantly larger toughness than the sum of its constituents?

A

Because a large amount of energy is consumed by fibre debonding and pullout.

29
Q

Wood is a _____, and has extremely ______ properties.

A

Wood is a composite, and has extremely anisotropic properties (different properties in different directions),

30
Q

Why is wood graded?

A

Because it is a natural product we cannot specify a particular set of mechanical properties (like with metals).

31
Q

If wood is a composite then what makes up the fibres and what is the matrix?

A

The long tubelike cells made of cellulose form the wood fibres, and are in a matrix made of lignin.

32
Q

Wood is both a fibre reinforced composite and a _______ composite.

A

Laminated.

33
Q

What are the two main directions of the wood fibres?

A

Parallel to the grain (longitudinal direction).

Perpendicular to the grain (both radial and tangential directions).

34
Q

How does a piece of wood fail if compression is applied parallel to the grain? (Defect free)

A

The grains will begin to buckle. As one grain buckles it makes it more likely that the one next to it will buckle in the same place, so the result is a co-ordinated buckling along a compression crease. This crease is at 45* due to the shear stresses involved.

35
Q

How does a piece of wood fail if tension is applied parallel to the grain? (Defect free)

A

The fibres begin to fracture, resulting in a jagged edge.

36
Q

How does a piece of wood fail if compression is applied perpendicular to the grain? (Defect free)

A

The fibres compress. Flattening out, as the cells do not offer much resistance to crushing.

37
Q

How does a piece of wood fail if tension is applied perpendicular to the grain? (Defect free)

A

The lignin matrix between the fibres begins to fail, allowing the fibres to pull apart.

38
Q

True/False: Knots in wood have a greater effect in tension than compression.

A

True. Compressive stresses can be transferred through defects like knots.

39
Q

What effect does the presence of knots have on the tensile strength of wood?

A
  1. Reduce cross sectional area supporting the load.
  2. Act as stress raisers (same as a notch).
  3. Distortion of the grain, around the knot, generates stress perpendicular to the grain.
40
Q

How is the moisture content of wood calculated?

A

Moisture Content = (Weight of wet wood - Weight of oven dried wood) / Weight of oven dried wood (x100%)

41
Q

What is free water?

A

Water that is inside the cell/fibre.

42
Q

What is bound water?

A

Water that is located within the cell/fibre walls.

43
Q

What kind of water evaporates first during drying?

A

Free water.

44
Q

What is the fibre saturation point?

A

The point at which all the free water has evaporated and bound water begins to evaporate instead.

45
Q

During drying wood shrinkage only occurs?

A

Beyond the fibre saturation point. Bound water evaporation causes shrinkage, free water evaporation does not.

46
Q

What is the moisture content of Green wood?

A

Above 19%.

47
Q

What is the moisture content of dry wood?

A

Below 19%.

48
Q

True/False: The strength of timber increases with decreasing moisture content.

A

True, though the gains after you dry to 19% moisture content are minimal.

Materials 121 (8 decks)

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