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Chapter 1-9 > Deck 12- Testing materials > Flashcards

Deck 12- Testing materials Flashcards

1
Q

What kind of material is bone?

A

Bone is a composite material.

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2
Q

What are the two substances that make bone?

A
  • Crystals of calicum phosphate which is a brittle ceramic material and which is strong in compression.
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3
Q

What are ceramic materials?

A

Ceramic materials are hard and brittle; they include china and pottery. Also includes modern engineering ceramics such as alumina and silicon carbide.

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4
Q

What are polymers?

A

Plastics such as polythene and polyester and natural materials such as cotton+ leather.

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5
Q

What are composite materials?

A

Composite materials are designed to combine the most desirable properties of two or more different materials.

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6
Q

What sort of forces are brittle materials good at withstanding?

A

Brittle materials are usually stronger in compression rather than tension.

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7
Q

Describe and go through the process of the notch test.

A

1) Cut a notch halfway along the back of the sample material.
2) Raise the hammer to a pre-determined height. This gives it potential energy.
3) Let go of the swinging hammer. Some of the energy is used to break the bone. Note the heigh to whichte hammer rises. This tells you how much potential energy is left. The difference is the energy required to break the sample.

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8
Q

What is the notch test a measure of?

A

It is a measure of the materials toughness because the toughness of a material is a measure of the energy required to create a new surface area.

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9
Q

What are brittle materials?

A

Brittle materials like glass fracture readily. Brittleness is caused by cracks spreading through a material.

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10
Q

What are the three different classes of materials?

A

1) Metal
2) Ceramic
3) Polymer

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11
Q

Give an example of a synthetic composite material.

A

Reinforced concrete.

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12
Q

Are the bricks of a house in compression or tension?

A

In compression, they are being squashed by the forces above and below.

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13
Q

What is a logarithmic scale?

A

When the scale increases by a factor.

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14
Q

When is it best to use logarithmic scales?

A

Logarithmic scale are best used to display quantities that differ by several orders of magnitude.

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15
Q

What produces strain?

A

Stress produces strain.

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16
Q

What is strain?

A

Strain is the fractional increase in length and does not depend on the original length.

Strain = x/L

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17
Q

What is young modulus?

A

A measure of how flexible or stiff a material is.

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18
Q

How do designers use young modulus?

A

Designers calculate how thick a beam or other building component must be to support any load it is likely to bear without bending to much.

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19
Q

What is hooke’s law?

A

The stretching of a spring is proportional to the stretching force you apply.

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20
Q

Draw the stress/strain graph of a material with a constant young modulus.

A
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21
Q

Draw a stress/strain graph for a flexible material anf give an example of a material.

A
  • A polymer
  • A steep graph(coming out from origin) should be plotted because little stress causes a lot of strain.
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22
Q

Draw the stress/strain graph for a stiff material.

A
  • Diamond/steel
  • A graph coming from the origin should be plotted with a shallow gradient, because a lot of stress causes little strain.
23
Q

What is a young modulus a measure of?

A

The young modulus is a property of the material not the specimen.

24
Q

What are the units of young modulus?

A
  • Units of young modulus are MNm-2 or MPa
  • For a stiff material GNm-2or GPa
25
Q

Why are the units of young modulus the same as the units for stress?

A

Because strain is a ratio of two lengths and stress is Nm-2.

26
Q

If strain has no units what represents strain on a graph?

A

It is given as a percentage (e.g extension is 1% of length)

27
Q

Draw the stress/strain graph for glass; draw it up to the point of fracture.

A
28
Q

What is elastic deformation?

A

When the stress on a material is released the material will return to its original shape.

29
Q

What is brittle fracture?

A

When a material breaks due to too much stress.

30
Q

What is plastic deformation?

A

When the stress on a material is removed the material does not return to its original shape.

31
Q

Draw the stress vs strain graph for mild steel.

A
32
Q

What is the main property of polymers?

A

Polymers tend to be elastic; they can stretch and bend.

33
Q

When can polymers not be elastic?

A
  • Some can be brittle such as Perspex
  • However, when heated, perspex becomes bendy.
34
Q

What class of material is usually strongest?

A

Ceramic

35
Q

What are stronger; Pure metals or alloys?

A

Alloys.

36
Q

A concrete block of cross-sectional area 0.01m2 is crushed when a force of 1MN is applied to it. SHow that its breaking stress is :

a) 100MNm-2
b) 100Nmm-2

A

a) 1/0.01 = 100MPa
b) 1x106/(0.01x106) = 100Nmm-2

37
Q

What are superconductors?

A

Conductors of electricity with no resistance at all.

38
Q

Do alloys conduct better or worse than pure metals?

A

Generally conduct less well.

39
Q

What are semiconductors?

A

Conductors of electricity, usually have a conductance of 1. Not very high.

40
Q

How much do insulators conduct?

A

They conduct very little, almost not at all. Usually have values of conductance of about 10-8-10-18.

41
Q

A piece of metal of conductance 1 has a same peice of metal of conductance 1 added on top of it. What is the new conductanceand resistivity of these two pieces of metals?

A

G is proportional to A so conductance is 2.

R is proportional to 1/A so resistance is 0.5 ohms.

42
Q

A block of metal of conductance 1 has another block added to the side of it. What is the conductance and resistance of the combination?

A

G is proportional to 1/L so conductance is 1/2.

R is proportional to L so reistance is 2.

43
Q

What are the units for conductivity?

A

Sm-1

44
Q

What are the units of resistivity?

A

Ohms m

45
Q

Give the units for the following properties:

Mass(of an object)

conductance

resistance

stiffness

Force to deform a metal bar

Force to break(window)

A

Kg

S

ohms

Nm-1

N

N

46
Q

Give the units for the following properties:

density

conductivity

resistivity

Young modulus

Yield stress

Breaking stress

A

Kg m-3

Sm-1

ohm m

Nm-2

Nm-2

Nm-2

47
Q

Why would it be better to have a range of values for which cotton breaks (in F/A) instead of just one value?

A

A thread of cotton breaks at different forces because a break usually starts at a weak point where some fibres are brocken, for this reason, breaking stress is not a well defined property.

48
Q

What is the problem with measuring young modulus?

A

The problem is most materials do not stretch very much.

49
Q

In young modulus experiments what gives the most uncertainty?

A

The largest uncertainty is likely to be in the measurement of the cross section of the wire. This is because cross-sectional area is proportional to the square of the wire diameter, so the percentage uncertainty in the cross-sectional area is double the percentage uncertainty in the diameter.

50
Q

Why is it hard to measure the resistivity of an insulator?

A

For insulators, the problem will be in detecting any electric current at all and if you get a conduction it is more likely to be through moisture of contamination or the surface of the material.

51
Q

What is the problem with measuring the conductivity/resistivity of a really good conductor?

A

For good conductors the problem is getting a specimen long enough and thin enough to have an appreciable resitance.

  • Making the specimen to thin will raise uncertainty.
52
Q

Give any systematic errors that might arise from measuring the resistivity or conductivity of a specimen.

A

For example, electrical contacts to the wire themselves have some resistance, which may make the resistance of the wire seem larger than it really is.

53
Q

With an insulator with high resistance, which is to high to measure any current from it, what can you assume?

A

You can assume the resistivity is smaller than this value.

54
Q
A

Chapter 1-9 (16 decks)

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