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MPI > 1: Physical Principles > Flashcards

1: Physical Principles Flashcards

1
Q

Circular field:

A

The magnetic field surrounding the flow of electric current.

In MPI, current flow in central conductor and has no magnetic poles present

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2
Q

Coercive force:

A

Negative/reverse applied magnetizing force (H) to reduce residual magnetizing force (B) to zero after saturation.

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3
Q

Direct contact magnetization:

A

Use of current passed through part via contact heads or prods to produce magnetic field.

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4
Q

Ferromagnetic:

A

A material that exhibits magnetic hysteresis and saturation, and who’s permeability is dependent on magnetizing force present.

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5
Q

Leakage field:

A

Magnetic field outside a part resulting from presence of discontinuity, change in magnetic permeability or change in parts cross-section.

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6
Q

Longitudinal field:

A

A magnetic field that results in magnetic poles.

Ex. Field that exists in a bar magnetic

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7
Q

Magnetic field:

A

Describes volume within and surrounding either a magnetized part or a current-carrying conductor.

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8
Q

Flux density (B):

A

Strength of magnetic field, expressed in flux lines per unit cross-sectional area.

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9
Q

Flux Lines or Lines of Force:

A

Conceptual representation of magnetic flux that can be illustrated by line pattern produced when iron filings are sprinkled on paper and laid over permanent magnet.

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10
Q

Hysteresis curve:

A

Curve describing behaviour of ferromagnetic materials that result from retentivity and coercive force being greater than zero.

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11
Q

Induced current magnetization:

A

Use of current induced in a part to produce a magnetic field.

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12
Q

Magnetic permeability (“micro symbol”):

A

Ease with which a ferromagnetic part can be magnetized.

Equal to ratio of flux density (B) produced to magnetizing force (H) inducing the magnetic field.

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13
Q

Magnetizing current (l):

A

Flow of either alternating/direct current used to induce magnetism into part being inspected.

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14
Q

Magnetizing force (H):

A

Magnetizing field applied to a ferromagnetic material to induce magnetization.

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15
Q

Residual magnetism:

A

Magnetic field that remains in part when the external magnetizing force has been reduced to zero.

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16
Q

Retentivity:

A

Property of a material to remain magnetized after the magnetizing force has been removed.

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17
Q

Saturation point:

A

Level of magnetism in a ferromagnetic material where magnetic permeability is equal to 1.

Level where an increase in magnetizing force (H) results in no greater increase in magnetic field (B).

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18
Q

Magnetic moment:

A

Strength of magnetic field

Ex. Compass

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19
Q

How do paired electrons spin?

A

In opposite direction which cancel each other out

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20
Q

Which 2 groups will not stay magnetized after magnetic field is removed?

A

Paramagnetic and diamagnetic

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21
Q

Describe paramagnetic:

A

Very weak, positive attraction.

Ex. Platinum, aluminum and manganese

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22
Q

Describe ferromagnetic:

A

High attraction

Ex. Iron, nickel, cobalt

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23
Q

Describe diamagnetic:

A

Very weak, negative attraction.

Ex. Copper, gold, mercury

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24
Q

What is a dipole?

A

Pair of equal and oppositely charged or magnetized poles separate by distance

Ex. North and south

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25
Q

When are magnet domains always present?

A

In ferromagnetic materials

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26
Q

What is the smallest detectable defect we can find?

A

1 mm

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27
Q

A magneto graph may also be referred to as: (3)

A
  1. Magnetic flux
  2. Flux lines
  3. Magnetic flux leakage field
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28
Q

7 characteristics of Magnetic Lines of Force:

A
  1. They never cross (want to repel each other)
  2. Seek path of least magnetic resistance (or reluctance)
  3. Most densely packed at poles of magnet
  4. Orient themselves north to south OUTSIDE magnet
  5. Orient themselves south to north INSIDE magnet
  6. ALWAYS form complete loop*
  7. Never leave/enter centre of magnet
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29
Q

What are the 2 other names for leakage field?

A

Flux leakage and leakage flux

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30
Q

What does a break in continuity cause?

A

Formation of new poles (opposite in polarity)

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31
Q

For successful MPI, what must the direction of magnetizing field do?

A

Cross plane of discontinuity at some angle

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32
Q

What happens when a ferromagnetic material is place in a magnetic field?

A

It will develop magnetic poles

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33
Q

Where do you want the polarity to be strong?

A

Around leakage field*

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34
Q

What is a good defect to specimen comparison while inspecting?

A

Defect 5x deeper than width

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35
Q

What are consequent poles and the 2 other names they could be referred to as?

A

Multiple/Concurrent poles.

When 2+ leakage fields are present simultaneously in magnetized area.

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36
Q

What are the 2 types of magnetic fields?

A

Longitudinal and circular

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37
Q

What is multi-directional magnetization?

A

Changing back and forth between longitudinal and circular

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38
Q

Magnetic fields are generally produced by one of the following (4):

A
  1. Magnetic Induction
  2. Direct Magnetization
  3. Indirect Magnetization
  4. Mechanical Induction
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39
Q

Where is magnetic induction produced?

A

In ferromagnetic material by some outside force (external).

Ex. Ferromagnetic material being brought into close contact with a permanent magnet

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40
Q

Describe indirect magnetization:

A

Running current through something - but not the part we’re inspecting.

Ex. Solenoid (coil) - electrical conductor

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41
Q

What does a coil do in MPI?

A

Turns circular magnetism into longitudinal magnetism.

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42
Q

Describe direct magnetism:

A

Current being passed directly through the part creating a circular field in part.

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43
Q

What is the advantage of a centre conductor?

A

Able to see inside defects as opposed to clamping heads (strength higher on the inside)

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44
Q

Another way to say lines of flux?

A

Lines of attraction

Longitudinal and polar

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45
Q

More permeable allows particles to become more….

A

Dense

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46
Q

What should we do if we don’t know the direction of defects?

A

Put current in at right angles and alternate between longitudinal and circular.

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47
Q

What is the “Right Hand Rule”?

A

Used to determine direction of magnetic field created by current flow.

Created at approx. 90 degrees to direction of current flow.

48
Q

Describe electrical current:

A

Movement of electrons through a conductor.

Measured in amps (group of electrons that move past given point in one second)

49
Q

The opposition to flow of electrons is called what?

A

Resistance

50
Q

What are the best 2 conductors?

A 
#1 Silver
#2 Copper
51
Q

Electrical resistance varies with what 3 things?

A
  1. Cross section of conductor
  2. Length of conductor
  3. Type of material
52
Q

What is electricity used for in MPI?

A

To induce magnetic fields.

Directly: clamping actual part
Ex. Coil or solenoid

53
Q

Where are electrons moving in AC and DC?

A

AC - most movement on surface

DC - flow down centre of conductor

54
Q

3 Phase Full Wave Rectified AC is used to test…

A

Large equipment

55
Q

How does DC move?

A

Not continuous but in skips (half wave)

56
Q

What does more permeability mean for holding onto charge?

A

Less of an ability to hold on to charge.

57
Q

Low permeability requires what type of force to make magnetic?

A

Stronger magnetizing force

58
Q

What is permeability measured in?

A

Henry’s per meter (H/m)

59
Q

What is magnetic saturation?

A

Determines the value of retentivity and measure of residual flux density

60
Q

If you’ve hit saturation point, what happens to retentivity?

A

Retentivity = residual point

*if it hasn’t then it’s just residual

61
Q

The different shapes of hysteresis loops represent what?

A

Different materials

62
Q

What colour range are we most sensitive to in darkened environments?

A

Yellow/green

63
Q

What is the primary goal of MPI?

A

CONTRAST

64
Q

Why is red often used?

A

Good compromise between contrast and sensitivity

65
Q

What is invisible light?

A

Ultraviolet light

66
Q

What type of UV light is used for fluorescent inspections? And what is it’s range?

A

UV-A (black light)

320-400 nm

67
Q

What is the range of UV-B and what is its wave?

A

280-320 nm

Mid wave

68
Q

What is the range of UV-C and what is its wave like?

A

100-280 nm

Short wave - most harmful

69
Q

What is the range of the electromagnetic spectrum?

A

400-700 nm***

70
Q

What can happen if you use too high of current in MPI?

A

Parts can be burned

71
Q

How are magnetic fields produced?

A

By negatively charged electrons orbiting nucleus as they move through space AND any time an electrical charge is in motion

72
Q

What are the smallest known permanent magnetic areas?

A

Magnetic domains (or dipoles) - approx 1 mm

73
Q

What are magnetic domains like in an unmagnetized state?

A

Randomly organized with zero bet magnetism

74
Q

What type of matter has north and south poles?

A

Every molecule of matter

75
Q

What is a simple way to magnetize an unmagnetized ferromagnetic material?

A

Rubbing a magnet against unmagnetized bar (causes domains to align)

76
Q

How are lines of force described?

A

Through a magnetograph

77
Q

What is the measure of force for a single flux line?

A

CGS: Maxwell - Greek letter phi
SI: Weber

78
Q

What unit describes flux density or number of flux lines per unit area?

A

CGS: Gauss (B)
SI: Tesla

79
Q

What does the presence of a leakage do to a uniform field?

A

Causes it to become distorted and extend into air beyond part surface

80
Q

For the best results in MPI, the direction of magnetizing field….

A

Moves at a right angle to suspected discontinuity

81
Q

Why do magnetic particles concentrate at leakage fields?

A

Magnetic attraction

82
Q

What does an air gap look like for permeability?

A

LOW permeability = high magnetic resistance

83
Q

What type of permeability do the particles have?

A

HIGH permeability = low magnetic resistance

84
Q

A magnetic pole is a place where….

A

A magnetic field can be detected exiting/entering material (always in pairs - why it’s called DIpole)

85
Q

What is a dipole?

A

Item with a magnetic pole on one end with a subsequent, opposite pole on other

86
Q

A horseshoe magnet is an example of what type of magnetic field?

A

Longitudinal

87
Q

How are longitudinal magnetic fields produced?

A

INDIRECTLY through use of carrying coil, electromagnetic or use of permanent magnets

88
Q

When multi-directional magnetism is necessary, what should happen with the strengths?

A

Circular and longitudinal fields should be balanced

89
Q

What are Vector/Resultant fields?

A

Combined/swinging field with different direction/intensity than other 2 primary fields

90
Q

If multi-directional magnetism is necessary but they differ in time/intensity…

A

Use Vector or Resultant fields

91
Q

What is the strength of a magnetic field measured in?

A

Amperes per meter (A/m) in SI (H)

CGS: oersted

92
Q

Describe induction:

A

Magnetism produced in a ferromagnetic material by some outside magnetizing force

93
Q

What is a solenoids primary use?

A

Being an electrical conductor

94
Q

What is the principle in indirect magnetism?

A

When an electrical current is passed through a conductive material, a magnetic field is produced at 90 degrees to direction of current flow

95
Q

What is indirect induction reserved for?

A

Inspections of tubes, ring shaped parts and parts containing passageways/hollow sections

96
Q

When indirect induction is performed right, what is the result?

A

Strong circular fields produced on inside as well as outer surface

97
Q

Who discovered MPI and through what type of induction?

A

William Hoke in the early 1900’s (MPI officially born in 1930s) through mechanical induction

98
Q

Describe mechanical induction:

A

A ferromagnetic part becomes magnetized due to directional action of cutters while being mechanically worked (machining, grinding, etc)

99
Q

For greatest sensitivity…

A

Magnetic lines of force should be as close to right angles to defect as possible

100
Q

What is a conductor?

A

Material with free electrons and allows electrical current to flow

101
Q

Electrical current is what?

A

The movement of electrons through a conductor (amps)

102
Q

What is current flow?

A

Unidirectional flow of electrons through a conductor (unit: l)

103
Q

If a material has low resistance, that makes it a ________.

A

Good conductor

104
Q

Describe Ohm’s Law:

A

One volt of electrical pressure (V) will force one ampere of current flow (l) through one Ohm (R) of electrical resistance?

V = I x R

105
Q

Who came up with Ohm’s Law?

A

Georg Simon Ohm, a German physicist & mathematician in 1826

106
Q

An advantage of DC over AC?

A

Able to locate subsurface discontinuities

However, AC can be rectified to do this

107
Q

What type of current is the maximum sensitivity for subsurface defects?

A

1/2 Wave Rectified AC or 1/2 Wave DC

108
Q

What does a hysteresis loop help us understand?

A

How much magnetizing force is needed to magnetize and how much residual will remain after

109
Q

What is the best METHOD to use?

A

Continuous method (magnetizing during distribution) over residual method (applying after force has been removed)

110
Q

As light intensity increases, the sensitivity to which colour spectrum does too?

A

Red/yellow portion

111
Q

As light intensity decreases, sensitivity to which colour spectrum INCREASES?

A

Blue/green portion

112
Q

Wavelengths of energy bellow 400 nm are considered what?

A

Ultraviolet

113
Q

How can you extend a mercury vapour arc lamps working life?

A

Leaving it on for duration of day so there will be little to no fluctuations in line voltage

114
Q

The extended bulb life of an LED black light provides how many extra hours?

A

30,000 hours or more

115
Q

The primary UV generation of a low pressure tube is how many nanometers?

A

Wavelength of approx. 250 nm which activates a phosphorus material then emits a range of 320-440 nm

116
Q

Advantage of a metal halide bulb compared to a standard hand held mercury vapour arc lamp?

A

Capable of producing up to 10x more UV-A intensity

MPI (10 decks)

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