1. Magnetic Particle Inspection TWI

2. Magnetism
Some natural materials strongly attract pieces of iron to themselves.
Such materials were first discovered in the ancient Greek city of Magnesia.
Magnets were utilised in navigation.
Oersted found a link between electricity and magnetism.
Faraday proved that electrical and magnetic energy could be interchanged.

3. Magnetic Particle Inspection (MT or MPI)
MT is a test method for the detection of surface and near surface defects in ferromagnetic materials.
Magnetic field induced in component
Defects disrupt the magnetic flux causing “flux leakage”.
Flux leakage can be detected by applying ferromagnetic particles

4. Permeability (μ)
Permeability can be defined as the relative ease with which a material may be magnetised.
It is defined as the ratio of the flux density (B) produced within a material under the influence of an applied field to the applied field strength (H)
μ =B/H

5. Permeability (μ)
On the basis of their permeability materials can be divided into 3 groups:
Diamagnetic
Paramagnetic
Ferromagnetic

6. Permeability (μ)
Diamagnetic: Permeability slightly below 1, weakly repelled by magnets.
Examples: Gold, Copper, Water
Paramagnetic: Permeability slightly greater than 1, weakly attracted by magnets.
Examples: Aluminium, Tungsten
Ferromagnetic: Very high permeability, strongly attracted by magnets.
Examples: Iron, Cobalt, Nickel

7. Domain theory Unmagnetized state Domains randomly orientated
A domain is a minute internal magnet
Each domain comprises 1015 to 1020 atoms
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
Unmagnetized state Domains randomly orientated

8. Domain Theory
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
Magnetized state Domains orientated in external magnetic field
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S

9. Domain Theory
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
Saturated state All domains orientated in strong external field
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S
N-S

10. N-S
N-S
N-S
N-S
N-S
N-S
Un-magnetised
N-S
N-S
N-S
N-S
N-S
N-S

11. Un-magnetised Magnetised N-S N-S N-S N-S N-S N-S N-S N-S N-S N-S N-S

12. Un-magnetised Magnetised Saturated N-S N-S N-S N-S N-S N-S N-S N-S N-S

13. Un-magnetised Magnetised Saturated Residual N-S N-S N-S N-S N-S N-S

14. Lines of Flux N S

15. Lines of flux
By convention they flow from North to South outside and South to North inside
They form closed loops
They never cross
They follow path of least resistance
Flux density is the number of lines of flux passing through a unit area.
Field strength is highest where where flux density is highest.

16. Electromagnetism + _ Direction of current flow
A current flows through a conductor and sets up a magnetic field around it
Field is at 90o to the direction of the electrical current
Direction of current flow + _
Direction of magnetic field

17. Right Hand Rule
Current
Field
Right Hand Rule + _

18. Coil Magnetisation _ + N S Changes circular field into longitudinal
Increases the strength of the field

19. Hysteresis
Place an un-magnetised piece of ferromagnetic material within a coil
Saturation point B+
Virgin curve
H +
H - B-

20. Hysteresis
B +
Residual magnetism
H -
H +
H +
B -

21. Hysteresis B+
H -
H +
H +
Coercive force
B -

22. Hysteresis
B +
H -
H +
H +
Negative saturation
point
B -

23. Hysteresis A B C F E D

24. Hysteresis
Hard ferromagnetic
Soft ferromagnetic

25. Permeability (µ) The ease with which a material can be magnetised
Opposite of reluctance (difficulty with which a material can be magnetised)
µ = B / H
Permeability of free space = µo
Relative Permeability (µr) = µ / µo

26. Relative Permeability (µr)
Paramagnetics Slightly > 1
Diamagnetics Slightly < 1
Ferromagnetics

27. Hard v Soft Ferromagnetics
Soft Hard
Typically Low carbon steel
High permeability
Easy to magnetise
Low residual magnetism
Typically high carbon steel
Lower permeability
More difficult to magnetise
High levels of residual magnetism

28. Definitions Magnetic field Region in which magnetic forces exist
Flux Total number of lines existing in a magnetic circuit
Flux Density Magnetic flux per unit area (measured in Tesla)

29. Principle of MPI : Flux Leakage
No Defect
Defect N S N S
Lines of flux follow the path of least resistance

30. LEAKAGE FIELDS
MAGNETIC SATURATION

31. Visibility of Flux Leakage
Depends on:
Depth of defect
Orientation of defect shape of defect
Size of defect
Permeability of material
Applied Field Strength
Contrast

32. Indications
Relevant Indications - Indications due to discontinuities or flaws
Non-Relevant Indications - Indications due to flux leakage from design features
Spurious Indications - Indications due incorrect inspection procedures

33. Defect Orientation N S
Defect at 90 degrees to flux : maximum indication

34. Defect Orientation N S
>30 Degrees to Flux: Acceptable indication

35. Defect Orientation N S
<30 Degrees to Flux : Weak indication

36. Defect Orientation N S Field N S Field Test 1 Test 2
MPI requires 2 tests at 90o to one another

37. Equipment

38. Permanent Magnet N S Longitudinal field between poles
Maximum sensitivity for defects orientated at 90º to a line drawn between poles N S

39. Permanent Magnet Advantages No power supply
No electrical contact problems
Inexpensive
No damage to test piece
Lightweight
Disadvantages
Direct field only
Deteriorate over time
No control over field strength
Poles attract detecting media
Tiring to use

40. Electromagnetism + _ Direction of current flow
A current flows through a conductor and sets up a magnetic field around it
Field is at 90o to the direction of the electrical current
Direction of current flow _ +
Direction of magnetic field

41. Coil Magnetisation _ + N S Changes circular field into longitudinal
Increases the strength of the field

42. Electromagnets
Maximum sensitivity for defects orientated at 90º to a line drawn between the poles
Magnetic Field
Induced in Core
by Electric Current
Passed Through Coil
Soft Iron
Laminated Core
Adjustable Legs
& Pole Pieces

43. Electromagnets Advantages AC,DC or rectified
Controllable field strength
No harm to test piece
Can be used to demagnetise
Easily removed
Disadvantages
Power supply required
Longitudinal field only
Electrical hazard
Poles attract particles
Legs must have area contact

44. Prods Current passed between 2 contacts.
Defects detected parallel to contacts
Field
Current
Defects

45. PROD METHOD

46. Prods Advantages AC,DC or rectified Controllable field strength
No poles attract particles
Control of amperage
Disadvantages
Arcing / damage to work piece
Transformer required
Current can be switched on without creating field
Good contact required
2 man operation

47. Flexible Cable Flexible, current carrying cable Used as Adjacent cable
Threading cable
Flexible coil

48. Flexible Cable Disadvantages Advantages
Difficult to keep cables in place
High currents required
Transformer required
Advantages
Simple to operate
No danger of burning
AC,DC or rectified
Current adjustable

49. Current Flow Current passed through sample Defects Current
Circular Field
Defects

50. Threading Bar
Current passed through brass bar placed between heads of bench unit
Circular field generated around bar
Sample hung from bar

51. Magnetic Flow
Magnetism passed through sample
Defects
Magnetism

52. Coil Magnetisation Changes circular field into longitudinal
Increases the strength of the field

53. Rigid Coil
Current passed through coil to generate a longitudinal field
Defects
Magnetism

54. MPI Equipment Portable Permanent magnet Electromagnet Prods
Flexible coil
Flexible cable
Clamps and leeches
Fixed
Current flow
Magnetic flow
Threader Bar
Rigid coil
Induced current

55. The Swinging Field Method
Uses two magnetic fields at 90º to each other applied alternately.

56. Testing Values
Page 64

57. Current Types Direct current (DC) Alternating current (AC)
Half wave rectified current (HWDC or HWRAC)
Full wave rectified (FWDC or FWRAC)

58. Direct Current + - Disadvantages No agitation Advantages
Less sensitive to surface defects
Advantages
Sub-surface defects
Availability from batteries

59. Alternating Current Disadvantages Advantages
Will not detect sub-surface defects
Advantages
Availability
Sensitivity to surface defects
Agitation of particles
Demagnetisation

60. Half Wave Rectified Current
Advantages
Penetration like DC
Agitation
Ease of production
High flux density for less power
Disadvantages
Sensitivity to surface defects lower than AC

61. Full Wave Rectified Current
Advantages
Penetration like DC
Agitation
Disadvantages
Sensitivity to surface defects lower than AC

62. 3 - PHASE FW RECTIFIED 2 1

63. RMS 16 12 8 4 -4

64. Direct Current: Field distribution

65. A.C. : Field distribution
SKIN EFFECT

66. SKIN EFFECT AC FIELD DC FIELD In order to achieve the same
LEAKAGE
DC FIELD
NO LEAKAGE
In order to achieve the same
sensitivity to shallow defects
a DC field must be far more powerful than a corresponding
AC field

67. Permanent Magnet and DC Electromagnet
Use the Lift Test
For pole spacing from 75 to 150mm kg N S

68. AC Electromagnets N / S Use the Lift Test
For pole spacing no more than 300mm kg N S /

69. PROD METHOD
Current passed through sample, typically: 5 Amps (rms) per mm of prod spacing

70. Flexible Coil
I = 3H(T + Y2 / 4T) for DC
I = 3H(10 + Y2 / 40) for AC Y

71. Adjacent Cable D D Defects located parallel to cable I = 4 d H
Return cable separated by 10d D D

72. Current Flow Defects Current Circular Field
Current passed through sample, typically:
I = H  diameter or
I = H x perimeter
For D/d = 1.5 or less, one shot only req’d
Defects
Current
Circular Field

73. Threading Bar I = H x perimeter
Increase the current (I) to increase R, the radius of the test zone. R R

74. Component placed within field and rotated for complete coverage
Threading Bar
Component placed within field and rotated for complete coverage

75. Field strength can be assessed using a “flux indicator”.
Magnetic Flow
Magnetism passed through sample
Defects
Magnetism
Field strength can be assessed using a “flux indicator”.

76. 0.4 H K 0.4 H K NI = N = L / D L / D x I Rigid Coil
N = Number of turns in coil
K = 32,000 for DC (typical)
22,000 for AC or FWR (typical)
11,000 for HWR (typical)
L/D = Length / Diameter

77. Rigid Coil Conditions
Cross section of test piece <10% of Coil (the fill factor)
Test piece must lie against side or bottom
The test zone is the part of the component which lies within the coil
L / D must be between 5 and 20

78. (Do not use with permanent magnets or DC electromagnets.)
FLUX INDICATORS
Check for adequate flux density and correct orientation with Flux Indicators.
(Do not use with permanent magnets or DC electromagnets.)

79. FLUX INDICATORS - COMMON TYPES
ASME
BERTHOLD PENETRAMETER
BURMAH CASTROL STRIPS

80. ASME V MAGNETIC FLUX INDICATOR
CONSISTS OF 8 STEEL PIE SEGMENTS BRAZED TOGETHER WITH COPPER FACEPLATE

81. ASME V MAGNETIC FLUX INDICATOR
STRONG
WELL DEFINED
INDICATION
AT 90 DEG TO FLUX
WEAKER
SLIGHTLY FUZZY
INDICATIONS
OBLIQUE TO FLUX
NO INDICATION
PARALLEL TO FLUX

82. Detecting Media

83. Dry Magnetic Particles
Iron powder or magnetic iron oxide (magnetite).
microns, rounded and elongated shapes
Colours vary for contrast against component
Can be used on hot surfaces
Poor particle mobility, HWDC best, DC or permanent magnets must never be used
Greater operator skill required
Difficult to apply to overhead surfaces especially in field conditions
Generally less sensitive than wet particles

84. Wet Magnetic Particles
Magnetic iron oxide (magnetite) or iron powder
microns rounded and elongated shapes
Colour contrast or fluorescent
Water or kerosene based
Concentration important
Good particle mobility
Easier to use
More sensitive

85. Detecting Media Magnetic Rubber
Fluorescence may degrade under UV(A), when exposed to acid and high temperatures

86. Demagnetisation

87. Check for removal with Field strength meter (magnetometer)
Demagnetisation
Removal of residual magnetisation
Required for:
Aircraft parts
Rotating parts
Components to be welded,machined or electroplated
Check for removal with Field strength meter (magnetometer)

88. How to Demagnetise? Flux
A constantly reversing and reducing magnetic field
Flux

89. Methods of Demagnetisation
Aperture type coil reversing stepped DC
Aperture type coil reducing AC
AC or reversing DC aperture type coil, withdraw component along the coil axis
AC electromagnet
Heating to above the Curie point (about 770C for steel)

90. MPI Practices

91. Test Methods
Continuous or Residual
Fluorescent or Visible
Wet or Dry

92. Continuous or Residual?
Continuous Method
Detecting media applied immediately prior to & during magnetisation.
Residual
Detecting media used after the applied field has been removed.
Requires high retentivity.
Less sensitive than continuous.
Useful for components like ball bearings

93. No special lighting required Higher concentration of particles
Fluorescent or Visible?
Visible
No special lighting required
Higher concentration of particles
Background paint may be required
Fluorescent
Detecting media dye coated
More sensitive
Less tiring for operators
Better for batch inspections

94. NB All surface defects form indications
But not all indications are caused by defects
Relevant indications…Linear 3:1
Non-relevant indications
Due to flux leakage but arising from design features
Changes in section
Changes in permeability
Grain boundaries
Forging flow lines
Spurious indications
Not due to flux leakage
Lint
Scale
Dirt
Hairs
Magnetic writing

95. Control and Maintenance Checks
To ensure equipment,ancillaries and materials are up to standard
Ink
Lighting conditions
Magnetising units

96. Control and Maintenance Checks
0.5
1.0
2.0
3.0
4.0
100
100 ml
1.0 ml
0.5 ml
Ink settlement

97. Ink Settlement Test Fluorescent Ink 0.1 - 0.3 % Non-Fluorescent Ink%

98. Control and Maintenance Checks
Ink settlement
Fluorescent ink check
Equipment performance check

99. Equipment Performance Checks
Current flow test piece
Magnetic flow test piece
Cracked component

100. Equipment Performance Checks
Steel Bar
Through Drilled Hole
Indication
Magnetic Flow Test Piece

101. Equipment Performance Checks
Aluminium Rod
Plastic Bush
Mild Steel
Disc
Through
Drilled
Holes
Current Flow Test Piece

102. Control and Maintenance Checks
Ink settlement
Fluorescent ink check
Equipment performance check
Viewing efficiency
Magnetising unit
Unit tank levels
Unit ammeters
Demagnetiser

103. Control Check Frequency
Settlement test Daily
Fluorescent intensity Weekly
Test piece Daily
Viewing efficiency Monthly

104. Maintenance Check Frequency
Magnetising units Weekly
Tank levels Daily
UV lamp Monthly
Ammeters 6 monthly
Demagnetiser 6 monthly

105. UV(A)

106. Electromagnetic Spectrum
X-rays & Gamma
Electric Waves
Microwaves
Ultra violet
Infra red TV
Light cm
Wavelength

107. A Damaged Black Light UV-B&C UV-C UV-B UV-A
Electromagnetic Spectrum
A Damaged Black Light UV-B&C
UV-C
UV-B
UV-A
ULTRAVIOLET VISIBLE
LIGHT LIGHT

108. Fluorescence Precautions Avoid looking directly at the lamp
UV-A Source : Mercury vapour arc lamp +
Filter
Precautions
Avoid looking directly at the lamp
Do not use if filter is cracked, damaged or incorrectly fitted

109. Fluorescence and the Electromagnetic Spectrum
Absorbs
Emits
ULTRAVIOLET VISIBLE
LIGHT LIGHT

110. Fluorescent v Colour Contrast
Fluorescent methods are more sensitive.
Less operator fatigue with fluorescent.
Background lacquer is not required.
Fluorescent properties will degrade if exposed to UV light, acids, alkalis or high temperature.
Background fluorescence is a problem on rough surfaces.
Some oils will produce strong background fluorescence.
Low background light levels are required.

111. Fluorescent v Colour Contrast Fluorescent Particles
Black Particles
Fluorescent Particles