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electrical engineering save > deck_5704902 > Flashcards

deck_5704902 Flashcards

1
Q

How does the charge of the electron change?

A

Electrons furthest from the nucleus (higher energy level) have the least negative charge, making they easier to remove.

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2
Q

How does the number of electrons stored in a shell increase?

A

2n^2 is the number of electrons stored and n the no. of the shell

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3
Q

Define molecule

A

Smallest physical unit of an element or compound, consisting of one or more like atoms in an element and two or more different atoms in a compound.

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4
Q

Give examples of primary bonds.

A

Ionic, covalent, metallic.

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5
Q

Give examples of secondary bonds

A

Van der Waals, Hydrogen, diopole-dipole

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6
Q

Give examples of ionic materials

A

Magnesium oxideSodium chloride

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7
Q

What are some of the properties of ionic materials?

A

High melting point. The ions can’t move so they insulate electricity

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8
Q

What is the equation that gives the force of attraction between ions?

A

F= q^2 4pir^2*ε

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9
Q

Describe the transfer of energy when ionic bonds are formed

A

Energy has to be put in to bring them close together.When they get close enough they pull them selves together and produce energy.

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10
Q

What is packing efficiency?

A

(Number of atoms per unit cell * volume of one atom) / ( volume of unit cell)

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11
Q

What is a body centered arrangement of atoms?

A

There is an atom in the middle and quarters in each corner.

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12
Q

What is a face centered arrangement of atoms?

A

Each face has half an atom and each corner a quarter.

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13
Q

What is the definition of the elastic modulus?

A

Resistance to deformation

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14
Q

What will experience pure stress?

A

An element in a thin walled tube that is being rotated.

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15
Q

Define strain

A

The amount a material is deformed relative to the original dimensions.

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16
Q

What is the difference between a brittle and a ductile material?

A

Ductile have large tensile strains before the point of rupture. Brittle have small strains up to the point of rupture.0.05 of strain is taken as the divide.

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17
Q

What is the elastic limit?

A

Limit beyond which the material will not go back to the original shape when the force is removed.

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18
Q

What is the yield point?

A

Point at which the material will have an appreciable elongation without a large increase in load.Stress at this point is yield strength.

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19
Q

What is the modulus of resilience?

A

Work done on a unit volume as the force is increased up to the elastic limit. It is a measure of the ability to absorb energy without creating a permanent distortion.

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20
Q

What is the modulus of toughness?

A

Area under the entire stress/strain curve. The ability to absorb energy without causing it to break.

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21
Q

What is BCC?

A

Body centered cubic. Atom is the centre and an eighth in each corner. A total of 2 in the unit.

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22
Q

What is FCC?

A

Face centered cubic. Half an atom in each face and an eighth in each corner. Total of 4 in a unit.Co-ordination number of 12

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23
Q

What is HCP?

A

Hexagonal close packed. The atoms in one layer position themselves in the empty spaces in adjacent layers.Co-ordination number =12

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24
Q

What is the relative ductility of the atomic arrangements?

A

FCC > BCC >HCP

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25
Q

What is the relative strength of the atomic arrangements?

A

HCP > BCC >FCC

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26
Q

What is the glass transition temperature?

A

Temperature below which an amorphous polymer is brittle like glass.

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27
Q

How do you find the proof strength?

A

Take a line parallel to the elastic loading line at 0.2% strain and where it intersects the stress-strain curve is the 0.2% proof stress

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28
Q

What is SC?

A

Simple cubic structure. An eighth of an atom in each corner of the unit. Rare due to poor packing density.

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29
Q

How does grain size control change strength?

A

Smaller the grain size the greater the fracture toughnessControlled during processing (casting, rolling, extrusion, heat treatment etc). Stronger as there is less space to move and more interactions between grains.

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30
Q

Solid solution strengthening

A

Add other metal atoms of different sizes. They will disrupt the structure and gives areas of tension and compression and make dislocation harder.

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31
Q

Phase balance control

A

The different phases can have different properties hence by changing the phase balance can change the strength level

32
Q

Precipitation strengthening

A

The distribution of large numbers of fine precipitates prevents dislocations moving past them. The applied stress has to push the dislocation line past the precipitates, which are pinning it.Achieved by introducing an impureity at high temperture. Introduce oxide to metal powder, compact and sinter

33
Q

Work hardening

A

Work hardening increases dislocation densities by orders of magnitude. The greater the dislocation density the closer dislocations are to each other – the more difficult is dislocation movement. FCC metals work harden more than BCC and HCP metalsRolling or shot-peening.

34
Q

Why are tensile tests not used to test ceramics?

A

They are brittle soDifficult to prepare samples with required geometryDifficult to grip samples without fracturing them

35
Q

Why is three point bending not used with ceramics?

A

The load is magnified at one point causing crushing and not a clean fracture.4 point is used.

36
Q

What is the effect of porosity on strength?

A

The smaller the porosity the better.

37
Q

What is the effect of temperature on a material?

A

a decrease in the elastic modulusa reduction in tensile strengthan increase in ductilityPolymers are more sensitive due to a lower melting point.

38
Q

What is the effect of strain rate?

A

Strain rate refers to how fast you deform the material, e.g. how fast you pull the sample in a tensile test.In general decreasing the strain rate has the same effect as increasing the temperature on the strength and ductility.

39
Q

Define hardness

A

Hardness is a measure of a material’s resistance to localised plastic deformation (e.g. a small dent or scratch).

40
Q

How might you measure hardness?

A

With a small indenter of fixed shape (eg cone or pyramid) which is forced into the surface of a material under controlled stress and rate of loading. Depth of indent (permanent plastic deformation) measured to give a hardness number which can be related to yield stress.

41
Q

What is toughness?

A

Toughness is a measure of the amount of energy absorbed by a material as it fractures.

42
Q

Why is a notch put in a material for fracture toughness?

A

To ensure no plastic deformation that would absorb energy.

43
Q

What is the DBTT

A

Ductile brittle transition temperture.Temperature below which a specimen changes from tough (ductile) yielding to brittle fracture.Not all materials have one

44
Q

What is the fracture thoughness test?

A

(K1C) test - sample with fatigue crack is tested in 3 point or 4 point bend at low strain rate

45
Q

What is the fracture toughness equation?

A
46
Q

If a crack in entirelywithin a material, what is crack length?

A

2a

47
Q

What equation can you use to find the stress in a pipe?

A

stress=(Pressure*radius)/ wall thickness

48
Q

How can you reduce fatigue?

A

High sy, UTS gives good fatigue limits. Therefore surface harden component, e.g. crankshafts in carsReduce applied tensile stresses either by design changes or put inResidual compressive stress e.g. reinforced concrete for constructionImprove surface finish e.g. polished hip implantsAvoid stress concentrations e.g. don’t use square corners but rounded ones on designRefine microstructure e.g. use a material with a smaller grain size

49
Q

What is fatigue?

A

The process of slow crack growth due to cyclic loading is fatigue

50
Q

What is the fatigue limit?

A

Fatigue limit - stress below this value does not cause any crack growthMaterials that do not show a fatigue limit show crack growth at all cyclic stresses.Ferrous alloys have one but aluminium and copper don’t.

51
Q

What are the important paris equations?

A
52
Q

What happens when two surfaces meet?

A

Only asperities touch meaning contact area is less than measured.

53
Q

What is the wear rate?

A

Volume lost / distance gone

54
Q

What is specific wear rate?

A

The height worn away= specific wear rate*distance gone

55
Q

Adhesive wear

A

When points fuse together and then are sheared off.

56
Q

Abrasive wear

A

–Asperities of the harder materials, or embedded hard particles, plough through the softer one, grinding off wear particles

57
Q

Define tensile strength.

A

Maximum stress on a stress-strain curve, i.e. the maximum stress a structure can withstand in tension

58
Q

Define fracture toughness

A

Fracture toughness: Critical stress for fracture of a cracked specimen of particular geometry in a 3 or 4 pt test at low strain rate.

59
Q

What is the grain size?

A

Region of separate crystal structure in a polycrystalline material, separated from others with grain boundaries.

60
Q

Define stress concentrations

A

Sharp change of section shape or hole or other geometrical feature which causes local increase in stress.

61
Q

What is wrong with hammering rivets into things?

A

Creates local stress concentrations and small cracks leading to brittle fracture.

62
Q

Why is proof strength not used with polymers?

A

Polymer are often not linearly elastic AND more importantly, their failure strains are far higher plus more variability between polymers.

63
Q

Why is quenched steel harder?

A

High temperature fcc form of iron is able to retain MORE carbon even though the packing factor (0.74) is higher than for bcc iron (0.68) – because of the shape of the spaces between the iron atoms in the crystal lattice. When the iron cools slowly from high temperature DIFFUSION of carbon out of the crystal takes place and fcc changes back to bcc. When iron is quenched the crystal change is not smooth and produces a new structure which is harder to deform (martensite). No time for diffusion.

64
Q

Explain briefly the available strengthening mechanisms in metals.

A

Grain size strengthening – smaller grain size increases strength (graph may be given) Solid solution strengthening – atoms of different size produce strain in crystal lattice Precipitate strengthening – small particles, due to alloying, present in material Work hardening – plastic deformation increases strength
Phase balance strengthening – presence of different crystals structures (e.g. a and b brass, ferrite & pearlite etc).

65
Q

When using the paris equation what are the limits used for integration?

A

Top one=final crack lengthBottom one=initialcrack length

66
Q

In the paris equation, what isΔδ?

A

The cyclic stress. Use units of mPa so don’t multiply by 106

67
Q

What is the paris graph?

A

Remember ΔK ≈ Δδ√(πa)

68
Q

What is high cycle fatigue?

A

a)High cycle fatigue Stresses are below the yield strength of the material ≥ 104 cycles to failure examples: rotating or vibratingsystems - wheels, axles, engine componentsThe most common

69
Q

What is low cycle fatigue?

A

Stresses are above yield strength but below tensile strength of the material ≤ 104 cycles to failure

70
Q

What are the close packed structures?

A

FCC and HCP

71
Q

What is a yield drop?

A

Common on low carbon steels. When the stress strain curve falls little after the elastic region. This gives an upper yield point and a lower yield point.

72
Q

Why do ceramics have a lower tensile strength than compressive strength?

A
73
Q

With fatigue, what does crack driving force depend on?

A

the crack driving force depends on the crack length and the applied cyclic stress

74
Q

How is an SN curve produced and for what purpose?

A

A smooth sample is tested at a set stress amplitude and the no. of the cycles to falure is measured.It is done again with a different sample at a different stress amplitude.Used to determine the fatigue limit (if there is one) and measure the fatigue life.

75
Q

What affects toughness?

A

Presence of a notch ( or other defects)Strain rateEnvironmental conditions e.g. temperature, chemicals

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